How tools overcame the chore of chewing

As early humans became bigger and brainier they required a lot more energy – and therefore food – to live. So why did we end up with smaller teeth and jaws than those of our earlier ancestors?

A new study says it was because of the palaeo diet our ancestors turned to around two million years ago, combined with the use of stone tools to tenderise raw meat.

Experts previously assumed that the teeth of early humans shrunk because we started cooking our food, rather than eating it raw.

However, the study, published in Nature, points out that cooking didn’t become widespread until around 500,000 years ago – 1.5 million years after Homo erectus began to emerge.

According to researchers Katherine Zink and Daniel Lieberman, both biologists at Harvard University, the reason for our decline in chewing ability has more to do with the rise of meat as a staple food, and the widespread use of tools to break it down.

“Eating meat and using stone tools to process food apparently made possible key reductions in the jaws, teeth and chewing muscles that occurred during human evolution,” Zink says.

Humans don’t spend a lot of time or energy on chewing and swallowing, compared to other species. We also have a smaller gut, despite requiring high amounts of energy to sustain our large brains and bodies.

Chimpanzees, by comparison, spend around half their day chewing. This was probably the case for our earlier primate ancestors, too.

Although meat is an excellent source of protein and energy, most animals don’t have the chewing ability to extract its calorific value. Early human ancestors used their large teeth and capable jaw bones to break down meat. However, according to the study, by the time Homo erectus walked the Earth we had found a better way to eat.

“Meat has a lot of nutrients, but it is also very elastic. You can think of it as being like a rubber band,” Zink explains.

“We can’t break it down with our flat, low-cusped teeth. But if you slice it up, then you do not need to use your teeth to break it down as much, and you swallow much smaller particles.”

It is thought that when meat entered the culinary scene, and became one-third of our diet, the amount of chewing required to get our daily energy needs dropped by 13%.

The study’s findings suggest that once tools became available to slice our meat and pound our vegetables, the amount of chewing was reduced by a further 5%, while the total force required to chew reduced by a whopping 12%.

This shift was particularly important for human development, because it resulted in shorter faces and smaller mouths, which increased our ability to talk to each other.

“With the origin of the genus Homo…we went from having snouts and big teeth and large chewing muscles to having smaller teeth, smaller chewing muscles, and snoutless faces,” explains Lieberman.

“Those changes, and others, allowed for selection for speech and other shifts in the head, like bigger brains.”

To identify the impact of stone tools on food processes, the researchers gathered groups of people into the lab to eat. The results were then analysed, including the chewed-up food.

Participants were given standard Lower Palaeolithic fare: one-third goat’s meat, and two-thirds underground vegetables such as beetroot, yam and carrot. The participants were monitored eating their meat raw, sliced, cooked and pounded – their movements analysed by instruments attached to each person’s jaw.

“The evolution of the ability to chew food into smaller particles gave mammals a big boost of extra energy,” says Lieberman.

“We humans have done something really remarkable: we eat even higher-quality foods than chimpanzees, and spend an order of magnitude less time chewing them.”