Origins of Modern Humans:

Multiregional or Out of Africa?

Around 30,000 years ago humans were anatomically and behaviorally similar throughout the world.

One of the most hotly debated issues in paleoanthropology (the study of human origins) focuses on the origins of modern humans, Homo sapiens.9,10,3,6,13,15,14 Roughly 100,000 years ago, the Old World was occupied by a morphologically diverse group of hominids. In Africa and the Middle East there was Homo sapiens; in Asia, Homo erectus; and in Europe, Homo neanderthalensis. However, by 30,000 years ago this taxonomic diversity vanished and humans everywhere had evolved into the anatomically and behaviorally modern form. The nature of this transformation is the focus of great deliberation between two schools of thought: one that stresses multiregional continuity and the other that suggests a single origin for modern humans.

Multiregional theory: Homo erectus left Africa 2 mya to become Homo sapiens in different parts of the world.

Understanding the issue

The Multiregional Continuity Model15 contends that after Homo erectus left Africa and dispersed into other portions of the Old World, regional populations slowly evolved into modern humans. This model contains the following components:

· some level of gene flow between geographically separated populations prevented speciation, after the dispersal

· All living humans derive from the species Homo erectus that left Africa nearly two million-years-ago

· Natural selection in regional populations, ever since their original dispersal, is responsible for the regional variants (sometimes called races) we see today the emergence of Homo sapiens was not restricted to any one area, but was a phenomenon that occurred throughout the entire geographic range where humans lived.

Out of Africa theory:
Homo sapiens arose in Africa and migrated to other parts of the world to replace other hominid species, including Homo erectus

In contrast, the Out of Africa Model13 asserts that modern humans evolved relatively recently in Africa, migrated into Eurasia and replaced all populations which had descended from Homo erectus. Critical to this model are the following tenets:

· after Homo erectus migrated out of Africa the different populations became reproductively isolated, evolving independently, and in some cases like the Neanderthals, into separate species

· Homo sapiens arose in one place, probably Africa (geographically this includes the Middle East)

· Homo sapiens ultimately migrated out of Africa and replaced all other human populations, without interbreeding

· Modern human variation is a relatively recent phenomenon
The multiregional view posits that genes from all human populations of the Old World flowed between different regions and by mixing together, contributed to what we see today as fully modern humans. The replacement hypothesis suggests that the genes in fully modern humans all came out of Africa. As these peoples migrated they replaced all other human populations with little or no interbreeding. To understand this controversy, the anatomical, archaeological, and genetic evidence needs to be evaluated.

Anatomical evidence

Sometime prior to 1 million years ago early hominids, sometimes referred to as Homo ergaster, exited Africa and dispersed into other parts of the Old World. Living in disparate geographical areas their morphology became diversified through the processes of genetic drift and natural selection.

· In Asia these hominids evolved into Peking Man and Java Man, collectively referred to as Homo erectus.

· In Europe and western Asia they evolved into the Neanderthals.
Neanderthals lived in quasi isolation in Europe during a long, relatively cool period that even included glaciations. Neanderthals are distinguished by a unique set of anatomical features, including:

· A large, long, low cranial vault with a well-developed double-arched browridge

· A massive facial skeleton with a very projecting mid-face, backward sloping cheeks, and large nasal aperture, with large nasal sinuses

· An oddly shaped occipital region of the skull with a bulge or bun

· Molars with enlarged pulp chambers, and large, often very heavily worn incisors

· A mandible lacking a chin and possessing a large gap behind the last molar

· A massive thorax, and relatively short forearms and lower legs

· Although short in stature they possessed robustly built skeletons with thick walled limb bones long clavicles and very wide scapulas.

Homo sapiens is a separate species from Neanderthals and other hominids

By 130,000 years ago, following a prolonged period of independent evolution in Europe, Neanderthals were so anatomically distinct that they are best classified as a separate species -- Homo neanderthalensis. This is a classic example of geographic isolation leading to a speciation event. In contrast, at roughly the same time, in Africa, a body plans essentially like our own had appeared. While these early Homo sapiens were anatomically modern they were not behaviorally modern. It is significant that modern anatomy evolved prior to modern behavior. These early sapiens were characterized by:

· a cranial vault with a vertical forehead, rounded occipital and reduced brow ridge
· a reduced facial skeleton lacking a projecting mid-face
· a lower jaw sporting a chin
· a more modern, less robustly built skeleton.

Hence, the anatomical and paleogeographic evidence suggests that Neanderthals and early modern humans had been isolated from one another and were evolving separately into two distinct species.

Homo sapiens exhibited technological skills around 50,000 years ago.

Archaeological evidence

Very interestingly, while Neanderthals and early Homo sapiens were distinguished from one another by a suite of obvious anatomical features, archaeologically they were very similar. Hominids of the Middle Stone Age of Africa (H. sapiens) and their contemporary Middle Paleolithic Neanderthals of Europe had artifact assemblages characterized as follows:

· little variation in stone tool types, with a preponderance of flake tools that are difficult to sort into discrete categories
· over long periods of time and wide geographical distances there was general similarity in tool kits
· a virtual lack of tools fashioned out of bone, antler or ivory
· burials lacked grave goods and signs of ritual or ceremony
· hunting was usually limited to less dangerous species and evidence for fishing is absent
· population densities were apparently low
· no evidence of living structures exist and fireplaces are rudimentary
· evidence for art or decoration is also lacking

The archaeological picture changed dramatically around 40-50,000 years ago with the appearance of behaviorally modern humans. This was an abrupt and dramatic change in subsistence patterns, tools and symbolic expression. The stunning change in cultural adaptation was not merely a quantitative one, but one that represented a significant departure from all earlier human behavior, reflecting a major qualitative transformation. It was literally a "creative explosion" which exhibited the "technological ingenuity, social formations, and ideological complexity of historic hunter-gatherers."7 This human revolution is precisely what made us who we are today.

The appearance of fully modern behavior apparently occurred in Africa earlier than anywhere else in the Old World, but spread very quickly, due to population movements into other geographical regions. The Upper Paleolithic lifestyle, as it was called, was based essentially on hunting and gathering. So successful was this cultural adaptation that until roughly 11,000 years ago, hominids worldwide were subsisting essentially as hunter-gatherers.
In the Upper Paleolithic of Eurasia, or the Late Stone Age as it is called in Africa, the archaeological signature stands in strong contrast to that of the Middle Paleolithic/Middle Stone Age. It was characterized by significant innovation:

·a remarkable diversity in stone tool types
·tool types showed significant change over time and space
·artifacts were regularly fashioned out of bone, antler and ivory, in addition to stone
·stone artifacts were made primarily on blades and were easily classified into discrete categories, presumably reflecting specialized use
·burials were accompanied by ritual or ceremony and contained a rich diversity of grave goods
·living structures and well-designed fireplaces were constructed
·hunting of dangerous animal species and fishing occurred regularly
higher population densities
·abundant and elaborate art as well as items of personal adornment were widespread
·raw materials such as flint and shells were traded over some distances

Homo sapiens of the Upper Paleolithic/Late Stone Age was quintessentially modern in appearance and behavior. Precisely how this transformation occurred is not well understood, but it apparently was restricted to Homo sapiens and did not occur in Neanderthals. Some archaeologists invoke a behavioral explanation for the change. For example, Soffer11 suggests that changes in social relations, such as development of the nuclear family, played a key role in bringing about the transformation.

Social or biological changes may account for "smarter" hominids

Klein7, on the other hand, proffers the notion that it was probably a biological change brought about by mutations that played the key role in the emergence of behaviorally modern humans. His biologically based explanation implies that a major neural reorganization of the brain resulted in a significant enhancement in the manner in which the brain processed information. This is a difficult hypothesis to test since brains do not fossilize. But it is significant that no changes are seen in the shape of the skulls between earlier and later Homo sapiens. It can only be surmised from the archaeological record, which contains abundant evidence for ritual and art, that these Upper Paleolithic/Late Stone Age peoples possessed language abilities equivalent to our own. For many anthropologists this represents the final evolutionary leap to full modernity.

Shortly after fully modern humans entered Europe, roughly 40,000 years ago, the Neanderthals began a fairly rapid decline, culminating in their disappearance roughly 30,000 years ago. Neanderthals were apparently no match for the technologically advanced fully modern humans who invaded Europe and evidence for interbreeding of these two types of hominids is equivocal.

Africans display higher genetic variation than other populations, supporting the idea that they were the first modern humans.

Genetic evidence

Investigation of the patterns of genetic variation in modern human populations supports the view that the origin of Homo sapiens is the result of a recent event that is consistent with the Out of Africa Model.

· Studies of contemporary DNA, especially mitochondrial DNA (mtDNA) which occurs only in the cellular organelles called mitochondria, reveal that humans are astonishingly homogeneous, with relatively little genetic variation.1,5

· The high degree of similarity between human populations stands in strong contrast to the condition seen in our closest living relatives, the chimpanzees.2 In fact, there is significantly more genetic variation between two individual chimpanzees drawn from the same population than there is between two humans drawn randomly from a single population. Furthermore, genetic variation between populations of chimpanzees is enormously greater than differences between European, Asian and African human populations.

· In support of an African origin for Homo sapiens the work of Cann and Wilson1 has demonstrated that the highest level of genetic variation in mtDNA occurs in African populations. This implies that Homo sapiens arose first in Africa and has therefore had a longer period of time to accumulate genetic diversity. Using the genetic distance between African populations and others as a measure of time, they furthermore suggested that Homo sapiens arose between 100,000 and 400,000 years ago in Africa.

