Download Unappetizing Experiment Explores Tools` Role in Humans` Bigger

Unappetizing Experiment Explores Tools’
Role in Humans’ Bigger Brains
Carl Zimmer
MATTER MARCH 9, 2016
Photo
The invention of cooking significantly contributed to early humans’ development, but using
stone tools to merely break down food into smaller pieces may also have helped, a new study
concluded.
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.