Current evidence suggests that snakes started to evolve less than 150 million years ago, but the debate about the evolutionary origins of snakes have not yet been definitively resolved, despite more than 100 years of research.
There are two competing theories. The first suggests that snakes evolved from land-based lizards which burrowed - eventually losing their fore-limbs and then the hind-limbs.
An alternative hypothesis, based on morphology, suggests the ancestors of snakes were related to Mosasaurs - extinct water-based reptiles from the Cretaceous era - which in turn are thought to have derived from Monitors (Varanidae) and Water Monitors (Lanthanotidae).
Recent DNA studies have not been able to definitively clarify their origin.
See Wikipedia
Also see :Snake venom evolutionplugin-autotooltip__plain plugin-autotooltip_bigSnake venom evolution
Although it's clear that venom systems can give significant advantages to predatory snakes and other animals, the evolutionary pathways which lead to their development are unknown.
"Oral venom systems evolved multiple times in numerous vertebrates enabling the exploitation of unique predatory niches.