· The low amount of genetic variation in modern human populations suggests that our origins may reflect a relatively small founding population for Homo sapiens. Analysis of mtDNA by Rogers and Harpending12 supports the view that a small population of Homo sapiens, numbering perhaps only 10,000 to 50,000 people, left Africa somewhere between 50,000 and 100,000 years ago.

· Scientists recently succeeded in extracting DNA from several Neanderthal skeletons.8 After careful analysis of particularly the mtDNA, but now also some nuclear DNA, it is apparent that Neanderthal DNA is very distinct from our own. In assessing the degree of difference between DNA in Neanderthals and modern humans, the authors suggest that these two lineages have been separated for more than 400,000 years.
Although in its infancy, such genetic studies support the view that Neanderthals did not interbreed with Homo sapiens who migrated into Europe. It is, therefore, highly likely that modern humans do not carry Neanderthal genes in their DNA.

Neanderthals and modern humans coexisted in some parts of the world for thousands of years. Neanderthals probably did not breed with modern humans but they borrowed some of their tools and skills.

Additional considerations

The chronology in the Middle East does not support the Multiregional Model where Neanderthals and anatomically modern humans overlapped for a long period of time.

· Cave sites in Israel, most notably Qafzeh and Skhul date to nearly 100,000 years and contain skeletons of anatomically modern humans. Furthermore, Neanderthal remains are known from sites such as the 110,000-year-old Tabun cave, which predates the earliest Homo sapiens by about 10,000 years in the region.

· The presence of Neanderthals at two other caves in Israel, Amud and Kebara, dated to roughly 55,000 years means that Neanderthals and Homo sapiens overlapped in this region for at least 55,000 years. Therefore, if Homo sapiens were in this region for some 55,000 years prior to the disappearance of the Neanderthals, there is no reason to assume that Neanderthals evolved into modern humans.

· Archaeological evidence from Europe suggests that Neanderthals may have survived in the Iberian Peninsula until perhaps as recently as 30,000 to 35,000 years ago. Fully modern humans first appear in Europe at around 35,000-40,000 years ago, bringing with them an Upper Paleolithic tool tradition referred to as the Aurignacian. Hence, Neanderthals and fully modern humans may have overlapped for as much as 10,000 years in Europe. Again, with fully modern humans on the scene, it is not necessary to have Neanderthals evolve into modern humans, further bolstering the view that humans replaced Neanderthals.

· The situation in southern France is, however, not quite as clear. Here, at several sites, dating to roughly 40,000 years there is evidence of an archaeological industry called the Châtelperronian that contains elements of Middle and Upper Paleolithic artifacts. Hominids from these sites are clearly Neanderthals, sparking speculation that the Châtelperronian is an example of Neanderthals mimicking the culture of modern humans. The lack of anatomical intermediates at these sites, suggests that if Neanderthals did encounter and borrow some technology from Homo sapiens, they did not interbreed.

· A potential 24,500-year-old Neanderthal/sapiens hybrid was announced from the site of Lagar Velho, Portugal.4 This 4-year-old has a short, squat body like a Neanderthal, but possesses an anatomically modern skull. There are a number of problems with interpreting this find as a Neanderthal/sapiens hybrid.14 First of all, as a hybrid it should have a mixture of traits throughout its body and not possess the body of a Neanderthal and skull of a modern human. For example, if we look at hybrids of lions and tigers they do not possess the head of one species and the body of the other, but exhibit a morphological mixture of the two species. Secondly, and more importantly, acceptance of this specimen as a hybrid would suggest that Neanderthal traits had been retained for some 6,000 to 10,000 years after Neanderthals went extinct, which is highly unlikely. This is theoretically unlikely since Neanderthal traits would have been genetically swamped by the Homo sapiens genes over such a protracted period of time.

· Proponents of the Multiregional Model, such as Milford Wolpoff, cite evidence in Asia of regional continuity. They see an evolutionary link between ancient Homo erectus in Java right through to Australian aborigines. A possible problem with this view is that recent dating of late surviving Homo erectus in Indonesia suggests that they survived here until 50,000 years ago, which is potentially when fully modern humans may have arrived in the region from Africa.

China may contain the best evidence for supporting the Multiregional Model. Here there are discoveries of a couple of skulls dated to roughly 100,000 years ago that seem to possess a mixture of classic Homo erectus and Homo sapiens traits. Better geological dating and more complete specimens are needed to more fully assess this possibility.

Conclusion
For the moment, the majority of anatomical, archaeological and genetic evidence gives credence to the view that fully modern humans are a relatively recent evolutionary phenomenon. The current best explanation for the beginning of modern humans is the Out of Africa Model that postulates a single, African origin for Homo sapiens. The major neurological and cultural innovations that characterized the appearance of fully modern humans has proven to be remarkably successful, culminating in our dominance of the planet at the expense of all earlier hominid populations.

Atmosphere

Atmosphere, mixture of gases surrounding any celestial object that has a gravitational field strong enough to prevent the gases from escaping; especially the gaseous envelope of Earth. The principal constituents of the atmosphere of Earth are nitrogen (78 percent) and oxygen (21 percent). The atmospheric gases in the remaining 1 percent are argon (0.9 percent), carbon dioxide (0.03 percent), varying amounts of water vapor, and trace amounts of hydrogen, ozone, methane, carbon monoxide, helium, neon, krypton, and xenon.

The mixture of gases in the air today has had 4.5 billion years in which to evolve. The earliest atmosphere must have consisted of volcanic emanations alone. Gases that erupt from volcanoes today, however, are mostly a mixture of water vapor, carbon dioxide, sulfur dioxide, and nitrogen, with almost no oxygen. If this is the same mixture that existed in the early atmosphere, then various processes would have had to operate to produce the mixture we have today. One of these processes was condensation. As it cooled, much of the volcanic water vapor condensed to fill the earliest oceans.

Chemical reactions would also have occurred. Some carbon dioxide would have reacted with the rocks of Earth’s crust to form carbonate minerals, and some would have become dissolved in the new oceans. Later, as primitive life capable of photosynthesis evolved in the oceans, new marine organisms began producing oxygen. Almost all the free oxygen in the air today is believed to have formed by photosynthetic combination of carbon dioxide with water. About 570 million years ago, the oxygen content of the atmosphere and oceans became high enough to permit marine life capable of respiration. Later, some 400 million years ago, the atmosphere contained enough oxygen for the evolution of air breathing land animals.

The water-vapor content of the air varies considerably, depending on the temperature and relative humidity. With 100 percent relative humidity, the water vapor content of air varies from 190 parts per million (ppm) at -40°C (-40°F) to 42,000 ppm at 30°C (86°F). Minute quantities of other gases, such as ammonia, hydrogen sulfide, and oxides of sulfur and nitrogen, are temporary constituents of the atmosphere in the vicinity of volcanoes and are washed out of the air by rain or snow. Oxides and other pollutants added to the atmosphere by industrial plants and motor vehicles have become a major concern, however, because of their damaging effects in the form of acid rain. In addition, the strong possibility exists that the steady increase in atmospheric carbon dioxide, mainly as the result of the burning of fossil fuels since the mid-1800s, may affect Earth’s climate.

Similar concerns are posed by the sharp increase in atmospheric methane. Methane levels have risen 11 percent since 1978. About 80 percent of the gas is produced by decomposition in rice paddies, swamps, and the intestines of grazing animals, and by tropical termites. Human activities that tend to accelerate these processes include raising more livestock and growing more rice. Besides adding to the greenhouse effect, methane reduces the volume of atmospheric hydroxyl ions, thereby curtailing the atmosphere’s ability to cleanse itself of pollutants.

The study of air samples shows that up to at least 88 km (55 mi) above sea level the composition of the atmosphere is substantially the same as at ground level; the continuous stirring produced by atmospheric currents counteracts the tendency of the heavier gases to settle below the lighter ones. In the lower atmosphere, ozone, a form of oxygen with three atoms in each molecule, is normally present in extremely low concentrations. The layer of atmosphere from 19 to 48 km (12 to 30 mi) up contains more ozone, produced by the action of ultraviolet radiation from the sun. Even in this layer, however, the percentage of ozone is only 0.001 by volume. Atmospheric disturbances and downdrafts carry varying amounts of this ozone to the surface of Earth. Human activity adds to ozone in the lower atmosphere, where it becomes a pollutant that can cause extensive crop damage.

The ozone layer became a subject of concern in the early 1970s, when it was found that chemicals known as chlorofluorocarbons (CFCs), or chlorofluoromethanes, were rising into the atmosphere in large quantities because of their use as refrigerants and as propellants in aerosol dispensers. The concern centered on the possibility that these compounds, through the action of sunlight, could chemically attack and destroy stratospheric ozone, which protects Earth’s surface from excessive ultraviolet radiation. As a result, industries in the United States, Europe, and Japan replaced chlorofluorocarbons in all but essential uses.

The atmosphere may be divided into several layers. In the lowest one, the troposphere, the temperature as a rule decreases upward at the rate of 5.5°C per 1,000 m (3°F per 3,000 ft). This is the layer in which most clouds occur. The troposphere extends up to about 16 km (about 10 mi) in tropical regions (to a temperature of about -79°C, or about -110°F) and to about 9.7 km (about 6 mi) in temperate latitudes (to a temperature of about -51°C, or about -60°F). Above the troposphere is the stratosphere. In the lower stratosphere the temperature is practically constant or increases slightly with altitude, especially over tropical regions. Within the ozone layer the temperature rises more rapidly, and the temperature at the upper boundary of the stratosphere, almost 50 km (about 30 mi) above sea level, is about the same as the temperature at the surface of Earth. The layer from 50 to 90 km (30 to 55 mi), called the mesosphere, is characterized by a marked decrease in temperature as the altitude increases.

