A long time ago, a population of apes lived in the middle of the jungle, eating the plentiful

plants on the ground and occasionally searching out rarer fruits in the trees.

But as the once-shallow Congo river in the middle of their range started to get wider

and deeper, the ape population was split in two: though the North and South of the river

contained the exact same kind of apes, they were now fatefully confined to separate environments.

From fossil evidence, we know those environments were ever-so-slightly different from each

other; it just so happened that the one South of the river had lots more plants on the ground

and the one North of the river had big mean gorillas.

And most researchers think that these two small differences caused the groups of apes to eventually split

into two species: one that was violent and one that was super-snuggly.

Why did they end up so different?

Well, if you were an ape north of the river, there would be lots fewer ground plants to

eat and big mean gorillas already eating them, so you'd be more likely to search out rarer

fruit up in the trees instead.

If you were more aggressive than other Northern apes, you'd be more likely to get more fruit

when you did find it.

And because you got more fruit, you'd not only be more likely to survive but you could

use that fruit as a way to attract a potential mate.

On the other hand, if you were an amiable ape living in the North, you'd likely get

less fruit and fewer opportunities to mate.

But South of the river, there would likely be plenty of ground plants to go around - and

no gorillas - so being aggressive wouldn't get you more food.

In fact, in a place where there was no reason to fight over food, being a jerk could make

you an outcast - an easier target for predators and much less likely to find a mate.

But if you were an amiable ape in the lush South, you'd likely make more friends, and

thus be less likely to be eaten by a predator and more likely to get to reproduce.

After thousands of generations, these slight changes in the advantageous-ness of certain

traits led to entirely new species: North of the river - gorilla country - was stocked

with aggressive apes likely to attack any outsiders they came across - these are chimpanzees.

And the lush South was packed with bohemian simians that took friendliness to such an

extreme that they were more likely to make love than war - these are bonobos.

Now, we aren't sure exactly how much the gorillas mattered versus the vegetation.

And there may have been other factors that contributed to the split.

But when the Congo River got big, it turned the apes into two species: one that solves

conflicts with fighting, and another that solves them by cuddling.

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