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Our closest genetic relative is the chimpanzee -- we share approximately 99% of our genomic makeup with the chimp. This is because we share a common ancestor. Humans and chimpanzees separated from each other approximately six million years ago, which is not so long in evolutionary terms.

So how do the diets of humans compare with the diets of chimpanzees and other great apes, given our common ancestry? More different than you might think. Plant materials comprise 87% to greater than 99% of the annual diet of great apes, according to research published in the Journal of Nutrition. This can be contrasted with the diet of present-day Americans, who tend to get about 70% of their protein from animal sources.

In fact, scientists speculate that one of the key reasons why humans took a different evolutionary trajectory than their primate relatives was due to their diet. Here's how it is explained by University of California, Berkeley anthropologist, Katharine Milton:

"Turning to animal source foods as a routine rather than occasional dietary component would have permitted the evolving human lineage to evade the nutritional constraints placed on body size increases in apes. Without routine access to animal source foods, it is highly unlikely that evolving humans could have achieved their unusually large and complex brain while simultaneously continuing their evolutionary trajectory as large, active and highly social primates. As human evolution progressed, young children in particular, with their rapidly expanding large brain and high metabolic and nutritional demands relative to adults would have benefited from volumetrically concentrated, high-quality foods such as meat."

It's somewhat surprising how much meat early humans likely ingested and how little meat is ingested by great apes.

Orangutans and gorillas, for instance, are estimated to take approximately 99% of their annual diet from plant sources. Much of this intake is of low quality, such as bark, mature leaves and unripe fruits. When primates do ingest non-plant material, it often comes in the form of inadvertently ingesting insects such as termites and ants. Incidentally, these insects contain trace nutrients that help primates a great deal.

It's rare, but not unheard of, for great apes to eat meat. For example, some communities of chimpanzees purposely eat termites and other insects, with some individuals occasionally hunting vertebrates. Generally speaking, it has been observed that only the most dominant chimpanzees consume significant amounts of vertebrate meat.

These dietary differences are reflected in the digestive tracts of great apes and humans. For instance, great apes tend to have much more developed hindguts (i.e., colons) than humans, which aids in the digestion of lower quality foods such as bulky plant material, insoluble fiber and seeds. Humans, on the other hand, have more developed foreguts (small intestines) which suggests an adaptation to foods that are nutritionally dense and highly digestible.

What's perhaps most interesting is how small differences in meat ingestion between chimpanzees and other great ape species such as gorillas and orangutans might be responsible for important behavioral distinctions observed between species.

For instance, gorillas and orangutans have been shown to be less active, less agile and less behaviorally nuanced than chimpanzees. They also lack the high level of social interaction seen in chimpanzees. It's possible that, due to their almost exclusively plant-based diet and corresponding digestive processes, these two species experience limited energy intake, which restricts the development of non-essential behaviors. In other words, the energy available to orangutans and gorillas may not be sufficient to support greater levels of activity and social behavior -- and meat might be to blame.