From investigations of the propagation and reflection of radio waves, it is known that beginning at an altitude of 60 km (40 mi), ultraviolet radiation, X rays, and showers of electrons from the sun ionize several layers of the atmosphere, causing them to conduct electricity; these layers reflect radio waves of certain frequencies back to Earth. Because of the relatively high concentration of ions in the air above 60 km (40 mi), this layer, extending to an altitude of about 1000 km (600 mi), is called the ionosphere. At an altitude of about 90 km (55 mi), temperatures begin to rise. The layer that begins at this altitude is called the thermosphere, because of the high temperatures reached in this layer (about 1200°C, or about 2200°F).
The region beyond the thermosphere is called the exosphere, which extends to about 9,600 km (about 6,000 mi), the outer limit of the atmosphere.

The density of dry air at sea level is about 1/800 the density of water; at higher altitudes it decreases rapidly, being proportional to the pressure and inversely proportional to the temperature. Pressure is measured by a barometer and is expressed in millibars, which are related to the height of a column of mercury that the air pressure will support; 1 millibar equals 0.75 mm (0.03 in) of mercury. Normal atmospheric pressure at sea level is 1,013 millibars, that is, 760 mm (29.92 in) of mercury. At an altitude of 5.6 km (about 3.5 mi) pressure falls to about 507 millibars (about 380 mm/14.96 in of mercury); half of all the air in the atmosphere lies below this level. The pressure is approximately halved for each additional increase of 5.6 km in altitude. At 80 km (50 mi) the pressure is 0.009 millibars (0.0069 mm/0.00027 in of mercury).

The troposphere and most of the stratosphere can be explored directly by means of sounding balloons equipped with instruments to measure the pressure and temperature of the air and with a radio transmitter to send the data to a receiving station at the ground. Rockets carrying radios that transmit meteorological-instrument readings have explored the atmosphere to altitudes above 400 km (250 mi). Study of the form and spectrum of the polar lights gives information to a height possibly as great as 800 km (500 mi).

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Atmosphere Diagram
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Cloud

I -INTRODUCTION
Cloud, condensed form of atmospheric moisture consisting of small water droplets or tiny ice crystals. Clouds are the principal visible phenomena of the atmosphere. They represent a transitory but vital step in the water cycle, which includes evaporation of moisture from the surface of the earth, carrying of this moisture into higher levels of the atmosphere, condensation of water vapor into cloud masses, and final return of water to the surface as precipitation.

II -FORMATION AND EFFECTS
The formation of clouds caused by cooling of the air results in the condensation of invisible water vapor that produces visible cloud droplets or ice particles. Cloud particles range in size from about 5 to 75 micrometers (0.0005 to 0.008 cm/0.0002 to 0.003 in). The particles are so small that light, vertical currents easily sustain them in the air. The different cloud formations result partly from the temperature at which condensation takes place. When condensation occurs at temperatures below freezing, clouds are usually composed of ice crystals; those that form in warmer air usually consist of water droplets. Occasionally, however, supercooled clouds contain water droplets at subfreezing temperatures. The air motion associated with cloud development also affects formation. Clouds that develop in calm air tend to appear as sheets or stratified formations; those that form under windy conditions or in air with strong vertical currents have a towering appearance.

Clouds perform a very important function in modifying the distribution of solar heat over the earth's surface and within the atmosphere In general, because reflection from the tops of clouds is greater than reflection from the surface of the earth, the amount of solar energy reflected back to space is greater on cloudy days. Although most solar radiation is reflected back by the upper layers of the clouds, some radiation penetrates to the surface of the earth, which absorbs this energy and reradiates it. The lower parts of clouds are opaque to this long-wave earth radiation and reflect it back toward earth.

The result is that the lower atmosphere generally absorbs more radiative heat energy on a cloudy day because of the presence of this trapped radiation. By contrast, on a clear day more solar radiation is initially absorbed by the surface of the earth, but when reradiated this energy is quickly dissipated because of the absence of clouds. Disregarding related meteorological elements, the atmosphere actually absorbs less radiation on clear days than on cloudy days.

Cloudiness has considerable influence on human activities. Rainfall, which is very important for agricultural activities, has its genesis in the formation of clouds. The marked effect of clouds on visibility at flight levels proved to be a major difficulty during the early days of the airplane, a hazard that was alleviated with the development of instrument flying, which permits the pilot to navigate even in the midst of a thick cloud. The sharp increase in consumption of electricity for lighting during cloudy days represents one of the major scheduling problems faced by the electric-power industry.

The first scientific study of clouds began in 1803, when a method of cloud classification was devised by the British meteorologist Luke Howard. The next development was the publication in 1887 of a classification system that later formed the basis for the noted International Cloud Atlas (1896). This atlas, considerably revised and modified through the years (most recently in 1956), is now used throughout the world.

III -CLASSIFICATION
Clouds are usually divided into four main families on the basis of their height above the ground: high clouds, middle clouds, low clouds, and clouds with vertical development that may extend through all levels. The four main divisions are further subdivided into genera, species, and varieties, which describe in detail the appearance of clouds and the manner in which they are formed. More than 100 different kinds of clouds are distinguishable. Only the primary families and most important genera are described below.

A -High Cloud
These are clouds composed of ice particles, found at average levels of 8 km (5 mi) or more above the earth. The family contains three principal genera. Cirrus clouds are isolated, feathery, and threadlike, often with hooks or tufts, and are arranged in bands. Cirrostratus clouds appear as a fine, whitish veil; they occasionally exhibit a fibrous structure and, when situated between the observer and the moon, produce halo phenomena. Cirrocumulus clouds form small, white, fleecy balls and wisps, arranged in groups or rows. Cirrocumulus and cirrus clouds are popularly described by the phrase “mackerel scales and mares' tails.”

B -Middle Clouds
These are clouds composed of water droplets and ranging in altitude from about 3 to 6 km (about 2 to 4 mi) above the earth. Two principal genera are included in the family. Altostratus clouds appear as a thick, gray or bluish veil, through which the sun or moon may be seen only diffusely, as through a frosted glass. Altocumulus clouds have the appearance of dense, fleecy balls or puffs somewhat larger than cirrocumulus. The sun or moon shining through altocumulus clouds may produce a corona, or colored ring, markedly smaller in diameter than a halo.

C -Low Clouds
These clouds, also composed of water droplets, are generally less than 1.6 km (1 mi) high. Three principal forms comprise this group. Stratocumulus clouds consist of large rolls of clouds, soft and gray looking, which frequently cover the entire sky. Because the cloud mass is usually not very thick, blue sky often appears between breaks in the cloud deck. Nimbostratus clouds are thick, dark, and shapeless. They are precipitation clouds from which, as a rule, rain or snow falls. Stratus clouds are sheets of high fog. They appear as flat, white blankets, usually less than 610 m (2000 ft) above the ground. When they are broken up by warm, rising air, the sky beyond usually appears clear and blue.

D -Clouds with Vertical Development
Clouds of this type range in height from less than 1.6 km (1 mi) to more than 13 km (8 mi) above the earth. Two main forms are included in this group. Cumulus clouds are dome-shaped, woolpack clouds most often seen during the middle and latter part of the day, when solar heating produces the vertical air currents necessary for their formation. These clouds usually have flat bases and rounded, cauliflowerlike tops. Cumulonimbus clouds are dark, heavy-looking clouds rising like mountains high into the atmosphere, often showing an anvil-shaped veil of ice clouds, false cirrus, at the top. Popularly known as thunderheads, cumulonimbus clouds are usually accompanied by heavy, abrupt showers.

An anomalous, but exceptionally beautiful, group of clouds contains the nacreous, or mother-of-pearl, clouds, which are 19 to 29 km (12 to 18 mi) high, and the noctilucent clouds, 51 to 56 km (32 to 35 mi) high. These very thin clouds may be seen only between sunset and sunrise and are visible only in high latitudes.

The development of the high-altitude airplane has introduced a species of artificial clouds known as contrails (condensation trails). They are formed from the condensed water vapor ejected as a part of the engine-exhaust gases.

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Rock (mineral)

I -INTRODUCTION
Rock (mineral), naturally occurring solid material consisting of one or more minerals. Minerals are solid, naturally occurring chemical elements or compounds that are homogenous, meaning they have a definite chemical composition and a very regular arrangement of atoms. Rocks are everywhere, in the ground, forming mountains, and at the bottom of the oceans. Earth’s outer layer, or crust, is made mostly of rock. Some common rocks include granite and basalt.

II -TYPES OF ROCKS
Rocks are divided into three main types, based on the ways in which they form. Igneous rocks are made of old rocks that have melted within the earth to form molten material called magma. Magma cools and solidifies to become igneous rocks. Sedimentary rocks form as layers of material settle onto each other, press together, and harden. Metamorphic rocks are created when existing rocks are exposed to high temperatures and pressures, and the rock material is changed, or metamorphosed, while solid.

A -Igneous Rock
Igneous rocks are rocks formed from a molten or partly molten material called magma. Magma forms deep underground when rock that was once solid melts. Overlying rock presses down on the magma, and the less dense magma rises through cracks in the rock. As magma moves upward, it cools and solidifies. Magma that solidifies underground usually cools slowly, allowing large crystals to form. Magma that reaches Earth’s surface is called lava. Lava loses heat to the atmosphere or ocean very quickly and therefore solidifies very rapidly, forming very small crystals or glass. When lava erupts at the surface again and again, it can form mountains called volcanoes.
Igneous rocks commonly contain the minerals feldspar, quartz, mica, pyroxene, amphibole, and olivine. Igneous rocks are named according to which minerals they contain. Rocks rich in feldspar and quartz are called felsic; rocks rich in pyroxene, amphibole, and olivine, which all contain magnesium and iron, are called mafic. Common and important igneous rocks are granite, rhyolite, gabbro, and basalt. Granite and rhyolite are felsic; gabbro and basalt are mafic. Granite has large crystals of quartz and feldspar. Rhyolite is the small-grained equivalent of granite. Gabbro has large crystals of pyroxene and olivine. Basalt is the most common volcanic rock.

