Cooking helped humans to develop
Learning to cook may have helped the human brain to grow, leading to the development of tools, culture and civilisation, say scientists.
Obtaining enough energy for a large brain by eating nothing but raw food simply takes up too much time, the study suggests.
The advent of cooking would have provided a much more efficient way of delivering calories to neurons, allowing the brain to expand.
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% 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 scientists, led by Suzana Herculano-Houzel, from the Federal University of Rio de Janeiro, wrote in the journal Proceedings Of The National Academy Of Sciences: "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. In line with this proposition, a cooked diet is preferred by extant (present day) non-human great apes."