Читать книгу The Power of Plagues - Irwin W. Sherman - Страница 19

It is now generally accepted that Africa was the cradle of humanity. The earliest evidence of hominids, that is, animals ancestral to modern humans and not closely related to other monkeys and apes, is found in Africa. The evidence for this comes from unearthed bones and teeth (fossils). The fossil record shows that one of our oldest ancestors—called Australopithecus—lived in Africa about 4.2 million to 3.8 million years ago. These early hominids, which split from the ape lineage (and were discovered in Kenya in1994), are named A. anamensis, and to judge from the structure of the teeth and the position of the opening where the spinal cord enters the skull, one can conclude that they were apelike humans, not apes. Our ancestor Australopithecus spent time in trees and basically behaved similarly to chimpanzees, or so we believe, since fossils provide no record of behavior. Whether A. anamensis walked on two feet is uncertain, but evidence for erect, upright posture in Australopithecus comes from bones discovered in Ethiopia and Tanzania that are 3.8 million to 3.0 million years old, from a species named A. afarensis. One of these finds, a small female, was discovered and named Lucy by Donald C. Johanson of the University of California at Berkeley. The limb structure and the way the hip joint and pelvis articulate make it clear that Lucy walked on two legs (Fig. 2.2). This was dramatically shown by Mary Leakey and her team, who discovered three sets of fossilized footprints left in wet volcanic ash some 3.2 million years ago. A. afarensis weighed about 75 lb and was not very “brainy,” its brain being no larger than the brains of living African great apes. When A. afarensis descended from the trees and stood upright with two feet firmly planted on the ground, not only did it affect posture, but it dramatically changed lifestyles and diets, and disease patterns began to be altered.

Descent from the trees to the ground placed the australopithecines into a new environment, an ecological niche that was very different from the forest canopy. This freed them from some diseases but allowed for the acquisition of new ones. For example, in the treetops australopithecines would have been bitten by mosquitoes that carried parasites acquired from other animals living in the canopy, but at ground level they would be exposed to other airborne bloodsuckers such as ticks and flies, or they would come in contact with different food sources and contaminated water. Their teeth were small and underdeveloped, as in modern human beings, and the canines, highly developed in existing ape species, were small like ours. We can infer from their teeth that these australopithecines probably chewed fruits, seeds, pods, roots, and tubers. Since no stone tools have been found associated with the fossils, it is believed that A. afarensis did not make or use durable tools or understand the use of fire. They were opportunistic scavengers or vegetarians. The life span of an australopithecine has been estimated to have been between 18 and 23 years.

Figure 2.2 Australopithecus reconstruction of Mr. and Mrs. Lucy.

Courtesy of Ken Mowbry, American Museum of Natural History

Beginning about 3 million years ago, the climate in Africa changed from tropical warm and wet to a more temperate cool and dry one, and as a consequence, the dense woodlands were replaced by more open grassy habitats, a savannah. This climate change presented a challenging environment for the woodland-dwelling australopithecines. Although we do not know whether the climate change triggered it, at about this same time, ~2.5 million to 1.8 million years ago, there appear in the fossil record several different kinds (species) of hominids, with two or three coexisting species in eastern and southern Africa. One of these species was A. boisei, a small-brained vegetarian, and the other was Homo habilis (Fig. 2.3b). The name Homo habilis, or “handy man,” is based on the fact that altered stones and animal remains have been found with the fossil bones. H. habilis was more than a scavenger and a gatherer. H. habilis was also a hunter who made and used stone tools: simple stone flakes, scrapers and “choppers” that were chipped from larger stones (Fig. 2.3a). (These stone tools, first found in Africa’s Olduvai Gorge, are called Oldowan tools.) The fashioning of tools suggests a great leap in human intelligence and begins the technological changes that would forever mark Homo as a tool maker and a tool user. H. habilis used the flake tools to cut up the carcasses of the animals that were killed; these were transported to a home base where the meat was fed upon. H. habilis, with a somewhat larger brain, was “smarter” than A. afarensis, but the fossil finds tell us nothing of the numbers of individuals, whether there was division of labor among males and females, or anything about their behavior. We speculate, however, that there were 50 to 60 individuals in a group living in an area of 200 to 600 square miles. We imagine that H. habilis lived at the edge of shallow lakes and in crude rock shelters.

