Snakes have no legs, so where did their legs go? Scientists still debate where snakes came from: some think burrowing lizards on land, others marine reptiles, and the fossil evidence is limited. As of January 2015, the world's oldest known snake fossil was Eophis underwoodi, about 167 million years old, from the Middle Jurassic. Only fragments were found, in a quarry in Oxfordshire, England, so scientists must work out what the animal was like from small pieces. The fossil Coniophis precedens, about 65 million years old, was called an extremely rare transitional form. Its study supports snakes arising after lizards grew long, limbless bodies, apparently for burrowing.
As of April 2000, scientists had described Haasiophis, a 95-million-year-old fossil snake with two legs. Some had thought legged fossil snakes linked snakes to ocean lizards called mosasaurs, but its describers found it was an advanced snake, like pythons and boas, so it could not show where snakes began. They also suggested limbs may have been lost several times within snakes. As of November 2019, a study of the fossil snake Najash found that legged snakes living nearly 100 million years ago still had a cheekbone, the jugal, that has almost vanished in modern snakes. The team also concluded that snakes kept small hind legs for the first 70 million years of their history. A 2015 fossil named Tetrapodophis was described as the first four-legged snake, about 120 million years old, but some scientists argue its crushed skull is too poorly preserved to be sure it was a snake.
Body shape can fool scientists building family trees. Burrowing lizards tend to evolve long bodies and reduced limbs even when they are only distantly related. As of April 2015, a team led by John Wiens warned that anatomy alone can mislead, and the team added DNA to the data. As of April 2015, a study combining living and fossil species found that the mosasaurs, giant sea lizards that died out with the dinosaurs, are close relatives of snakes. No mosasaur DNA exists, so the answer came from the tree's pattern. An international team built the largest, most comprehensive evolutionary tree of snakes and lizards, covering nearly 1,000 species, and linked snakes' big spread of kinds to the last 66 million years.
As of August 2016, Bangor University researchers had changed the Oct4 gene in mouse embryos to test how snakes got so long, and the mice grew more trunk vertebrae. That supported the idea that a change in when Oct4 turns on and off helped make snakes' long trunks. As of November 2026, University of Florida researchers had traced, in 2016, snakes' lost legs to mutations in a genetic switch, called an enhancer, that turns on the Sonic hedgehog gene during limb formation. Pythons and boas keep vestiges of leg structures. A wild snake is best left alone: it will usually go its own way. If one is near a home, a grown-up can call animal control.
Words to know
- enhancer: genetic switch
- vestiges: small leftovers
- mosasaurs: giant sea lizards
- fossil: ancient remains
