Humans have the digestive physiology of omnivores (like bears, pigs, chimps, birds, etc for example) Human digestive physiology is actually very much like a carnivore, but with a bigger large intestine. This is because we are omnivores, and have evolved to eat BOTH plants AND animals. Humans don't have big teeth, claws, etc because we didn't evolve from lions and wolves. For millions of years our ancestors used tools such as spears, arrows, etc to hunt their prey. So we don't need big teeth and claws.
"The digestive systems of humans, dogs, mice, horses, kangaroos and great white sharks are, to a first approximation, virtually identical. If you look more carefully however, it becomes apparent that each of these species has evolved certain digestive specializations that have allowed it to adapt to a particular diet. "
"These differences become particularly apparent when you compare a carnivore like a dog with a herbivore like a goat or a horse. Goats and horses evolved from ancestors that subsisted on plants and adapted parts of their digestive tracts into massive fermentation vats which enabled them efficiently utilize cellulose, the major carbohydrate of plants. "
"In contrast, dogs evolved from animals that lived on the carcasses of other animals, and have digestive systems that reflect this history - extremely small fermentation vats and essentially no ability to utilize cellulose. Bridging the gap between carnivores and herbivores are omnivores like humans and pigs, whose digestive tracts attest to a historical diet that included both plants and animals."
"In many ways, humans have guts like other apes. Food travels slowly through the intestines to allow time to absorb nutrients from fibrous plant foods. The human gut differs from those of other apes in one important way - it is far shorter. Mammals with short guts tend to be carnivores, because animal foods are easily digested. With meat, the nutrition comes in smaller, low fibre packages, which means less processing time is needed in the gut. Eating meat may also help to reduce the effects of undigestible or poisonous substances in the plant foods that are eaten. Modern humans have smaller guts than would be expected if we were ordinary apes, which supports the archaeological evidence that our ancestors embarked on a meat-rich diet."
Babies, Brains and Bone Marrow
By Prof. Leslie Aiello and Claire Imber, Department of Anthropology, University College London
"Broken animal bones found in the Turkana Basin, Kenya, show that big changes had occurred with the appearance of the species Homo erectus around 1.8 million years ago. Cutmarks on the bones show clearly that Homo erectus was able to get to animal carcasses before other scavengers, and feed on the remains. Some of the long bones have been carefully smashed to get to the marrow inside. Stone tools found at this time are larger, more regular and more symmetrical than the simple tools found earlier. At the same time, the climate in East Africa was becoming drier and cooler, and the landscape was covered with less forest. It was not a coincidence that these developments all happened at the same time. Overcoming the challenges of the changing environment led directly to the evolution of some fundamental human characteristics.
The australopithecines, like modern apes, probably relied on plant foods for most of their nutritional needs. They seem to have lived in forests close to rivers, environments which became increasingly scarce as the climate became drier and more seasonal. As a result, the important forest foods that apes traditionally rely on were not available all year round. Homo erectus had to find new food sources to fill the nutritional gap.
Apes rely on sugar-rich fruits as a source of energy and vitamins, with other important vitamins, minerals and protein coming from young plant shoots. Some apes eat animal foods like insects, grubs and small mammals occasionally, which supplement their protein intake. Studies of the isotope composition of some australopithecine fossils indicate that they too were omnivorous. Indeed, there have even been cut-marks found on animal bones dating from 2.5 million years ago, associated with a late australopithecine, Australopithecus garhi. Evidence suggests that Homo erectus relied much more on animal foods as a regular part of their diet.
Meat is highly nutritious, containing vital minerals, vitamins and protein building blocks. Some parts of the carcass like the brain, internal organs and bone marrow contain precious fats, in short supply on the savanna. In particular, bone marrow contains long chain fatty acids that are important for brain growth and development.