Figure 2.3a Oldowan tools used by Homo habilis, Courtesy Didier Descouens, CC-BY-SA 4.0

There is no fossil record of the parasites that afflicted H. habilis since their soft bodies have disintegrated over time, but we do know that with meat eating came an increase in parasitism. As these nomadic hunters encountered new prey, they met new parasites and new vectors of parasites. The result was zoonosis; that is, animal infections were transmitted to humans. What were these zoonotic infections? We surmise that the parasites of H. habilis were those acquired from the wild animals that were killed and scavenged. The butchered meat might have had parasites such as the bacteria anthrax and tetanus, the roundworm that causes trichinosis, and a variety of intestinal tapeworms. H. habilis would probably have been bitten by mosquitoes, ticks, mites, and tsetse flies, and probably also had head lice. H. habilis also may have suffered from viral diseases such as the mosquito-transmitted yellow fever, as well as non-vector-borne viruses that cause hepatitis, herpes, and colds, and he may have had spirochete infections such as yaws. It is doubtful, but H. habilis could also have been infected with the parasites that cause sleeping sickness, malaria, and leprosy. They certainly must have been infected with filaria, pinworms, and blood flukes, but probably did not have typhus, mumps, measles, influenza, tuberculosis, cholera, chickenpox, diphtheria, or gonorrhea. At the time when H. habilis roamed the African savannah, the human population was quite small, consisting of about 100,000 individuals, and we expect that rates of human-to-human transmission of parasites were low.

Figure 2.3b Diorama in the Nairobi National Museum of Homo habilis,

CC-BY 2.0, https://www.flickr.com/photos/ninara/17147417090/; CC-BY 2.0 license

Figure 2.3c Acheulean tools used by Homo erectus, Courtesy Didier Descouens

Figure 2.3d Diorama of H. ergaster the “African equivalent” to fossils of H. erectus. Alamy Stock Photo.

Roughly 1.6 million years ago (or ~1.8 million years ago in Africa), H. habilis was replaced by H. erectus (meaning “erect man”) (Fig. 2.3d). (In Africa, H. erectus is equivalent to fossils that have been named H. ergaster.) H. erectus was close to modern humans in body size, and its skull capacity suggests that it was somewhat larger-brained than H. habilis—but still with barely half the capacity of modern humans. H. erectus had smaller cheek teeth, suggesting that they were omnivores; they were smaller-faced; they developed a culture characterized by living in caves; and they hunted game animals using bifaced flake stone tools fashioned into “hand axes.” This stone tool technology (called Acheulean tools) (Fig. 2.3c) allowed H. erectus to process more completely the harder parts of animals and plants by grinding, crushing, splitting, and cutting up these before eating. As such, these stone tools represented a technological advance and served as extensions of the hands and teeth to break down food before digestion. H. erectus was able to start fires and made use of fire to cook the food. The H. erectus population was now somewhat less than a million. According to the “long journey” hypothesis, about 2 million years ago the H. erectus populations began to move out of Africa via the Middle East, but climate and geography prevented them from turning west, and so they took a more southerly route into present-day China and Indonesia. Then they turned north and moved west again across the more central parts of Europe and Asia. The earliest fossil remains of H. erectus were found in Indonesia (Java) by Eugene Dubois in 1891 and so were named Java Man; 2 decades later, when Davidson Black found similar fossils in caves in China, they became known as Peking Man. The cave sites in China are ~500,000 years old, and the last of these were abandoned ~230,000 years ago.

