Meat was a fundamental part of the human diet as long ago as 1.5 million years, a study of a Tanzanian skull fragment shows.
The two-inch skull fragment was found at the Olduvai Gorge - sometimes called 'the cradle of mankind' - and belonged to a two-year-old child.
It shows signs of porotic hyperostosis associated with anemia, which the research team believes was likely caused by a diet suddenly lacking in meat.
"Meat eating has always been considered one of the things that made us human, with the protein contributing to the growth of our brains," says Charles Musiba of the University of Colorado.
"Our work shows that 1.5 million years ago we were not opportunistic meat eaters, we were actively hunting and eating meat."
The most likely scenario is that, after weaning, the infant's diet became deficient in vitamin B12 and B9.
"He was not getting the proper nutrients and probably died of malnutrition," says Musiba.
The implication is that human beings were already adapted to meat-eating at this time, and to such an extent that the lack of it could lead to serious medical conditions.
The movement from a scavenging, largely plant-eating lifestyle to a meat-eating one is widely believed to have provided the protein needed to grow our brains and give us an evolutionary boost.
"Meat eating is associated with brain development," says Musiba. "The brain is a large organ and requires a lot of energy. We are beginning to think more about the relationship between brain expansion and a high protein diet."
Humans are one of the few surviving species with such a large brain to body size ratio. Chimpanzees, our closest relatives, eat little meat and have a far smaller brain capacity than humans.
"That separates us from our distant cousins," says Musiba. "The question is what triggered our meat eating? Was it a changing environment? Was it the expansion of the brain itself? We don't really know."