Reconstruction of the ancestral crown-group snake based on this study. Illustration by Julius Csotonyi.

Snakes are a diverse bunch, with more than 3400 species around today. Some are tiny, like the 4-inch Barbados threadsnake. Some are (or were) huge, like pythons, anacondas, and the ancient school bus-sized Titanoboa. Some kill with constriction, others with a blend of toxins. They live in forests, grasslands, and deserts, and some are even at home in the open ocean. What was the snake at the root of this complex family tree like? A team of Yale researchers gives us the best and most complete model yet in a new study.

Reconstructing the evolutionary history of snakes has historically been a tough gig because of a lack of useful fossils. Over the last decade, though, scientists have uncovered fossil snakes that are more complete and better preserved than what had been available before, giving us a clearer look at how snakes have changed (or stayed the same) over time. With that information finally available, the Yale team decided to piece together “when, where and how snakes originated.” 

The researchers, led by Allison Hsiang, analyzed and compared the genetic data and anatomical features of 73 snake species, both living and extinct, to construct a comprehensive snake family tree. They also used a method called “ancestral state reconstruction” to infer when and where in the tree certain traits appeared, which gave them an idea of how the animals’ most recent common ancestor would have looked and behaved. 

Their tree suggests that snakes originated around 130 million years ago on what was then the supercontinent of Gondwana. The earliest snakes likely lived in warm, moist forests, were active at night, hunted by stealth and, like most of their descendants, swallowed small vertebrates whole. These ancient snakes probably didn’t differ much in appearance from modern ones, except for one notable feature: they had legs. 

Early snakes likely retained “tiny hindlimbs” from their own ancestors, complete with ankles and toes. These vestigial limbs probably weren’t used for moving around, though—even in their earliest days, snakes had to slither.