Around 1.9 million years ago, something extraordinary happened to the chimp-like hominids called Homo erectus. Their brains began to enlarge, becoming double the size of those of chimpanzees. Several theories are beginning to coalesce about why this happened. One is that early people began to eat more and better meat around this time, which allowed more calories to be consumed faster. This led to a shrinking of gastrointestinal organs and an increase in brain size that essentially traded guts for gray matter.

Our big brains need this extra energy. Modern humans eat about the same number of calories as other primates that approximate their weight, but we suck up an average of 25 percent of our body's energy expenditure, compared with the 8 percent sucked up by apes. Human babies use 60 percent of their energy to feed their heads.

Anthropologists have assumed that H. erectus ate their burgers and steaks raw, since most early fire pits discovered so far date back about 500,000 years, with the oldest, in Israel, dating back 790,000 years. Charred stones and tools associated with human sites have been discovered that date back as much as 1.5 million years, but these might have been naturally occurring fires.

Now Harvard University's Richard Wrangham has provided some evidence that the very distant ancestors of America's top chefs indeed may have learned to cook their antelope and rabbit. Cooking makes both plants and meat softer and easier to chew, providing more calories with less effort. What's more, human teeth got smaller and duller at around this time, which is the opposite of what would have happened if people had had to rip and chew lots of raw meat.

Wrangham worked with Stephen Secor, an animal physiologist from the University of Alabama at Tuscaloosa, who ran experiments to test the energy required by pythons to consume cooked versus raw meat; Secor also ran experiments on mice to gauge the impact of cooked versus raw meat. The snakes used almost 25 percent less energy chowing down cooked meat; the mice gained more weight and grew slightly longer. The fast turnaround in the mice indicates that cooked meat might have had a quick and dramatic evolutionary impact on early people.

Reducing the time and energy required to chew and digest raw meat means more energy available for other uses--such as feeding a voracious brain that's getting bigger and bigger. Wrangham also thinks that the modern rise in the consumption of cooked meat may contribute to the obesity epidemic; the same goes for processed food, which is even easier to eat and digest. Wrangham presented his findings at a recent paleoanthropology meeting in Cambridge, in the United Kingdom.

Paleoanthropologists are excited by Wrangham's findings and provocative ideas, but the absence of definitive proof of campfires appearing at the same time that human brains doubled in size is a problem. Many still believe that humanity's first cooked meal came much later--about 800,000 to 500,000 years ago, when the human noggin began growing again, expanding by about 30 percent into the modern-size brain.

One question that I'd like to ask evolutionary biologists and paleoanthropologists is why the huge expansion of our brains led to such seemingly unique traits in humans like advanced language skills and acute self-awareness. Would these same traits develop in other mammals if they were fed a diet of broiled beef over several generations? I wonder how many generations of mice it would take to replicate what happened to us--that is, I'd like to see if mouse brains double or triple in size, and also what our furry little friends would do with all that extra neural material.

Citation:

Science 15 June 2007:Vol. 316. no. 5831, pp. 1558 - 1560 DOI: 10.1126/science.316.5831.1558