Scientists led by Suzana Herculano-Houzel, from the Federal University of Rio de Janeiro found the advent of cooking would have provided a much more efficient way of delivering calories to neurons, allowing the brain to expand, the Daily Mail reported.
Without the invention of cooking humans would have to spend nine hours or more every day eating raw food only to support the 86bn neurons in their brains.
Fewer hours spent foraging for food would also have freed up more time for social interaction and creative tasks. This probably contributed further to the evolution of a large and complex brain.
The Brazilian scientists calculated the metabolic needs of both present-day great apes and a range of different early human species.
Large apes with energy-hungry bodies, such as gorillas, are already at the limit of achievable brain size with a raw-food diet, the research showed.
Gorillas spend around eight hours a day eating, and sometimes almost as much as 10 hours.
For a gorilla to have a brain corresponding to 2 per cent of its body mass - as is the case in humans - it would have to devote more than another two hours a day to feeding.
The study showed that three early human species, Homo habilis, Australopithecus afarensis and Paranthropus boisei, would all have had to spend more than seven hours a day eating raw food to support their brain size.
Cooking was probably invented by Homo erectus, thought to be a direct ancestor of modern humans, which lived up to 1.8 million years ago.
"The advent of cooking food greatly increases the caloric yield of the diet, as a result of the greater ease of chewing, digestion and absorption of foods," researchers said.
"In line with this proposition, a cooked diet is preferred by extant (present day) non-human great apes," they wrote.
"Although the earlier addition of raw meat to the diet of earlier hominins may also have contributed to increase its caloric content, raw meat is difficult to chew and ingest, whereas cooked meat is easier to chew and has a higher caloric yield," they said.
"Cooking would have also increased the time available for social and more cognitively demanding activities," they added.
The study was published in the journal Proceedings Of The National Academy Of Sciences.
