The Power of Plagues. Irwin W. Sherman
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with other potential hosts, thereby reducing the possibilities for virus transmission by direct contact. Similarly, the development of diarrhea in a person with the disease cholera or Salmonella infection (which causes “food poisoning”) facilitates the dispersal of these intestinal microbes via fecally contaminated water and food, and in the absence of diarrhea parasite transmission would be reduced.
AIDS is a consequence of an increase in the virulence of HIV. The enhancement in HIV virulence is believed to have resulted from accelerated transmission rates due to changes in human sexual behavior: the increased numbers of sexual partners was so effective in spreading the virus that human survival became less important than survival of the parasite. As the various kinds of plagues are considered in greater detail in subsequent chapters, recognition of the evolutionary basis for virulence may suggest strategies for public health programs. Clean water may thus favor a reduction in the virulence of waterborne intestinal parasites (such as cholera), and clean needle exchange and condom use would both reduce transmission and lessen HIV virulence. But some contend that this indirect mechanism may be too weak and too slow to reduce virulence substantially, and that a better approach could be direct selection by targeting the virulence factor itself. For example, immunization that produces immunity against the toxin produced by the diphtheria microbe also results in a decline in virulence. Future efforts will determine which strategy is the better means for effective “germ” control to improve the public’s health.
1 *See: Cells and Their Structure in the Appendix
2
Plagues, the Price of Being Sedentary
Figure 2.1 Hollywood’s view of Australopithecus as seen in the movie 2001: A Space Odyssey. Turner Entertainment Co. Licensed by Warner Bros Entertainment Inc. All rights reserved, Alamy Stock Photo
In Stanley Kubrick’s classic film 2001: A Space Odyssey, Richard Strauss’s music (“Thus Spake Zarathustra”) provides a haunting and frightening background to the sequence of scenes that represent the dawn of humanity. The sun rises on a barren African savannah. A band of squat, hairy ape-men appear; they eat grass. Though herds of tapirs graze close by, the ape-men ignore them, since the means and the tools necessary to attack or kill the tapirs have not yet been developed. These ape-men are vegetarians who forage for roots and edible plants. On the dawn of the second day, the ape-men are seen huddled around a water hole; the landscape is littered with bones. One, the leader of the group, picks up a bone, smashes the skeleton of an antelope, and then the bone is used to kill a tapir (Fig. 2.1). Shortly thereafter, the raw pieces of tapir flesh are eaten and shared by other hairy apelike creatures, members of the clan. At the dawn of the third day, the meat-eating, tool-using man-apes drive off a neighboring band of apelike creatures. Bone tools used for killing animal prey are now used to threaten and drive off rival tribes. In slow motion, accompanied by the slowly building tones of Strauss’s music, the leader of the man-apes flings his weapon, a fragmented piece of bone, into the air. It spins upward, twisting and turning, end over end. There is a jump cut of 4 million years into the future, and the bone dissolves into a white, orbiting space satellite. Kubrick’s science fiction film has been described as a countdown to tomorrow, a visual masterpiece and a compelling drama of human evolution. Absent from the film is an examination of how the enlightened roving bands of early apelike humans settled down and become increasingly disease-ridden. Here is that part of the story.
Becoming Human, Becoming Parasitized
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.