Snakes are thought to have evolved from four-legged reptiles, and the oldest snake fossils are dated at 150 million years old
Four groups of legged snakes lived in the cretaceous period, though their limbs were small and were no longer used. Most scientists think that snakes evolved from an as-yet-undiscovered terrestrial lizard, the probable ancestor of monitor lizards.
Early reptiles are thought to have experienced a disruption in the expression of the genes that were responsible for growing limbs and the supporting vertebrae, or back bones.
The scientific community is split, however, with some researchers thinking it was the other way around. The simple body plans of early snakes may have been a stepping-stone to the more complex designs seen in lizards. A 2015 study compared snake and skeletons and found their vertebrae to be more complex than expected. If snake evolution was a process of simplification, then the vertebrae might be more simply shaped. As it stands, snake spines are divided into the same number of sections as lizard spines, though research is ongoing.