Suddenly – and silently – you all cross the border. You fan out, still silent, looking for the adolescent males in this tribe. When you find them, you kill them. As for the rest, you scare them off with your screams and your charges, and, at the end, they’ve lost some of their own territory – and trees and plants and delicious grubs – while you’ve got just a little bit more. And you return, triumphant, with the bodies of your enemies, which you eat, with your tribe, in a victory dinner.
This all sounds horrid and nasty and mean and just not cricket. The scourge of war, as familiar to us today as it would have been to our most distant human ancestors. But war begins before we did, an inheritance which came to us from those species which came before us.
How do we know that ‘war’ stretches this far back into our past? A paper published in Current Biology and reported in THE ECONOMIST described how primatologists have seen this precise, coordinated, warlike behavior among chimpanzees, in their natural habitats in the rain forests of central African. The scene I just described isn’t ten million years old, or even ten thousand, but current. Chimpanzees wage war. This kind of tribal warfare is exactly what was commonplace in New Guinea and the upper reaches of Amazonia until relatively recently – certainly within the last few billion seconds. War is a behavior common to both chimpanzees and humans – so why wouldn’t it be something we inherited from our common ancestor?
War. What’s it good for? If you win your tiny tribal war for a tiny bit more territory, you’ll gain all of the resources in that territory. Which means your tribe will be that much better fed. You’ll have stronger immune systems when you get sick, you’ll have healthier children. And you’ll have more children. As you acquire more resources, more of your genes will get passed along, down the generations. Which makes you even stronger, and better able to wage your little wars. If you’re good at war, natural selection will shine upon you.
What qualities make you good at war? You’re good at war if you and your tribe can function effectively as a unit. To do that, you must be able to coordinate your activities to attack (or defend) territory. We know language skills don’t go back millions of years, so our pre-human ancestors did this the old-fashioned way, with gestures and grunts and an ability to get into the heads of the other members of the tribe. That’s the key skill: if you can get into one another’s heads, you can think as a group. The better you can do that, the better you will do in war. The better you do in war, the more offspring you’ll have. That skill, reinforced by natural selection, transforms, over thousands of generations, into evolution. With every generation you get better at knowing what your tribe is thinking.
This is the beginning of the social revolution.
All the way back here, before we looked anything like human, we grasped the heart of the matter: we must know one another to survive. If we want to succeed, we must know each other well. There are limits to this knowing, particularly with our small-brained ancestors. Knowing someone well takes a lot of brain capacity, and soon that fills up. When it does, you can’t know everyone around you intimately. As that happens the tribe grows increasingly argumentative, confrontational, eventually fracturing into two independent tribes. All because of a communication breakdown.
There’s strength in numbers; if I can manage a tribe of thirty while all you can manage is twenty, I’ll defeat you in war. There’s pressure, year after year, to grow the tribe, and, quite literally, to stuff more people into the space between your ears. For many generations that pressure leads nowhere; then there’s a baby born with just a small genetic difference, one which allows just a bit more brain capacity, so it can manage one or two or three more people — a small difference with a big impact. Genes that lead to success in war get passed along very rapidly; soon everyone holds a few more people inside their heads. But that capability comes with a price. Those pre-humans have slightly bigger brains, within slightly bigger heads. They need to eat more to keep those bigger brains well-fed. And those big heads would eventually prove very problematic.