B -Sedimentary Rock
Sedimentary rock forms when loose sediment, or rock fragments, hardens. Geologists place sedimentary rocks into three broad categories: (1) clastic rocks, which form from clasts, or broken fragments, of pre-existing rocks and minerals; (2) chemical rocks, which form when minerals precipitate, or solidify, from a solution, usually seawater or lake water; and (3) organic rocks, which form from accumulations of animal and plant remains. It is common for sedimentary rocks to contain all three types of sediment. Most fossils are found in sedimentary rocks because the processes that form igneous and metamorphic rocks prevent fossilization or would likely destroy fossils.

The most common types of clastic rocks are sandstone and shale (also known as mudrock). Sandstone is made from sand, and shale is made from mud. Sand particles have diameters in the range 2.00 to 0.06 mm (0.08 to 0.002 in), while mud particles are smaller than 0.06 mm (0.002 in). Sand and mud form when physical or chemical processes break down and destroy existing rocks. The sand and mud are carried by wind, rivers, ocean currents, and glaciers, which deposit the sediment when the wind or water slows down or where the glacier ends. Sand usually forms dunes in deserts, or sandbars, riverbeds, beaches, and near-shore marine deposits. Mud particles are smaller than sand particles, so they tend to stay in the wind or water longer and are deposited only in very still environments, such as lake beds and the ocean floor.

Sedimentary rock forms when layers of sand and mud accumulate. As the sediment accumulates, the weight of the layers of sediment presses down and compacts the layers underneath. The sediments become cemented together into a hard rock when minerals (most commonly quartz or calcite) precipitate, or harden, from water in the spaces between grains of sediment, binding the grains together. Sediment is usually deposited in layers, and compaction and cementation preserve these layers, called beds, in the resulting sedimentary rock.

The most common types of chemical rocks are called evaporites because they form by evaporation of seawater or lake water. The elements dissolved in the water crystallize to form minerals such as gypsum and halite. Gypsum is used to manufacture plaster and wallboard; halite is used as table salt.
The most common organic rock is limestone. Many marine animals, such as corals and shellfish, have skeletons or shells made of calcium carbonate (CaCO3). When these animals die, their skeletons sink to the seafloor and accumulate to form large beds of calcium carbonate. As more and more layers form, their weight compresses and cements the layers at the bottom, forming limestone. Details of the skeletons and shells are often preserved in the limestone as fossils.

Coal is another common organic rock. Coal comes from the carbon compounds of plants growing in swampy environments. Plant material falling into the muck at the bottom of the swamp is protected from decay. Burial and compaction of the accumulating plant material can produce coal, an important fuel in many parts of the world. Coal deposits frequently contain plant fossils.

C -Metamorphic Rock
Metamorphic rock forms when pre-existing rock undergoes mineralogical and structural changes resulting from high temperatures and pressures. These changes occur in the rock while it remains solid (without melting).

The changes can occur while the rock is still solid because each mineral is stable only over a specific range of temperature and pressure. If a mineral is heated or compressed beyond its stability range, it breaks down and forms another mineral. For example, quartz is stable at room temperature and at pressures up to 1.9 gigapascals (corresponding to the pressure found about 65 km [about 40 mi] underground). At pressures above 1.9 gigapascals, quartz breaks down and forms the mineral coesite, in which the silicon and oxygen atoms are packed more closely together.
In the same way, combinations of minerals are stable over specific ranges of temperature and pressure. At temperatures and pressures outside the specific ranges, the minerals react to form different combinations of minerals. Such combinations of minerals are called mineral assemblages.

In a metamorphic rock, one mineral assemblage changes to another when its atoms move about in the solid state and recombine to form new minerals. This change from one mineral assemblage to another is called metamorphism. As temperature and pressure increase, the rock gains energy, which fuels the chemical reactions that cause metamorphism. As temperature and pressure decrease, the rock cools; often, it does not have enough energy to change back to a low-temperature and low-pressure mineral assemblage. In a sense, the rock is stuck in a state that is characteristic of its earlier high-temperature and high-pressure environment. Thus, metamorphic rocks carry with them information about the history of temperatures and pressures to which they were subjected.

The size, shape, and distribution of mineral grains in a rock are called the texture of the rock. Many metamorphic rocks are named for their main texture. Textures give important clues as to how the rock formed. As the pressure and temperature that form a metamorphic rock increase, the size of the mineral grains usually increases. When the pressure is equal in all directions, mineral grains form in random orientations and point in all directions. When the pressure is stronger in one direction than another, minerals tend to align themselves in particular directions. In particular, thin plate-shaped minerals, such as mica, align perpendicular to the direction of maximum pressure, giving rise to a layering in the rock that is known as foliation. Compositional layering, or bands of different minerals, can also occur and cause foliation. At low pressure, foliation forms fine, thin layers, as in the rock slate. At medium pressure, foliation becomes coarser, forming schist. At high pressure, foliation is very coarse, forming gneiss. Commonly, the layering is folded in complex, wavy patterns from the pressure.

III -THE ROCK CYCLE
The rock cycle describes how rocks change, or evolve, from one type to another. For example, any type of rock (igneous, sedimentary, or metamorphic) can become a new sedimentary rock if its eroded sediments are deposited, compacted, and cemented. Similarly, any type of rock can become metamorphic if it is buried moderately deep. If the temperature and pressure become sufficiently high, the rock can melt to form magma and a new igneous rock.

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I -INTRODUCTION
Mineral Deposit, concentrated, natural occurrence of one or more minerals. Mineral deposits can form within any kind of rock and consist of any type of mineral. They are valuable economically because they contain high concentrations of metallic and nonmetallic elements or other valuable materials that are essential to an industrial society.

The concentration of a mineral in a mineral deposit is critically important in determining whether it can be mined profitably. For the mining of metals, concentration in a mineral deposit is measured two ways. The grade depends on the percentage by weight of a metal in a mineral deposit. This percentage is measured by dividing the weight of the metal by the weight of the rock. The concentration factor (also called enrichment factor) is the number of times more abundant a metal is in a mineral deposit than it is in average crustal rock. The concentration factor is measured by dividing a mineral deposit’s grade by the average grade of crustal rocks for that metal. A concentration factor of ten, for example, means that a metal is ten times more abundant in a particular deposit than in the earth’s crust.

If a metal is to be mined profitably, it must have attained a minimum concentration factor—otherwise, the amount of that metal acquired will be too small to pay for the mining process. Minimum concentration factors vary from one metal to the next. Iron, which is relatively abundant in the earth’s crust, typically requires a concentration factor of between 5 and 10. Gold and silver, however, require concentration factors in excess of 2,000. The term ore describes rock that contains high enough concentrations of a metal to be mined profitably.

The accessibility of a mineral deposit also plays an important role in determining the cost-effectiveness of mining. In general, deposits that reside deeper in the crust are more difficult and more expensive to mine. Consequently, the minimum required concentration factor increases with the difficulty of extraction.

II -PROCESSES OF SEGREGATION
Geological processes, such as melting and crystallizing of igneous rocks as well as erosion and deposition, sometimes separate and concentrate minerals. At other times, these processes mix and dilute them. Any process that separates and concentrates minerals is called a process of segregation.


A -Magmatic Processes
During cooling and crystallization of a magma, minerals with a high temperature of crystallization form early and may settle to the floor of the magma chamber. These early-formed minerals, such as pyroxene or olivine, tend to be relatively rich in iron and magnesium and poor in silicon and oxygen when compared to the entire magma. They also typically contain no potassium or aluminum. Consequently, minerals with lower temperatures of crystallization that form later tend to be relatively rich in potassium, aluminum, silicon, and oxygen, but poor in iron and magnesium. This process, called fractional crystallization, segregates minerals.

Fractional crystallization can lead to valuable mineral deposits because many rare and valuable elements form mineral crystals either early or late in the crystallization process. For example, when magmas have compositions with abundant chromium, the mineral chromite crystallizes early and can form deposits on the floor of the magma chamber. Extensive chromite deposits are mined in the Bushveld Complex of South Africa and in the Stillwater Complex of Montana, United States. In other magmas, the latest-forming mineral crystals may contain a variety of rare elements such as beryllium, lithium, boron, molybdenum, and uranium. These deposits are called pegmatites. Numerous well-known pegmatites are scattered throughout the western United States.

B -Hydrothermal Processes
Hydrothermal processes involve the transportation of elements dissolved in hot water and the subsequent precipitation, or crystallization, of minerals when the water cools. In some cases, the elements precipitate in their native states, such as pure gold or copper. More often, however, they precipitate as sulfide minerals, including pyrite (iron sulfide), galena (lead sulfide), sphalerite (zinc sulfide), cinnabar (mercury sulfide), and chalcopyrite (copper sulfide). Hydrothermal processes are particularly effective at segregating minerals because the fluids typically contain only a small variety of dissolved elements. Hydrothermal processes are responsible for most of the world’s metallic mineral deposits such as gold, silver, lead, and copper.

Hydrothermal fluids originate in several different ways. Some originate from magmas that have water dissolved in them. As the magma cools and crystallizes, the water is excluded from the growing crystals and separates from the magma. Such fluids will be very hot and rich with elements dissolved from the magma. Other sources of hydrothermal fluids include circulating groundwater that comes into contact with hot rock, or seawater circulating through seafloor sediments that interacts with newly created volcanic rock on the ocean floor. These fluids typically migrate away from their heat sources along fractures and cool. This cooling causes some minerals to precipitate.

