Extinction: A Brief History of an Idea
A saber-toothed cat. One of the millions of species now extinct. (Courtesy: Los Angeles County) |
Once evolution is accepted as the central theme of the history of Life on Earth, the question arises "where is everybody", that is, where are all the creatures that have evolved and whose altered remains we find in the rocks as "fossils." We now know, of course, that they are extinct. Gone. When talking about eukarya, we can make an estimate that the typical life span of a species is around 1 million years. With eukarya flourishing by 1 billion years ago, we can estimate that we now see only the last 1 million year's worth, that is, about 1/1000 of the total number of species. This calculation is a bit hokey, of course. It assumes that the number of species is the same through time, and that their life span stays constant. (What do you think about that?). Nevertheless, it allows the pregnant (and much quoted) statement that 99.9 of all species that ever lived are now extinct. Probably not far off, give or take a factor of 10 in the fraction of survivors (99 to 99.99).
Extinction may be natural, but it did not come easily to the naturalists. There was a presumption, at the time these things were first contemplated, that the world was created perfect. If so, there is no place for extinction - it made no sense to envisage a Creator who put living things in place only to destroy them later, as though some mistake had been made. If a fossil had no living counterpart, one could always assume that such a counterpart might be found in the depths of the ocean, which were as yet very poorly explored two hundred years ago.
A major turning point in the question about the reality of extinction came with a discovery by the French zoologist Baron Georges Leopold Cuvier (1769-1832), the greatest vertebrate anatomist of his time. He showed conclusively that the elephant remains found in Europe were not from animals closely related to the present elephants in the tropics, but were from an extinct form, the mammoth. The mammoth was big enough to find, were it still alive. No-one had reported seeing one. Most likely, it was indeed extinct. Soon it became clear that a host of other large mammals (cave bears, lions, hyenas, rhinos, bison) are extinct as well. (Similar extinctions were subsequently demonstrated for the Americas.) First the Great Flood, and then the Great Ice Age were made responsible by the naturalists of the middle of the 19th century. But by the beginning of the 20th century geologists realized that the impact of human hunters had something to do with these extinctions, and that the biggest and final effect came only about 12,000 years ago. This is especially true for North America, where the "Pleistocene megafauna" disappeared shortly after the arrival of humans from eastern Asia.
Extinction did not come as a surprise, when Cuvier proposed it. Earlier, based on the sequence of deposits in the Paris Basin, Alcide d'Orbigny had proposed a sequence of alternating quiet periods each with its flourishing marine fauna, punctuated by catastrophe, whereby a new set of species replaced the old one. Catastrophism, however, was in low esteem since the ideas of James Hutton (1729-1797) were taking hold. Hutton proposed that the observable processes are sufficient to explain Earth's features, and there is no need to take recourse to catastrophic events. The first important textbook writer of geology, Sir Charles Lyell (1797-1875) whole-heartedly agreed and this set the tone. Catastrophe was out. Darwin surmised that extinctions simply came from becoming rarer, presumably from losing out in the struggle for survival.
The problem was that the gradualist point of view (extinction as a process in the background of Life's history) could not readily explain the more drastic changes at certain rather well-defined times in Earth history. One of these - the one with the largest known impact on eukarya - is the end of the Permian, when more than 90 percent of all species are thought to have gone extinct. Another one, closer by and more familiar, is the end of the Cretaceous, which saw the demise of the dinosaurs (and all other large animals). The record of this particular event is well-preserved in marine sediments, including deep-sea sediments, and the course of history has been reconstructed in some detail. There is now little doubt that the impact from an asteroid (some 10 km across) resulted in conditions that were unbearable for many organisms. This finding, by the father and son team Louis and Walter Alvarez and their co-workers (published in 1980) brought catastrophism back from hiding, and put it prominently into the history of Life. We - the living organisms on this planet - are at the mercy of errant wanderers within the solar system. As long as we are lucky, they fall into the Sun or into Jupiter. When we are not, there is trouble on Earth.
The likelihood of getting hit from space must enter any calculation estimating the survival time of an advanced civilization (such as might engage in space travel). In contrast, the microbes clearly have less to fear from such catastrophes. There is usually some place, in the ground or deep in the water, where the environment remains favorable for survival. For the same reason, it is nonsense to suppose that human activities can seriously threaten Life on Earth.
Many mountain slopes in southern California show a strangely patterned patchwork, like a garment made from rags. This is the result of brush fires. Where there has been no fire for some time, there is a dense cover of chaparral. Areas of recent fire show fresh growth of grass and the first sprouts of leaves below the charred skeletons of bushes, next to the ground. Intermediate stages between full chaparral and grass cover have their own communities of plants, with their own specific hues of green, olive and yellow.
Taken as a whole, because of the occasional disturbance by fire, the slopes have many more plants than they would without the disturbance. Thus, disturbance helps create and maintain diversity. (Of course, there can be too much of it: where harsh conditions prevail routinely, as on mountain tops in the Sierras, only few species can survive.)
There is no question that the same principle "disturbance increases diversity" is also valid through geologic time. When one set of species becomes extinct, through whatever mechanism, physical or biological, others will rise to fill the ecologic space. Seen over the entire interval, then, there are more species than would be otherwise, without disturbance.
Extinction as a natural phenomenon was first established by Georges Cuvier in 1796, when he presented his paper "On the species of living and fossil elephants" at a public lecture in Paris. He argued that the mammoth is a new species of elephant and that it is extinct. If it were still alive, we should see it. The fact that extinction is for real changed the level of discussion about the history of life. If things go extinct, they must also be created. If extinction is real, the world is not perfect but in constant flux.