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Image:
JOHN GURCHE
SALAD DAYS:
Australopithecus afarensis, a human
ancestor, forages for plant foods
in an African woodland some 3.5 million
years ago. |
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Food
for Thought
Dietary change was a driving
force in human evolution.
By William R. Leonard
December 2002
© 1996-2003 Scientific American, Inc.
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We
humans are strange primates.
We walk on two legs, carry around enormous brains and
have colonized every corner of the globe. Anthropologists
and biologists have long sought to understand how our
lineage came to differ so profoundly from the primate
norm in these ways, and over the years all manner of
hypotheses aimed at explaining each of these oddities
have been put forth. But a growing body of evidence
indicates that these miscellaneous quirks of humanity
in fact have a common thread: they are largely the result
of natural selection acting to maximize dietary quality
and foraging efficiency. Changes in food availability
over time, it seems, strongly influenced our hominid
ancestors. Thus, in an evolutionary sense, we are very
much what we ate.
Accordingly, what we eat is yet another way in which
we differ from our primate kin. Contemporary human populations
the world over have diets richer in calories and nutrients
than those of our cousins, the great apes. So when and
how did our ancestors' eating habits diverge from those
of other primates? Further, to what extent have modern
humans departed from the ancestral dietary pattern?
Scientific interest in the evolution of human nutritional
requirements has a long history. But relevant investigations
started gaining momentum after 1985, when S. Boyd Eaton
and Melvin J. Konner of Emory University published a
seminal paper in the New England Journal of Medicine
entitled "Paleolithic Nutrition." They argued
that the prevalence in modern societies of many chronic
diseases -- obesity, hypertension, coronary heart disease
and diabetes, among them--is the consequence of a mismatch
between modern dietary patterns and the type of diet
that our species evolved to eat as prehistoric hunter-gatherers.
Since then, however, understanding of the evolution
of human nutritional needs has advanced considerably
-- thanks in large part to new comparative analyses
of traditionally living human populations and other
primates -- and a more nuanced picture has emerged.
We now know that humans have evolved not to subsist
on a single, Paleolithic diet but to be flexible eaters,
an insight that has important implications for the current
debate over what people today should eat in order to
be healthy.
To appreciate the role of diet in human evolution, we
must remember that the search for food, its consumption
and, ultimately, how it is used for biological processes
are all critical aspects of an organism's ecology. The
energy dynamic between organisms and their environments
-- that is, energy expended in relation to energy acquired
-- has important adaptive consequences for survival
and reproduction. These two components of Darwinian
fitness are reflected in the way we divide up an animal's
energy budget. Maintenance energy is what keeps an animal
alive on a day-to-day basis. Productive energy, on the
other hand, is associated with producing and raising
offspring for the next generation. For mammals like
ourselves, this must cover the increased costs that
mothers incur during pregnancy and lactation. |
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 Image:
JOHN GURCHE |
SKELETAL
REMAINS indicate that our ancient
forebears the australopithecines were bipedal
by four million years ago. In the case of
A. afarensis (right), one of the earliest
hominids, telltale features include the
arch in the foot, the nonopposable big toe,
and certain characteristics of the knee
and pelvis. But these hominids retained
some apelike traits-- short legs, long arms
and curved toes, among others-- suggesting
both that they probably did not walk exactly
like we do and that they spent some time
in the trees. It wasn't until the emergence
of our own genus, Homo (a contemporary representative
of which appears on the left), that the
fully modern limb and foot proportions and
pelvis form required for upright walking
as we know it evolved.
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The type of environment a creature inhabits will
influence the distribution of energy between these
components, with harsher conditions creating higher
maintenance demands. Nevertheless, the goal of
all organisms is the same: to devote sufficient
funds to reproduction to ensure the long-term
success of the species. Thus, by looking at the
way animals go about obtaining and then allocating
food energy, we can better discern how natural
selection produces evolutionary change.
