Meat eating & human evolution: a review of research into diet and evolution
Ironically, our greatest achievement as a species may be applying our enlarged brain and our technology to recreating the diet we instinctively ate a million years ago.
Imagine the primordial jungle. Imagine many kinds of primates, including anthropoids (chimps, gorillas and early humans) foraging for fruits and protein-rich leaves in the canopy of the arboreal forest. This story begins more than 55 million years ago but it has been the life-long study of Dr.Katherine Milton, professor of anthropology at the University of California. Her quest for links between diet and evolution is shared by David Popovich, a doctoral student in nutritional sciences at the University of Toronto, who also sees a connection between diet and human development of vision, depth perception, memory, speech, dexterity and social behaviour.
We have been given the impression that our early ancestors were closer to carnivores than they were to plant eating animals. The degree of meat in the early hominid diet is a matter of controversy and the more conservative view sees evidence for including small amounts of meat in the ancestral diet through opportunistic foraging and scavenging. The ancestors in question lived long before any modern human predecessors. The National Geographic Society’s recent report on Neanderthal life in glaciated Europe, for example, cites evidence of cannabilism and reliance on hunting for food. However, these primate cousins were relatively recent in hominid history. Our original ancestors predate them by eons, long before the last great ice age. The early hominids were much more similar to modern day chimpanzees and gorillas.
Most of us think of a chimp’s life as being fairly carefree. Dr. Milton was surprised when she was observing a troupe of chimps and noticed that, instead of sitting around in the tree branches and eating what was nearby, they hurriedly sought out specific foods, rejecting a number of delicious looking leaves in order to move on. When they found an acceptable specimen, they did not gorge themselves. Instead, they seemed driven to obtain a mixture of fruits and leaves from a variety of plant species. On the spot, Dr. Milton decided to devote her career to studying how these animals met their nutritional needs.
Chimps in the wild today face many challenges to obtaining a sufficient variety of plant material – similar challenges were likely faced our distant ancestors. For starters, many plants have developed outer coatings to discourage hungry herbivores. These outer layers contain chemical compounds that taste terrible and sometimes are lethal.
In addition, the fibrous content of plants, which we call “fibre” or “roughage,” resists breakdown by mammalian digestive enzymes. Excessive intake of fibre is troublesome because when fibre goes undigested, it provides no energy for the feeder. The trick is to do a better job of digesting the fibre. At the University of Toronto, David Popovich has been studying the micro-nutrient content of the wild vegetation consumed by gorillas. He has found that much of the energy and nutrient value that gorillas are able to derive from such a diet comes from colonic fermentation. Their studies on human subjects have shown that humans may also be able to rely on colonic fermentation. Thus, a diet consisting of substantial quantities of fruits, vegetables and nuts – no pasta or starches – will provide adequate protein, B-12 and amino acids (the building blocks of protein). Gorillas and chimpanzees have little trouble digesting cellulose thanks to the presence of the ciliate Troglodytella in their intestines. However, chimps and gorillas in captivity begin to lose their Troglodytella when they are fed cooked food. Thus, it is reasonable to assume that humans lost their intestinal cilia when they started cooking with fire.
Another concern with such a diet is finding time to forage. Primates cannot concentrate on just a few plant sources because, even if the fibre could be well-digested, many plant foods are low in one or more of the required nutrients, such as vitamins or amino acids. Fruits tend to be rich in easily digested forms of carbohydrate and relatively low in fibre, and provide little protein. Given that primates in the arboreal canopy do not cultivate protein-rich beans and vegetables, they rely heavily on efficient access to a wide variety of preferred fruits and leaves to achieve adequate protein.
Developing a better memory for the exact location of favoured trees, the shortest routes between them and a timetable for when they would likely be fruit-bearing would definitely favour survival. A larger brain would no doubt support these activities as well as group communication. Today, spider monkeys comb the forest for fruit by dividing into small, changeable groups. Their expanded mental capacity helps them recognize members of their own social unit and learn the meanings of different food-related calls.
The inherent complexities of the plant food niche could have been a factor in increasing the longevity of primates. Neither apes nor humans can rely on their relatively poor senses of taste and smell to detect toxicity, so they require several years of adolescence to learn which foods are safe and nutritious. This may be why humans are one of the longest living animals on earth.
Dr. Milton claims that the crafty Homo sapiens were better equipped to solve the dietary problems wrought by changing environmental conditions. Expansion of brain power in combination with growth in body size and reduction in the jaw and teeth, are evidence of achievement of a high quality diet. Without the high quality diet, the increased body size simply produces a slow moving, fairly sedentary and unsociable ape, like present-day orangutans and gorillas. Dental patterns among fossils of hominids support evidence of a high quality, plant-based diet. The decreased mass of the jaw and teeth signify that either our ancestors were eating less fibrous, easier-to-chew foods or they were processing them to remove material that would be hard to digest.
Some researchers have proposed that modification in dental structures resulted partly from specialization in hunting and scavenging. However, electron microscope examination of bones collected from early hominid sites reveals that our ancestors most likely scavenged bones that were already ravaged by carnivores. While the amount of meat consumed by our distant ancestors is still hotly debated, there is consensus that the Pleistocene diet consisted overwhelmingly of vegetable material. While chimpanzees are known to kill, this behaviour is not necessarily dietary but ritualistic and their diet is at least 94% plants and fruits.
Wild chimps take in 100 grams of fibre each day, much more than the 10 grams or less that the average North American ingests today. Dr. Milton’s studies have shown that the chimpanzee gut is strikingly similar to the human gut in the efficiency with which it processes fibre. According to Dr. Milton, our digestive tract does not seem to be greatly modified from that of the common ancestor of apes and humans, which was undoubtedly a predominately herbivorous animal.
While there is no authoritative recommendation for the daily intake of fibre, the small amount ingested daily by most Canadians is far less than we need to remain healthy. According to David Popovich, captive gorillas are dying in zoos of the same arterial sclerosis afflicting human cardiac patients because the zoos are unaware of the gorillas’ reliance on fibre. Dr. David Jenkins, known as the father of the “fibre movement” in Canada and Director of the Clinical Risk Factor Modification Centre at St. Michael’s Hospital, continues to make a strong case for vegetarianism as the optimum human diet.
2002 Hans Dehmelt. What is the optimal diet of the anthropoid primate homo sapiens? (212 Kb pdf file) Department of Physics, University of Washington, Seattle, March 18, 2002
1993 Milton, Katherine. “Diet and Primate Evolution,” Scientific American, pp. 86-93.
1996 Popovich, David. Interviews with graduate researcher in the Department of Nutritional Sciences at the University of Toronto to prepare this article (May, 1996).
1997 David G. Popovich, David J. A. Jenkins, et all. “The Western Lowland Gorilla Diet Has Implications for the Health of Humans and Other Hominoids.” The Journal of Nutrition, Vol. 127, No. 10 October 1997, pp. 2000-2005.
1988 Potts, Richard. “On an early hominid scavenging niche.” Current Anthropology, 29(1):153-155.
1984 “Home bases and early hominids.” American Scientist, 72: 338-347.
1988 Shipman, Pat. Scavenging or hunting in early hominids: theoretical framework and tests. American Anthropologist, 88(1): 27-43.
1984 Stalh, Ann A. Brower. “Hominid dietary selection before fire.” and Adrian Kortlandt, “Commentary,” Current Anthropology 25(2): 151-168 .
From November/December ’96 Lifelines