Evidence for evolution

Comparative anatomy compares the structure of organs. Homologous organs have the same origin and structure but may have different functions: the human arm, whale flipper, bat and bird wings – all have humerus, radius, ulna, wrist bones and finger bones; in plants a pea tendril and a barberry spine are modified leaves. Homology shows a common ancestor (divergent evolution). Analogous organs have similar functions but different origins: the wings of a butterfly and a bird, a cactus spine (leaf) and a hawthorn spine (shoot); they result from adaptation to a similar environment (convergence). Vestigial organs were fully developed in ancestors and are now reduced: the side toes of a horse, the pelvic bones of a whale; in humans the coccyx (tailbone), wisdom teeth and ear muscles (the appendix has also shrunk and has other functions). Atavism is the return of an ancestral trait in some individuals: a tail-like outgrowth or thick hair cover present from birth in humans (very rare), side toes in horses. Palaeontology: fossils show the history of life; transitional forms link groups: Archaeopteryx (about 150 million years ago) – toothed, long-tailed, with feathers – has reptile and bird features; Tiktaalik (about 375 million years ago) lies between fish and the first land vertebrates; the horse phylogenetic series runs from small many-toed ancestors through three-toed forms (for example Hipparion) to the present one-toed horse; whale ancestors show the move from land hoofed animals to water. Fossils lie in order in strata: simple and ancient in deep layers, more complex and closer to modern forms above.