Eat Meat, Or Eat All Day

"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?"

http://www.washingtonpost.com/national/health-science/sorry-vegans-eatin
g-meat-and-cooking-food-is-how-humans-got-their-big-brains/2012/11/26/3d
4d36de-326d-11e2-bb9b-288a310849ee_story.html

This article is a great read, and I recommend it in its entirety.  It falls right in line with the excellent book Catching Fire, which details the process described in this article in much greater detail.  Another juicy cut:

"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.""

Indeed so - in fact, among those on a completely raw food diet, over one third become infertile.  However, for you men who would like to be rid of that pesky six drive, a raw vegan diet seems to do the trick nicely.