Monday, December 17, 2007 12/17/07-Cooking and Brain Size

In today's excerpt--raw versus cooked. Since brain tissue requires 22 times the food energy that skeletal muscle does, Homo erectus would have had to chew raw food for six hours each day to obtain enough food energy to sustain its brain size. This fact has led to Harvard anthropologist Richard Wrangham's controversial theory that fire and cooking were the necessary steps that allowed for the evolution of larger brains:

" 'I tend to think about human evolution through the lens of chimps,' Wrangham remarks. 'What would it take to convert a chimpanzee-like ancestor into a human?' Fire to cook food, he reasoned, which led to bigger bodies and brains. And that is exactly what he found in Homo erectus, our ancestor that first appeared 1.6 million to 1.9 million years ago. H. erectus's brain was 50 percent larger than that of its predecessor, H. habilis, and it experienced the biggest drop in tooth size in human evolution. 'There's no other time that satisfies the expectations that we would have for changes in the body that would be accompanied by cooking,' Wrangham says.

"The problem with his idea: proof is slim that any human could control fire that far back. Other researchers believe cooking did not occur until perhaps only 500,000 years ago. ...

"So Wrangham did more research. He examined groups of modern hunter-gatherers all over the world and found that no human group currently eats all their food raw. Humans seem to be well adapted to eating cooked food: modern humans need a lot of high-quality calories (brain tissue requires 22 times the energy of skeletal muscle); tough, fibrous fruits and tubers cannot provide enough. Wrangham and his colleagues calculated that H. erectus (which was in H. sapiens's size range) would have to eat roughly 12 pounds of raw plant food a day, or six pounds of raw plants plus raw meat, to get enough calories to survive. Studies on modern women show that those on a raw vegetarian diet often miss their menstrual periods because of lack of energy. Adding high-energy raw meat does not help much, either--Wrangham found data showing that even at chimps' chewing rate, which can deliver them 400 food calories per hour, H. erectus would have needed to chew raw meat for 5.7 to 6.2 hours a day to fulfill its daily energy needs. When it was not gathering food, it would literally be chewing that food for the rest of the day. ... [Animals] expend less effort breaking down cooked food than raw. Heat alters the physical structure of proteins and starches, thereby making enzymatic breakdown easier.

"Wrangham's theory would fit together nicely if not for that pesky problem of controlled fire. Wrangham points to some data of early fires that may indicate that H. erectus did indeed tame fire.

Rachael Moeller Gorman, "Cooking Up Bigger Brains," Scientific American, January 2008, pp. 102-104.


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