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Ernährungsmedizin |
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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 |
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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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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