Becoming Bipeds
Without exception, living nonhuman primates habitually
move around on all fours, or quadrupedally, when
they are on the ground. Scientists generally assume
therefore that the last common ancestor of humans
and chimpanzees (our closest living relative)
was also a quadruped. Exactly when the last common
ancestor lived is unknown, but clear indications
of bipedalism -- the trait that distinguished
ancient humans from other apes -- are evident
in the oldest known species of Australopithecus,
which lived in Africa roughly four million years
ago. Ideas about why bipedalism evolved abound
in the paleoanthropological literature. C. Owen
Lovejoy of Kent State University proposed in 1981
that two-legged locomotion freed the arms to carry
children and foraged goods. More recently, Kevin
D. Hunt of Indiana University has posited that
bipedalism emerged as a feeding posture that enabled
access to foods that had previously been out of
reach. Peter Wheeler of Liverpool John Moores
University submits that moving upright allowed
early humans to better regulate their body temperature
by exposing less surface area to the blazing African
sun.
The list goes on. In reality, a number of factors
probably selected for this type of locomotion.
My own research, conducted in collaboration with
my wife, Marcia L. Robertson, suggests that bipedalism
evolved in our ancestors at least in part because
it is less energetically expensive than quadrupedalism.
Our analyses of the energy costs of movement in
living animals of all sizes have shown that, in
general, the strongest predictors of cost are
the weight of the animal and the speed at which
it travels. What is striking about human bipedal
movement is that it is notably more economical
than quadrupedal locomotion at walking rates.
Apes, in contrast, are not economical when moving
on the ground. For instance, chimpanzees, which
employ a peculiar form of quadrupedalism known
as knuckle walking, spend some 35 percent more
calories during locomotion than does a typical
mammalian quadruped of the same size -- a large
dog, for example.
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BRAINS GREW BIGGER and hence more
energetically demanding -- over time. The
modern human brain accounts for 10 to 12
percent more of the body's resting energy
requirements than the average australopithecine
brain did.
Click here for full-size image
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 Image:
CORNELIA BLIK |
Differences in the settings in which humans and
apes evolved may help explain the variation in
costs of movement. Chimps, gorillas and orangutans
evolved in and continue to occupy dense forests
where only a mile or so of trekking over the course
of the day is all that is needed to find enough
to eat. Much of early hominid evolution, on the
other hand, took place in more open woodland and
grassland, where sustenance is harder to come
by. Indeed, modern human hunter-gatherers living
in these environments, who provide us with the
best available model of early human subsistence
patterns, often travel six to eight miles daily
in search of food.
These differences in day range have important
locomotor implications. Because apes travel only
short distances each day, the potential energetic
benefits of moving more efficiently are very small.
For far-ranging foragers, however, cost-effective
walking saves many calories in maintenance energy
needs -- calories that can instead go toward reproduction.
Selection for energetically efficient locomotion
is therefore likely to be more intense among far-ranging
animals because they have the most to gain.
For hominids living between five million and 1.8
million years ago, during the Pliocene epoch,
climate change spurred this morphological revolution.
As the African continent grew drier, forests gave
way to grasslands, leaving food resources patchily
distributed. In this context, bipedalism can be
viewed as one of the first strategies in human
nutritional evolution, a pattern of movement that
would have substantially reduced the number of
calories spent in collecting increasingly dispersed
food resources.
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Image: DAVID BRILL
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ROBUST
AUSTRALOPITHECINES
like A. boisei (left)
had pronounced adaptations to eating tough,
fibrous plant foods. H. erectus (right),
in contrast, evolved to eat a softer,
higher-quality diet -- one that most likely
featured meat regularly.
