Evidence for Evolution
The fact that evolution has taken place can be established by taking several kinds of evidences that are available. These evidences can be either 'direct' or 'indirect'. Direct evidences interestingly are provided by organisms, which have now become extinct, while indirect evidences are available from the study of organisms that are existing today - extinct organisms.
Evidences from Palaeontology
The branch of biology, which deals with the study of extinct organisms that are now available only in the form of fossils, is called
palaeontology. The study of fossils indicates the structural features of organisms that existed then. A comparison of this with the existing forms gives a clear indication as to how evolution has taken place.
Fossils are the organic remains of plants and animals that existed long, long ago. An entire organism or a part of its body may have become buried in the deeper sediments of the earth resulting in the formation of fossils.
Fossil Formation
Since traces of animals are found deeply embedded in the solid rock, the study of fossils includes the study of how these were formed. There are three large classes of rocks
Sedimentary rocks
, which were formed by packing together of sediment such as, mud and sand.
Igneous rocks
which were at one time the molten lava
Metamorphic rocks
, which were at one time sedimentary or igneous, but distorted by heat and pressure of overlying rocks.
Fossils are available only in sedimentary rocks.
Since more recent rock layers are on the top, the oldest fossils are found in the deepest rocks. In many places old rocks have become exposed due to erosion and weathering. Although sedimentary rocks are our major sources of fossil information, they are not the only source. Fossils have also been found in the tarpits, in amber (the fossilised resins of evergreen trees) and in ice. Excellently preserved insects have been found in amber while the entire Woolly Mammoths have got preserved in ice.
By arranging the fossils in the order in which they have appeared on earth many fossil series have been established. Such series demonstrate as to how various forms of life have changed gradually over a period of millions of years. As a result, the evolution of a number of animals such as horse, camel and elephant, has been clearly understood. Plenty of fossil evidence is now available so as to establish human evolution too. Wherever it is possible to determine the age of a rock, the age of a fossil enclosed in it has also been established. Conversely, where the age of the fossil is known, the age of the rocks can be directly determined. This process of finding out the age of a fossil is known as
dating of fossils. The most effective way of dating is to measure the amount of radioactive material in the rock or soil. Half-life periods are calculated on the basis of the ratio of the radioactive material and its products of decay. Dating of fossils has provided an accurate determination of the age. The most commonly used radioactive materials are uranium and carbon-14. Even potassium-argon is being used.
Through extensive study of fossils, scientists, geologists and palaentologists have been able to construct a story of life in the form of a
geological time scale. It lists the major geological periods of time and the types of life forms that existed then. These major periods of time are called eras. The eras have been subdivided into periods and periods have been subdivided into epochs. The following table summarises the major geological conditions and the biological features of different eras.
A well-known example of a fossil is the ancient bird Archaeopteryx. It showed a number of features like wings, feathers and beak seen in the present day birds. It also showed certain features like presence of teeth, presence of a long tail, and scales on the body, characteristics that are seen in reptiles. Thus, Archaeopteryx represents an extinct link between reptiles and birds.
Types of fossils
Evidences from Homologous Organs
One of the indirect evidences for evolution comes from the study of homologous organs in organisms that exist today. Homologous organs are those, which have the same basic structure, but different functions. For example, an examination of the forelimb skeleton in different groups of land vertebrates reveals that the number and arrangement of bones remains the same. However, the forelimbs are used for diverse functions in different groups. In frogs, forelimbs are not generally used for locomotion or swimming. In a lizard, forelimbs are as prominently used for locomotion as the hind limbs. In birds, they are modified into wings. In mammals, they show further diversity. A bat's patagium, a man's arm, a horse's fore leg and a whale's flippers serve totally different functions, but possess the same basic structure. Such a structural homology indicates a probable common ancestry for the different groups.
Evidences from Embryology
Embryology is another branch, which offers some indirect evidences for evolution.
An embryologist by name Haeckel, proposed the idea 'ontogeny recapitulates phylogeny' which means that the evolutionary history (phylogeny) of a species is indicated by the developmental stages (ontogeny) that it passes through. In the course of development from a zygote into an adult, the various embryonic stages are thought to represent the various ancestral stages of that species. In the development of frog, the tadpole stage almost resembles a fish, indicating its fish ancestry. In birds, the young chick at the time of hatching has sedimentary teeth, which are lost subsequently, indicating the reptilian ancestry. Similarly, human embryos in their various stages of development resemble fish, chick, rabbit and monkey embryos. All mammals develop gills in their embryonic stages and lose them later. Even a whale loses them although they live in water. This is suggestive of a fish-like ancestry for all these groups.
Evidences from vestigial Organs
There are some structures in the body of organisms, which have no apparent use. Such structures are described as 'vestigial'. Vestigial organs indicate that they must have been present in a form in which they were highly functional in the ancestral forms. The human body is described as 'a living museum of vestigial organs'. More than 100 vestigial structures have been identified. A nictitating membrane, the vermiform appendix, the vestigial tail, the external ear muscles, to name a few, are vestigial structures in the human body. Flightless birds have vestigial flight muscles. The python has vestigial hind limbs. All these and many more clearly indicate that such structures have lost their significance in the course of evolution.
Evidences from geographical distribution
The distribution of plants and animal species in different geographical areas of the world is also a form of indirect evidence to show that evolution has taken place. For example, Australia was, until recently, populated only with marsupial mammals and it had no placental mammals. The inference is that Australia was once connected with the rest of the world and got separated (continental drift) after mammals had begun to evolve, but before placental mammals had emerged.
Biochemical Evidences for Evolution
One of the strong indirect evidence for evolution comes from the study of Biochemistry of living organisms. Bio-chemically living organisms show considerable similarity. All organisms contain the same type of molecules, and almost all organisms control themselves through information contained in DNA. Not only that, the genetic code that could take infinite forms, is the same in every organism studied. The mechanism of protein synthesis is largely the same in all living organisms studied so far. These and other biochemical studies strongly suggest a common ancestry for all forms of life.