When the populations of H. erectus left Africa, some of their parasites went with them, but only those that could be transmitted directly from person to person. Those vectors that remained restricted to Africa, such as the species of mosquito, snail, and fly that transmit diseases such as filariasis, blood fluke disease, and sleeping sickness, respectively, did not follow the migratory path. Indeed, even today they remain diseases that are characteristic of Africa. But as H. erectus encountered new environments with new kinds of animals, they were subjected to sources of new parasites; with an increase in the number of humans living in more-restricted geographic environments, the probability for large-scale infections was enhanced.

Tool making and tool use, as well as human cooperation, made hunting possible. Together they contributed to further increases in the size of the human population, and over time H. erectus evolved into humans closely resembling us. A half-million years ago the human populations of Africa and those in Europe and Asia began to diverge from one another. Some 200,000 years ago the fossil record shows individuals who were larger-brained; those with a more graceful face, found in southwestern Europe and dated to 40,000 years ago, were called Cro-Magnon man. In a fit of hubris, Carolus Linnaeus gave them the scientific name Homo sapiens, literally “wise man.” H. sapiens not only used the stone technology of H. erectus but also made tools from bone and antlers. They made artistic carvings and cave paintings, kept records on bone and stone, played music on simple wind instruments, adorned themselves with jewelry, and buried their dead in ritual ceremonies; their living sites were highly organized and stratified, and they hunted and fished in groups. The intermittent technological advances in tool making seen with H. habilis and H. erectus were constantly refined. Clearly, they were our immediate ancestors. At this time the human population numbered about a million individuals.

In western Europe, human skeletons were found first in the Neander Valley of Germany in 1856; they were called Neanderthals. Subsequently, Neanderthal fossils were found in the Middle East and parts of western Asia. They date from between 190,000 and 29,000 years ago. Some archeologists have classified them as a separate species,

Figure 2.4a Neanderthal man in profile; Neanderthal woman cleaning a reindeer skin, (both) Wellcome Library, London (CC-BY 4.0)

Homo neanderthalensis. The Neanderthals, who previously have been depicted as brutish cavemen (Fig. 2.4a and b), have been “modernized” on the basis of a brain size slightly greater than our own, and they left evidence showing that they cared for the sick and performed ritual burials (Fig. 2.4c). Their stone tools, however, were cruder than those of Cro-Magnon man. Within a few thousand years the Cro-Magnons, with their superior weapons and other advanced cultural practices, had completely displaced the Neanderthals. In Africa there are skeletal remains that are more modern than those of Neanderthals, dating back 200,000 years. Thus, nearly a quarter of a million years ago, populations of H. sapiens lived in Africa, Asia, and Europe. Their migration into the Americas took place ~35,000 to 12,000 years ago when they crossed the land bridge at the Bering Strait from Asia into the Americas. Forty thousand years ago humans moved into Australia. Thus, over the past 5 million years, new hominid species have emerged, coexisted, competed, and colonized new environments, in some instances succeeding, in others becoming extinct. The fossil record is of necessity incomplete, and much will be learned from future anthropological digs, but what is certain is that we did not arrive by a straight-line descent from the apes; we are not the single topmost limb in the hominid evolutionary tree, but simply one of its many branches.

Figure 2.4b A 1953 B- grade movie poster representing a monster-like Neanderthal man. Courtesy of Popcorn Posters.

Figure 2.4c Neanderthal Family, Reconstruction. Ian Tattersall, American Museum of Natural History

Hunter-gatherers, unable to preserve and store fruits, vegetables, and meat, were forced to roam over large distances in search of wild edible plants and to hunt down game animals and find sources of drinking water. Moving from place to place, these nomadic bands were not surrounded by heaps of rotting meat or feces, and exposure to parasite-infested waters was limited. Though the hunter-gatherers did come together in groups, the size of their populations was small, and so diseases of crowds requiring human-to-human transmission were absent. Based on what we know about modern hunter-gatherer societies like those in present-day New Guinea, the Australian aborigines, and the Kalahari bushmen, we believe our hunter-gatherer ancestors were a relatively healthy lot. Gradually, however, conditions would change as the size of human populations increased and people adopted sedentary habits—living for extended periods of time in permanent or semipermanent settlements. This would, over time, dramatically increase the incidence of human disease.