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. Yet how and when they evolved remains poorly understood. Up to now, most research on venom evolution has focused strictly on the toxins. However, using toxins present in modern day animals to trace the origin of the venom system is difficult, since they tend to evolve rapidly, show complex patterns of expression, and were incorporated into the venom arsenal relatively recently.
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Comparative transcriptomics revealed that the network is conserved between venom glands of snakes and salivary glands of mammals. This suggests that while these tissues have evolved different functions, they share a common regulatory core, that persisted since their common ancestor."

Source : PNAS 118 (14)

Also see :Snake evolutionplugin-autotooltip__plain plugin-autotooltip_bigSnake evolution

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.