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Food for Thought?

They're dirty, hairy, and ugly, but tubers may have sparked our evolutionary leap to bigger human brains. Michael M. Torrice digs for clues. Illustrated by Noel Sirivansanti and Cecelia Azhderian.

Illustration: Noel Sirivansanti

On the arid plains of Tanzania, Nathaniel Dominy watches the Hadza tribe dig for its dinner. Hadza women, carrying their babies in slings, thrust long wooden poles tapered to a point into the dry dirt. They scrape at the soil, unearth what look like basketball-sized boulders, and place them over small, fast-burning fires. After a few minutes of roasting, a Hadza grandmother splits the charred orbs to expose the softened flesh of the nutritious, starchy tubers.

For 50,000 years, this ancient tribe has hunted tubers. Meanwhile, in the United States, we load our Thanksgiving tables with hefty mounds of mashed potatoes and piles of candied yams. Something compels people to pull up the gnarled subterranean parts of plants and call them dinner.

Following the dictum “You were what you ate,” some anthropologists study our extinct ancestors through prehistoric dinners. Dominy, an anthropologist at UC Santa Cruz, is one of a group of researchers now focusing on the role tubers may have played in early human diets. These scientists believe the buried vegetables fueled one of the greatest leaps in our evolution: the growth of larger, smarter brains.

Because big brains need big calories, anthropologists have long debated which foods fed our hungrier minds. Through fieldwork on tubers in sub-Saharan Africa and genetic analysis of ape spit, Dominy has tried to bolster the hypothesis that around the time of this major evolutionary leap, our ancestors dined mainly on the humble tuber. His research pushes against the prevailing theory among anthropologists that our brains' caloric jolt came from meat.

“I came into these studies with a relatively neutral view. I just thought these would be cool ways to test the idea,” Dominy says. “But now I really see the value of a tuber.”

Monkey catcher

Dominy's path to his tuber epiphany began a decade ago in the rainforests of Costa Rica, where he watched for falling monkeys. That summer he was tagging along with anthropologist Mark Teaford, his undergraduate advisor at Johns Hopkins University, as Teaford studied the teeth and diets of the forests' wild monkeys. When a monkey—or any animal—chews food, the meal leaves tiny scuffs and craters in its teeth. By studying marks on the teeth of living animals, anthropologists can learn what our ancestors ate from the wear on their fossils.

Photo: Michael M. Torrice

Anthropologist Nate Dominy displays tubers and an early hominid skull at UC Santa Cruz.

The student and his teacher spent their days searching for monkeys. When they found one, an expert in Teaford's team would shoot a tranquilizer dart into the trees as Dominy waited for the animals to pass out and drop from the branches. “I was the guy with the net to catch the monkey,” Dominy says. “I loved it. I was a young guy and it was a lot of fun. . . . That's how I got interested in diet, foods, and monkey behavior.”

Diet provides a glimpse into the bigger picture of how an animal lived, says Craig Stanford, an anthropologist at the University of Southern California. The food our ancestors ate can explain when they scavenged for food, whether they traveled in large groups, or how vulnerable they were to predators. An early human that munched leaves would have had a very different routine from one that hunted big game on the African savanna.

And then there are our big brains. Our brains have grown continuously since our ancestors and their ape relatives parted ways, evolutionarily speaking, more than six million years ago. Around two million years ago, brain size spiked. This moment in time was a turning point. The ape-like creatures Australopithecus and Paranthropus transformed into more humanoid animals.

Anthropologists refer to this new group as Homo (as in Homo sapiens, the scientific name for us). They had larger bodies and brains than their predecessors, along with smaller teeth and guts. Homo erectus, the first major animal in this new group, left Africa and began to conquer Asia.

Brains, especially big ones, are hungry organs. The modern human brain burns 20 percent of the body's total energy, whereas the heart needs only 5 percent. Blood vessels weave through brain tissue, ferrying nutrients to power its constant calculations. “There's the old common wisdom that on a cold day, you lose a huge amount of heat through your head,” says William Leonard, an anthropologist at Northwestern University. “That's what we're talking about here.”

To stoke the fires in their brains' bigger boiler rooms, Leonard says Homo erectus must have eaten meals that packed a greater caloric punch. These dietary changes could have been made in two ways: Either they chose better foods, or they processed their old foods to extract more calories.

