Throughout the human lineage, both the gut and the brain have been composed of metabolically expensive tissue—that is, they require a disproportionate amount of energy to function properly. According to one theory, as the human brain evolved to be bigger the gut had to shrink, to leave more energy available for the brain.

Source: www.americanscientist.org Read the full article by clicking on the link. There are some wonderful pictures there depicting the evolution of the size of our brain and gut compared to primates.

Evidence of meat-eating among our distant human ancestors is hard to find and even harder to interpret, but researchers are beginning to piece together a coherent picture.
by Briana Pobiner

Over the course of six million years of human evolution, brain size increased 300 percent. Our huge, complex brains can store and process decades worth of information in split seconds, solve multifactorial problems, and create abstract ideas and images. This would have been a big advantage to early humans as they were spreading out across Africa and into Asia just under two million years ago, encountering unfamiliar habitats, novel carnivore competitors, and different prey animals. Yet our large brains come at a cost, making childbirth more difficult and painful for human mothers than for our nearest evolutionary kin. Modern human brains take up only about 2 percent of our body weight as adults, but use about 20 percent of our energy. Such a disproportionate use of resources calls for investigation. For years, my colleagues and I have explored the idea that meat-eating may have played a role in this unusual aspect of human biology.

In 1995, Leslie Aiello and Peter Wheeler developed the expensive tissue hypothesis to explain how our huge brains evolved without bringing about a tremendous increase in our rate of metabolism. Aiello, then of University College London, and Wheeler, then of Liverpool John Moores University, proposed that the energetic requirements of a large brain may have been offset by a reduction in the size of the liver and gastrointestinal tract; these organs, like the brain, have metabolically expensive tissues. Because gut size is correlated with diet, and small guts necessitate a diet focused on high-quality food that is easy to digest, Aiello and Wheeler reasoned that the nutritionally dense muscle mass of other animals was the key food that allowed the evolution of our large brains. Without the abundance of calories afforded by meat-eating, they maintain, the human brain simply could not have evolved to its current form.