How much time and effort do you spend chewing?
Although you probably enjoy a few leisurely meals every day, chances are that you spend very little time and muscular effort chewing your food. That kind of easy eating is very unusual. For perspective, our closest relatives, chimpanzees, spend almost half their day chewing, and with much greater force.
When and how did eating become so easy? And what were its consequences?
According to a new Harvard study, our ancestors between 2 and 3 million years ago started to spend far less time and effort chewing by adding meat to their diet and by using stone tools to process their food. The researchers estimate that such a diet would have saved early humans as many as 2.5 million chews per year, and made possible further changes that helped make us human. The study is described in a March 9 paper published in Nature.
One of the biggest puzzles in human evolution is how species such as Homo erectus evolved smaller teeth, smaller faces, and smaller guts, and yet managed to get more energy from food to pay for their bigger brains and bodies before cooking was invented. “What we showed is that…by processing food, especially meat, before eating it, humans not only decrease the effort needed to chew it, but also chew it much more effectively” said Katie Zink, the first author of the study, and a lecturer working in the lab of Daniel Lieberman, the Edwin M. Lerner II Professor of Biological Sciences.
By changing their diets to include just 33 percent meat, and processing their food – slicing meat and pounding vegetables – before eating, Zink and Lieberman found that the muscular effort required per chew and the number of chews required per day was reduced by almost 20 percent. They also found that by simply slicing meat with the sorts of simple tools available more than 2 million years ago, humans were able to swallow smaller, more easily digestible pieces than would have been possible without using tools.
“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 said.
But testing a process as basic as chewing isn’t as easy – or as attractive – as it might sound.
“What Katie did was creative but sometimes, frankly, a little stomach-churning,” Lieberman said. “Not only did she have people come into the lab, chew raw meat and other foods, and spit them out, but then she had to analyze the stuff.”
It wasn’t just any food – or any meat – that subjects noshed on.
To approximate the toughness and texture of the game that early humans ate, Zink and Lieberman (after much experimentation) settled on using goat – which subjects chewed raw while Zink used instruments attached to their jaw to measure the effort involved.
In each trial, volunteers were given, in random order, a selection of foods prepared in several ways – raw, sliced, pounded and cooked goat, as well as several vegetables, including carrots, beets and yams. After chewing each morsel until they would normally swallow, subjects spit out the food. Zink then spread the individual food particles out onto a tray, photographed them, and digitally measured their sizes.