When minerals form a precipitate within open fractures, the resulting deposit is called a vein. During the late 19th and early 20th centuries, miners exploited veins of highly concentrated gold throughout the western United States. Two well-known examples are Cripple Creek in Colorado, and Bullfrog in Nevada. Besides cooling, other causes of precipitation include sudden decreases in pressure or reactions with the surrounding rock. When precipitation occurs at the earth’s surface, the minerals form hot springs deposits, such as the deposits at Yellowstone National Park.
Precipitation of minerals can also occur in microscopic networks of fractures or pore spaces to form mineral deposits that are disseminated, or spread widely, throughout the rock. Disseminated deposits typically display much lower concentration factors than vein deposits. Some are so extensive, however, that their huge volumes make up for the low concentrations. Many of the copper mines in Arizona and Utah, and many of the gold mines in Nevada, are in disseminated deposits.

C -Evaporation Processes
When water containing dissolved minerals evaporates, the minerals will precipitate. Deposits of minerals formed in this way are called evaporites. Evaporite deposits can form on land in enclosed arid basins. Incoming water cannot exit except by evaporation. Because the incoming water also carries dissolved minerals, the basin continually receives additional minerals, and the resulting deposit can be quite thick. Land-based evaporites currently are forming in desert lakes in the American states of California, Nevada, and Utah, and in the Dead Sea between Israel and Jordan.

Evaporite deposits also form in tropical seas or bays connected to the open ocean through narrow passages. Seawater flows through the narrow passages to replace water lost through evaporation. Because the incoming water is salty, the basin continually receives additional sea salts. If the concentration of salts is high enough, the minerals will precipitate. If the conditions persist for a long time, the resultant deposits can be very thick. In the western United States, a thick layer of marine evaporites formed more than 200 million years ago during the Permian Period.

Some examples of common evaporite minerals are halite (sodium chloride), gypsum (calcium sulfate), and borax (sodium borate). Many evaporite deposits are mined for use in table salt, fertilizers, wallboard, plaster, detergents, and fluxes.

D -Residues of Weathering Process
Chemical weathering causes minerals to decompose into clays and other materials. This weathering typically leads to the removal of all material that does not resist weathering. In regions of especially intense weathering, such as the tropics, virtually everything except oxides of aluminum and iron becomes weathered and is eventually removed. Through this process of weathering and removal of the nonresistant material, aluminum and iron oxides form a concentrated residue. These residues, if extensive enough, can be mined for aluminum and iron.

Bauxite is a rock made from aluminum oxide residues and is the principal ore of aluminum. The world’s leading producers of bauxite, the countries Surinam, Jamaica, and Guyana, are all located in the tropics. Commercial bauxite deposits that occur outside of the tropics, such as in the United States, the former Soviet Union, and China, indicate that those regions once experienced tropical weathering conditions.

E -Depositional Processes
Some mineral deposits form in river beds because running water tends to segregate dense minerals. Rivers deposit grains that are either larger or denser first, and then carry grains that are either smaller or lighter farther downriver. Relatively dense minerals or metals, such as cassiterite (a source of tin), diamond, or gold, erode from their sources and get deposited with the heavier, coarser grains. The sites of deposition are most frequently the gravel or sandbars that form on the inside bends of meandering rivers. Mineable deposits of these materials are called placer deposits.

Placer mining has provided humankind with more than half of its gold. Well-known placer deposits include gravels formed about 40 million years ago during the Eocene Epoch in California, the discovery of which helped fuel the 1849 California Gold Rush. Much of this placer gold originally eroded from hydrothermal vein deposits of gold associated with igneous intrusions in western Nevada. Precambrian deposits in South Africa, formed more than 500 million years ago, are the largest known placer gold deposits in the world.

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Pest Control

I -INTRODUCTION
Pest Control, any of a wide range of environmental interventions that have as their objective the reduction to acceptable levels of insect pests, plant pathogens, and weed populations. Specific control techniques include chemical, physical, and biological mechanisms. Despite all the control efforts used, pests annually destroy about 35 percent of all crops worldwide. Even after food is harvested, insects, microorganisms, rodents, and birds inflict a further 10 to 20 percent loss, bringing the total destruction to about 40 or 50 percent. With so many areas of the world facing serious food shortages, researchers seek to reduce this loss by improving pest control.

II -CHEMICAL CONTROLS
The chemical agents called pesticides include herbicides (for weed control), insecticides, and fungicides. More than half the pesticides used in the United States are herbicides that control weeds. The United States Department of Agriculture (USDA) estimates indicate that 86 percent of U.S. agricultural land areas are treated with herbicides, 18 percent with insecticides, and 3 percent with fungicides. The amount of pesticide used on different crops also varies. For example, in the United States, about 67 percent of the insecticides used in agriculture are applied to two crops, cotton and corn; about 70 percent of the herbicides are applied to corn and soybeans, and most of the fungicides are applied to fruit and vegetable crops.

Most of the insecticides now applied are long-lasting synthetic compounds that affect the nervous system of insects on contact. Among the most effective are the chlorinated hydrocarbons DDT, chlordane, and toxaphene, although agricultural use of DDT has been banned in the United States since 1973. Others, the organophosphate insecticides, include malathion, parathion, and dimethoate. Among the most effective herbicides are the compounds of 2,4-D (2,4-dichlorophenoxyacetic acid), only a few kilograms of which are required per hectare to kill broad-leaved weeds while leaving grains unaffected.

Agricultural pesticides prevent a monetary loss of about $9 billion each year in the United States. For every $1 invested in pesticides, the American farmer gets about $4 in return. These benefits, however, must be weighed against the costs to society of using pesticides, as seen in the banning of ethylene dibromide in the early 1980s. These costs include human poisonings, fish kills, honeybee poisonings, and the contamination of livestock products. The environmental and social costs of pesticide use in the United States have been estimated to be at least $1 billion each year. Thus, although pesticides are valuable for agriculture, they also can cause serious harm.

Indeed, the question may be asked—what would crop losses be if insecticides were not used in the United States, and readily available nonchemical controls were substituted? The best estimate is that only another 5 percent of the nation's food would be lost. Many environmentalists and others advocate organic farming as an alternative to heavy chemical pesticide use.

III -NONCHEMICAL CONTROLS
Many pests that are attached to crop residues can be eliminated by plowing them underground. Simple paper or plastic barriers placed around fruit trees deter insects, which can also be attracted to light traps and destroyed. Weeds can be controlled by spreading grass, leaf, or black plastic mulch. Weeds also may be pulled or hoed from the soil.

Many biological controls are also effective. Such insect pests as the European corn borer, Pyrausta nubilalis, and the Japanese beetle, Popillia japonica, have been controlled by introducing their predators and parasites. Wasps that prey on fruit-boring insect larvae are now being commercially bred and released in California orchards. The many hundreds of species of viruses, bacteria, protozoa, fungi, and nematodes that parasitize pest insects and weeds are now being investigated as selective control agents.
Another area of biological control is breeding host plants to be pest resistant, making them less prone to attack by fungi and insects. The use of sex pheromones is an effective measure for luring and trapping insects.

Pheromones have been synthesized for the Mediterranean fruit fly, the melon fly, and the Oriental fruit fly. Another promising pest-control method is the release of sterilized male insects into wild pest populations, causing females to bear infertile eggs. Of these techniques, breeding host-plant resistance and using beneficial parasites and predators are the most effective. Interestingly, the combined use of biological and physical controls accounts for more pest control than chemical pesticides.

Integrated pest management (IPM) is a recently developed technology for pest control that is aimed at achieving the desired control while reducing the use of pesticides. To accomplish this, various combinations of chemical, biological, and physical controls are employed. In the past, pesticides were all too often applied routinely whether needed or not. With IPM, pest populations as well as beneficial parasite and predator populations are monitored to determine whether the pests actually present a serious problem that needs to be treated. If properly and extensively employed, IPM might reduce pesticide use by as much as 50 percent, while at the same time improving pest control. If this goal were achieved, the environmental problems would be minimized, and significant benefits would result for farmers and society as a whole.

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Protein

I -INTRODUCTION

Protein, any of a large number of organic compounds that make up living organisms and are essential to their functioning. First discovered in 1838, proteins are now recognized as the predominant ingredients of cells, making up more than 50 percent of the dry weight of animals. The word protein is coined from the Greek proteios, or “primary.”

Protein molecules range from the long, insoluble fibers that make up connective tissue and hair to the compact, soluble globules that can pass through cell membranes and set off metabolic reactions. They are all large molecules, ranging in molecular weight from a few thousand to more than a million, and they are specific for each species and for each organ of each species. Humans have an estimated 30,000 different proteins, of which only about 2 percent have been adequately described. Proteins in the diet serve primarily to build and maintain cells, but their chemical breakdown also provides energy, yielding close to the same 4 calories per gram as do carbohydrates.

Besides their function in growth and cell maintenance, proteins are also responsible for muscle contraction. The digestive enzymes are proteins, as are insulin and most other hormones. The antibodies of the immune system are proteins, and proteins such as hemoglobin carry vital substances throughout the body.

II -NUTRITION
Whether found in humans or in single-celled bacteria, proteins are composed of units of about 20 different amino acids, which, in turn, are composed of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. In a protein molecule these acids form peptide bonds—bonds between amino and carboxyl (COOH) groups—in long strands (polypeptide chains). The almost numberless combinations in which the acids line up, and the helical and globular shapes into which the strands coil, help to explain the great diversity of tasks that proteins perform in living matter.

