Extinction

Extinction is the permanent disappearance of an entire species. (A species is a group of closely related, physically similar living organisms that can breed with one another.) A species goes extinct when every single one of its kind has died. Extinctions have been happening since life first began on Earth. However, human activity has greatly accelerated the pace of extinction, and many believe that a great extinction may be taking place today.

While some use the word extinction to describe what might better be described as "local or regional extinction," to most it means the complete and total elimination of a certain species from the face of Earth. Others describe extinction as the loss of a species and its replacement by an evolved version. However, although a particular species may no longer exist, the fact that it adapted or changed so much that it became a distinctly different species means that it never died out completely. Scientists know from fossils that the earliest ancestor of today's horse stood less than 2 feet (0.61 meters) high. Although that species of horse no longer exists, in a way it does, because it slowly and gradually evolved into the very large horse that is common today. If the smaller horse had died out completely and never had a chance to adapt and change and to pass on those changes to its descendants, then it would have gone extinct.

A NATURAL PART OF EARTH'S LIFE CYCLE

Extinction is an ongoing feature of the Earth's ever-changing ecosystem. It is usually caused by major environmental or climate changes. Humans also indirectly cause extinction by the effects their activities have on the environment. Hunting, habitat destruction, and pollution are some of the ways that humans have driven species to extinction. Also, a phenomenon known as "coextinction" can take place when the disappearance of one species of plant or animal dooms the existence of others that are entirely dependent upon it. It is believed that the saber-toothed tiger of prehistoric times died out completely when its sole prey, the mastodon, went extinct.

Throughout the enormously long time that life has existed on Earth, millions of species have simply come and gone. It is estimated that more than 90 percent of all the species that ever existed during Earth's history are now extinct. This is known from the fossil record, which reveals that a number of mass extinctions took place; huge events that resulted in the destruction of large numbers of different species. (A fossil is the remains of a living thing that has tuned into rock and therefore has been preserved.) The different layers, or strata, of Earth's crust represent different times in Earth's history. By this record, we know that the earliest mass extinction was about 650,000,000 years ago during a time called the Precambrian period when life on Earth consisted mostly of algae (plants or plantlike organisms that contain chlorophyll and other pigments that trap light from the Sun) floating on oceans. Scientists think that about 70 percent of the algae species were killed by a major ice age (a period where glaciers covered much of the Earth). Another mass extinction, probably caused by a climate change and considered the largest ever, occurred about 250,000,000 years ago, killing about 90 percent of all land and sea creatures. However, the best-known extinction killed the dinosaurs and many other species during the Cretaceous period about 65,000,000 years ago. Scientists are now fairly certain that this event was caused by a meteorite that crashed into the Earth, sending a vast cloud of dust into the atmosphere and blotting out all sunlight. Most think that the food chain was disrupted since plants could not grow without light, resulting in the death of herbivores (plant-eaters) and the eventual death of carnivores (flesheaters). Scientists do not agree as to whether the dense cloud would have cooled the atmosphere or warmed it, but either scenario would have changed the environment so quickly that plant and animal life would have had no time to adapt. Finally, the Ice Age that occurred during the Pleistocene era (between 1,600,000 and 10,000 years ago) caused the destruction of many plants and animals. This was a time of extremely diverse animal life, and mammals rather than reptiles were the dominant species.

HUMANS INCREASE EXTINCTION RATE

Since the evolution (the process by which gradual genetic change occurs) of humans about 10,000 years ago, there have been dramatic changes in Earth's biosphere (all the parts of Earth that make up the living world). With the invention of agriculture (farming) and the domestication (taming) of some animals, humans spread throughout the world and began to use its resources. Some ecologists argue that early humans hunted certain species into extinction, but probably more have been simply crowded out by countless human populations who have taken over the species' habitat. For example, it is known that at least 200 plant species that were native to North America have become extinct only in the past few thousand years. Worldwide, it is estimated that as many as 20 percent of all bird species have disappeared since the evolution of humans.

LUIS WALTER ALVAREZ

American physicist (a person specializing in the study of physics) Luis Alvarez (1911–1988) was a physicist who had wide-ranging interests and abilities that led him to suggest that the extinction of dinosaurs was caused indirectly by an asteroid that struck Earth. During World War II (1939–45), he was involved with radar and the atomic bomb, and in 1968 he won the Nobel Prize for Physics.

