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 monkeyor any
animalchews 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
leavesthe 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 unreliableor 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 ratswhich are neither mole nor ratare 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
volcanoesthe piles of red dirt left behind by these courteous
scavengersas 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. Requires QuickTime Player
Dominy learned that
plenty of root-like vegetables were edible for early humansbut 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 genomewe 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 tubersmole 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 grainsthat 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
tubersa position Wrangham also now holds.
The
way the debate has been framed istoo 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 caloriesfrom 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.
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 methodthe
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 journalistwithout the help of neon-colored
books.
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.
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.