Probably back in some previous biology course you learned that snakes evolved from lizards, and that the first snakes weren't venomous and then along came more advanced snakes, the venomous snakes.

也许在以前的生物课上，你学过蛇是由蜥蜴进化而来的，而且最初的蛇是没有毒的，后来出现了更高级的蛇，也就是毒蛇。