Question: What happened when gorillas eventually evolved into humans, but some stayed as gorillas or monkeys?

Sam Tazzyman answered on 13 Jun 2011:

This is a good question, because we can really get to understand about evolution by thinking about this. In fact when people say that humans are descended from gorillas (or chimps, or monkeys) they don’t actually really mean that gorillas turned into humans but some stayed as gorillas. As you have questioned, this would be strange – why would some stay as gorillas?

What actually happens is this: there was some other ancient species that no longer exists. It probably looked a bit like a gorilla and a bit like a human. There were probably some populations of this species that lived in the jungle in the mountains. In these populations, being a bit more gorilla-like than human-like was better, and so over many thousands of years they gradually became gorillas. In the populations that lived on the plains, meanwhile, being human-like was better than being gorilla-like, and so over many thousands of years they became humans. So it wasn’t that gorillas turned into humans: instead there was something that was around before there were either gorillas or humans, and that thing gradually turned into gorillas in some places and humans in other places.

You can see the whole family tree of the primates (primates means monkeys, lemurs which live in Madagascar, and apes like humans and gorillas) here:

http://whozoo.org/mammals/Primates/primatephylogeny.htm

If you look at the bit where humans and gorillas are, you can follow the tree back to see where the two species split. You can also see that chimps and bonobos split from the human bit of the tree after the gorillas did, which shows us that humans are more closely related to chimps and bonobos than to gorillas.