“…human brains are fairly typical primate brains; they just became unusually large. “

….it turns out also that the idea that brains got large early on in human evolution is incorrect as well. We now know that:

  • humans and chimpanzees split maybe around six to seven million years ago
  • and the very earliest hominins, those are creatures that are more closely related to humans than to chimpanzees, had really small brains.
  • In fact, early Australopithecus, like Lucy, also had quite small brains. Even the early members of the genus Homo had small brains.
  • Some tools first started appearing around 2.6 million years ago, and those hominins have slightly larger brains than Australopithecus.

But if you actually factor out the effects of body size, what’s called their encephalization quotient (the ratio of brain size to body size for what you expect for a mammal of a body size versus what you actually got) it was actually not that much bigger than chimpanzees or early Australopiths. To put it into perspective:

  • an EQ of one means that your brain size is exactly the size of a brain you predict for your body size.
  • Chimpanzees have an EQ of 2.1
  • humans have an EQ of about f5.1.
  • Australopiths have EQ’s of about 2.5
  • the earliest members of the genus Homo have EQ’s of about 3.0 to 3.3.

Their brains are a little bit bigger than a chimpanzee’s, but not hugely so, and it wasn’t until long after the genus Homo evolved that brains actually started getting really, really large. So increases in brain size were not really an early event in human evolution, and in fact, they didn’t occur until after hunting and after the invention of hunting and gathering, and not even until cooking and various other technological inventions, which gave us the energy necessary to have really large brains.

Brains are very costly. Right now, just sitting here, my brain (even though I’m not doing much other than talking) is consuming about 20- 25 percent of my resting metabolic rate. That’s an enormous amount of energy, and to pay for that, I need to eat quite a lot of calories a day, maybe about 600 calories a day, which back in the Paleolithic was quite a difficult amount of energy to acquire. So having a brain of 1,400 cubic centimeters, about the size of my brain, is a fairly recent event and very costly.


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