Humans' big brains is being attributed to the consumption of cooked food, scientists have revealed.
According to one controversial evolutionary theory, early humans developed a taste for cooked food around 2 million years ago, and this set in motion a series of changes that made us utterly different from any other animal.
Now, scientists have presented fresh evidence in support of the idea - and it all comes down to how you chew.
Christopher Organ of Harvard and Brown University in Providence, Rhode Island, and Charles Nunn and Richard Wrangham at Harvard University had predicted that if humans are uniquely adapted to eating cooked food, then we should spend far less time chewing than other primates, as cooked food tends to be softer than raw food.
To test this, they gathered data from various primate species and looked at the correlation between chewing time and body size, taking into account how the different species were related to each other.
A primate species of our size should, in theory, spend 48 per cent of the waking day chewing, they found. Yet on average we chew for less than 10 per cent of the day, says Organ.
The team found that we have small molars for our body size.
"In H. erectus the molars are considerably smaller than in the earlier hominids," New Scientist quoted Leslie Aiello, president of the Wenner Gren Foundation for Anthropological Research in New York, as saying.
For Wrangham, cooking is the explanation. Around 1.8 to 2 million years ago, he says, H. erectus or perhaps an immediate ancestor acquired a taste for food that had accidentally fallen into a fire.
Because the cell walls in cooked food are already partially broken down, it needs less chewing and is easier to digest. Wrangham argues the additional energy humans gained allowed them to evolve bigger brains and build complex social relationships. He points out that the fossil record suggests the size of hominin brains grew rapidly around this time.