Every now and again, life on Earth faces a crisis. At least five times in the past 540 million years half or more of all species have been wiped out in a short space of time. These mass extinctions are important punctuation marks in the history of life, as once-dominant groups are swept away and replaced with new ones. What triggers this wholesale regime change? How does life recover? And are we in the middle of a mass extinction of our own making?
Given how important mass extinctions are to understanding the history of life, it may seem surprising that no one was much interested in the idea until the 1970s. Of course, the great Victorian palaeontologists such as Richard Owen and Thomas Huxley were aware that dinosaurs and other ancient creatures were extinct, but they did not see any role for sudden, dramatic events.
Following Charles Darwin, they argued that extinction was a normal process: species originated at some point by splitting from existing species, and at some point they died out.
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This mindset can be traced back to Charles Lyell, who in the 1830s argued that the foundation of sane geology was uniformitarianism. This holds that “the present is the key to the past”: all geological phenomena can be explained by processes we see today, extrapolated over enormous periods of time.
In fact, until quite recently, geologists were conditioned against seeing any evidence of major crises. Woe betide anyone who believed in past impacts and explosions, the marks of an unscientific catastrophist! Until the 1950s geologists even denied that the Earth had been hit by meteorites, arguing, for example, that in Arizona was a volcanic collapse feature.
“Until quite recently, geologists were conditioned against seeing evidence of major crises of any kind”
This all began to change in the 1960s, a time of ferment and revolution for geologists when ideas of an immobile Earth were rejected in favour of the dynamic reality of plate tectonics.
That decade also saw the birth of impact geology. Gene Shoemaker of the California Institute of Technology in Pasadena identified rare minerals, such as coesite and stishovite, in the floor of Meteor Crater, and argued that these were evidence of an impact. At the time such minerals were unknown in nature and had only been created in the lab using enormous temperatures and pressures.
Shoemaker also investigated a large circular depression called , Germany. There he found coesite and stishovite, along with suevite, a type of rock composed of partially melted material. The depression is now considered to be an impact crater some 16 million years old.
Around the same time, palaeontologist Norman Newell of Columbia University in New York began building the case that the fossil record contained evidence of large-scale extinctions. With his work the concept of mass extinctions began to gain currency. Even so, when Luis Alvarez at the University of California, Berkeley, and his colleagues proposed in 1980 that the dinosaurs had been killed off by an asteroid impact the world was still not ready to believe it. Opposition to the idea was substantial, and it took another decade to convince the world that this massive catastrophe really happened.
What is a mass extinction?
Extinction is a normal part of evolution. Species come and go continually – around 99.9 per cent of all those that have ever existed are now extinct. The cause is usually local. For example, a lake might dry up, an island might sink beneath the waves or an invasive species might outcompete another. This normal loss of species through time is known as the background rate of extinction. It is estimated to be around 1 extinction per million species per year, though it varies widely from group to group.
The vast majority of species meet their end in this way. Most dinosaurs did not die out in the asteroid strike – after 165 million years of evolution, hundreds or thousands of species had already been and gone.
Sometimes many species disappear together in a short time. At the end of the ice ages 11,000 years ago, for example, mammoths, woolly rhinos, cave bears and other large mammals adapted to cold conditions died out across Europe and North America. There have been many such “extinction events” through the history of life.
Occasionally extinction events are global in scale, with many species of all ecological types – plants and animals, marine and terrestrial – dying out in a relatively short time all over the world. This is a mass extinction.
There is no exact definition of a mass extinction. The loss of 40 to 50 per cent of species is about the norm, but this is only the upper end of a spectrum of extinction events. There is no set timescale either: some extinctions happen relatively quickly, like the KT event, others take several million years, as in the late Ordovician. It depends on the cause (see page vi).
The big five (or is it six, or seven?)
We now recognise that there have been several mass extinctions over the past 600 million years – the period over which macroscopic life has existed in relative abundance. The first of these was about 540 million years ago, at the end of the Neoproterozoic era (see geological timescale), when the enigmatic Ediacaran animals disappeared. Some palaeontologists also identify the late Cambrian as another time of mass extinction.
Three further mass extinctions punctuate the Palaeozoic era. The late Ordovician, between 450 and 440 million years ago, saw substantial losses among the dominant animals of the time: trilobites, brachiopods, corals and graptolites. The late Devonian mass extinction, beginning around 375 million years ago, was another long and drawn out affair. Armoured fish known as placoderms and ostracoderms disappeared, and corals, trilobites and brachiopods suffered heavy losses. The Palaeozoic ended with the enormous end-Permian mass extinction (see page v).
Another 50 million years or so passed before the next mass extinction, at the end of the Triassic. Fish, molluscs, brachiopods and other marine groups saw substantial losses, while extinctions on land opened the way for the dinosaurs. They dominated for 135 million years before being wiped out in the most recent extinction, the Cretaceous-Tertiary (KT) event (see page iv).