This supports the idea that snakes and venomous lizards evolved from a common venomous ancestor, and after connecting the DNA dots, Fry and his colleagues traced venom to a single origin 200 million years ago.

"That's also when the small, bite-sized animals were starting to exist. Any time there's a new food source you see the emergence of a new predatory trick," Fry told LiveScience. "In this case, venom was the new trick."

Which came first, the TV dinner or the microwave?

The common ancestor had venom glands on both its upper and lower jaws. Since then, snakes have evolved to having glands on just their upper jaw – glands on the lower would make it difficult to swallow prey.

A very intelligent design decision...

The Surprising Origin of Venom Revealed - Yahoo! News