Our taste for processed foods is nothing new, early humans processed meat and vegetables two million years ago.
A diet of processed food also encouraged evolutionary changes that shrank the size of our jaw and teeth to the proportions we have today.
Jawline changes emerged long before our ancestors discovered how to make fires and roast the bush meat and root vegetables that made up our diet.
The ape-like heavy jaw and teeth were more or less gone by the time Homo erectus appeared two million years ago, said Katherine Zink and Daniel Lieberman of Harvard University's Department of Human Evolutionary Biology.
By then our ancestors looked more or less like modern humans, taller with bigger brains and increased daily nutritional requirements to match.
But even as our daily calorie intake rose, our teeth and jaws got smaller and bite force was half of what it was in our earlier cousins.
The assumption is the extra calories arrived as meat, probably making up a third of the daily energy intake, the authors write in the journal Nature.
At this stage cooking can be ruled out as evidence for this did not begin appearing until about 500,000 years ago.
So stone tool processing of food became the most likely method used to increase food intake to feed our growing brains while shrinking our “masticatory apparatus”, say the scientists.
Chewing power
Homo erectus used stone tools to slice and grind meat making it easier to chew and digest. They also pounded tough veg so that it required fewer chews and less chewing power for us to swallow.
Zink and Lieberman set up experiments to test the benefits of food processing and measure the difference between sliced and chopped foods compared to unprocessed.
They fed volunteers what would have passed as five-star cuisine in the Lower Paleolithic – goat, jewel yams, carrots and beetroots. The scientists measured the muscular effort required for chewing and how well the food was broken up before swallowing.
The difference between processed and unprocessed was striking. Assuming a third of energy intake was meat, then slicing would save about two million chews a year, a 13 per cent reduction. Chewing force was also lower by 15 per cent, the researchers found.
There were similar declines in chewing and force when the veg was pounded with rocks before consumption, although meat – even tough goat – was always easier to chew down than the root veg.
This leads the authors to believe that our evolved masticatory features “would have been initially made possible by the combination of using stone tools and eating meat”, they write.
