Snakes and legs

The Epoch Times is reporting the appearance of a snake with hindlegs in Shandong, China. Such reappearances of long-lost traits are called "atavisms", and in this case it appears this specimen has silenced genes that cause limb buds to stop growing and be resorbed. Quite a number of snakes form hind limb buds but lose them as they develop, while pythons retain spurs into adulthood and use them for mating.

What's interesting about this is not that atavisms exist from time to time, because we know now a lot more about the developmental triggering on genes and the conservation of genes across long distances of evolutionary time. What is interesting is that it means that the classification of snakes as Tetrapods is not silly.

A joke among taxonomists, or some taxonomists at any rate, is that snakes have four limbs in an unusual way (by not having them). Philosophically this might seem a dumb thing to say, but snakes are, in fact, derived tetrapods, and it's not that their limbs are present by not existing, but that the genes and developmental processes that generate limbs exist in snakes but are highly derived from their legged ancestor.

Many snakes have a pelvic girdle, and Cretaceous snakes have been found with hind limbs. It is thought that snakes separated from other reptiles in the Cretaceous, and they differentiated into the diversity we see today after the extinction of dinosaurs in the Paleocene.

Classification in modern biology relies on historical groups. A "clade" is a single species and all of its descendents. Since snakes arose within the clade of four legged organisms, they are tetrapods whether they have four limbs or none, since to remove them from that clade would make it an arbitrary group. Likewises, whales and dolphins are mammals despite not having hair or hind limbs (although whales also often have vestigial or atavistic hind limbs). Atavisms are an indication of the ancestry of a species or group, and show us that the only natural way to classify is to base our classes on evolutionary history.

Hat tip: John Latter.