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proved methods of extracting DNA from still older fossilized bone now appear close at hand. With them, we may begin building the family tree from a root that was alive when the human family was young.

Epilogue since this article was first published, further genetic work on the mi-tochondrial DNA sequences of three Neandertal specimens upholds our conclusions about the lack of a mixture between ancient and modern Homo sapiens. Furthermore, whole mitochondrial genome sequencing—all 16,569 base pairs from more than 50 donors—gives more precise resolution to the timescale of our emergence. It now seems that the earliest migration out of Africa is closer to 120,000 years ago than 200,000 years ago—more recent, yet still within the range we had originally estimated. S3

Mitochondrial DNA and Human Evolution. Rebecca L. Cann, Mark Stoneking and Allan C. Wilson in Nature, Vol. 325, No. 6099, pages 31-36; January 1-7, 1987.

Mitochondrial DNA. M. Stoneking and A. C. Wilson in The Colonization of the Pacific: A Genetic Trail. Edited by Adrian V. S. Hill and Susan W. Serjeantson. Oxford University Press, 1989. Mitochondrial DNA Sequences in Single Hairs from a Southern African Population. Linda Vigilant, Renee Pennington, Henry Harpending, Thomas D. Kocher and Allan C. Wilson in Proceedings of the National Academy of Sciences USA, Vol. 86, No. 23, pages 9350-9354; December 1989.

Sequence Evolution of Mitochondrial DNA in Humans and Chimpanzees. T. D. Kocher and A. C. Wilson in Evolution of Life. Edited by S. Osawa and T. Honjo. Springer-Verlag, Tokyo, 1991.

IGHT 2003 SCIENTIFIC AMERICAN, INC.

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 hind limb features required for upright walking evolved. These include the fully modern limb and foot proportions and pelvis morphology.

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 homi-nid 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 pri-mates—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 environ-ments—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, this must cover the increased costs that mothers incur during pregnancy and lactation.

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, which ensures 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

WHEN THEY ARE on the ground, living nonhuman primates typically move around on all fours, or quadrupedally. 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 bipedal-ism evolved abound in the paleoanthro-pological 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

Overview/Diet and Human Evolution

The characteristics that most distinguish humans from other primates are largely the results of natural selection acting to improve the quality of the human diet and the efficiency with which our ancestors obtained food. Some scientists have proposed that many of the health problems modern societies face are consequences of a discrepancy between what we eat and what our Paleolithic forebears ate.

Yet studies of traditionally living populations show that modern humans are able to meet their nutritional needs using a wide variety of dietary strategies. We have evolved to be flexible eaters. The health concerns of the industrial world, where calorie-packed foods are readily available, stem not from deviations from a specific diet but from an imbalance between the energy we consume and the energy we expend.

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 bi-pedalism 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. 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. Se-

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