The process of evolution allows for in that the survival of the fittest tends to assist in population growth and diversity but extinguishes the weak. By extension or analogy, extinctions related to the evolutionary process affect not only plants and animals but culture as well, and all that it entails. In 1846, Justus Liebig observed that the success of a population or community is depen­dent upon an intricate complex of environmental conditions and that any condition that approaches or exceeds the limit of tolerance for the organism or population in question may be said to be a controlling factor. By exceeding the limits of toler­ance, extinction can occur with innovations, inventions, communications, ethnicities, lan­guages, economies, and civilizations. The Stanley Steamer became extinct when the internal com­bustion engine gained in popularity early in the 20th century. What ever became of the 13-key Marchant calculator so popular with statisticians in the 1950s? For that matter, how many communicate in Latin anymore? There were soci­etal things that now no longer exist, or are no longer active, or have gone out of use, or have become ineffective: The incandescent lightbulb, so ubiquitous today, may become extinct in the 21st century as more energy-efficient alternatives become available to a world increasingly con­cerned with dwindling energy supplies.

During the past couple of billion years, life has evolved from simple one-celled creatures, such as the amoeba, into complicated complex multicellu­lar life forms or living environments. Along the way, several mass extinction events have occurred. One of the two largest mass extinctions was caused by the breakup of Pangea at the end of the Paleozoic era, known as the Great Dying, when about 90% of all marine species and 75% of all land animals became extinct. The other was at the end of the era, when massive igneous eruptions and the impact of a great meteor created a highly toxic planetary environment, resulting in the extinction of about 75% of all marine species and, on land, the total extinction of the dinosaurs. The limit of tolerance was definitely exceeded in both cases.

In the past 500 million years there have been at least five or six major extinction events, and about the same number of smaller ones. A variety of condi­tions, including igneous (volcanic) eruptions and meteorites, have contributed to these extinction events. Toxic gases entering the atmosphere from extensive igneous eruptions, resulting in acid rain and acid deposition, have the ability to kill plant and animal species. This has occurred historically and is also taking place at the present time. Then, too, global cooling and have resulted in extinctions by merely exceeding the limit of toler­ance through significant increases or decreases in temperature. Along with the associated temperature change is the change in the amount of available moisture. Climatic stress is a very real thing as large areas exceed tolerance levels for the more sensitive species.

We will learn more about global cooling after the present global warming period ends; that is, if we are in another interglacial stage, as many scien­tists believe. With regard to global warming it is likely that emissions of methane gas (CH4) in the sea floor, soils, and melting ice caps contribute to “greenhouse” gases in greater amounts than car­bon dioxide. A large release of methane is often related to surface igneous activities, which are related to convection currents in the mantle, which in turn are related to plate tectonics. Such large emission of methane in the past may again take place in the future.

Extinction events, large and small, are continu­ally being studied, particularly since the more recent glacial ice began retreating. These events tend to coincide with humankind exploring the land in areas where the ice was retreating, as well as other areas of the world. It is known that cer­tain species of horses, camels, sloths, elephants, glyptodonts, peccary, antelope, and bison disap­peared together over 8,000 years ago. The extinc­tion of the mammoth occurred about 11,000 years ago. Certain birds also became extinct about the same time. And stratigraphic evidence, including volcanic ash, suggests that extinction of many spe­cies of tortoises occurred about 70,000 years ago, showing that climatic change and volcanic erup­tions are both involved. Many archaeological sites show that extinction events closely followed the arrival of humankind. Very simply, civilization tends to shrink the range needed for other species: The Colorado squawfish, Carolina parakeet, California condor, Everglade kite, Eskimo curlew, and Labrador duck, among others, have become extinct in the past 200 years. These are not new observations about the diminishing wilderness and decimation of natural areas. Research continues as currently endangered species and the extent of their ranges are at the mercy of humankind.

A comparison of extinctions in the natural set­ting with those in the cultural domain is rendered difficult by the tendency to think in terms of human time rather than geologic time. Natural extinction events take thousands of years or even longer, while cultural extinctions occur in rapid succession with each technological innovation or societal upheaval. At the present time, as scientists continue to search for answers to the questions surrounding biological extinction, the paradigm of cultural extinctions may yield information useful in under­standing and responding to these phenomena.

Richard A. Stephenson

See also Archaeopteryx; Coelacanths; Dinosaurs; Extinction and Evolution; Extinctions, Mass; Fossil Record; Fossils, Interpretations of; Ginkgo Trees; K-T Boundary; La Brea Tar Pits; Trilobites


Further Readings

Chernicoff, S., & Whitney, D. (2007). Geology (4th ed.). Upper Saddle River, NJ: Prentice Hall.

Merritts, D., De Wet, A., & Menking, K. (1998). Environmental geology. New York: Freeman.

Wright, H. E., Jr., & Frey, D. G. (Eds.). (1965). The quaternary of the United States. Princeton, NJ: Princeton University Press.

What do you think?

Thought Experiments

Thought Experiments

Extinction and Evolution

Extinction and Evolution