Over the last million years a group of primates began walking upright. This genus, Homo, (our species, Homo sapiens sapiens, evolved about 200,000 years ago) is distinguished by a number of physiological and morphological adaptations to their environment. Importantly, a change in diet from our primate relatives appears to have been the key change that drove our recent evolution.
What was the paleo diet of the genus Homo?
Stephan Guyunet provides this analysis:
"All we know is that they ate some meat. Although humans eventually became top-level predators, we also don't know whether these early humans were actively hunting, or simply scavenging what other predators left behind-- perhaps using their tools to access gristle, brain, and marrow inaccessible to other animals.
At the same time as tool-marked bones appear in the archaeological record, early humans began undergoing a remarkable physical transformation, which represented (in large part) a progressive genetic adaptation to a new subsistence strategy. Our brain doubled in volume, our gut became smaller, and the proportion of small intestine to large intestine increased. Our teeth and jaws became smaller and less robust (Daniel Lieberman. The Story of the Human Body. 2013).
What does this signify? The consensus is that these changes occurred in response to a shift toward a so-called "high-quality" diet. This means a diet that has a higher calorie density and contains less fiber, relative to the typical primate diet of leaves and low-calorie fruit (the latter is not at all suitable for a modern human). The small intestine is what breaks down and absorbs protein, carbohydrate, and fat, while the large intestine ferments fiber to extract calories from it. The shift from a large-intestine-dominant gut to a small-intestine-dominant gut signifies a shift from getting most calories from intestinal fiber fermentation, to getting most calories from direct absorption of protein, carbohydrate, and fat."