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The theories that explain the evolution are hypothetical. There is no practical proof for them. In fact, it is not possible for anybody to observe even a single change in favor of evolution that occurs in the body of organism as our life span is too short to notice such slow changes. Hence scientists collected evidences from different branches of biology. Some of them are:
a) Evidences from Comparative anatomy
b) Evidences from Paleontology 

a) Evidences from Comparative anatomy/ Anatomical evidences {vestigial structures}
The organs which were functional in the ancestors but non-functional and reduced in the descendants are called vestigial organs. Presence of vestigial organs is the most convincing evidence in favor of organic evolution and also supports the concept of disuse proposed by Lamarack.
E.g. Hind limbs in python, hind limbs and pelvic girdle in whale , wings of flightless birds, vermiform appendix, coccyx, plica semilunaris, hair on the body, etc., in humans.

b) Evidences from Paleontology/Palaeontological evidences
Paleontology is the study of prehistoric life through fossils.Fossils are the remnants of plants or animals that were preserved in the layers of the earth and have been excavated from the soil.They are of various types like moulds, casts, petrifications, coprolites, actual remains of animals preserved in ice, etc. They support the idea that life has gradually evolved on the earth. The biologists and paleontologists have found the fossils of many transitional forms which link all the major groups of vertebrates.
E.g. Eusthenopteron between fishes and amphibians, Seymouria between amphibians and reptiles, Archaeopteryx between reptiles and birds, Cynognathus between reptiles and mammals etc.

[The age of fossil is calculated by using Carbon¹⁴, Uranium²³⁸ and Pottasium⁴⁰. Of these, as C¹⁴ is commonly used to determine the age of comparatively recent and this method is called Radio Carbon Dating method]
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