Many anthropologists think our ancestors picked the first option and discovered the nutritional benefits of meat. Meat is a denser source of calories than fruits, seeds, and leaves—the plant diets of apes and our more ape-like ancestors. Moreover, fossils show signs of our carnivorous past. Archaeologists have found fossilized animal bones with cuts made by stone tools dating back two million years. This evidence for butchering coincides with our species' great cranial leap forward. But fossils only tell researchers that our ancestors started chopping up animals around the time of Homo erectus, not how frequently they ate meat. Some anthropologists think hunting or scavenging meat would have been unreliable—or even too risky to make it a staple in early human meals.

In 1999, anthropologist Richard Wrangham of Harvard University offered an alternative hypothesis. He proposed that the transition from ape-like to human-like species was fueled mainly by roasted plant foods. By cooking the plants, our ancestors made their food more digestible and thus unleashed more nutrients and calories. According to Wrangham's hypothesis, most of those vegetables were tubers and their root-like cousins.

Formally known as “underground storage organs,” or USOs, tubers are caloric goldmines. They're “like safety-deposit boxes for these plants,” Dominy says. “When times are good, you deposit your excess resources in there and get prepared for the dry season.” Tuber plants littered the dry fields of sub-Saharan Africa. Few other animals ate them, making them the perfect meal to fall back on when other foods were scarce.

Illustration: Cecelia Azhderian

Dominy also believes that hunting for the vegetables would have helped promote brain growth. “It takes something special to eat a tuber,” Dominy says. While other animals rely on their sense of smell to find their dinners, early humans searching for tubers needed to be amateur botanists to remember which plants buried the most nutritious meals. They also had to manufacture tools to dig up the plants. Only a smart animal could be a taxonomist and a toolmaker, Dominy says. So as our ancestors' diet became more tuber-focused, natural selection would have favored smarter animals.

According to this tuber hypothesis, our Sudoku-puzzle-solving brains are the descendents of a human-like ape whose growling stomach led her to pull up a flower and roast its roots. But, unlike meat, there wasn't strong physical evidence that our ancestors ate those roots. Dominy set out to find it.

Digging tubers

In the summer of 2005, Dominy and his graduate student Justin Yeakel camped along the Okavango River Delta in Botswana. At night, they slept in canvas tents listening to the grunts of hippopotami. Scraps of fabric covered big holes in the tents' roofs. Above their heads, the culprits dangled: the 20-pound giant cucumber-like fruits of the sausage tree. “If one of those fruits falls on you,” Yeakel says, “you're dead.”

The UCSC duo were hunting tubers as part of Dominy's multi-pronged test of the tuber hypothesis. Their goal was to understand how edible tubers are. If tubers were a staple of early human diets, then our ancestors had to be able to chew them easily. Anthropologists already had grasped the chewing power of Australopithecus and Paranthropus by comparing their fossils to the jaws and teeth of living animals. But no one had collected data on the chewing forces needed to break down a tuber.

So for two months, Dominy and Yeakel traveled from Tanzania to Kenya to Botswana to South Africa, testing the toughness and hardness of 98 varieties of the root vegetables. To find their quarry, the researchers often traveled with local plant experts who pointed out flowers known to hide tubers under the soil. When they went searching alone, the anthropologists relied on animals to lead them to the vegetables. For instance, mole rats—which are neither mole nor rat—are tuber specialists. The blind rodents live in underground societies similar to ant colonies, where they dig around hoping to bump into tubers to gnaw on. When they do find their dinner, they finish their meals by packing dirt against the chewed part of the vegetable to preserve the plant. Dominy and Yeakel used what the South Africans call “volcanoes”—the piles of red dirt left behind by these courteous scavengers—as tuber beacons.

Once they dug up a new tuber, the two sprang into action. As Dominy tried to identify the tuber, Yeakel cut it into small cubes. Dominy then brought out a mechanical chewer. The laptop-sized device estimated the forces jaws and teeth needed to exert during chewing. As he turned a crank, Dominy slowly lowered a metal tooth onto each tuber sample like a slow guillotine. A pointed tooth cracked the tubers to measure toughness, like chewing taffy. A flat tooth squished the tubers to measure hardness, like biting a lollipop.

