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.
Based on their experiments, Dr. Zink and Dr. Lieberman concluded that, long before the invention of cooking, stone tools could have made it easier for hominins to eat raw meat and tubers, conserving precious energy.
“It was surprising to us how effective it was,” Dr. Lieberman said. He and Dr. Zink reported the results of their experiment Wednesday in the journal Nature.
Stone tools used to process meat and vegetables could have influenced the evolution of hominins.
Big, strong teeth, for instance, may have become less important to their survival. “That can help explain the reduction of the face long before the evidence of cooking,” Dr. Lieberman said. The extra energy could have helped to drive the evolution of larger hominin brains.
Dr. Lieberman and Dr. Zink do not dismiss the importance of cooking. It killed food pathogens, made it possible to store food longer and destroyed toxins. But they argue that the advantages of the cooking fire were preceded by those of stone tools.
“It is a very clever piece of research,” said William R. Leonard, an anthropologist at Northwestern University. “I think they make a strong case that cooking was not critical” to the transformation of early hominins, he added.
Dr. Wrangham disagrees. There is no evidence, he said, that hominins actually smashed tubers to eat them, nor do any living hunter-gatherers engage in the practice.
And even if early hominins did smash uncooked tubers, Dr. Wrangham said, he doubts that they got enough nutrition from them to keep a modern human healthy. He points to studies of people who eat only raw foods that link the diet to difficulty with pregnancy.
“The average woman on a diet like that with incredibly high-class agricultural foods cannot have a baby,” Dr. Wrangham said.
Kenneth Sayers, an anthropologist at the Language Research Center at Georgia State University, agreed that stone kitchen tools may have played a part in human evolution.
But he cautioned that the energy hominins put into eating went well beyond chewing. “Food was not presented on a plate,” he said. Hominins went to great trouble just to acquire something to eat.
Dr. Zink did not disagree. “This is just one little step in what hopefully be some broader body of knowledge,” she said