Evidences from Comparative Anatomy

Comparative anatomy studies structural similarities between groups of organisms as indicators of evolutionary relationship. Homologous structures share a common origin/basic plan but may serve different functions — vertebrate forelimbs are the classic example: the humerus, radius, ulna, carpals, metacarpals and phalanges of a human hand, bird wing, bat wing, whale flipper, horse leg and cat leg all follow the same underlying skeletal blueprint despite performing very different jobs, illustrating divergent evolution. The thorn of Bougainvillea (a defence structure) and the tendrils of Cucurbita/Pisum sativum (climbing supports) are a plant example of the same principle. Analogous structures, by contrast, look or function alike but arose independently and share no common structural origin — insect and bird wings both enable flight but are built completely differently, illustrating convergent evolution; the eyes of an octopus and of mammals, and the flippers of penguins and dolphins, are further examples, as is the storage function shared by sweet-potato roots and potato stems despite arising from different plant organs. Vestigial organs are structural remnants that were functional in an ancestor but have become non-functional through disuse — the human vermiform appendix, a shrunken remnant of the caecum still functional for cellulose digestion in herbivores like the rabbit, is the textbook case; the coccyx, wisdom teeth, ear muscles, body hair, male mammae and the nictitating membrane of the eye are further human examples. A connecting link shows characters of two different groups, marking a tra …