Luis Walter Alvarez was born in San Francisco, California, the son of a medical researcher and physician. His paternal grandfather was originally from Spain, but had run away to Cuba before making his fortune in real estate in California. Young Alvarez attended school in San Francisco, but when his father accepted a position at the Mayo Clinic in Rochester, Minnesota, he attended high school there. After enrolling in the University of Chicago in 1928 to study chemistry, he soon came to love physics and switched his major. Alvarez stayed at Chicago through his bachelor's, master's, and the doctorate degree he received in 1936. He then joined the faculty at the University of California at Berkeley, where he remained until his retirement in 1978.

At Berkeley, Alvarez soon was given the title "prize wild idea man" by his colleagues because of his involvement in such a wide variety of research activities. His earliest research was in the area of nuclear physics and cosmic rays, and when World War II broke out in Europe he worked at Massachusetts Institute of Technology's radiation laboratory on radar (a method of detecting distinct objects) and helped develop a narrow beam radar system that allowed airplanes to land in bad weather. He was also involved in the Manhattan Project to develop the world's first nuclear weapons. Since he helped develop the bomb's detonating device, Alvarez flew aboard a B-29 airplane that followed the Enola Gay when it dropped the first atomic bomb on Hiroshima, Japan. Returning to Berkeley after the war, Alvarez built a huge bubble chamber that could track extremely short-lived particles. It was for this invention that he won the Nobel Prize for Physics in 1968.

In 1980, he and his son, Walter, who was a professor of geology at Berkeley, accidentally discovered a band of sedimentary rock in Italy that contained an unusually high level of the rare metal iridium. Dating techniques set the age of this layer at about 65,000,000 years old. Similar iridium-rich layers were later found in other parts of the world. This led the father-son team to propose a theory regarding the extinction of dinosaurs that occurred 65,000,000 years ago. They then proposed that the iridium had come from an asteroid that struck Earth and sent huge volumes of smoke and dust (including the iridium) into Earth's atmosphere. They suggested that the cloud produced by the asteroid's impact covered Earth for so long that it blocked out sunlight, causing widespread death of plant life. This loss of plant life in turn brought about the extinction of dinosaurs who fed on the plants. While this overall theory has found favor among many scientists and has been confirmed to some extent by additional findings, it is still the subject of much debate, although most now agree with the first half of the hypothesis (the asteroid impact).

While the threat of extinction has always existed in nature, the actual time it takes to happen has rapidly speeded up in modern times. The difference today is the overwhelming pressure placed upon the environment by human activity. Habitat destruction, pollution, and over-harvesting are some of the reasons why today, some ecologists argue that we are driving species to extinction at a rate between 1,000 and 10,000 times faster than has ever happened before. The American biologist Edward O. Wilson (1929– ) currently estimates that the world may have already lost one-fifth of its present species. Today's human interference in nature is probably the most destructive since much of it is the result of chemical pollutants that easily enter an organism's reproductive system and cause a wide range of birth defects. Habitat destruction also accounts for a great amount of loss as the rain forests of the world, which contains a vast variety of plant and animals species, are steadily being cut down.

COUNTRIES TRY TO CURB EXTINCTION RATES

Many countries are trying to control the effects of human activity, which can be most devastating but is also most manageable. Laws have been passed in many countries that protect certain habitats from destruction and individual species from being hunted. Zoos are changing from places that exhibit animals to places that keep an endangered species alive. One such success story is the resurgence of the American bison. Where it once dominated the plains from Alaska and western Canada to the United States and northern Mexico, the American bison was so ruthlessly hunted that by 1899 there were fewer than 1,000 left. When guarded preserves were established and breeding programs begun, the numbers of plains bison rose to more than 50,000 and the species is no longer threatened by extinction. The bison resurgence however, required an enormous and expensive effort on a national scale, and would be impossible to duplicate for every valued species that is threatened. Instead, it is easier and more efficient to regulate the activities that threaten the diversity of life on Earth in the first place. While this may sound simple however, it often involves very sensitive issues of economics, which many times takes priority over the protection of plant and animal species.

[See alsoEndangered Species; Habitat; Pollution ]