Increased meat in ancient hominin diets, beginning about 3 million years ago, meant that guts (large and small intestines) could become shorter because they could extract more expeditious calories in shorter lengths. This change in diet was directly responsible for the steady development of bigger brains beginning at this time. But what other factors might have helped the natural selection for shorter guts besides meatier diets? One profound influence may have been the diversity of flora, primarily bacteria, in hominin’s large and small intestines, mouth, skin, urinary tracts, and genital areas. And not just the diversity of this flora but its abundance. It has been estimated that there may be as many as 100,000,000,000,000 (one hundred trillion) bacterial cells in modern humans, and although bacterial cells are about 1,000 times smaller than typical human cells, their aggregate weight (1,500 gms) is about the same as the human brain (1,350 gms) or liver (1,600 gms). These bacterial cells are also amazingly complex as to date over 3.3 million different genes have been identified contrasted with less than 25,000 protein-coding genes in the rest of the human body.