Once meat was a regular part of the diet, it could be bulked out with raw calories from any source. Little would have been available in the dry landscape inhabited by Homo erectus. The kinds of plants that survive in these conditions often produce underground tubers as storage organs that cache their surplus energy as starch. Tubers are good energy foods, but have little other nutritional value. This doesn't matter if meat was available to provide the protein and minerals.
In many ways, humans have guts like other apes. Food travels slowly through the intestines to allow time to absorb nutrients from fibrous plant foods. The human gut differs from those of other apes in one important way - it is far shorter. Mammals with short guts tend to be carnivores, because animal foods are easily digested. With meat, the nutrition comes in smaller, low fibre packages, which means less processing time is needed in the gut. Eating meat may also help to reduce the effects of undigestible or poisonous substances in the plant foods that are eaten.
Modern humans have smaller guts than would be expected if we were ordinary apes, which supports the archaeological evidence that our ancestors embarked on a meat-rich diet. There are even more important repercussions connected with this fundamental change in dietary strategy. The evolution of a smaller gut freed up energy that could be used elsewhere. The most obvious place to benefit from an increased share of the body's energy budget is the brain. The fossil record shows that Homo erectus brains were about twice as big as those of the australopithecines.
So, meat in the diet also allowed the evolution of a larger brain, but that produced further problems. Homo erectus babies would have had to be born relatively under-developed, before the brain grew too large to allow easy childbirth. Their infants, like ours, would have needed long periods of care and feeding before they could survive alone and feed themselves. The burden for this extended care period would fall initially on the mother, who has the energetic costs of pregnancy and lactation to bear. The brain is almost full size by 1 year old, although the body still has a long way to go, so a lot of the expense comes before weaning.
Big-brained babies provide the impetus to develop another key characteristic of human life - food sharing. The huge burden of child raising means the mother needs to find help somewhere. The most likely people to help her out are perhaps her post-reproductive female relatives and her partner, both of whom have strong genetic interest in the child. By supplementing the mother's diet throughout pregnancy and lactation, when she might be hampered in her own food-gathering, they can help to ensure the health of mother and child.
Meat and bone marrow would be especially valuable during this period. A mother who was provisioned would deplete her own reserves less with each pregnancy. If she is healthier, then the time taken to get pregnant again would decrease, and women who were helped by kin would have higher reproductive success overall than women who didn't. The families that shared food would be more successful than those that did not, and the behaviour would be likely to spread.
But why grow a big brain at all? It is likely that the new technologies and strategies needed to exploit animal foods produced increased cognitive demands as well. Digging for tubers, developing new tools, learning how to use them, and co-operating in groups to protect a kill from scavengers would all be novel situations for Homo erectus. They had to get smarter to cope successfully, and at this stage in human evolution, a large brain probably meant a smarter individual. The changing environment provided our ancestors with both problems and their solutions. The evolutionary responses made at this point in human prehistory were crucial in producing some of the key characteristics we recognise as human.
Aiello, L.C. and Wheeler, P. (1995) The Expensive-Tissue Hypothesis. Current Anthropology 16: 199-221.
Bunn, H.T. (1994) Early Pleistocene hominid foraging strategies along the ancestral Omo river at Koobi-Fora, Kenya. Journal of Human Evolution 27: 247-266.
Key, C.A. and Aiello, L.C. (1999) The Evolution of Social Organisation. In: 'The Evolution of Culture', (Eds: Dunbar, R., Knight, C., and Power, C.)
O'Connell, J.F., Hawkes, K., Jones, N.G.B. (1999) Grandmothering and the evolution of Homo erectus. Journal of Human Evolution 36: 461-485.
Milton, K. (1999) A hypothesis to explain the role of meat eating in human evolution. Evolutionary Anthropology 8: 11-21.
Wrangham, R.W., Jones, J.H., Laden, G., Pilbeam, D., Conklin Brittain, N. (1999) The raw and the stolen - Cooking and the ecology of human origins. Current Anthropology 40: 567-594.