To synthesize its life-essential proteins, each species needs given proportions of the 20 main amino acids. Although plants can manufacture all their amino acids from nitrogen, carbon dioxide, and other chemicals through photosynthesis, most other organisms can manufacture only some of them. The remaining ones, called essential amino acids, must be derived from food. Nine essential amino acids are needed to maintain health in humans: leucine, isoleucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine, and histidine. All of these are available in proteins produced in the seeds of plants, but because plant sources are often weak in lysine and tryptophan, nutrition experts advise supplementing the diet with animal protein from meat, eggs, and milk, which contain all the essential acids.

Most diets—especially in the United States, where animal protein is eaten to excess—contain all the essential amino acids. (Kwashiorkor, a wasting disease among children in tropical Africa, is due to an amino acid deficiency.) For adults, the Recommended Dietary Allowance (RDA) for protein is 0.79 g per kg (0.36 g per lb) of body weight each day. For children and infants this RDA is doubled and tripled, respectively, because of their rapid growth.

III -STRUCTURE OF PROTEINS
The most basic level of protein structure, called the primary structure, is the linear sequence of amino acids. Different sequences of the acids along a chain, however, affect the structure of a protein molecule in different ways. Forces such as hydrogen bonds, disulfide bridges, attractions between positive and negative charges, and hydrophobic (“water-fearing”) and hydrophilic (“water-loving”) linkages cause a protein molecule to coil or fold into a secondary structure, examples of which are the so-called alpha helix and the beta pleated sheet. When forces cause the molecule to become even more compact, as in globular proteins, a tertiary protein structure is formed. When a protein is made up of more than one polypeptide chain, as in hemoglobin and some enzymes, it is said to have a quaternary structure.

IV -INTERACTION WITH OTHER PROTEINS
Polypeptide chains are sequenced and coiled in such a way that the hydrophobic amino acids usually face inward, giving the molecule stability, and the hydrophilic amino acids face outward, where they are free to interact with other compounds and especially other proteins. Globular proteins, in particular, can join with a specific compound such as a vitamin derivative and form a coenzyme, or join with a specific protein and form an assembly of proteins needed for cell chemistry or structure.

V -FIBROUS PROTEINS
The major fibrous proteins, described below, are collagen, keratin, fibrinogen, and muscle proteins.

A -Collagen
Collagen, which makes up bone, skin, tendons, and cartilage, is the most abundant protein found in vertebrates. The molecule usually contains three very long polypeptide chains, each with about 1000 amino acids, that twist into a regularly repeating triple helix and give tendons and skin their great tensile strength. When long collagen fibrils are denatured by boiling, their chains are shortened to form gelatin.

B -Keratin
Keratin, which makes up the outermost layer of skin and the hair, scales, hooves, nails, and feathers of animals, twists into a regularly repeating coil called an alpha helix. Serving to protect the body against the environment, keratin is completely insoluble in water. Its many disulfide bonds make it an extremely stable protein, able to resist the action of proteolytic (protein-hydrolyzing) enzymes. In beauty treatments, human hair is set under a reducing agent, such as thioglycol, to reduce the number of disulfide bonds, which are then restored when the hair is exposed to oxygen.

C -Fibrinoge
Fibrinogen is a blood plasma protein responsible for blood clotting. With the catalytic action of thrombin, fibrinogen is converted into molecules of the insoluble protein fibrin, which link together to form clots.

D -Muscle Proteins
Myosin, the protein chiefly responsible for muscle contraction, combines with actin, another muscle protein, forming actomyosin, the different filaments of which shorten, causing the contracting action.

VI -GLOBULAR PROTEINS
Unlike fibrous proteins, globular proteins are spherical and highly soluble. They play a dynamic role in body metabolism. Examples are albumin, globulin, casein, hemoglobin, all of the enzymes, and protein hormones. The albumins and globulins are classes of soluble proteins abundant in animal cells, blood serum, milk, and eggs. Hemoglobin is a respiratory protein that carries oxygen throughout the body and is responsible for the bright red color of red blood cells. More than 100 different human hemoglobins have been discovered, among which is hemoglobin S, the cause of sickle-cell anemia, a hereditary disease suffered mainly by blacks.

A -Enzymes
All of the enzymes are globular proteins that combine rapidly with other substances, called substrate, to catalyze the numerous chemical reactions in the body. Chiefly responsible for metabolism and its regulation, these molecules have catalytic sites on which substrate fits in a lock-and-key manner to trigger and control metabolism throughout the body.

B -Protein Hormones
These proteins, which come from the endocrine glands, do not act as enzymes. Instead they stimulate target organs that in turn initiate and control important activities—for example, the rate of metabolism and the production of digestive enzymes and milk. Insulin, secreted by the islands of Langerhans, regulates carbohydrate metabolism by controlling blood glucose levels. Thyroglobulin, from the thyroid gland, regulates overall metabolism; calcitonin, also from the thyroid, lowers blood calcium levels. Angiogenin, a protein structurally determined in the mid-1980s, directly induces the growth of blood vessels in tissues.

C -Antibodies
Also called immunoglobulins, antibodies (see Antibody) make up the thousands of different proteins that are generated in the blood serum in reaction to antigens (body-invading substances or organisms). A single antigen may elicit the production of many antibodies, which combine with different sites on the antigen molecule, neutralize it, and cause it to precipitate from the blood.

D -Microtubules
Globular proteins can also assemble into minute, hollow tubes that serve both to structure cells and to conduct substances from one part of a cell to another. Each of these microtubules, as they are called, is made up of two types of nearly spherical protein molecules that pair and join onto the growing end of the microtubule, adding on length as required. Microtubules also make up the inner structure of cilia, the hairlike appendages by which some microorganisms propel themselves.

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Vertebrate

I -INTRODUCTION
Vertebrate, animal with a backbone, or spinal column, made of interlocking units called vertebrae. This strong but flexible structure supports the body and anchors the limbs, and it also protects the nerves of the spinal cord. Vertebrates include fish, amphibians, and reptiles, as well as birds and mammals. In all vertebrates, the spinal column forms part of a complete internal skeleton. Unlike the hard external skeleton covering an insect, which is periodically shed as the insect grows, a vertebrate’s internal skeleton can grow gradually along with the rest of the body.

Vertebrates make up only about 2 percent of the animal species, and they belong to just 1 of more than 30 phyla, or overall groups, in the animal kingdom. Despite this, vertebrates occupy a dominant position in almost all habitats and are by far the most familiar animals. When asked to name an animal at random, most people identify a type of vertebrate.

There are several reasons why vertebrates are so successful and so noticeable. One has to do with their size. Invertebrates—that is, animals without backbones, such as worms, shellfish, and insects—tend to be small and slow moving. This is because they lack effective ways to support a large body and the muscles needed to power it. Vertebrates, on the other hand, have evolved a much more versatile support system. Their skeletons can be adapted for use in many different ways and work just as well in an animal weighing 4 tons as in one weighing 113 g (4 oz). As a result, vertebrates have been able to develop bigger, faster bodies than invertebrates.

Vertebrates also have highly developed nervous systems. With the help of specialized nerve fibers, they can react very quickly to changes in their surroundings, giving them a competitive edge.

II -CHARACTERISTICS
In nearly all vertebrates, bone gives the skeleton its strength. Bone is a living tissue composed of hard mineral salts produced by specialized cells. Unlike an oyster’s shell or a grasshopper’s body case, bone can strengthen after it has reached full size, and it can be repaired if it breaks. The only vertebrates that do not have this kind of skeleton are cartilaginous fish, a group that includes sharks, skates, and rays. As their name suggests, the skeletons of these species are made of cartilage, a rubbery tissue that other vertebrates have mainly in their joints.

A vertebrate's spinal column is held together by strong ligaments, but the faces of adjoining vertebrae are separated by elastic pads called intervertebral disks. These disks allow a small amount of movement at each joint, and as a result the entire spine can bend. How far the spine bends depends on the number of vertebrae that compose it and how they are shaped. Frogs, for example, can have as few as nine vertebrae, and their backbones hardly bend at all. Humans have 33 vertebrae, making us fairly flexible, and some snakes have more than 400, enabling them to shape their bodies into coils.

Besides the backbone, vertebrates share many other physical features. Their bodies are more or less bilaterally symmetrical (divisible into two equal halves), with sense organs concentrated in the head. Most vertebrates have jaws, and their brains are usually protected by a bony case called the cranium. Most also have limbs, but the shapes and uses of vertebrate limbs vary enormously. Fish typically have several paired fins and a large finned tail, but all other vertebrates either have four limbs or are descended from ancestors that had four. Four-limbed animals, known as tetra pods, use their limbs to swim, walk, run, and fly.

Although vertebrates do not have external skeletons, they often have other anatomical features that protect the surface of their bodies. Most fish and reptiles have a covering of hard scales, while birds and mammals have feathers or hair. Feathers and hair are not as tough as scales, but they have other functions apart from physical protection. One of the most important is insulation. By regulating the heat generated inside the body, such coverings allow birds and mammals to remain active in a wide range of temperatures.
Nearly all vertebrates breed by sexual reproduction, either laying eggs or giving birth to live young. The few exceptions to this rule include animals such as North American whiptail lizards, which can breed without mating in a process known as parthenogenesis. In several species of these lizards, males have never been found.

III -TYPES OF VERTEBRATES
There are over 40,000 species of vertebrates, which scientists classify into five groups: (1) fish, (2) amphibians, (3) reptiles, (4) birds, and (5) mammals. Scientists divide fish into three groups based on their anatomy: jawless fish, cartilaginous fish, and bony fish. The other vertebrate groups are made up of tetrapods, which have lungs and generally live on land.

