Consider that 60% of the energy expended by a resting baby is consumed by the baby’s brain. A resting adult brain uses 25% of its energy. Compare this to the average ape brain, which uses only 8% of the apes energy. In short, having a big brain requires a lot of energy.
The June 15, 2007 edition of Science presents the article “Food for Thought: Did the First Cooked Meals Helped Fuel the Dramatic Evolutionary Expansion of the Human Brain?” [Most of the articles of Science are only available to paid subscribers online]. Richard Wrangham, a Harvard primatologist, realized that when we started pre-digesting our food by heating it, this allowed us to spend less time digesting the food. From this observation, he realized that this would have given our ancestors a big evolutionary advantage. “With cooking, we should see major adaptive changes.” What would you do with that extra energy made available by chewing less? How about things like cave painting or writing poetry are inventing airplanes?
Wrangham proposed cooking as one of the answers to a long-standing riddle in human evolution: where did humans get the extra energy to support their large brains? “Even small differences in diet can have big effects on survival and reproductive success,” Wrangham states. This article points out that australopithecines, (which lived from 4 million to 1.2 million years ago) had brains the size of chimpanzee brains. About 1.9 million years ago, the brain of H. erectus showed dramatic growth to twice the size of the chimpanzee’s brain. It was about this time that archaeological sites showed that H erectus was moving animal carcasses into the campsites for communal eating. “It’s teeth, jaws and guts all got smaller.”
One prevailing argument is that H. erectus were simply better hunters. Wrangham argues, however, that we learned to cook between 1.9 million to 1.6 million years ago, and our brains grew suddenly. Studies show that “cooking gelatinizes the matrix of collagen in animal flesh and opens up tightly woven carbohydrate molecules and plants to make them easier to absorb.” Whereas chimpanzees spent five hours per day chewing food, hunter gatherers who cook only spent one hour chewing each day. It’s important to note that H erectus had smaller teeth than that of his ancestors. The lack of the large teeth characteristic of its ancestor is additional evidence that H. erectus had learned to cook.
Other scientists are showing enthusiasm about Wrangham’s proposal. There’s a problem, however. There is no clear evidence that anybody was cooking that long ago. The article details solid evidence for cooking 250,000 years ago in southern Europe and indicates that there is evidence of controlled fire 790,000 years ago in Israel. Wrangham notes, however, “that evidence for fire is often ambiguous and argues that humans were roasting meat and tubers around the campfire as early as 1.9 million years ago.
There’s also a classical approach to the question of where we got extra energy that does not necessarily require the cooking hypothesis. The theory is that we began to eat more meat. Eating more meat allowed our guts to shrink, which saves energy for digestion. This is called the “expensive tissue hypothesis,” proposed in 1995 by Leslie Aiello. This hypothesis is now receiving solid experimental support.
Intuitively, at least, this makes sense. And I think cooking meals would have even more implications than you touched on here. Having a place with a fire and the setup necessary to cook meals would establish more of a "home base" for our ancestors, I would think, which might therefore make them safer from wandering off into predatory danger or simply getting lost. It might also make us more social, as it would spur our ancestors to sit around the fire together, eventually gathering around it to sing or tell stories. And if cooking afforded them more time to make cave paintings, as you said Erich, then it would have helped lead to the explosion of symbols and the use of language, a very heavy brain activity.
Yet it seems like we could replace "cooking" with "fire" in this scenario. Fire would lead to cooking, which would save us time, give our brains more energy, and so forth, but fire would also lead to a few other changes. With fire, our ancestors could stay awake later at night because they could see and stay warm. This extra time, too, would make the explosion of language and art easier. But as you said, we just don't know when prehumans began using fire.
Fire also helps with hardening spears and arrow heads.
Cooking, on the other hand, doesn't increase the energy available from meat; in fact it *decreases* it. Cooking can act as to preserve meat to a limited extent, though, as does drying.
But the idea that you'll get a net energy gain through not having to chew the meat so much or work so hard to digest it is ludicrous. Raw meat doesn't require chewing, just biting or tearing off pieces and swallowing them. (That's how I just ate a raw steak for lunch.)
Admittedly, cooking *does* let you eat starches that would otherwise be toxic or indigestible, though. But this isn't as high quality of a food as raw meat is, so it's doubtful that it's the origin of our bigger brains.