Although this isn’t the first such assertion from archaeologists and evolutionary biologists, the new studies demonstrate that it would have been biologically implausible for humans to evolve such a large brain on a raw, vegan diet and that meat-eating was a crucial element of human evolution at least a million years before the dawn of humankind.
Calories to grow our brains
At the core of this research is the understanding that the modern human brain consumes 20 percent of the body’s energy at rest, twice that of other primates. Meat and cooked foods were needed to provide the necessary calorie boost to feed a growing brain.
One study, published last month in the Proceedings of the National Academy of Sciences, examined the brain size of several primates. For the most part, larger bodies have larger brains across species. Yet humans have exceptionally large, neuron-rich brains for our body size, while gorillas — three times as massive as humans — have smaller brains with one-third the neurons. Why?
The answer, it seems, is the gorillas’ raw, vegan diet (devoid of animal protein), which requires hours upon hours of eating to provide enough calories to support their mass.
Researchers from Brazil, led by Suzana Herculano-Houzel, a neuroscientist at the Federal University of Rio de Janeiro, calculated that adding neurons to the primate brain comes at a fixed cost of approximately six calories per billion neurons.
For gorillas to evolve a humanlike brain, they would need an additional 733 calories a day, which would require two more hours of feeding, the authors wrote. A gorilla already spends as much as 80 percent of the tropics’ 12 hours of daylight eating.
Similarly, early humans eating only raw vegetation would have needed to munch for more than nine hours a day to consume enough calories, the researchers calculated. Thus, a raw, vegan diet would have been unlikely, given the danger and other difficulties of gathering so much food.
Cooking makes more foods edible year-round and releases more nutrients and calories from both vegetables and meat, Herculano-Houzel said.
“The bottom line is, it is certainly possible to survive on an exclusively raw diet in our modern day, but it was most likely impossible to survive on an exclusively raw diet when our species appeared,” Herculano-Houzel told LiveScience.
The study puts an upper limit on how big a brain is able to grow while on a premodern raw, vegan diet. But the researchers could not determine when daily cooking began. Was it about 250,000 years ago, when humans were nearly fully evolved with big brains, which is supported by archaeological findings? Or was it about 800,000 years ago, when prehumans began their most dramatic brain-growth spurt, an era for which there is little archaeological evidence of controlled fires for cooking?