Humans may have started eating meat as long as 1.5 million years ago, much earlier than previously thought.
The evidence is in the form of a child's skull fragment showing abnormalities linked to anaemia caused by a lack of B-vitamins.
It suggests that when the child was alive, around 1.5 million years ago, meat-eating was common enough to result in nutritional deficiency if it was lacking.
Carnivores get the essential vitamins B12 and B6, or folate, by consuming other animals. The liver contains large amounts of folate.
Herbivores can obtain folate from green vegetables while B12 is produced by gut microbes that help them digest cellulose.
At some stage in their evolution, humans lost their cellulose-processing ability. The appendix is all that is left of an intestinal pouch which fulfils this function in plant-eating animals.
The child fossil, from an early human species such as Homo habilis or Homo ergaster, was discovered at Olduvai Gorge in Tanzania.
Scientists believe the child, who was younger than two, may have died while beginning to eat solid food. Alternatively, if the child was still suckling, its mother may have been deficient in B vitamins through lack of meat.
The evidence, reported in the online journal Public Library of Science ONE, suggests regular meat-eating was a part of human life many thousands of years before it was previously thought to have emerged.
"Early humans were hunters, and had a physiology adapted to regular meat consumption at least 1.5 million years ago," said the authors led by Dr Manuel Dominguez-Rodrigo, from the Museo de los Origenes in Madrid, Spain.