Click
here for full-size image
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Big Brains and Hungry Hominids
No sooner had humans perfected their stride than the
next pivotal event in human evolution -- the dramatic
enlargement of the brain -- began. According to the
fossil record, the australopithecines never became much
brainier than living apes, showing only a modest increase
in brain size, from around 400 cubic centimeters four
million years ago to 500 cubic centimeters two million
years later. Homo brain sizes, in contrast, ballooned
from 600 cubic centimeters in H. habilis some two million
years ago up to 900 cubic centimeters in early H. erectus
just 300,000 years later. The H. erectus brain did not
attain modern human proportions (1,350 cubic centimeters
on average), but it exceeded that of living nonhuman
primates.
From a nutritional perspective, what is extraordinary
about our large brain is how much energy it consumes--
roughly 16 times as much as muscle tissue per unit weight.
Yet although humans have much bigger brains relative
to body weight than do other primates (three times larger
than expected), the total resting energy requirements
of the human body are no greater than those of any other
mammal of the same size. We therefore use a much greater
share of our daily energy budget to feed our voracious
brains. In fact, at rest brain metabolism accounts for
a whopping 20 to 25 percent of an adult human's energy
needs -- far more than the 8 to 10 percent observed
in nonhuman primates, and more still than the 3 to 5
percent allotted to the brain by other mammals.
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By using estimates of hominid body size
compiled by Henry M. McHenry of the University
of California at Davis, Robertson and I
have reconstructed the proportion of resting
energy needs that would have been required
to support the brains of our ancient ancestors.
Our calculations suggest that a typical,
80- to 85-pound australopithecine with a
brain size of 450 cubic centimeters would
have devoted about 11 percent of its resting
energy to the brain. For its part, H. erectus,
which weighed in at 125 to 130 pounds and
had a brain size of some 900 cubic centimeters,
would have earmarked about 17 percent of
its resting energy -- that is, about 260
out of 1,500 kilocalories a day -- for the
organ.
How did such an energetically costly brain
evolve? One theory, developed by Dean Falk
of Florida State University, holds that
bipedalism enabled hominids to cool their
cranial blood, thereby freeing the heat-sensitive
brain of the temperature constraints that
had kept its size in check. I suspect that,
as with bipedalism, a number of selective
factors were probably at work. But brain
expansion almost certainly could not have
occurred until hominids adopted a diet sufficiently
rich in calories and nutrients to meet the
associated costs.
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Image:
I. DEVORE Anthro-Photo File
EARLY COOKING
of plant foods, especially tubers, enabled
brain expansion, argue Richard Wrangham
of Harvard University and his colleagues.
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Eating more animal foods is one way of boosting the
caloric and nutrient density of the diet, a shift that
appears to have been critical in the evolution of the
human lineage. But might our ancient forebears have
improved dietary quality another way? Richard Wrangham
of Harvard University and his colleagues recently examined
the importance of cooking in human evolution. They showed
that cooking not only makes plant foods softer and easier
to chew, it substantially increases their available
energy content, particularly for starchy tubers such
as potatoes and manioc. In their raw form, starches
are not readily broken down by the enzymes in the human
body. When heated, however, these complex carbohydrates
become more digestible, thereby yielding more calories.
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The researchers propose that Homo erectus was
probably the first hominid to apply fire to food, starting
perhaps 1.8 million years ago. They argue that early
cooking of plant foods (especially tubers) enabled this
species to evolve smaller teeth and bigger brains than
those of their predecessors. Additionally, the extra
calories allowed H. erectus to start hunting -- an energetically
costly activity -- more frequently.
Comparative
studies of living animals support that assertion. Across
all primates, species with bigger brains dine on richer
foods, and humans are the extreme example of this correlation,
boasting the largest relative brain size and the choicest
diet [see "Diet and Primate Evolution," by
Katharine Milton; Scientific American, August 1993].
According to recent analyses by Loren Cordain of Colorado
State University, contemporary hunter-gatherers derive,
on average, 40 to 60 percent of their dietary energy
from animal foods (meat, milk and other products). Modern
chimps, in comparison, obtain only 5 to 7 percent of
their calories from these comestibles. Animal foods
are far denser in calories and nutrients than most plant
foods. For example, 3.5 ounces of meat provides upward
of 200 kilocalories. But the same amount of fruit provides
only 50 to 100 kilocalories. And a comparable serving
of foliage yields just 10 to 20 kilocalories. It stands
to reason, then, that for early Homo, acquiring more
gray matter meant seeking out more of the energy-dense
fare. |
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Image: HELLIO & VAN
INGEN Photo Researchers, Inc.
