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.