There was a time when people did not know about extinctions. Before the theory of evolution, before it was understood what fossils represented, it was thought that all forms of life that ever existed still existed; they could still be found somewhere on the planet. Eventually the fossil evidence became overwhelming, and it was recognized that many forms of life had died out. There are no more mastodons, mammoths, saber-toothed tigers, or dinosaurs. No more trilobites or ammonites.
Usually the process of extinction is gradual, but five times in the more than 5 billion years of Earth’s fossil record, it has been sudden, drastic, and catastrophic. The first of these mass extinctions, at the end of the Ordovician around 444 million years ago, saw the end of about 85% of all marine species. Most life was still confined to the ocean, so this was a huge bottleneck in the evolutionary tree. Another, bigger bottleneck occurred at the end of the Permian, when 90% of all life died off. The asteroid that ended the Cretaceous period 65 million years ago killed around three-quarters of all species.
The causes of these mass extinctions are varied. Global glaciation or other climate change, (either from astronomical variations or changes in biological activity), ocean acidification, massive volcanic eruptions, and asteroid impact have all been implicated.
Kolbert makes the case that we are in the midst of the sixth great extinction in Earth’s history, and that humans are causing it. There is a proposal before the International Commission on Stratigraphy to define the current epoch as the Anthropocene, the age of humans. We have altered the earth at a geologically significant scale. We have transformed a third to half of the planet’s land surface, most major rivers have been dammed or diverted, artificial fertilizer production exceeds natural fixing of nitrogen, and species numbers are in decline. We are altering the composition of the atmosphere. According to one study, only 22% of available land can be counted as wild, and even these wildlands are cut by pipelines, seismic lines, power lines and forest roads. “Every wild place has, to one degree or another, been cut into and cut off.”
Usually said to have begun with the industrial revolution, the Anthropocene may have started much earlier, when early humans began spreading across the earth. People seem to have driven to extinction the mega-fauna they encountered, killing off the large animals faster than they could reproduce. Now, we are seeing the disappearance or endangerment of many species, including frogs, corals, and bats; tropical birds, trees and insects; and, if ocean acidification continues to increase with increased atmospheric CO2, any marine organism with a calcium carbonate shell.
After each extinction event, surviving life forms have evolved, diversified into new species. But adaptation to new conditions takes time, and the rate of change caused by humans is unprecedented, too fast for some species, leading to a world with far less bio-diversity. We don’t have a very good record of sustainable development. We have a long history of entering a new area, cutting down too many trees, killing too many animals, and ruining the land for future use. The changes we ourselves have caused may lead to our own great extinction.
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