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Extinctions have occurred throughout the history of the Earth; extinction is in fact a critical component in the theory of evolution and is attributed to natural selection, random factors, or catastrophic events. The Earth has survived five mass extinctions—including one that destroyed the dinosaurs—all of which have shaped the world as we know it today.
- End-Cretaceous Extinctions
- Pleistocene/Holocene Extinctions
- What Was the Cause?
- Late Prehistoric and Historic Period
- Extinction Matters
- A Sixth Mass Extinction?
The fossil record reveals that the history of life has been a process of speciation, or the diversification of organisms into new species, accompanied by extinction, or the disappearance of existing species. Speciation and extinction are equal partners in evolution. In neo-Darwinian theory, genetic variation and natural selection are the primary agents in evolution. Natural selection favors genetic variation that improves an organism’s chances of surviving to reproduce within its environment. Successful variation is passed on to the next generation. Its frequency in the population increases from generation to generation until it eventually replaces the earlier, less advantageous trait. Acquisition of new traits sometimes enables organisms to invade new habitats. In neo-Darwinism, the environment is the prime source of selection. Therefore, environmental change—or change of environment, in the case of habitat invasion—is considered the chief cause of species evolution. New environmental conditions change selective pressures operating on populations of organisms. Populations must adapt to new circumstances or perish without descendants. In addition to natural selection, random factors and rare, catastrophic events also appear to play a role in extinctions.
Earth is some 4.6 billion years old. Geologists divide this time into eras, periods, epochs, and ages. Life first appeared on Earth late in the Precambrian era some 3.5 billion years ago. The majority of species that evolved since that first appearance are now extinct. Through time, species generally disappear individually in “background extinctions.” But during at least five mass extinction events, huge numbers of species perished together over (geologically) short spans. By eliminating some previously successful species and allowing other, formerly minor, groups to expand and diversify, each mass extinction restructured the biosphere.
The Cretaceous (114 to 65 million years ago [mya]) was the final period in the age of dinosaurs. At its close, virtually all of these great beasts—together with more than 50 percent of Earth’s species— disappeared suddenly and entirely. Many scholars attribute their extinction to climate change, but the disappearances were not gradual, and the dinosaurs appear to have been diversifying up to the end. The physicist Luis Alvarez and his colleagues (1980) note that rock layers dating to the Cretaceous boundary commonly contain a thin clay layer of iridium, a rare metal resembling platinum, and spherules or tektites, spherical bits of glassy rock formed from molten silicate droplets. They interpret this as evidence that one or more mighty asteroids or comets struck Earth around 65 million years ago. They hypothesize that the impact generated a giant vapor cloud (out of which the spherules condensed), sent shock debris across the globe, created an immense fireball, and cast a huge plume of dust into the atmosphere. As this dust plume ascended, atmospheric winds spread it into a globe-embracing cloud that blocked sunlight from the Earth’s surface for a year or more. In this scenario, mass extinctions resulted from both the initial impact and the disruption of the food chain that followed. Perhaps the collision also ignited vast global fires that added huge smoke plumes to the sun-blocking haze of the windblown impact debris. A crater between 200 and 300 kilometers in diameter of the proper age and aspect that has been identified in the subsurface of the Yucatán Peninsula of Mexico may be the impact point. Asteroid collisions may have caused the earlier mass extinctions of the Late Devonian, the Permio-Triassic, and the Late Triassic as well. Whatever its cause, the Cretaceous extinctions ended the dinosaurs’ domination and provided empty ecological space that was ultimately filled by the mammals, a hitherto insignificant class of vertebrates.
The Pleistocene epoch (1.8 mya to 8000 BCE) was preceded by the Pliocene and followed by the Holocene epoch (8000 BCE to present). It is called the Ice Age because continental-scale glacial ice sheets cyclically advanced and retreated during this time. Near its close, anatomically modern people (Homo sapiens sapiens) evolved, probably in Africa, and spread throughout most of the Earth. Although background extinctions occurred throughout the epoch, Pleistocene animal life remained rich. Then, between 10,000 BCE and 6000 BCE, extinctions occurred on a massive scale. “Megafauna,” species with large adult body sizes and body weights over 45 kilograms (100 pounds), were most severely affected.
Although extinctions occurred on all continents, they did not occur on them uniformly. The number of megafaunal species that disappeared in the New World was much higher than elsewhere. During the 4,000 years after 10,000 BCE, North and South America lost the Colombian mammoth, mastodon, horse, camelops, Shasta ground sloth, saber-toothed cat, and seventy other genera. Extinction rates in Europe and Eurasia were lower. Nonetheless, the woolly mammoth, woolly rhinoceros, Irish elk, and other cold-adapted creatures disappeared after 14,000 BCE. Musk oxen, steppe lions, and hyenas disappeared there but survived elsewhere. The distribution of horses and steppe bison became greatly restricted. Late Pleistocene extinction rates were lowest in Africa. Megafaunal extinctions that occurred there, frequently at the beginning of the epoch, were modest in number at its close.
