Human ancestral diets changed substantially approximately four to five million years ago with major climatic changes creating open grassland environments.
We developed a larger brain balanced by a smaller, simpler gastrointestinal tract requiring higher-quality foods based around meat protein and fat.
Anthropological evidence from cranio-dental features and fossil stable isotope analysis indicates a growing reliance on meat consumption during human evolution.
Study of hunter-gatherer societies in recent times shows an extreme reliance on hunted and fished animal foods for survival.
Optimal foraging theory shows that wild plant foods in general give an inadequate energy return for survival, whereas the top-ranking food items for energy return are large hunted animals.
Numerous evolutionary adaptations in humans indicate high reliance on meat consumption, including poor taurine production, lack of ability to chain elongate plant fatty acids and the co-evolution of parasites related to dietary meat.
Without meat, said Milton, it's unlikely that proto humans could have secured enough energy and nutrition from the plants available in their African environment at that time to evolve into the active, sociable, intelligent creatures they became. Receding forests would have deprived them of the more nutritious leaves and fruits that forest-dwelling primates survive on, said Milton.
"Stone tools and fossil bones--the latter commonly displaying distinctive cut-marks produced when a carcass is dismembered and stripped of edible flesh with a sharp-edged stone flake--are found together on many Plio-Pleistocene archaeological sites, convincing proof that by at least 2.0 to 2.5 Ma [million years ago] before present (BP) these early hominids did in fact eat meat (Bunn 1986; Isaac and Crader 1981). In contrast, plant remains are absent or exceedingly rare on these ancient sites and their role in early hominid diet, therefore, can only be guessed on the basis of their known importance in contemporary forager diets, as well as their potential availability in Plio-Pleistocene environments (for example, see Peters et al. (1984); Sept (1984). Thus few today doubt that early hominids ate meat, and most would agree that they probably consumed far more meat than did their primate forebears. Instead, most studies nowadays focus primarily on how that meat was procured; that is, whether early hominids actively hunted animals, particularly large-bodied prey, or scavenged carcasses...I fully concur with the view that meat was a regular and important component of early hominid diet. For this, the archaeological and taphonomic evidence is compelling."
When Hardy et al wrote their paper, they got widespread public acclaim from the communities that laud the benefits of carbohydrates (even though, scientifically speaking, there aren't any). Fortunately there are some clever minds out there who could spot the flaws in the paper and they managed to come up with this one.
Barry Groves Goes Through the Logic of Human Digestive Systems
Agriculture cost us as a species, big time. We’ve spent the last 500 generations—or less, less than 0.4% of our evolutionary history —after we spent more than 100,000 generations (99.9+%) on a mostly meat and fat diet—eating a diet that is demonstrably and increasingly unnatural to our species. Pre-agricultural paleolithic hunters and gatherers derived most of their calories from about 100-200 different species of wild animals (meat and fat being as much as 90% of their dietary economy)…and to a lesser degree fibrous vegetables, greens and nuts/fruits. Suddenly, all of about 17 species of plants today, most of which are entirely new to the human diet—are providing 90 percent of the world’s food supply. That’s an insanely huge flip-flop, and it has NOT been a healthy one. The toll this has taken upon our health, not to mention the health of the planetary environment is immeasurable and tragic.
Paleopathology and the Origins of the Paleo Diet - Dr. Mike Eades