NEANDERTAL
MEALS consisted
mostly of meat (from, for example, reindeer),
according to analyses of bone chemistry.
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Fossils, too, indicate that improvements
to dietary quality accompanied evolutionary
brain growth. All australopithecines had
skeletal and dental features built for processing
tough, low-quality plant foods. The later,
robust australopithecines -- a dead-end
branch of the human family tree that lived
alongside members of our own genus -- had
especially pronounced adaptations for grinding
up fibrous plant foods, including massive,
dish-shaped faces; heavily built mandibles;
ridges, or sagittal crests, atop the skull
for the attachment of powerful chewing muscles;
and huge, thickly enameled molar teeth.
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(This
is not to say that australopithecines never ate
meat. They almost certainly did on occasion, just
as chimps do today.) In contrast, early members
of the genus Homo, which descended from the gracile
australopithecines, had much smaller faces, more
delicate jaws, smaller molars and no sagittal
crests-- despite being far larger in terms of
overall body size than their predecessors. Together
these features suggest that early Homo was consuming
less plant material and more animal foods. |
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To
reconstruct what early humans ate, researchers have
traditionally studied features on their fossilized teeth
and skulls, archaeological remains of food-related activities,
and the diets of living humans and apes. Increasingly,
however, investigators have been tapping another source
of data: the chemical composition of fossil bones. This
approach has yielded some especially intriguing findings
with regard to the Neandertals.
Michael Richards, now at the University of Bradford
in England, and his colleagues recently examined isotopes
of carbon (13C) and nitrogen
(15N) in 29,000-year-old Neandertal
bones from Vindija Cave in Croatia. The relative proportions
of these isotopes in the protein part of human bone,
known as collagen, directly reflect their proportions
in the protein of the individual's diet. Thus, by comparing
the isotopic "signatures" of the Neandertal
bones to those of other animals living in the same environments,
the authors were able to determine whether the Neandertals
were deriving the bulk of their protein from plants
or from animals.
As to what prompted Homo's initial shift toward the
higher-quality diet necessary for brain growth, environmental
change appears to have once more set the stage for evolutionary
change. The continued desiccation of the African landscape
limited the amount and variety of edible plant foods
available to hominids. Those on the line leading to
the robust australopithecines coped with this problem
morphologically, evolving anatomical specializations
that enabled them to subsist on more widely available,
difficult-to-chew foods. Homo took a different path.
As it turns out, the spread of grasslands also led to
an increase in the relative abundance of grazing mammals
such as antelope and gazelle, creating opportunities
for hominids capable of exploiting them. H. erectus
did just that, developing the first hunting-and-gathering
economy in which game animals became a significant part
of the diet and resources were shared among members
of the foraging groups. Signs of this behavioral revolution
are visible in the archaeological record, which shows
an increase in animal bones at hominid sites during
this period, along with evidence that the beasts were
butchered using stone tools. |
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These changes in diet and foraging behavior
did not turn our ancestors into strict carnivores;
however, the addition of modest amounts
of animal foods to the menu, combined with
the sharing of resources that is typical
of hunter-gatherer groups, would have significantly
increased the quality and stability of hominid
diets. Improved dietary quality alone cannot
explain why hominid brains grew, but it
appears to have played a critical role in
enabling that change. After the initial
spurt in brain growth, diet and brain expansion
probably interacted synergistically: bigger
brains produced more complex social behavior,
which led to further shifts in foraging
tactics and improved diet, which in turn
fostered additional brain evolution.
