Book 1 Reading and Speaking (1108795), страница 24
Текст из файла (страница 24)
As the African continent grew drier, forests gave way to grasslands, leavingfood resources patchily distributed. In this context, bipedalism can be viewed as one of the first strategies in humannutritional evolution, a pattern of movement that would have substantially reduced the number of calories spent incollecting increasingly dispersed food resources. Indeed, modern human hunter-gatherers living in theseenvironments, who provide us with the best available model of early human subsistence patterns, often travel six toeight miles daily in search of food.Big Brains and Hungry HominidsNo sooner had humans perfected their stride than the next pivotal event in human evolution—the dramaticenlargement of the brain—began.
According to the fossil record, the australopithecines never became muchbrainier than living apes, showing only a modest increase in brain size, from around 400 cubic centimeters fourmillion years ago to 500 cubic centimeters two million years later. Homo brain sizes, in contrast, ballooned from600 cubic centimeters in H. habilis some two million years ago up to 900 cubic centimeters in early H. erectus just300,000 years later. The H. erectus brain did not attain modern human proportions (1,350 cubic centimeters onaverage), but it exceeded that of living nonhuman primates.From a nutritional perspective, what is extraordinary about our large brain is how much energy itconsumes—roughly 16 times as much as muscle tissue per unit weight.
We therefore use a much greater share ofour daily energy budget to feed our voracious brains. In fact, at rest brain metabolism accounts for a whopping 20to 25 percent of an adult human’s energy needs—far more than the 8 to 10 percent observed in nonhumanprimates, and more still than the 3 to 5 percent allotted to the brain by other mammals.How did such an energetically costly brain evolve? One theory, developed by Dean Falk of Florida StateUniversity, holds that bipedalism enabled hominids to cool their cranial blood, thereby freeing the heat-sensitivebrain of the temperature constraints that had kept its size in check.
I suspect that, as with bipedalism, a number ofselective factors were probably at work. But brain expansion almost certainly could not have occurred untilhominids adopted a diet sufficiently rich in calories and nutrients to meet the associated costs.Comparative studies of living animals support that assertion. Across all primates, species with bigger brainsdine on richer foods, and humans are the extreme example of this correlation, boasting the largest relative brainsize and the choicest diet. 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 fromthese comestibles. Animal foods are far denser in calories and nutrients than most plant foods. It stands to reason,then, that for early Homo, acquiring more gray matter meant seeking out more of the energy-dense fare.Fossils, too, indicate that improvements to dietary quality accompanied evolutionary brain growth. Allaustralopithecines 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 owngenus—had especially pronounced adaptations for grinding up fibrous plant foods, including massive, dish-shapedfaces; heavily built mandibles; ridges, or sagittal crests, atop the skull for the attachment of powerful chewingmuscles; and huge, thickly enameled molar teeth.
(This is not to say that australopithecines never ate meat. Theyalmost certainly did on occasion, just as chimps do today.) In contrast, early members of the genus Homo, whichdescended from the gracile australopithecines, had much smaller faces, more delicate jaws, smaller molars and nosagittal crests—despite being far larger in terms of overall body size than their predecessors. Together thesefeatures suggest that early Homo was consuming less plant material and more animal foods.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 continueddesiccation 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, evolvinganatomical specializations that enabled them to subsist on more widely available, difficult to chew foods.
Homotook a different path. As it turns out, the spread of grasslands also led to an increase in the relative abundance ofgrazing 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 asignificant part of the diet and resources were shared among members of the foraging groups.
Signs of thisbehavioral revolution are visible in the archaeological record, which shows an increase in animal bones at hominidsites during this period, along with evidence that the beasts were butchered using stone tools.These changes in diet and foraging behavior did not turn our ancestors into strict carnivores; however, theaddition of modest amounts of animal foods to the menu, combined with the sharing of resources that is typical ofhunter-gatherer groups, would have significantly increased the quality and stability of hominid diets.
Improveddietary quality alone cannot explain why hominid brains grew, but it appears to have played a critical role inenabling that change. After the initial spurt in brain growth, diet and brain expansion probably interactedsynergistically: bigger brains produced more complex social behavior, which led to further shifts in foraging tacticsand improved diet, which in turn fostered additional brain evolution.55A Movable FeastThe evolution of H. erectus in Africa 1.8 million years ago also marked a third turning point in humanevolution: the initial movement of hominids out of Africa. Until recently, the locations and ages of known fossil sitessuggested that early Homo stayed put for a few hundred thousand years before venturing out of the motherlandand slowly fanning out into the rest of the Old World.
Earlier work hinted that improvements in tool technologyaround 1.4 million years ago—namely, the advent of the Acheulean hand ax—allowed hominids to leave Africa. Butnew discoveries indicate that H. erectus hit the ground running, so to speak. Rutgers University geochronologistCarl Swisher III and his colleagues have shown that the earliest H. erectus sites outside of Africa, which are inIndonesia and the Republic of Georgia, date to between 1.8 million and 1.7 million years ago. It seems that the firstappearance of H. erectus and its initial spread from Africa were almost simultaneous.The impetus behind this newfound wanderlust again appears to be food. What an animal eats dictates to alarge extent how much territory it needs to survive. Carnivorous animals generally require far bigger home rangesthan 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 thesmaller, more vegetarian australopithecines did. Exactly how far beyond the continent that shift would have takenH. erectus remains unclear, but migrating animal herds may have helped lead it to these distant lands.Modern QuandariesJust as pressures to improve dietary quality influenced early human evolution, so, too, have these factorsplayed a crucial role in the more recent increases in population size. Innovations such as cooking, agriculture andeven 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, humansbegan 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 ofcrop species to make “better” fruits, vegetables and grains. Similarly, the development of liquid nutritionalsupplements and meal replacement bars is a continuation of the trend that our ancient ancestors started: gainingas 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. Butperhaps the strongest testament to the importance of energy- and nutrient-rich foods in human evolution lies in theobservation that so many health concerns facing societies around the globe stem from deviations from the energydynamic that our ancestors established.
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