The first evidence lies in the fossil sites. Where hominid remains are found, so also are animal bones - at times in their thousands. If we were not meat-eaters, why is that?
Secondly, although modern hunting tribes do eat plants, they have fire. Without it, there are very few plant foods with sufficient calorific value that we could have digested.
There were fruits, of course, but there is not one prehistoric site in all Africa that indicates forests extensive enough to have supplied sufficient fruit to meet the needs of its inhabitants.
Indeed, there is agreement that our ancestors did not dwell in forests at all but on the savannahs where there were vast plains of grass. However, grass is of no value to our digestive system. Even to live off fleshier leaves would require the much more highly specialized digestive systems of other primates.
Compare the shape of the gorilla against that of the man. The area between the chest and the legs of the gorilla is much greater than the same part of the man. This is because the gorilla, a herbivore, needs a much larger digestive system. The walls of all plant cells are made of cellulose, a form of dietary fiber. There is no enzyme in the human digestive system that will break it down.
And with the cell walls intact, the nutrients in the cells cannot be digested. Passing unaffected straight through the gut, therefore, all the nutrients in the plant would be ejected as waste.
Studies conducted on monkeys have led to the suggestion that the seeds of the grass could have supplied us with the energy we required (6). However, if this were the case, why is it that we cannot eat them now without cooking them first?
Seeds, the staples such as rice, wheat, maize and beans, play an important part in our lives today. All of them, however, must be cooked before we can eat them in any quantity. Seeds and berries are a plant’s reproductive system.
Many are designed to attract animals to eat them but there would be little point in this if the seeds were digested. No, they are indigestible - deliberately, designed to pass through the animal to be defecated and take root elsewhere. Two means only are available to make them digestible: cooking and grinding.
Before fire was harnessed, the only means by which the seeds could have been rendered digestible would have been by pounding them and breaking down the plant cell walls, but no archaeologist has ever found a Stone Age tool for this job.
If chewing were the method used to do the job, a very large proportion of the seeds would escape and, passing through the body undigested, end up in the feces. Hominid feces, or coprolites as they are called, have been found and studied in detail (7).
Older coprolites from Africa contain no plant material. Relatively recent ones from North America have included just about everything that could remotely be called edible: from eggshells and feathers to seeds and vegetable fibers. But these remains occur only after the Paleoindians had mastered fire, and even then, seeds had passed through undigested and unharmed. Thus there is no doubt that seeds cannot have been a natural part of their diet.
Homo erectus began to appreciate the value of fire around 350,000 years ago (8). It is true that if our ancestors had started cooking grain then, we could have evolved and adapted to it by now. However, cooking grain is not as easy as cooking meat. You cannot hang it in a chunk over the fire or lie it in the embers. To cook grain and other seeds, you need a container of some sort. The oldest known pot is only 6,800 years old. In evolutionary terms, that was only yesterday.
For any reliance on cooking, you also need a controlled fire. Although hearths have been discovered that are 100,000 years old, these are relatively rare. European Neanderthal coprolites from around 50,000 years ago, before the use of fire, contain no plant material whatsoever.
It was not until Cro-Magnon’s colonization of Europe, some 35,000 years ago, that hearths became universal. However, even then they were used merely for warmth, not for cooking plants. At the time, Europe was in the grip of a succession of ice ages.
For some 70,000 years there were long, cold winters and short, cool summers. Cro-Magnon and his Eurasian ancestors cannot have eaten plants - for most of the year there weren’t any! He ate meat or he died. And he ate that meat raw.
Fats And Brain Size
The evidence was already overwhelming that we could not be a vegetarian species. However, in 1972 the publication of two independent investigations really nailed the lid on the vegetarian hypothesis’s coffin. The first concerned fats (9).
About half our brain and nervous system is composed of complicated, long-chain, fatty acid molecules. The walls of our blood vessels also need them. Without them we cannot develop normally. These fatty acids do not occur in plants.
Fatty acids in a simpler form do but they must be converted into the long-chain molecules by animals - which is a slow, time-consuming process. This is where the herbivores come in. Over the year, they convert the simple fatty acids found in grasses and seeds into intermediate, more complicated forms that we can convert into the ones that we need.
Our brain is considerably larger than that of any ape. Looking back at the fossil record from early hominids to modern man, we see a quite remarkable increase in brain size. This expansion needed large quantities of the right fatty acids before it could have occurred.
It could never have occurred if our ancestors had not eaten meat. Human milk contains the fatty acids needed for large brain development - cow’s milk does not. It is no coincidence that in relative terms, our brain is some fifty times the size of a cow’s.
The vegetarian will be dismayed to learn that while soy bean is rich in complete protein, and grains and nuts also combine to provide complete proteins, none contains the fats that are essential for proper brain development.
Although the eating of fats today is believed by some to be a cause of heart disease (erroneously, see The Cholesterol Myth), we know that our ancestors ate large amounts of fat. Animal skulls are broken open and the brains scooped out; long bones likewise are broken for their marrow content. Both brain and marrow are very rich in fat.
Nice one! 
