BERKELEY-- Human ancestors who roamed the dry and open savannas of Africa about 2 million years ago routinely began to include meat in their diets to compensate for a serious decline in the quality of plant foods, according to a physical anthropologist at the University of California, Berkeley. It was this new meat diet, full of densely-packed nutrients, that provided the catalyst for human evolution, particularly the growth of the brain, said Katharine Milton, an authority on primate diet. Without meat, said Milton, it's unlikely that proto humans could have secured enough energy and nutrition from the plants available in their African environment at that time to evolve into the active, sociable, intelligent creatures they became. Receding forests would have deprived them of the more nutritious leaves and fruits that forest-dwelling primates survive on, said Milton. Her thesis complements the discovery last month by UC Berkeley professor Tim White and others that early human species were butchering and eating animal meat as long ago as 2.5 million years. Milton's article integrates dietary strategy with the evolution of human physiology to argue that meat eating was routine. It is published this month in the journal "Evolutionary Anthropology" (Vol.8, #1). Milton said that her theories do not reflect on today's vegetarian diets, which can be completely adequate, given modern knowledge of nutrition. "We know a lot about nutrition now and can design a very satisfactory vegetarian diet," said Milton, a professor in the Department of Environmental Science, Policy & Management. But she added that the adequacy of a vegetarian diet depends either on modern scientific knowledge or on traditional food habits, developed over many generations, in which people have worked out a complete diet by putting different foods together. In many parts of the world where people have little access to meat, they have run the risk of malnutrition, said Milton. This happened, for instance, in Southeast Asia where people relied heavily on a single plant food, polished rice, and developed the nutritional disease, beriberi. Closer to home, in the Southern United States, many people dependent largely on corn meal developed the nutritional disease, pellagra. Milton argues that meat supplied early humans not only with all the essential amino acids, but also with many vitamins, minerals and other nutrients they required, allowing them to exploit marginal, low quality plant foods, like roots - foods that have few nutrients but lots of calories. These calories, or energy, fueled the expansion of the human brain and, in addition, permitted human ancestors to increase in body size while remaining active and social. "Once animal matter entered the human diet as a dependable staple, the overall nutrient content of plant foods could drop drastically, if need be, so long as the plants supplied plenty of calories for energy," said Milton. The brain is a relentless consumer of calories, said Milton. It needs glucose 24 hours a day. Animal protein probably did not provide many of those calories, which were more likely to come from carbohydrates, she said. Buffered against nutritional deficiency by meat, human ancestors also could intensify their use of plant foods with toxic compounds such as cyanogenic glycosides, foods other primates would have avoided, said Milton. These compounds can produce deadly cyanide in the body, but are neutralized by methionine and cystine, sulfur-containing amino acids present in meat. Sufficient methionine is difficult to find in plants. Most domesticated grains - wheat, rice, maize, barley, rye and millet - contain this cyanogenic compound as do many beans and widely-eaten root crops such as taro and manioc. Since plant foods available in the dry and deforested early human environment had become less nutritious, meat was critical for weaned infants, said Milton. She explained that small infants could not have processed enough bulky plant material to get both nutrients for growth and energy for brain development. "I disagree with those who say meat may have been only a marginal food for early humans," said Milton. "I have come to believe that the incorporation of animal matter into the diet played an absolutely essential role in human evolution." Milton's paper also demonstrates that the human digestive system is fundamentally that of a plant-eating primate, except that humans have developed a more elongated small intestine rather than retaining the huge colon of apes - a change in the human lineage which indicates a diet of more concentrated nutrients. ###