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A
Movable Feast
The evolution of H. erectus in Africa 1.8 million
years ago also marked a third turning point in
human evolution: the initial movement of hominids
out of Africa. Until recently, the locations and
ages of known fossil sites suggested that early
Homo stayed put for a few hundred thousand years
before venturing out of the motherland and slowly
fanning out into the rest of the Old World. Earlier
work hinted that improvements in tool technology
around 1.4 million years ago--namely, the advent
of the Acheulean hand ax -- allowed hominids to
leave Africa. But new discoveries indicate that
H. erectus hit the ground running, so to speak.
Rutgers University geochronologist Carl Swisher
III and his colleagues have shown that the earliest
H. erectus sites outside of Africa, which are
in Indonesia and the Republic of Georgia, date
to between 1.8 million and 1.7 million years ago.
It seems that the first appearance of H. erectus
and its initial spread from Africa were almost
simultaneous. |
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The impetus behind this newfound wanderlust again appears
to be food. What an animal eats dictates to a large
extent how much territory it needs to survive. Carnivorous
animals generally require far bigger home ranges than
do herbivores of comparable size because they have fewer
total calories available to them per unit area.
Large-bodied and increasingly dependent on animal foods,
H. erectus most likely needed much more turf than the
smaller, more vegetarian australopithecines did. Using
data on contemporary primates and human hunter-gatherers
as a guide, Robertson, Susan C. Antón of Rutgers
University and I have estimated that the larger body
size of H. erectus, combined with a moderate increase
in meat consumption, would have necessitated an eightfold
to 10-fold increase in home range size compared with
that of the late australopithecines -- enough, in fact,
to account for the abrupt expansion of the species out
of Africa. Exactly how far beyond the continent that
shift would have taken H. erectus remains unclear, but
migrating animal herds may have helped lead it to these
distant lands.
As humans moved into more northern latitudes, they encountered
new dietary challenges. The Neandertals, who lived during
the last ice ages of Europe, were among the first humans
to inhabit arctic environments, and they almost certainly
would have needed ample calories to endure under those
circumstances. Hints at what their energy requirements
might have been come from data on traditional human
populations that live in northern settings today. The
Siberian reindeer-herding populations known as the Evenki,
which I have studied with Peter Katzmarzyk of Queen's
University in Ontario and Victoria A. Galloway of the
University of Toronto, and the Inuit (Eskimo) populations
of the Canadian Arctic have resting metabolic rates
that are about 15 percent higher than those of people
of similar size living in temperate environments. The
energetically expensive activities associated with living
in a northern climate ratchet their caloric cost of
living up further still. Indeed, whereas a 160-pound
American male with a typical urban way of life requires
about 2,600 kilocalories a day, a diminutive, 125-pound
Evenki man needs more than 3,000 kilocalories a day
to sustain himself. Using these modern northern populations
as benchmarks, Mark Sorensen of Northwestern University
and I have estimated that Neandertals most likely would
have required as many as 4,000 kilocalories a day to
survive. That they were able to meet these demands for
as long as they did speaks to their skills as foragers.
Modern
Quandaries
Just
as pressures to improve dietary quality influenced early
human evolution, so, too, have these factors played
a crucial role in the more recent increases in population
size. Innovations such as cooking, agriculture and even
aspects of modern food technology can all be considered
tactics for boosting the quality of the human diet.
Cooking, for one, augmented the energy available in
wild plant foods. With the advent of agriculture, humans
began to manipulate marginal plant species to increase
their productivity, digestibility and nutritional content
-- essentially making plants more like animal foods.
This kind of tinkering continues today, with genetic
modification of crop species to make "better"
fruits, vegetables and grains. Similarly, the development
of liquid nutritional supplements and meal replacement
bars is a continuation of the trend that our ancient
ancestors started: gaining as much nutritional return
from our food in as little volume and with as little
physical effort as possible.
