If this isn't b8... The way species evolve is through mutations. You know, like people being born with an extra finger or being born albino? Those are mutations. Now, when a helpful mutation appears (for example, opposable thumbs) the specimen with the mutation has a higher chance of surviving to reproduce. If it passes on the mutation to its young, they too have a higher chance of survival, and so on and so forth. Eventually, if a group of individuals is isolated for long enough, they'll develop mutations that help them survive in their environment. This is why different monkeys exist, because they're specially suited to deal with their selective environments. Speed this up and eventually different species form until eventually you get apes then humans. Get it?