A -Jawless Fish
Jawless fish are the only living vertebrates that have never evolved jaws. There are about 50 species—a tiny fraction of the world's total fish—and they are instantly recognizable by their suckerlike mouths. Eels, lampreys, and hagfish are examples of jawless fish.

B -Cartilaginous Fish
Cartilaginous fish do have jaws and use them to deadly effect. Numbering about 1,000 species, they include sharks, skates, and rays, as well as chimaeras, also known as ratfish. Cartilaginous fish are widespread throughout the world's oceans. Most skates and rays feed on or near the seabed, but sharks typically hunt in open water.

C -Bony Fish
Bony fish are some of the most successful vertebrates alive today. These animals can be found in a vast variety of habitats, from coral reefs and the deep-sea bed to lakes hidden away in caves. As their name indicates, bony fish have a skeleton made of bone, and most also have an air-filled sac called a swim bladder that keeps them buoyant. At least 24,000 species of bony fish have been identified, and many more probably await discovery. Common bony fish include salmon, sturgeon, and cod.

D -Amphibians
Amphibians make up the smallest of the four groups of tetrapods, with about 4,000 species. Most amphibians, such as frogs and toads, live in damp habitats. Like fish, the majority of amphibians reproduce by laying eggs. Amphibians usually lay their eggs in water, because they dry out quickly in air. The eggs produce swimming, fishlike young called tadpoles, which develop limbs and lungs as they mature.

E -Reptiles
Compared to amphibians, reptiles are much more fully adapted to life on land. They have scaly, waterproof skin, and they either give birth to live young or lay eggs with waterproof shells. There are about 7,000 species alive today, including snakes, alligators, and turtles. During the age of the dinosaurs, about 230 million to 65 million years ago, reptiles outnumbered all other land vertebrates put together.

F -Birds
Birds evolved from flightless reptiles but underwent some major changes in body form during their evolution. Of the roughly 10,000 species alive today, most have lightweight, air-filled bones, and all have a unique and highly efficient respiratory system that is found in no other group of vertebrates.

G -Mammals
Mammals are the only vertebrates that raise their young by feeding them on milk produced by the mother’s body, and the only ones that have teeth that are individually specialized for particular functions. Mammal species number about 4,600, and they include the largest animals on land and in the sea. Dogs, bears, monkeys, whales, and humans are all mammals.

IV -THE ORIGIN OF VERTEBRATES
Biologists believe that vertebrates evolved over millions of years from animals similar to today’s lancelets, which burrow in sand on the seabed and filter food from the water. Lancelets possess certain traits similar to vertebrates, including a reinforcing structure called a notochord that runs the length of the body. In a lancelet the notochord is the only hard part of the body, and it allows the animal to wriggle without losing its shape. In most vertebrates, the notochord is lost during early development, and its role is taken over by bone. The characteristics shared by lancelets and vertebrates cause scientists to classify them together in the chordate phylum.

Scientists do not know exactly how the transition from lancelet to vertebrate occurred. Fossils of fishlike animals found in China indicate that vertebrates evolved at the start of the Cambrian Period, an interval of geologic time that began about 570 million years ago. These fish lacked a bony skeleton and teeth (scientists propose that their skeletal structures were made of cartilage), but they did have gill slits and a muscle arrangement similar to today’s fish. Once vertebrates evolved hard body parts, they began to leave more fossilized remains. Fish called ostracoderms, which had bony plates covering their bodies, first appeared in the late Cambrian Period, about 500 million years ago. Like present-day lampreys and hagfish, ostracoderms had no jaws. They probably fed by sucking water into their mouths and then swallowing any food it contained.

With the evolution of jaws, vertebrates acquired a valuable new asset in the struggle for survival, one that enabled them to collect food in a variety of different ways. Jaws first appeared in fish about 420 million years ago, during the mid-Silurian Period. Unlike earlier vertebrates, jawed fish developed complex internal skeletons and paired fins, which helped them maneuver as they pursued their food or escaped from their enemies. Over time, evolution has produced vertebrates with many different body types and behaviors. As a result, vertebrates can now be found in almost every part of the world.

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Invertebrate

I -INTRODUCTION
Invertebrate, any animal lacking a backbone. Invertebrates are by far the most numerous animals on Earth. Nearly 2 million species have been identified to date. These 2 million species make up about 98 percent of all the animals identified in the entire animal kingdom. Some scientists believe that the true number of invertebrate species may be as high as 100 million and that the work of identifying and classifying invertebrate life has only just begun.

Invertebrates live in a vast range of habitats, from forests and deserts to caves and seabed mud. In oceans and lakes they form part of the plankton—an immense array of miniature living organisms that drift in the surface currents. Invertebrates are also found in the soil beneath our feet and in the air above our heads. Some are powerful fliers, using wings to propel themselves, but others, particularly the smallest invertebrates, float on the slightest breeze. These tiny invertebrates form clouds of aerial plankton that drift unseen through the skies.

Although the majority of invertebrates are small, a few reach impressive sizes. The true heavyweights of the invertebrate world are giant squid, which can be over 18 m (60 ft) long and can weigh more than 2,000 kg (4,000 lb). The longest are ribbon worms, also known as nemerteans, whose pencil-thin bodies can grow up to 55 m (180 ft) from head to tail. At the other end of the size scale, animals called rotifers rank among the smallest invertebrates of all. Some species may reach 3 mm (0.12 in) in size, but most are less than 0.001 mm (0.00004 in), smaller than the largest bacteria.

II -PHYSICAL CHARACTERISTICS
Due to their numbers and variety, invertebrates share only a single trait in common: the absence of a backbone. Many invertebrates have no hard body parts at all. These soft-bodied invertebrates, which include earthworms, keep their shape by maintaining an internal pressure, similar to the air pressure within an inflated balloon. However, having a soft body has disadvantages, one of which is that it leaves animals vulnerable to attack from predators.

To defend against predators, other invertebrates have evolved exoskeletons, hard outer coverings such as the shells found in clams and mussels and the body cases that surround adult insects. As well as protecting the animal, these exoskeletons also provide anchorage for muscles. On land, a body case is also useful because it prevents the water that bathes internal structures from evaporating. As a result the animal does not dry up and die. Arthropods, animals with a hard, outer skeleton and a jointed body and limbs, make up the single largest group of invertebrates. Arthropods include insects, crustaceans, and arachnids, such as spiders and ticks.

Invertebrates have two basic body plans. Some invertebrates, such as corals and sea anemones, have a circular body plan arranged around a central mouth, similar to the way spokes radiate out from the hub of a wheel. This type of body plan is known as radial symmetry. Animals with radial symmetry often spend their adult lives fastened in one place, like the sea anemone that attaches to a rock, waiting for food to pass by. By contrast, invertebrates that move in search of food, such as flatworms, have an elongated body plan known as bilateral symmetry. Invertebrates with bilateral symmetry have right and left halves that mirror each other, and they typically have a definite front and back end. They have a head that often contains one or more pairs of eyes, together with organs that can taste, smell, or touch. However, major sense organs are often found on other body parts among some invertebrates. Katydids, for example, have hearing organs on their front legs, just below knee like joints.

Compared to vertebrates (animals with backbones), most invertebrates have simple nervous systems, and they behave almost entirely by instinct. This system works well most of the time, even though these animals cannot learn from their mistakes. Moths, for example, repeatedly flutter around bright lights, even at the risk of getting burned. Notable exceptions are octopuses and their close relatives, which are thought to be the most intelligent animals in the invertebrate world. Studies have shown that these animals have the ability to learn. In some experiments they have solved simple puzzles, such as opening containers to retrieve food.

Invertebrates differ from each other internally in a wide variety of ways. Some have respiratory organs, circulatory systems, and excretory organs for getting rid of waste. The simplest invertebrates, such as placozoans, survive with few or no specialized organs at all. These animals absorb what they need from their surroundings—a way of life that works only in watery habitats and only with small animals.

III -TYPES OF INVERTEBRATES
Zoologists (scientists who study animals) classify invertebrates into about 30 major groups, known as phyla. These phyla vary enormously in the number of species they contain. Arthropods (phylum Arthropoda) are the invertebrate phylum with the most species—more than one million known species and countless more awaiting discovery. The mollusks (phylum Mollusca) make up the second largest group of invertebrates, with at least 50,000 species. Among the simplest invertebrates are the sponges (phylum Porifera). Other major invertebrate phyla include the cnidarians (phylum Cnidaria), echinoderms (phylum Echinodermata), and several different groups of worms, including flatworms (phylum Platyhelminthes), roundworms (phylum Nematoda), and annelids (phylum Annelida).

Arthropods live in every habitat on Earth from mountaintops to hydrothermal vents, springs of hot water located on the deep ocean floor. Surrounded by protective exoskeletons, arthropods have tubular legs that bend at flexible joints. This unique characteristic sets them apart from all other invertebrates, and it enables them to hop, walk, and run.

Insects dominate the arthropod phylum. Making up 90 percent of all arthropods, insects have a strong claim to be the most successful animals in the world. On land, they live in almost every habitat, aided by their small size and, for many, their ability to fly. They also live in fresh water, but remarkably, they have failed to colonize the sea. Some zoologists believe this is because crustaceans have already exploited this habitat to its fullest.
Mollusks make up the second largest group of invertebrates. Even by invertebrate standards mollusks are extremely varied. Mollusks include snails, clams, octopuses, and squid, as well as some lesser-known animals, such as chitons and monoplacophorans. Some mollusks, such as bivalves, are sedentary animals, while others such as squid are jet-propelled predators that are the swiftest swimmers in the invertebrate world. Most sedentary mollusks are filter feeders—that is, they feed on tiny organisms that they strain from water. Other mollusks, including snails and other gastropods, scrape up their food using a radula—a ribbonlike mouthpart that is unique to mollusks and covered with rows of microscopic teeth.