Overall,
that strategy has evidently worked: humans are here
today and in record numbers to boot. But perhaps the
strongest testament to the importance of energy- and
nutrient-rich foods in human evolution lies in the observation
that so many health concerns facing societies around
the globe stem from deviations from the energy dynamic
that our ancestors established. For children in rural
populations of the developing world, low-quality diets
lead to poor physical growth and high rates of mortality
during early life. In these cases, the foods fed to
youngsters during and after weaning are often not sufficiently
dense in energy and nutrients to meet the high nutritional
needs associated with this period of rapid growth and
development. Although these children are typically similar
in length and weight to their U.S. counterparts at birth,
they are much shorter and lighter by the age of three,
often resembling the smallest 2 to 3 percent of American
children of the same age and sex.
In the industrial world, we are facing the opposite
problem: rates of childhood and adult obesity are rising
because the energy-rich foods we crave -- notably those
packed with fat and sugar -- have become widely available
and relatively inexpensive. According to recent estimates,
more than half of adult Americans are overweight or
obese. Obesity has also appeared in parts of the developing
world where it was virtually unknown less than a generation
ago. This seeming paradox has emerged as people who
grew up malnourished move from rural areas to urban
settings where food is more readily available. In some
sense, obesity and other common diseases of the modern
world are continuations of a tenor that started millions
of years ago. We are victims of our own evolutionary
success, having developed a calorie-packed diet while
minimizing the amount of maintenance energy expended
on physical activity.
The magnitude of this imbalance becomes clear when we
look at traditionally living human populations. Studies
of the Evenki reindeer herders that I have conducted
in collaboration with Michael Crawford of the University
of Kansas and Ludmila Osipova of the Russian Academy
of Sciences in Novosibirsk indicate that the Evenki
derive almost half their daily calories from meat, more
than 2.5 times the amount consumed by the average American.
Yet when we compare Evenki men with their U.S. peers,
they are 20 percent leaner and have cholesterol levels
that are 30 percent lower.
These differences partly reflect the compositions of
the diets. Although the Evenki diet is high in meat,
it is relatively low in fat (about 20 percent of their
dietary energy comes from fat, compared with 35 percent
in the average U.S. diet), because free-ranging animals
such as reindeer have less body fat than cattle and
other feedlot animals do. The composition of the fat
is also different in free-ranging animals, tending to
be lower in saturated fats and higher in the polyunsaturated
fatty acids that protect against heart disease. More
important, however, the Evenki way of life necessitates
a much higher level of energy expenditure.
Thus, it is not just changes in diet that have created
many of our pervasive health problems but the interaction
of shifting diets and changing lifestyles. Too often
modern health problems are portrayed as the result of
eating "bad" foods that are departures from
the natural human diet -- an oversimplification embodied
by the current debate over the relative merits of a
high-protein, high-fat Atkins-type diet or a low-fat
one that emphasizes complex carbohydrates. This is a
fundamentally flawed approach to assessing human nutritional
needs. Our species was not designed to subsist on a
single, optimal diet. What is remarkable about human
beings is the extraordinary variety of what we eat.
We have been able to thrive in almost every ecosystem
on the earth, consuming diets ranging from almost all
animal foods among populations of the Arctic to primarily
tubers and cereal grains among populations in the high
Andes. Indeed, the hallmarks of human evolution have
been the diversity of strategies that we have developed
to create diets that meet our distinctive metabolic
requirements and the ever increasing efficiency with
which we extract energy and nutrients from the environment.
The challenge our modern societies now face is balancing
the calories we consume with the calories we burn. |
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WILLIAM
R. LEONARD is a professor of anthropology at Northwestern
University. He was born in Jamestown, N.Y., and received
his Ph.D. in biological anthropology at the University
of Michigan at Ann Arbor in 1987. The author of more
than 80 research articles on nutrition and energetics
among contemporary and prehistoric populations, Leonard
has studied indigenous agricultural groups in Ecuador,
Bolivia and Peru and traditional herding populations
in central and southern Siberia.
©
1996-2003 Scientific American, Inc. |
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