Video (20.8 mb): Anthropologist Nate Dominy describes his fieldwork in Africa and demonstrates his lab studies on tubers. Shot and edited by Michael M. Torrice.
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Dominy learned that plenty of root-like vegetables were edible for early humans—but not all to the same extent. The animals that preceded Homo erectus were best adapted to chew on corms and bulbs, cousins of the tuber. Corms are a group of hard and brittle vegetables like water chestnuts, perfect for the teeth of Paranthropus. (The anthropologist Louis Leakey called this animal “nut-cracker man.”) Bulbs are softer onion-like plant parts that Australopithecus could have eaten easily.

Today the Hadza tribe regularly eats harder tubers, such as the long twisted root called //ekwa hasa (the two backslashes represent clicks in the Hadza language). They transform these inedible tubers into dinner with a simple step: cooking. With his mechanical chewer, Dominy determined that just five minutes of roasting softens the vegetables by 50 percent. The Hadza then chew this cooked root, break it into fibrous wads, and spit it out.

How do the Hadza get nutrients if they spit out the wads? Humans, along with other apes, have digestive enzymes in their mouths. An enzyme called amylase chops up starch, the major nutrient in tubers and roots, into small sugars that our bodies use. In 2007, Dominy and collaborators from Arizona State University discovered an intriguing relationship between human genetics and the amount of amylase in our spit. The gene for this enzyme pops up several times in the human genome—we have, on average, five to six copies. Dominy and his team studied 50 university students and determined that the people with more copies of the gene also had more of the amylase enzyme in their saliva.

Dominy then studied chimpanzees and gorillas to see if they followed this trend. Unlike docile college students, chimpanzees are more dangerous. “Almost all chimpanzee keepers that I know are missing bits of their fingers because they get bit off,” Dominy says. So Dominy traveled to a reserve in Auburn, California, for retired Hollywood chimpanzees to collect spit samples without sacrificing his digits. He found that chimps and gorillas have only two copies of the gene and smaller amounts of amylase in their saliva than humans have. Because the chimp and gorilla diets of fruits and leaves don't contain much starch, they don't need as much amylase, Dominy reasoned.

During evolution, Dominy believes, humans added more copies of the amylase gene and increased the amount of the digestive enzyme in their mouths. This evolutionary change made humans better adapted to eat starchier diets, possibly from dining on more tubers.

Back in Africa during tuber-hunting breaks, Dominy and Yeakel traveled to South African museums in search of fossilized mole rat teeth. They wanted to answer a conundrum about our ancestors' molars. Anthropologists had recently found a specific chemical mark called an isotope pattern on the fossilized teeth of Australopithecus and Paranthropus. “Isotope patterns record what you eat,” Yeakel says. Our ancestors' teeth had preserved chemical clues about the food they ate.

Dominy wondered whether the isotope pattern could have come from tubers. He decided to study an animal that eats only tubers—mole rats. At many archaeological sites, fossilized mole rats lie beside our ancestor's ancient bones, indicating that the animals roamed common turf. The chemical signals on ancient mole rat teeth matched the signals on the teeth of our ancestors. So at the same time and place, Dominy concluded, mole rats and humans were possibly eating the same diet of tubers.

Meat vs. potatoes

Wrangham, the Harvard anthropologist who helped launch the tuber hypothesis, thinks Dominy's studies have bolstered the argument for the vegetables' key role in human evolution. “Nate's got a wonderful eye for discovering the kind of data to test these ideas,” Wrangham says. “It's exactly the kind of data we need more of.”

However, Craig Stanford of the University of Southern California has not been swayed. The problem, he says, is the lack of hard evidence for tuber eating. Meanwhile, the fossilized remains of butchered animals from two million years ago fill museums. “The bottom line is you have a body of evidence for meat-eating that is empirical and physical and real and voluminous,” Stanford says. “Then you have some circumstantial evidence and some well-thought-out speculations to support the [tuber] issue.”

Dominy doesn't debate that our ancestors butchered and ate meat, but he questions the frequency. He wonders how easy it would have been for early Homo animals to hunt and scavenge meat with unsophisticated tools. “Only 20 to 30 percent of the Hadza diet [today] comes from meat,” Dominy says. “And they have language, they have technology, and they have iron-tipped and poison-tipped arrows.” It's hard to imagine that our more primitive ancestors ate meat as frequently, Dominy says.