What Was the Cause?
Rapid climate change at the Pleistocene’s close is a possible cause. During the last 128,000 years of the epoch, the Northern Hemisphere experienced long continental glacial advances punctuated by shortterm ice contractions or interstadials. Glaciation terminated abruptly about 10,000 BCE and was followed by rapid deglaciation and a warmer, moister climate. During this interstadial, European, Eurasian, and North American forests expanded northward, severely curtailing the range and population sizes of plains megafauna like the horse, mammoth, bison, and woolly rhinoceros. This interstadial ended about 9000 BCE, and glacial conditions returned for another thousand years. Extinctions were not entirely coeval with this deglaciation, however. Further, rapid climatic changes had occurred often during the Pleistocene without triggering similar waves of extinction. Presumably, most Pleistocene species had evolved physiological tolerance for climatic change or could migrate in the face of it. Further, some species, most notably the horse, became extinct in North America but survived in Eurasia to be reintroduced to the New World in historic times. If the environment of North America had been fatal to the horse at the end of the Pleistocene, would the wild descendants of Spanish mustangs be able to roam the western United States with such success today?
While climatic change cannot explain the Pleistocene extinctions, perhaps “anthropogenic overkill,” human hunting on a limitless scale, does. Two kinds of evidence suggest this: (1) the asymmetry of worldwide extinctions and (2) the stratigraphic association between human stone tools and the bones of extinct species. Variation in the pace of extinction in Africa, Eurasia, and the New World may reflect the different arrival times of fully modern humans on each continent. Megafauna and the hominids evolved together in Africa over the last four to six million years and anatomically fully modern Homo sapiens sapiens probably evolved there some 150,000 years ago. Some suggest that the extinction rate of African megafauna was low by the terminal Pleistocene because the species had evolved behavioral means of coping with human predation by that time. Anatomically fully modern Homo sapiens sapiens did not appear in Europe until around 35,000 years ago. The shorter period of association between European game animals and these new human predators may account for the greater rate of megafaunal extinctions in Europe compared to Africa.
The stratigraphic association between the stone tools and the bones of extinct megafauna found in the terminal Pleistocene age archaeological sites is also taken as evidence of human “overkill.” The vast quantities of mammoth bones recovered from the celebrated kill sites in the Ukraine indicate Eurasian hunters slaughtered prodigious numbers of these beasts there. Such evidence is strongest in North America where direct and primary association between the distinctive lanceolate-shaped Paleo-Indian stone projectile points and megafauna is well documented. In North America, the extinction of key elements of Pleistocene megafauna, including ground sloths, camelopses, tapirs, mastodons, various species of bison, mammoths, and horses between 10,000 and 8000 BCE is clearly correlated with the abrupt appearance of the Paleo-Indians between 9500 and 9000 BCE. If the specialized Paleo-Indian hunters were the first people to arrive in the New World, they would have found a hunter’s paradise filled with game but empty of human competitors. That paradise would have been unlike anything in the Old World as the animals in it had not evolved the behaviors they needed for dealing with cunning, two-legged human predators. As Paleo-Indian peoples grew in number, they would have expanded southward and across the continent in an advancing wave. By employing game drives and other wasteful hunting strategies that indiscriminately killed large numbers of game, these early hunters may have extinguished megafauna on an ever-widening front. It seems plausible that anatomically modern humankind—practicing predation on the large and profligate scale reflected at kill sites like Solutré, Dolni Vestonice, and Olson-Chubbuck—tipped the balance against megafauna already stressed by changing climate. If these sites do indeed reflect human connivance in “megacide,” they provide a cautionary tale: once destroyed, species do not come back.
The extinction of the megafauna must have forced big-game-hunting peoples to realign their subsistence. The archaeologist Mark Cohen (1977) asserts that, by reducing available game, the extinctions initiated a “food crisis” after around 8000 BCE that forced people in Eurasia, Europe, Africa, and the Americas to expand their subsistence bases and exploit a greater range of species and habitats. Hitherto neglected species like fish, crabs, turtles, mollusks, land snails, migratory waterfowl, and rabbits became a routine part of the diet, and plants were exploited systematically for fruit, tubers, nuts, and seeds. Ultimately this food crisis may have stimulated the development of agriculture.