Sponges have many unique characteristics that set them apart from other kinds of animal life. They are the only animals with skeletons made of microscopic mineral spikes and the only ones that feed by pumping water through hollow pores. Some of their cells are remarkably like free-living protozoans called collar flagellates. To evolutionary biologists, this resemblance strongly suggests that sponges and other invertebrates arose from protozoan-like ancestors.

Cnidarians include jellyfish, sea anemones, and corals. Their bodies have two layers of cells, a central digestive cavity, and a mouth surrounded by stinging tentacles. Most cnidarians are quite small, but the largest jellyfish—a species from the North Atlantic Ocean—can grow over 2 m (7 ft) across, with tentacles over 30 m (100 ft) long.

Among the major phyla, the echinoderms are the most distinctive and unusually shaped. They include starfish, sea urchins, and sea cucumbers and are the only animals with a five-pointed design. They live in the sea and move with the help of tiny fluid-filled feet—another feature found nowhere else in the animal world.

Zoologists recognize several different groups of worms. The phylum known as flatworms contains the simplest animals possessing heads. Nerves and sense organs are concentrated in the head. Most flatworms are paper-thin and live in a variety of wet or damp habitats, including the digestive systems of other animals. Roundworms represent another phylum. They are more complex than flatworms, with cylindrical bodies and mouthparts designed to pierce their food. Although flatworms have digestive systems with only one opening, the roundworm digestive system runs from the mouth straight through its body to an excretory opening—a body plan shared by more advanced invertebrates as well as vertebrates.

Although roundworms are extremely abundant, they often go unseen. So, too, do many worms that live exclusively in the sea, such as spoonworms (phylum Echiura), peanut worms (phylum Sipuncula), and pogonophores (phylum Pogonophora). Annelids are a large group of worms that contain some more familiar species. Among them are earthworms—annelids that feed by burrowing through the soil. An earthworm’s body is divided into repeated segments or rings, a feature shared by annelids as a whole.

IV -REPRODUCTION AND LIFE CYCLE
Invertebrates display a wide variety of methods of reproduction. Some invertebrates reproduce by asexual reproduction, in which all offspring are genetically identical to the parent. Asexual reproduction methods include fragmentation, in which animals divide into two or more offspring, and budding, in which animals sprout buds that break away to take up life on their own. The majority of invertebrates reproduce sexually. The genes from two parents recombine to produce genetically unique individuals. For most invertebrates, sexual reproduction involves laying eggs. With a few exceptions, such as scorpions and spiders, most invertebrates abandon their eggs as soon as they are laid, leaving them to develop on their own.

When invertebrate eggs hatch, the animals that emerge often look nothing like their parents. Some are so different that, in the past, zoologists mistook them for entirely new species. Young like this are known as larvae. As they grow up, larvae change shape, a process known as metamorphosis. A larval stage enables invertebrates to live in different habitats at different stages of their lives. For example, adult mussels live fastened to rocks, but their larvae live floating among plankton. By having larvae that drift with the currents, mussels are able to disperse and find homes with new food sources for their adult life.

The change from larva to adult is quite gradual in many invertebrates, such as crabs and lobsters, but in insects it can be much more abrupt. Caterpillars, the larvae of butterflies and moths, often live for several months, but they take just a few days to turn into adults. During the transition stage, known as the pupa, the caterpillar’s body is broken down and reassembled, forming an adult insect that is ready to breed.

Most invertebrates are short-lived animals, but slow-growing species often break this rule. Wood-boring beetles can live well into their teens, while queen termites can live 40 years or more. But in the invertebrate world, the real veterans live in the sea. Growth lines on bivalve shells suggest that some clams can live to be 400 years old or more. An age of about 200 years has been claimed for pogonophoran worms living around hydrothermal vents in the darkness of the deep seafloor.

V -EVOLUTION
As the simplest animals, invertebrates date back to the time when animal life first began in ancient shallow seas. Zoologists are uncertain when this was, because the first invertebrates were small and soft-bodied and left no direct fossil remains. However, some scientists believe that strange patterns preserved in sedimentary rocks dating back to 1 billion years ago may be the fossilized tracks and burrows of ancient invertebrates. Other scientists, studying genetic material in living animals, believe that the earliest invertebrates may have appeared even earlier and may already have begun to separate into different phyla before 1 billion years ago.

The oldest recognized fossils of invertebrates date back to the close of the Precambrian period, about 550 million years ago. The best known of this fossil finds, from the Ediacaran Hills in southern Australia, include animals that look like jellyfish and annelid worms. Zoologists disagree about their status. Some think that they might well be ancestors of animals alive today, but others believe they belong to a group of invertebrates that eventually became extinct.

With the start of the Cambrian period 542 million years ago, invertebrate life evolved with almost explosive speed. Due to the appearance of the first invertebrates with exoskeletons, the fossil record provides a rich record of invertebrate life in the Cambrian period. By the time the Cambrian period ended 488 million years ago, all the invertebrate phyla alive today were established.

Between that time and the present, invertebrates spread through the seas and also invaded land. Scientists believe that the first land dwellers were almost certainly arthropods, including the forerunners of wingless insects. During the Carboniferous period, which began 359 million years ago, flying insects appeared, including giant dragonflies with a wingspan of up to 75 cm (30 in). But on land the great expansion of invertebrate life occurred during the Cretaceous period, which started 145 million years ago. Flowering plants first evolved in this period, enabling insects to exploit a whole new source of food and triggering a huge growth in insect life that has continued to this day.

While many invertebrates flourished, some of the most successful groups of invertebrates in the fossil record nonetheless became extinct. Giant sea scorpions and trilobites were types of arthropods that thrived for much of the Paleozoic era, about 270 million years ago, but were unable to survive the great mass extinction at the end of the Permian period 251 million years ago. Ammonites (mollusks related to today’s octopuses and squids) fared better. They first appeared during the Silurian period about 440 million years ago and lived into the Mesozoic era, only to vanish at the same time as the dinosaurs, about 65 million years ago. Their intricate massive spiral shells were often superbly preserved as fossils, some measuring almost 2 m (7 ft) across.

VI -IMPORTANCE OF INVERTEBRATES
The continued prominence of invertebrates, measured by their great diversity and abundance, indicates that these animals have adapted to their ecosystems over millions of years. In so doing, invertebrates have become necessary to the health of Earth’s ecology. For instance, all ecosystems support one or more food chains that form food webs. Each chain begins with plants, known as primary producers, which convert light energy into food. Primary producers are eaten by primary consumers, and secondary consumers eat the plant-eating primary consumers. Decomposers derive their energy from the dead remains of plants and animals. Invertebrates occupy several niches in this food web, acting as primary consumers, secondary consumers, and decomposers.

Many invertebrates have a direct and invaluable impact on their environment. For example, the common earthworm burrows deep below the surface, consuming soil along the way. Coiled soil masses known as casts are excreted from the worm’s digestive system, making the soil more fertile. The earthworm’s burrowing action continually moves mineral-rich soil to the surface, which improves plant growth. The burrowing action also aerates soil, enhancing drainage. In another example, as honey bees, butterflies, and moths flit from flower to flower collecting nectar, they inadvertently transport pollen from the male reproductive structure of one flower to the female reproductive structure of another flower. Known as pollination, this leads to the fertilization of the plant’s seeds—an essential stage in the process of reproduction.

Other invertebrates form mutually beneficial partnerships with other animals. For example, some crabs form alliances with sea anemones, which they fasten to their backs. In this alliance, the crab is protected from predators by the anemone’s stinging tentacles. The anemone, in turn, receives food particles as the crab tears up meat from the animals it consumes. As the crab grows, it periodically sheds its body case. Before doing so, it removes the anemone, and then afterwards puts it back in place.

Humans sometimes share a troubled relationship with invertebrates. A number of invertebrate organisms cause many parasitic diseases in humans and farm animals. These parasites survive by feeding and reproducing inside a host, often causing internal destruction. Some of the most damaging parasites include the flatworm Schistosoma, which causes schistosomiasis; the roundworms that cause hookworm infection; and the roundworm larvae of Trichinella spiralis that cause trichinosis. Other invertebrates are agricultural pests, destroying plant crops. Insects such as leaf beetles, flea beetles, and young caterpillars feed on the leaves, stems, roots, and flowers of plants. Sucking insects, including aphids, leafhoppers, and scales, remove plant sap, weakening the plants. Sucking insects can also spread disease-causing viruses and bacteria to plants. The larvae and adult stages of some roundworms are parasites of plants. Using specialized structures called stylets, these roundworms pierce plants at the roots to extract cell content, killing the plant.

Although invertebrates can cause problems for humans, they are more often beneficial. In many cultures, invertebrates such as squid, octopuses, cuttlefish, clams, mussels, crabs, and lobsters are considered popular food items. Scientists use invertebrates for a variety of experiments that have profound benefits for human health. Scientists have performed delicate surgery on the glandular systems of caterpillars and roaches to uncover clues to the function of glands in humans. In other experiments, scientists have given spiders different types of drugs and observed the animals as they created spider webs. The different pattern of spider webs offered a way to distinguish and measure the effects of various drugs.

The vinegar fly Drosophila melanogaster, also known as the fruit fly, has long been the standard test subject in the field of genetics. In the 1910s and 1920s American geneticist Thomas Hunt Morgan used the vinegar fly to demonstrate that genes lie in a linear fashion on chromosomes, establishing the chromosomal basis of inheritance. In early 2000 studies of vinegar flies continued to advance the field of modern genetics when researchers sequenced the fly’s entire genetic makeup, or genome. The techniques used to reveal the vinegar fly genome were then applied to the efforts to decode the human genome.

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