New evidence suggests that our ancestors have been cooking and processing food as far back as 1.9 million years ago. This could explain why humans have small teeth, as we don't need to spend our time chewing as much as animals.

The study was published Monday in the journal Proceedings of the National Academy of Sciences.

Four extinct hominids, humans, chimpanzees and modern apes were examined, as researchers measured their tooth sizes and body masses. The scientists found that chimpanzees spend 10 times more time chewing compared to humans.

Cooking softens and processes food to make eating much easier and reduce chewing time. Had our ancestors not cooked, we'd be eating nearly half of the day instead of just 5 percent that we spend today.

We see a dramatic shift in the tooth size of Homo erectus, which means it was likely responding to a history already of eating cooked and processed food, Harvard University's researcher Chris Organ told LiveScience. If you're cooking your food you have many more hours of your day free, and you can spend those hours doing other things.

Eating processed food means there is more surface area from which the gut can absorb nutrients, resulting in more calories and less gut time to digest calories.

This extra time and calories likely had a large impact on the evolution of modern humans, and even the evolution of language and social lives, since you can't eat with your mouth full, according to researchers.

So the next question is, have researchers found hard evidence of ancestors using fire pits dating back so far?

There isn't a lot of good evidence for fire. That's kind of controversial, Organ said. That's one of the holes in this cooking hypothesis. If those species right then were cooking you should find evidence for hearths and fire pits.

Cooking would have been done over a hearth or fire pit, and mashing with stones. The oldest evidence of hominids using fire goes back 1 million years.

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