Stanford disagrees and says the tool evidence in the fossil record is strong. “You can go to some of these fossil sites and literally step out of a Land Rover and your feet just crunch stone tools everywhere,” he says. He also notes that researchers disagree about the amount of tubers in the diet of today's Hadza tribe.

Based on his studies of meat-eating chimpanzees, Stanford believes that dining on meat created smarter animals through social pressures. Because hunting meat was difficult and catching prey would have been infrequent, some animals may have bartered for others' food. As our ancestors moved to a more carnivorous diet, Stanford says, more intelligent animals had an advantage because they could better navigate the new social landscape.

For Stanford, the tuber advocates still have more work to convince him and other anthropologists: “Nobody should be writing this into a textbook.”

Grains of truth

Back in his office, a small room the size of a monk's cell, Dominy discusses his next angle of attack to sway the tuber skeptics: the tiny particles of starch molecules found in plants, called starch grains. Anthropologists have found these grains on prehistoric stone tools and in the fossilized plaque on our ancestors' teeth. Each plant family has unique grain shapes and sizes, making it easy to trace each particle to its source. Dominy is planning another African tour to collect starch grains from different tubers so he can match modern starch particles to those found in the fossil record. “The idea now is to identify the starch grains…that the animals actually put into their mouths,” Dominy says. “Some of them could have been tubers, and that would be some incredibly direct evidence of this hypothesis.”

While Dominy accumulates this new evidence and the meat faction sticks by its fossils, Northwestern's William Leonard offers a compromise. He thinks our more human-like relatives fed their bigger brains by hunting for calorie-rich meat and cooking tubers—a position Wrangham also now holds.

“The way the debate has been framed is…too either-or, black-and-white,” Leonard says. “The hallmark of human evolution is our ability to increase the quality of our diet and our ability to make a meal in any environment.” He points to the development of agriculture 10,000 years ago. By selectively breeding plants and animals, we created higher quality versions of foods we had found in the wild, he says.

Even today, Leonard notes, we still pursue bigger calories—from bioengineering more nutritious crops to bulking up chickens with hormones. These new technologies, like our ancestors' digging sticks and stone tools, are part of our ongoing quest to nourish our big brains.

Story ©2009 by Michael M. Torrice. For reproduction requests, contact the Science Communication Program office.



Michael M. Torrice
S.B. (chemistry) Massachusetts Institute of Technology
Ph.D. (chemistry) California Institute of Technology
Internship: Science, Washington, D.C.

Long before I encountered a distillation set-up or a pipette, I read a thin hot-pink book, How to Think Like a Scientist. Fixated on the title, I absorbed this introduction to the scientific method—the path that scientists follow from question to conclusion. Many thicker dull-colored books later, I was at a lab bench studying proteins in the brain that translate the chemical chatter of our thoughts. Although the science fascinated me, I enjoyed the last step of the scientific method the most: communicating results. From discussing my own data to explaining discoveries by other scientists, I had found the thrill of science writing. Now I'm leaving the lab and learning to think like a journalist—without the help of neon-colored books.

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Noel Sirivansanti
B.S. (molecular environmental biology) UC Berkeley
Internship: Annual Reviews of Science, Palo Alto, CA

Growing up in the Bay Area, I've always been sympathetic to nature and the environment. Add to that my love of science, exploration, colors, and making things with my hands, and you get the makings of a science illustrator. Discipline and the Science Illustration Program taught me how to become a more effective illustrator. I am grateful to be in a field where almost every experience, from studying biology, to taking hikes in the rainforests of Costa Rica, to listening to music while dissecting a bird, can provide material for my drawings. I look forward to weaving together more pictures from what I learn about nature and science, and sharing them with you.

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Cecelia Azhderian
B.S. (aquatic biology) and B.A. (studio art) UC Santa Barbara
Internship: UC Berkeley Gump South Pacific Research Station

Art and science have always been two compelling passions in my life, in and out of the classroom. Much of my time as a student has been spent studying some aspect of nature and then recording it in various drawings and paintings. But when it came time to enter the job force I chose science, thinking it would be the more sensible pursuit and that art would just have to be a lifetime hobby. However, after three years working as a full-time biologist, I realized I needed a change. Discovering science illustration and becoming involved in this UCSC graduate program has proved to be the perfect fit and has provided a balanced career path surrounded by like-minded and inspiring people.