As individuals, we rarely think about extinctions in our daily lives, unless you watch a documentary on television or on the internet, see an advertisement how dinosaurs are part of the recipe for oil products, or take a class in your local college or university. However, extinctions have been significant catalysts for change. Some originated from the exosphere (outer space), changes in the lithosphere (volcanism, earthquakes, tsunamis) or other changes in the atmosphere (climatic changes), but the common theme is the radical transformation of our biosphere. How does that evidence give us insight to dramatic changes to life on Earth?
- Extinctions of species have occurred gradually and continuously throughout the history of life.
- … species and groups of species gradually disappear, one after another, first from one spot, then from another, and finally from the world.
- … the complete extinction of the species of a group is generally a slower process than their production: if the appearance and disappearance be represented … by a vertical line of varying thickness the line is found to taper more gradually at its upper end, which marks the progress of extermination….
- Sudden disappearances of many species, now called mass extinctions, did not actually occur. Although the Cretaceous-Tertiary eventwas well-known in Darwin’s day, Darwin was convinced that sudden disappearances of species from the fossil record were due solely to unrecognized gaps in the temporal record.
- With respect to the apparently sudden extermination of whole families or orders, as of Trilobites at the close of the Paleozoic period [Permian mass extinction] and of Ammonites at the close of the secondary period [K-T mass extinction], we must remember what has already been said on the probable wide intervals of time between our consecutive formations; and in these intervals there may have been much slower extermination.
- Like his geologist colleague Charles Lyell, Darwin was contemptuous of those who thought extinctions were caused by great catastrophes. … so profound is our ignorance, and so high our presumption, that we marvel when we hear of the extinction of an organic being; and as we do not see the cause, we invoke cataclysms to desolate the world, or invent laws on the duration of the forms of life!
- Species extinction is usually, though not always, caused by the failure of a species in competition with other species. That is, causes of extinction are generally biological, not physical.
- The inhabitants of each successive period in the world’s history have beaten their predecessors in the race for life, and are, insofar, higher in the scale of nature….
- If … the Eocene inhabitants … were put into competition with the existing inhabitants, … the Eocene fauna or flora would certainly be beaten and exterminated; as would a secondary [Mesozoic] fauna by an Eocene, and a palaeozoic fauna by a secondary fauna.
- … each new variety, and ultimately each new species, is produced and maintained by having some advantage over those with which it comes into competition; and the consequent extinction of the less-favoured forms almost inevitably follows.
- The extinction of species (and larger groups) is closely tied to the process of natural selection and is thus a major component of progressive evolution. In some passages of the Origin, Darwin seems to have seen extinction as part of natural selection; in others, as an inevitable outcome.
- … extinction and natural selection … go hand in hand.
- The extinction of species and of whole groups of species, which has played so conspicuous a part in the history of the organic world, almost inevitably follows on the principle of natural selection; for old forms will be supplanted by new and improved forms.
- Thus, as it seems to me, the manner in which single species and whole groups of species become extinct accords well with the theory of natural selection.
The Role of Extinction
Was Darwin correct? Maybe not. The “Big Five” extinctions, cannot be so clearly defined, but rather appear to represent the largest (or some of the largest) of extinction events on Earth. Each of these major events were significant in reduction of life on this planet. Research indicates that impact to the biosphere produced extinctions of families, genera and species. What survived those cataclysms became the foundation those next levels of evolutionary plant and animal development.
- Cretaceous–Tertiary extinction event happened about 65.5 million years ago. It is estimated that about 75% of species became extinct. The majority of non-avian dinosaurs became extinct during that time. Mammals and birds emerged as dominant land vertebrates in the age of new life.
- Triassic–Jurassic extinction event happened about 205 million years ago. Most non-dinosaurian archosaurs, most therapsids, and most of the large amphibians were eliminated, leaving dinosaurs with little terrestrial competition. Large amphibians also survived until the Cretaceous in Australia.
- Permian–Triassic extinction event happened about 251 million years ago. This was Earth’s largest extinction killed 96% of all marine species and an estimated 70% of land species, including insects. The “Great Dying” had enormous evolutionary significance: on land, it ended the primacy of mammal-like reptiles.
- Late Devonian extinction happened about 360-375 million years ago. A prolonged series of extinctions eliminated about 70% of all species. This extinction event lasted perhaps as long as 20 million years ago, and there is evidence for a series of extinction pulses within this period. Ordovician–Silurian extinction event happened about 440–450 million years ago. Together they are ranked by many scientists as the second largest of the five major extinctions in Earth’s history in terms of percentage of genera that went extinct.
Why is this an important topic when discussing Sustainability? Historically, nature has rebounded from those events, even in some cases by as long as 30 million years to adjust the biosphere. However, with the stress mankind is placing on the Earth, the biosphere is having difficulty digesting harmful man-made products, let alone the level of waste that is deposited daily.
Death is one thing; and in the birth is something else. … At the present rate of extinction we may lose 20% of all the species on the planet within the next 20 to 40 years, most of these are in the tropical rain forest. In the United States, the present global warming projections are correct, we face losses of 20% over 20,000 plant species. … The loss of evolutionary potential is being called the death of birth.
Nature, it seems, is a healing organism that extracts fuel from calamities to feed the surviving life forms. The shock (e.g., asteroid or comet impact, global volcanism, glaciation) to the environment happens so quickly, species become stressed and cannot adapt. That natural consumption of organic waste revitalize and restores the “natural balance” of the environment. Those who survive the episode, continue to live in a restructured biosphere, and those events sometimes are constructive and sometimes destructive to the evolution of life. But we are living in an age of “unnatural” waste production and research suggests that significant problem contributes to the extinctions of species today, including ours.
 Raup, David M.; The Role Of Extinction In Evolution, Proc. Nati. Acad. Sci. USA, Vol. 91, pp. 6758-6763, July 1994, Colloquium Paper, founded on the Origins of Species by Charles Darwin
 Paul Hawken, The Ecology of Commerce, HarperBusiness publishing, 1993, 2nd edition 2010, page 36
- Top 10 Mass Extinctions (listverse.com)
- Angry birds? These lost species have every right to be… (guardian.co.uk)