Extinctions

The origins of photosynthesis, eukaryotes, sex and multicellularity are probably the most important events in evolutionary history since the origin of life itself. Some people would add the origins of animals, around 600 million years ago, because the familiar structure of food chains, predator and prey, hunter and hunted, dates from this time. The antecedents of animals might have been the curious multicellular organisms of the late Precambrian era, the so-called "Ediacara fauna". Others might add the origins of plants, apparently as a result of symbiosis between algae and fungi around 450 million years ago, because this presaged the spread of life from sea to land. These events have received more coverage in popular books than the topics we have examined in this chapter, because fossil evidence tells us far more about them. But deciding which events are "important" is a matter of viewpoint. From the point of view of the archaea, hardly any of the events listed in this chapter have mattered at all; only photosynthesis was significant for the archaea - and that for largely negative reasons.

From an anthropocentric rather than an archaeacentric stance, the periods of mass extinction were also salient events in life's history. The fossil evidence tells us that the five most recent extinction episodes occurred at intervals of roughly a hundred million years, but it says nothing unequivocal about the causes. The best-known hypothesis blames large asteroid impacts on the Earth. Astronomical data support the view that large asteroid impacts afflict the Earth at roughly hundred-million year intervals. An asteroid of about 10 km diameter almost certainly struck the Earth some 65 million years ago, coinciding with the most recent mass extinction, which ended the age of the dinosaurs: this marked the Cretaceous-Tertiary (CT) boundary. This impact must have raised a massive dust-cloud or a vast amount of water vapour, depending whether the asteroid struck land or sea, so following the immediate devastation there would have been severe global climatic changes that lasted for many years.

There are two difficulties with this hypothesis. First, there is no compelling evidence that asteroid impacts coincided exactly with any mass extinction other than the CT boundary. Second, many of the species that went extinct at the CT boundary disappeared from the fossil record a million or two years before the asteroid struck.

An alternative hypothesis hinges on the melting of mantle rocks caused by the collision or fragmentation of tectonic plates. When this happens, the affected parts of the Earth's surface are flooded with molten rock, forming "flood basalts" such as the Deccan Traps of northern India and the Siberian plain. Flood basalt eruptions are not single events but continue sporadically over hundreds of thousands of years of tectonic trauma. They cause gross local destruction of life and release millions of tons of carbon dioxide and sulphur dioxide into the atmosphere. The former gas heats the Earth (the greenhouse effect) and the latter cools it; both lower the atmosphere's oxygen content; and both cause acid rain that poisons land and sea. Flood basalt eruptions coincide with three of the five known major extinctions. The Deccan Traps were formed in the late Cretaceous. The Siberian plain was formed at the end of the Permian, the third and biggest of the known extinctions. The Permian extinction occurred when the ancient supercontinent, Pangea, began to break up. The fossil and geological record shows that the sea was acidified, the atmosphere was depleted of oxygen and there was widespread glaciation. All these are plausible consequences of flood basalt eruptions caused by the fracturing of the giant land-mass; collectively, they might account for the loss of 19 out of every 20 extant species that the fossil record indicates. The third known correlation connects a flood basalt plain on either side of what is now the Atlantic with the late Triassic extinction, which occurred about 230 million years ago.

Perhaps no single cause explains every mass extinction. As we suggested earlier in the chapter, oxygen pollution might have caused a mass extinction when photosynthesis first evolved. Man-made pollution is now contributing to another. The rate of species extinction during the past few decades has probably been greater than at any time in the 3,800 million year history of life. The cause this time lies in the poor control of the activities of the most intelligent species of all.

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