Evidence of Evolution Part 2
Darwin’s second line of evidence comprised the developmental and structural remnants of past ancestry that we find in living species.
- A structure or organ is vestigial if it has diminished in size or usefulness in the course of evolution. Vestigial structures are markers of evolutionary descent. For example, boa constrictors, which are descended from four-legged reptiles, grow tiny hind legs. Duckbill platypuses, which are descended from extinct platypus species that had teeth as adults, grow and re-absorb teeth before birth. In human beings, the vermiform appendix marks descent from mammals that had a much larger sac in this position and used it to digest their high-cellulose diet (as many species, including other primates, still do).
- Recent studies of the human genome provide more evidence. Our genome contains non-functional DNA, including many inactive “pseudo-genes” that were functional that were functional in our ancestors. Why do humans, unlike most mammals, require vitamin C in our diet? Because primates cannot synthesize vitamin C from simpler chemicals. Yet we still carry all the genes for synthesizing vitamin C. The gene used for the last step in the synthesis was inactivated by mutations 40 million years ago. Probably because synthesis was unnecessary in fruit eating primates.
