Naturally_Speaking_L_N_Shevyrdyaeva (Л.Н. Шевырдяева - Naturally Speaking & Listening), страница 17

Exercise 7. Are the following statements true of false, according to the text? Explain your opinion, correct the false statements.

1. Mental decline with age is caused by loss of neurons.

2. Healthy aging is accompanied by a considerable loss of memory.

3. Nerve cells can both age and regenerate with time.

4. Neurons respond very slowly to the change of environment and lifestyle.

5. Age can bring some improvement of mental activity.

Exercise 8. Summarize all the information about age-related transformations in the human organism discussed in this unit and speak on aging and the current research in this sphere.

Unit 15. Food

Then I commended mirth, because a man hath no better thing under the sun, than to eat, and to drink, and to be merry.

Ecclesiastes 8:15:

One should eat to live, not live to eat.

Cicero

There is no love sincerer than the love of food.

George Bernard Shaw

Exercise 1. What do you know about food?

1. What nutrients are necessary for human life?

2. Why are vitamins and minerals necessary?

3. How much food should people consume? What does the daily norm of food consumption depend on?

4. What is generally understood as “healthy diet”?

5. What food products are considered healthy and unhealthy? Why?

6. What factors can lead to overweight and obesity?

7. What role have dietary habits played in human evolution?

8. How did cooking food affect human physiology?

Exercise 2. The article below deals with the role of dietary habits in human evolution. As you read the text, find answers to the questions:

1. What were the main dietary shifts in human evolution?

2. What changes have they led to?

Food for Thought

Dietary change was a driving force in human evolution

By William R. Leonard   

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 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.

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?

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. The type of environment a creature inhabits will influence the distribution of energy into maintenance energy (which keeps an animal alive on a day-to-day basis) and productive energy (which, on the other hand, is associated with producing and raising offspring covering the increased costs that mothers incur during pregnancy and lactation). 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.

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. 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.

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. 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.

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.

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. 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. 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. 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. (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.

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.

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.

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.

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. 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.