Humans on fast lane to early extinction

MORE THAN 99% of documented living species are now extinct. Extinction is a natural and inevitable process that is compensated for by ‘speciation’, the development of new species.

But occasionally through the millennia the natural background rate for extinctions spikes sharply upwards. These are called Great Extinction Events.

The evidence for extinction events is based on fossils so it largely ignores microbial life, which doesn’t fossilise well, and concentrates heavily on marine life, which provides the best fossils. Most scientists recognise five major events in the past half a billion years, when larger and harder creatures have been around.

The most recent, and probably the best known, happened about 65.5 million years ago. It killed the dinosaurs, which allowed birds, mammals and eventually ourselves to flourish. The most probable cause is impact from a large meteorite or a comet, though some experts believe massive volcanic activity was responsible.

There were four earlier events in which more than 50% of species died. The biggest one, in which about 90% of species died, was around 250 million years ago (Ma). Again, the majority of experts believe it was caused by the impact of a large object from space.

There was another somewhere between 365 and 370 Ma and the second largest was 440 to 450 Ma — there is some evidence that this one was actually two events relatively close together in geological time. It seems to have been caused by a sudden cooling of the earth’s surface — though this begs the question of what caused the cooling.

Of course, there must have been other extinction events earlier than this which can’t be documented from the fossil record. When cyanobacteria changed the composition of the atmosphere by pumping oxygen into it 2.4 billion years ago most anoxic life forms must have perished, with the survivors retreating to unfashionable locations like my septic tank.

This is an interesting event because the cause was biotic — it was a life form, the oxygen producing cyanobacteria, that caused the crisis.

From this background information we can see that the sixth great extinction event is unusual in the context of the last half billion years because the cause is also biotic. This event is just getting under way and is caused by a species rather inappropriately named Homo sapiens. Human activity is currently causing the extinction rate to accelerate to a stage where it qualifies as a great extinction event.

The overall reason for this is that there are too many people on the planet. 10,000 years ago, when agriculture was first developed, there were somewhere between one and 10 million humans. Today there are over six billion, rising rapidly.

We have transformed the landscape, over-exploited other species, polluted and spread alien species around the globe. The result is that other living creatures are now disappearing at 10,000 times the natural background extinction rate. This rate increases every year so this will probably end up as the greatest extinction event of all time.

We are such a wise species we are called Homo sapiens. Wisdom gives us the unique ability to learn from history. And the history of extinction events has some interesting lessons. They are not all bad news. If something hadn’t dropped out of the sky and killed off the dinosaurs we wouldn’t be here today. After this extinction event life will go on. There will be a gap when the planet is strangely empty, but then new species will evolve.

But one thing is certain, we won’t be around to see them. All of this is going to happen over quite a short space of time. This is why conservation and environmental protection are important. Why we must sometimes step back and look at the bigger picture.

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