Evidence of evolution from comparative anatomy

The argument

Comparative anatomy — the study of similarities and differences in the body structures of different species — provides strong evidence for evolution by revealing patterns that only common descent explains. The clearest example is the homologous structure: the same underlying skeletal plan, modified for very different uses, appearing across related species. The forelimbs of a human arm, a whale's flipper, a bat's wing and a cat's leg look and function quite differently, yet beneath the surface they share the same arrangement of bones — one upper bone, two lower bones, and a set of wrist and digit bones — inherited from a common ancestor and adapted by evolution for grasping, swimming, flying and walking. A designer starting fresh would have no reason to build a flipper and a wing from the same bones; shared ancestry explains exactly why they would. The argument is reinforced by vestigial structures — features that are reduced and functionless remnants of organs that were useful in an ancestor. Humans carry a tailbone and an appendix, whales and snakes retain tiny remnants of hip and leg bones, and flightless birds have stunted wings. These make sense only as leftovers of evolutionary history, structures inherited from ancestors in which they served a purpose and since rendered redundant. Their existence is puzzling on any view that each species was independently designed, but expected if species descend with modification from earlier forms. From this standpoint, the body plans of living things are a record of their shared ancestry, written in homologies and vestiges that trace the branching tree of life. Comparative anatomy reveals organisms to be modified versions of common templates, exactly as descent with modification predicts. Because comparative anatomy supports the theory in this way, this argument holds, the theory of evolution by natural selection is correct.

Premises

Counter-arguments

Critics from a design perspective reply that homology can be read as common design — a designer reusing an efficient limb template — so shared bone patterns underdetermine common descent versus common design. This is a minority objection; the scientific consensus reads homology as strong evidence for descent. On vestigial structures, they note that several organs once called functionless remnants (the appendix, the coccyx) have since been found to have roles, so specific 'useless leftover' claims have needed revision. On the mainstream view these caveats don't overturn the nested pattern of homology and genuine vestiges, which remains powerful evidence for evolution — but they flag that the inference from similarity to descent, and blanket 'functionless' assertions, carry assumptions worth stating.

Rejecting the premises

[Rejecting P1] Design critics argue homologous limb bones can be read as a reused design template, so shared anatomy underdetermines descent versus design (a minority objection; the consensus reads homology as strong evidence for descent). [Rejecting P2] Several structures once called vestigial (the appendix, the coccyx) have been found to have functions, so 'functionless remnant' claims have needed revision. [Rejecting P3] On the mainstream view these caveats don't overturn the nested pattern of homology and genuine vestiges, which remains powerful evidence for evolution; they flag interpretive assumptions rather than refuting it.

Further reading

https://www.nap.edu/read/6024/chapter/4