(Nov. 23rd, 2009) Celebrated for his revolutionary insight into the origins of species, Charles Darwin was also well aware that living species could entirely disappear “one after another, first from one spot, then from another, and finally from the world”. In this first report, Jeremy Garwood presents a short history on the scientific discovery of extinction and its possible causes.
For a change, our initial knowledge of extinction pre-dates Darwin by a half-century. Extinction was already a hot topic during Charles Darwin’s formative years, sparked by the 18th century discovery of the fossilised remains of plants and animals that had ceased to live on earth many millions of years ago.
From 1796 onwards, Georges Cuvier, the great French paleontologist, argued that fossils represented organisms that were extinct. He based his thesis of extinction on detailed anatomical comparisons that emphasized differences between fossil and living forms.
Darwin embraced Cuvier’s explanation and then extended it by proposing that extinction was a by-product of evolution. In his book, “On the Origin of Species”, Darwin presented his own view of species extinction, arguing that it is usually caused by the failure of a species in competition with other species. For Darwin, the causes of extinction were biological and closely tied to the process of Natural Selection. It had occurred gradually and continuously throughout the history of life.
But in this respect, Darwin did not accept Cuvier’s argument that mass extinctions had occurred. Studying the fossil records, Cuvier claimed they proved “the existence of a world previous to ours, destroyed by some kind of catastrophe.” Instead, Darwin argued sudden mass disappearances of species from the fossil record were simply due to imperfect fossil records.
Today, there are still only around 250,000 known fossil species. This might seem a lot, but since virtually all plant and animal species that have ever lived on the earth are extinct, it in fact only represents a small subset of the estimated 4 billion species that have lived in the geologic past. However, although “extinctions have been almost as common as origination” (David M. Raup, Biological Extinction in Earth History, Science, 1986), nevertheless “extinctions are clustered in time.”
Despite a gradual, ongoing “background” extinction, seen as the normal or spontaneous rate of replacement of one species by another, it is now accepted that on several occasions some pretty nasty things have happened to earth’s living conditions. There have been at least five mass extinctions in the history of life on earth - the terminal Ordovician (440 million years ago), late Devonian (365 million years ago), terminal or late Permian (250 million years ago), terminal Triassic (215 million years ago), and terminal Cretaceous (65 million years ago). The last of these saw the extinction of the dinosaurs, but the largest extinction event is thought to be the Permian with the elimination of up to 96% of the then-existing species.
So, what caused mass extinctions? Something physically devastating! But what? The most commonly cited causes have included changes in sea level and climatic deterioration, perhaps resulting from changes in the earth’s orbit or volcanic activity. However, one that has excited the popular imagination for centuries is that of a collision by a large comet or asteroid. Ironically, strong scientific argument for this as a real possibility (rather than mere doomsday astrology), came in 1973 from Harold Urey, father of primordial “soup theory” (see LT online Editorial 85 or LT04/09), who claimed that such collisions could have provided the “spark” that first started life on earth out of a chemical broth.
Evidence for this as a principal cause of the last mass extinction event (65 million years ago) was presented in 1980 by Luis and Walter Alvarez who demonstrated the presence of enriched extraterrestrial matter, notably iridium, at just this point in the fossil records. In 1991, Mexico’s Yucatan Peninsula was shown by Alan Hildebrand to be the favoured site for the impact of a large asteroid, 10 km across, at the end of the Cretaceous period. This resulted in the creation of the 180 km wide Chicxulub Crater, generating megatsunamis and clouds of superheated dust, stream and ash. Not very pleasant.
However, even without the aid of extraterrestrial rocks, mankind may already have created the conditions for a sixth mass extinction event – “Some 2,000 species of Pacific Island birds (about 15 percent of the world total) have gone extinct since human colonization. Roughly 20 of the 297 known mussel and clam species and 40 of about 950 fishes have perished in North America in the last century. The globe has experienced similar waves of destruction just five times in the past,” wrote Philip Levin on “The Real Biodiversity Crisis” (American Scientist, 2002).
Various figures have been given for the long term extinction rate averaged over earth’s biological history, e.g. around 9 species per million per annum, but compared to this pre-human “background” rate, the current rate of species extinction is conservatively estimated to be at least 100 times higher (some claim up to 10,000 times higher!). For example, if the loss of tropical rainforests remained at 1% per year, the projected losses of species could reach 1-5% per decade writes Ilkka Hanski in “The world that became ruined” (EMBO Reports, 2008).
Nevertheless, there is a striking difference about the latest wave of species extinction. Unlike the biological causes advanced to explain the gradual background extinction rate, or the handful of physical catastrophes that have been associated with mass extinction events, this is the first time that economics has been shown to be responsible. (An article on ‘The economics of extinction’ will appear in LT01/2010.)
In the second part of this online series, Jeremy Garwood will look at the scientific debate about which species to save from extinction or those that should be abandoned forever.