Scientific American Feature
Article December 2002 issue
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.
WL: We humans are strange primates.
WL: We walk on two legs, carry around enormous
brains and have colonized every corner of the globe.
WL: 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.
WL: 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.
WL: 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.
WL: Accordingly, what we eat is yet another
way in which we differ from our primate kin.
WL: Contemporary human populations the
world over have diets richer in calories and nutrients than those of our
cousins, the great apes.
WL: Scientific interest in the evolution
of human nutritional requirements has a long history.
WL: 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.
WL: Since then, however, understanding
of the evolution of human nutritional needs has advanced considerably--
WL: ... thanks in large part to new comparative
analyses of traditionally living human populations and other primates--and
a more nuanced picture has emerged.
WL: 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.
WL: To appreciate the role of diet in human
WL: 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.
WL: 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.
WL: 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.
WL: ... bipedalism can be viewed as one
of the first strategies in human nutritional evolution ...
WL: 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].
WL: 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).
WL: Modern chimps, in comparison,
obtain only 5 to 7 percent of their calories from these comestibles.
WL: Animal foods are far denser
in calories and nutrients than most plant foods.
WL: 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.
WL: It stands to reason, then,
that for early Homo, acquiring more gray matter meant seeking out more
of the energy-dense fare.
WL: Fossils, too, indicate that improvements
to dietary quality accompanied evolutionary brain growth.
WL: (This is not to say that australopithecines
never ate meat. They almost certainly did on occasion, just as chimps
WL: As to what prompted Homo's initial
shift toward the higher-quality diet necessary for brain growth
WL: The continued desiccation of the African
landscape limited the amount and variety of edible plant foods available
WL: 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.
WL: ... the addition of modest amounts
of animal foods to the menu ... would have significantly increased the
quality and stability of hominid diets.
WL: 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.
WL: 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, ... 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.
WL: -- 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.
WL: 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.
WL: 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.
WL: 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.
WL: Cooking, for one, augmented the energy
available in wild plant foods.
WL: 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.
WL: Overall, that strategy has evidently
worked: humans are here today and in record numbers to boot.
WL: 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
WL: 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.
WL: 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
WL: Obesity has also appeared in parts
of the developing world where it was virtually unknown less than a generation
WL: We are victims of our own evolutionary
WL: 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.
WL: 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.
Again, we see that anthro-apologists have abandoned real science, facts, and logic in their fanciful creative writing efforts.