Snakes are crafty and diverse. Some catch their prey by waiting in ambush. Some actively hunt. Some are venomous. Some kill by constriction. Some use mimicry to fool would be predators. Some even fly. According to Genesis, some snakes talk.
Having never had a conversation with a snake, I am a bit skeptical that this ever happened. To be fair, there are examples of animal communication: whales and birds sing, bees dance, some great apes have learned basic sign language, parrots can talk, and this dog is capable of expressing complex human emotions in English. Unfortunately all of these examples fall well short of what the Genesis serpent demonstrates. The whales, birds, and bees don’t exactly speak to humans, apes have to be taught the language and even then their ability is limited, parrots mimic human speech and haven’t proven themselves as adept conversationalists, and that dog’s owners are just hearing what they want to hear.
In Genesis, the serpent is not only capable of creating human language sounds, but he’s also able to think critically and argue with some eloquence. Language, like all thought, is done in the brain. The encephalization quotient (an intelligence predictor based on relative brain size) for reptiles is about one tenth of that of mammals. For some perspective on how wide that gap is, a sheep’s encephalization quotient is about one tenth of a human’s. If snakes can talk, and brains make language possible, it might seem reasonable to expect a lot of talking animals. This is obviously not the case. Snakes clearly cannot talk.
There are a few ways to rationalize the talking serpent at the expense of reason. The most common is that the serpent was actually Satan in the form of a snake, but as Greg pointed out, there isn’t any evidence to support this theory. Perhaps this serpent is from a species that’s now extinct and no fossil record has survived. That would certainly be convenient. Maybe God made an advanced race of snake-men. Their punishment (Gen. 3:144) makes more sense in this context. It’s possible Eve ate some psychedelic mushrooms and had a bad trip. Or maybe the author of Genesis did.
The snake brain does not provide sufficient cognitive power for our serpent antagonist to have the conversation in Genesis 3. It’s more likely that the serpent is content to warm itself in the sun and lie in wait for an unsuspecting rodent. The human brain, in contrast, is one of the most amazing results of evolution. It’s capable of incredible abstraction and creativity: mathematics, logic, music, language, poetry, and as clearly demonstrated in Genesis, fiction.