Late Prehistoric and Historic Period
Extinctions at the hands of humans continued in the late Prehistoric and Historic periods through (1) habitat fragmentation and destruction, (2) the introduction of predators or foreign competitors (particularly on islands), (3) overhunting, and, more recently, (4) hunting or gathering wild organisms for market sale. By late prehistory, few of the world’s habitats had not felt the impact of humankind. Nonetheless, all impact was not the same. While that of the hunter-gatherers was modest, agricultural systems transformed the landscape. Extinction of species through habitat loss has chiefly been due to the spread of agriculture and the competition between indigenous fauna and domestic animals. Yet domesticants are not the only new arrivals that generate extinctions. On islands, numerous bird species have vanished following the human introduction of rats, mongooses, or snakes. The brown tree snake, brought to Guam around 1950, extirpated nine of the thirteen native forest birds on the island. Species extinction has also resulted from intentional human persecution. A dramatic example is found in the archaeological record of New Zealand. Long isolated from mainland Asia, New Zealand supported a unique array of large, ground-dwelling, flightless birds called moa. Moa species ranged from the size of turkeys to giant, ostrich-like creatures ten feet or more in height. Having evolved in the absence of predators, they probably had little fear of the Polynesian colonists who probably arrived on the island sometime in the thirteenth century. Within one hundred years, the eleven species of moa had been hunted to extinction.
Human persecution is especially likely to result in extinction when species are exploited for the market. The passenger pigeon inhabited eastern North America in uncounted numbers; their migrations darkened the skies for days. In the nineteenth century these birds were systematically harvested for market, and the species was extinct by 1914. Similar fates befell the great auk, the dodo, and numerous other bird species. An equally grim fate faces chimpanzees and gorillas, currently sold as “bush meat” in parts of Africa. Rhinoceroses are nearly extinct because ground rhino horn is regarded as an aphrodisiac in Asia. Marine fisheries worldwide are threatened by systematic market and sport fishing. Future extinctions may include cod, tuna, blue marlin, swordfish, and some species of whale. The dismal list goes on and on.
Extinctions have affected world history profoundly. First, the mass extinction of the dinosaurs at the end of the Cretaceous period allowed the diversification of the mammalian order and, ultimately, the evolution of humans. That distant extinction event marks the nascent beginning of human history. Second, by forcing great changes in subsistence, the great extinctions in the Late Pleistocene may have triggered a “food crisis” that led to agriculture. Third, Late Pleistocene extinctions greatly impoverished the fauna of the New World. In the absence of key domesticable animal species, New World agriculture came to be based on plants. Old World peoples, living in an environment less impoverished by extinctions, domesticated many animal species and, as a result, were plagued by zoonoses, infectious diseases like smallpox and flu derived from them. As long as they remained isolated from the Old World, Native Americans remained free of such diseases. Following the arrival of Europeans and Africans, however, their lack of exposure and immunity laid them open to massive “virgin land infections” that contributed directly to their defeat and large-scale genetic replacement by peoples from the Old World. Late Pleistocene animal extinctions patterns thus helped to determine the human population makeup of the Americas.
A Sixth Mass Extinction?
Present rates of extinction appear to be between 1,000 and 10,000 times the rates seen through most of geological time. The possibility of global warming darkens the picture further as changing climatic patterns may cause extinctions by disrupting species distribution and abundance. Is the world on the brink of a sixth mass extinction event? And, if so, can anything be done to avert it? In the near term, international efforts at saving such critical habitats as the world’s forests and ocean fisheries must be intensified. Introduction of foreign species into new environments must be curtailed and the protection of endangered species increased. Such protection is never cheap or easy, but experience with the California condor, the sea otter, the American bison, the whooping crane, and other species indicates that it can be done. Simultaneously, zoos and aquariums must act decisively to breed endangered species in their charge. Reproductive and developmental biology can help. Genome banking of sperm from endangered species is promising as are more experimental approaches like somatic cell cloning. Chinese biologists are seeking to increase giant panda populations using nuclear transfer techniques involving bears as egg donors and surrogate mothers. Attempts at genetically restoring vanished species like the Tasmanian tiger and the mammoth are in the offing.
Whatever we do, we must recognize that extinctions in the present age are caused by the inordinate success of our own species. Therefore, controlling our populations and bridling our destructive impulses are essential. Global economic underdevelopment intensifies both of these problems. Poor people do not kill rhinoceroses or hunt endangered apes out of malice. They do so to relieve their poverty. Stopping these practices demands eliminating the economic inequalities that necessitate them. No small task this, but, if we fail in it, the final impact of extinction on world history may be the elimination of our own species.
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