Plant to animal ratio
The specific plant to animal food ratio in the Paleolithic diet is also a matter of some dispute. The mean diet among modern hunter-gatherer societies is estimated to consist of 64-68% of animal calories and 32-36% of plant calories, with animal calories further divided between fished and hunted animals in varying proportions (most typically, with hunted animal food comprising 26-35% of the overall diet). As part of the so-called Man the Hunter paradigm, this ratio was used as the basis of the earliest forms of the Paleolithic diet by Voegtlin, Eaton and others. To this day, many advocates of the Paleolithic diet consider high percentage of animal flesh to be one of the key features of the diet.
However, great disparities do exist, even between different modern hunter-gatherer societies. The animal-derived calorie percentage ranges from 25% in the Gwi people of southern Africa, to 99% in Alaskan Nunamiut. The animal-derived percentage value is skewed upwards by polar hunter-gatherer societies, who have no choice but to eat animal food because of the inaccessibility of plant foods. Since those environments were only populated relatively recently (for example, paleo-Indian ancestors of Nunamiut are thought to have arrived to Alaska no earlier than 30,000 years ago), such diets represent recent adaptations rather than conditions that shaped human evolution during much of the Paleolithic. More generally, hunting and fishing tend to provide a higher percentage of energy in forager societies living at higher latitudes. Excluding cold-climate and equestrian foragers results in a diet structure of 52% plant calories, 26% hunting calories, and 22% fishing calories. Furthermore, those numbers may still not be representative of a typical Stone Age diet, since fishing did not become common in many parts of the world until the Upper Paleolithic period 35-40 thousand years ago, and early humans' hunting abilities were relatively limited[dubious – discuss], compared to modern hunter-gatherers, as well (the oldest incontrovertible evidence for the existence of bows only dates to about 8000 BCE, and nets and traps were invented 22,000 to 29,000 years ago.)
An extreme version of this line of thought posits that, up until the Upper Paleolithic, humans were frugivores (fruit eaters), who supplemented their meals with carrion, eggs, and small prey such as baby birds and mussels, and, only on rare occasions, managed to kill and consume big game such as antelopes. In his book The Third Chimpanzee, Jared Diamond describes how he was invited on a hunt by a tribe in New Guinea who had retained Stone Age technology and habits of thought in the 20th century. The day's total bag was two baby birds ("weighing about one-third of an ounce each"), a few frogs, and a lot of mushrooms. Although the men of the tribe frequently boasted of the large animals they had killed, when pressed for details, they admitted that large animals were killed only a few times in a hunter's career. Unlike the Stone Age humans, these people did have bows and arrows, and their stone tools were far more advanced than the stone tools found on prehistoric sites, so Professor Diamond thinks it unlikely that prehistoric hunters could have enjoyed a much higher success rate than present day hunter-gatherer tribes.
This view is supported by the studies of higher apes, particularly chimpanzees. Chimpanzees are closest to humans genetically, sharing more than 98% of their DNA code with humans, and their digestive tract is functionally very similar to that of humans. Chimpanzees are primarily frugivores, but they are sometimes described as opportunistic carnivores, which means that they could and would consume and digest animal flesh, given the opportunity. However, their actual diet in the wild is about 95% plant-based, with the remaining 5% filled with insects, eggs, and baby animals. Some comparative studies of human and higher primate digestive tracts do suggest that humans have evolved to obtain greater amounts of calories from animal foods, allowing them to shrink the size of the gastrointestinal tract, relative to body mass, and to increase the brain mass instead. Nevertheless, modern humans' digestive organs and mechanisms, such as dentition, stomach pH, and gut size, remain much closer to chimpanzees than to obligate carnivores or true omnivores such as bears and raccoons.