Sixth Extinction

Extinction events have struck life throughout its entire 3.5-billion-year history on earth. Each time, life has managed to rebound. Ever since there have been complex forms of life on earth—the expanse of geologic time spanning the past 600 million years or so—five major mass extinctions, global in scale, have devastated the earth's ecosystems and sent many species to extinction: the Late Ordovi-cian (approximately 440 million years ago); Late Devonian (approximately 360 million years ago); the Permo-Triassic crisis (245 million years ago—the largest of all so far, with perhaps as many as 96 percent of all species on life becoming extinct); Late Triassic (approximately 210 million years ago); and the Late Cretaceous (65 million years ago— the famous K-T event). Numerous less global extinction events occurred in between these major global events. All such mass extinction events were caused either by extraterrestrial impact, global climate change, or some mixture of physical factors.

We now live in what many geologists and biologists consider the Sixth Extinction (also known as the "biodiversity crisis"); according to Harvard biologist E. O. Wilson, the earth is losing more than 30,000 species a year (approximately 3 per hour)—a rate of loss equal to the great global mass extinctions of the past. The cause this time is different from any in the past: the cause this time is a single biological species, ourselves, species Homo sapiens. Because of the profound changes in our ecological status, brought about by the increased reliance on culture in human ecology and culminating in the origin of agriculture some

10,000 years ago, the global population of humans has recently surpassed 6 billion—and with that explosion in our numbers has come a transformation of the planet's surface through the spread of agriculture, urbanization, and other physical transformations of terrestrial habitats; the spread of alien species, including pathogens; and simple overharvesting of the world's fisheries and timberlands. Together with pollution, these factors stand out as the leading cause of death of so many species of plants, animals, fungi, and microbes each year the world over. Sustainable development, conservation, and stabilization of human population numbers seem to be the best hope of curbing this current wave of mass extinctions.

The Sixth Extinction has so far developed in two distinct phases. Phase I began when our species, Homo sapiens, began to spread out of our homeland of Africa just less than 100,000 years ago. By that time, humans were accomplished hunters as well as gatherers of edible plants and fungi. According to the archaeological record, wherever human beings invaded, ecosystems were profoundly disrupted, and many animals quickly succumbed to extinction. This wave of extinction accompanying the migration of Homo sapiens was for the most part likely caused by overhunting, especially in regions that earlier hominid species had never reached. In addition, some paleontologists have suggested that people brought disease-causing microorganisms with them as well, along with pets, and that these additional factors may well have played a role in driving native animals extinct.

Perhaps the most famous victim of Homo sapiens-induced extinction in this first wave of the Sixth Extinction was none other than our collateral kin, the Neanderthals (Homo nean-derthalensis) of western Europe. Whether by direct competition, or perhaps even open warfare, Neanderthals were gone within 5,000 to

7,000 years of the appearance of modern humans in Europe, some 38,000 years ago.

Archaeologists dispute the earliest dates of the peopling of the Americas, but modern humans did not arrive in North America in any significant numbers until 12,500 B.C.E. Some archaeological finds document evidence of hunting and butchering (for example, points embedded in rib cages of extinct bison; scrapings on bones of mammoths and mastodons). It was right after humans arrived in numbers in North America that most of the great species of Ice Age mammals became extinct: wooly rhino, wooly mammoth, American mastodon, giant bison, and so forth. In the Caribbean, humans arrived about 8,000 years ago—and soon thereafter the larger mammals of the Caribbean islands, including some monkey species, became extinct. One final example—human colonization of Madagascar only 2,000 years ago—shows the correlation again working: elephant birds, a species of pygmy hippo, and some of the larger species of lemurs all succumbed to extinction immediately thereafter. Extinction of birds and other species in the Pacific also reflects the relatively recent spread of humans to the remote islands of the Pacific Ocean.

Phase II of the Sixth Extinction overlaps with Phase I, the expansion of modern humans around the globe. Phase II began with the invention of agriculture and the immediate changes that agriculture brought to the human ecological condition. Phase II started slowly, as human population growth has exploded exponentially mostly in the last few centuries. But in these past few hundred years, the explosive growth in human population numbers— together with the rapid growth of industrialization and the even newer technologies of communication—has prompted major transformations of the terrestrial landscape for agriculture and logging. Coupled with the spread of alien species, pollution, and overharvesting (all consequent on the fact that humans are no longer dependent upon the productivity of local ecosystems), these factors have combined to trigger the enormous increase in the rate of species loss that has come to be called the "Sixth Extinction."

—Niles Eldredge

See also: Agriculture and Biodiversity Loss: Industrial Agriculture; Agriculture, Origin of; Alien Species; Conservation, Definition and History; Ecological Status of Modern Humans; Extinction, Direct Causes of; Mass Extinction; Pollution; Population Growth, Human; Sustainable Development; Urbanization

Bibliography

Dobson, Andrew P. 1996. Conservation and Biodiversity. New York: Scientific American Library; Eldredge, Niles. 1997. Dominion. Berkeley: University of California Press; Eldredge, Niles. 1998. Life in the Balance: Humanity and the Biodiversity Crisis. Princeton: Princeton University Press; Stanley, Steven M. 1987. Extinction. New York: Scientific American Books; Wilson, Edward O. 1993. The Diversity of Life. Cambridge: Harvard University Press.

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