The sudden disappearance of the dinosaurs 66 million years ago is just one of a number of mass extinctions that have occurred in the history of life on Earth. Species become extinct all the time – that is the nature of evolution. But at certain points there have been huge spikes in the rate of extinction.
If we define a mass extinction as the sudden loss of 50 per cent or more of animal species, then there have been five mass extinctions over the past 540 million years. Many other significant extinction events have fallen short of 50 per cent.
These mass extinctions have been identified via the fossils of multicellular organisms, so it is possible that there were earlier ones involving single-celled organisms, which leave little in the way of fossil traces. We do know that around 2.4 billion years ago the accumulation in the atmosphere of oxygen – pumped out by more and more photosynthesizing microorganisms – proved fatal to vast numbers of other microorganisms for whom the gas was poisonous.
The causes of more recent mass extinctions are not always so clear. Any plausible explanation must account for the fact that, although many groups of animals die out, many others survive. Although a catastrophic event may deliver the final blow, this may be preceded by a long build-up of environmental pressures on certain groups of animals. This is the so-called ‘press/pulse’ model.
Three different types of catastrophe, or ‘pulse’, are regarded as the likeliest candidates. We know that in the past there have been periods of massive volcanic activity, filling the atmosphere with dust for years on end. This would cut out much sunlight and inhibit photosynthesis, and hence the food supply at the base of nearly all food chains. Volcanic eruption also blasts large amounts of sulphur dioxide and carbon dioxide into the atmosphere. The sulphur dioxide would result in poisonous acid rain, and the carbon dioxide in global warming.
The second candidate is a fall in sea levels, most likely the result of global cooling, as happens during ice ages – when more of the oceans’ water is locked up in icecaps. Falling sea levels reduce the area of the continental shelves, the most productive zone of the oceans, and would also have disrupted weather patterns.
The final candidate is the most dramatic. In this scenario, a large asteroid or comet strikes the Earth. The massive explosion would create a hugely destructive shockwave, and possibly also megatsunamis and extensive forest fires. As in the volcanic scenario, the atmosphere would fill with smoke and dust, blocking out sunlight and so disrupting food chains. If the object hit sulphur-rich rocks, this could bring widespread acid rain. It is now widely accepted that a large asteroid did hit the Earth about 66 million years ago, but debate goes on as to whether this was the sole cause of the extinction of the dinosaurs.
Mass extinctions give a spur to evolution, leaving many ecological niches empty. With the extinction of the dinosaurs, a group of small, inconspicuous animals that had been around for over 150 million years seized the opportunity to thrive, diversify and radiate across the planet. These were the mammals.
The final cause?
We may be in the midst of another mass extinction. Some scientists estimate that up to 140,000 species (many of them as yet unrecorded plants and invertebrates) now become extinct every year. The cause? Human activity.