We have actually evolved at dietary generalists, able to eat both vegetation and meat. It is also true that complete folivores like gorillas have extended guts that are used to digest cellulose-something that we do not have. The hominids that were herbivore dietary specialists (the Robust australopithecines) died out when in competition with our direct ancestors Homo habilis. In fact, there seems to be a correlation between our larger brains and reduced gut size. Digestive systems require a lot of calories when used to digest nothing but vegetative foods. Higher quality (meaning more calories per gram) meat foods allowed for a reduction in gut size and more calories to be expended in the equally costly expanding brain. In addition, many of our digestive enzymes are there to digest meat products.

There is archaeological evidence that as early as Homo habilis we scavenged the remains of animals. Primarily brain and bone marrow. They are high fat, high calorie parts that could be gotten with little effort with stone tools.