A Snake--With Legs! 

Python with vestigial appendage (hind limb bud). 

©Don & Pat Valenti/DRK PHOTO

Snakes don't have legs, right?  Wrong--look closely!  Pythons and boa constrictors have tiny hind leg bones buried in muscles toward their tail ends.  Such features, either useless or poorly suited to performing specific tasks, are described as vestigial.  They are also intriguing evidence of the evolutionary histories of species. 

Vestigial legs are a clue that snakes descended from lizards.  Over 100 million years ago, some lizards happened to be born with smaller legs, which, in certain environments, helped them move about unencumbered.  As generation after generation survived and reproduced, this new form flourished.  Over time, all members of the group were born with shorter legs, and eventually with no legs at all.  Almost.  The ancestor of boas and pythons retained very small vestigial legs, a trait passed on to its descendants, including the reticulated python seen here. 

Blind Cave Tetra Fish (Astyanax fasciatus mexicanus)

©Ken Lucas / Visuals Unlimited

Why Do Good Eyes Go Bad? 

Cave-dwelling tetra fish (Astyanax mexicanus) are blind; they have small vestigial eyes that do not work.  Then why have them at all?  Biologists have long struggled to explain how natural selection could fully account for such degenerations, and recently they have found another possible answer: Genetic mutations that hamper eye development also may increase the number of taste buds.  Thus, mutations that happened to give the fish an advantage in tasting and smelling--a huge benefit in a dark environment--might also have inadvertently, and harmlessly, caused the degeneration of their eyes. 

Human Vestiges

Human Vestiges: Erector pili (muscles that cause goose bumps), body hair, coccyx (tail bone), wisdom teeth, appendix

©AMNH

Humans also have vestigial features, evidence of our own evolutionary history.  The appendix, for instance, is believed to be a remnant of a larger, plant-digesting structure found in our ancestors.