Unappetizing Experiment Explores Tools’ Role in Humans’ Bigger Brains

Scientists recently turned Harvard’s Skeletal Biology Laboratory into a pop-up restaurant. It would have fared very badly on Yelp.

Katherine D. Zink, then a graduate student, acted as chef and waitress. First, she attached electrodes to the jaws of diners to record the activity in the muscles they use to chew food. Then she brought out the victuals.

Some volunteers received a three-course vegetarian meal of carrots, yams or beets. In one course, the vegetables were cooked; in the second, they were raw and sliced; in the last course, Dr. Zink simply served raw chunks of plant matter.

Other patrons got three courses of meat (goat, in this case). Dr. Zink grilled the meat in the first course, but offered it raw and sliced in the second. In the third course, her volunteers received an uncooked lump of goat flesh.

In some of the trials, the volunteers chewed the food until it was ready to swallow and then spat it out. Dr. Zink painstakingly picked apart those food bits and measured their size.

“If that was all my dissertation was, I would have quit graduate school,” Dr. Zink said. “It was as lovely as it sounds.”

Dr. Zink persevered, however, because she was exploring a profound question about our origins: How did our ancestors evolve from small-brained, big-jawed apes into large-brained, small-jawed humans?

Scientists studying the fossil record have long puzzled over this transition, which happened around two million years ago. Before then, early human relatives — known as hominins — were typically about the size of chimpanzees, with massive teeth and a brain only a third the size of humans’ current brains.

But with the emergence of species like Homo erectus, hominins grew to about our current height, with brains twice as large as those of their forebears.

In the late 1990s, Richard W. Wrangham, an anthropologist at Harvard, proposed that cooking was the key. Once hominins learned to use fire, he suggested, they roasted meat and starchy tubers they dug out of the ground.

Cooked food was easier to chew and digest, and hominins no longer needed big teeth to grind tough plants. Better yet, the extra calories they received helped fuel hungry neurons and, eventually, bigger brains.

While the oldest known hearths date to only 400,000 years ago, Dr. Wrangham argued that hominins cooked long before then. At first, they might simply have used natural fires to cook food before mastering the art of making one themselves.

Yet cooking was not the only way hominins prepared food. As long as 3.5 million years ago, scientists have found, hominins were making stone tools. Cut marks on mammal bones suggest that the tools were used to carve meat from carcasses.

Early hominins also made so-called hammerstones, which some researchers have speculated were used to smash nuts and other food.

Dr. Zink and her adviser at Harvard, Daniel E. Lieberman, wondered if stone tools helped hominins digest meat and starchy tubers long before cooking was invented. To explore that possibility, they opened their unappetizing cafe.

The findings? First, raw meat was impossible for the subjects to chew into smaller pieces. “It’s like chewing gum,” Dr. Zink said.

But slicing raw meat into smaller pieces allowed the volunteers to grind it further into bits small enough to swallow. (The test subjects spat out the raw meat to avoid food poisoning.)

Cooked meat actually demanded more chewing, but it could be ground into even smaller particles that were digested with less effort.

Slicing raw vegetables did not make it easier for the volunteers to eat them, the scientists found — but pounding on the vegetables did. Cooking made the vegetables even easier to consume.