Larsen 2003 wrote an article about meat eating and its implications in human evolution. Humans (like all primates) have always eaten meat. However, 2+mya this was done by scavenging other kills and hunting small game. 2-1.7mya a shift happened, humans began hunting in groups and they changed drastically in that time period. The extra protein, fat, iron, zinc, vitamin A and B12 increased male body size by 44% and female size by 52% (thus decreasing sexual dimorphism). By 100 tya, meat eating was the primary food source for many groups of humans living throughout the globe. About 10 tya, animal and plant domestication began. This lead to staple foods from plants and an increasingly sedentary lifestyle with associated health problems. People shrank in size and remained smaller until the 18th century! If brain weight, body size, and overall health are all positively correlated with eating meat (and this means wild game or organic meat, not a $2 hot dog at the corner stand), then eat more of it. And remember to hit the gym, our ancestors had to hunt for food.