Some 90 per cent of species that have ever lived on earth have vanished from the planet. But extinction is not just confined to history; it's believed that we're experiencing another wave of mass extinction right now: the Holocene extinction event.

At a glance
Extinction has been happening since the dawn of time. It occurs for many different reasons: destruction of habitat by the elements, for example, or relentless hunting by hungry predators.

It's generally agreed that there have been five major extinction events during the last 550 million years. The Ordovician extinction event is dated around 438 million years ago and may have been caused by the onset of an ice age. Most of earth's complex life forms lived in the seas, and the change in temperature destroyed many of their natural habitats. This event led to around a quarter of marine families being wiped out.

The Devonian extinction occurred some 360 million years ago and also saw a quarter of marine families becoming extinct, including much of the world's coral reefs. Its causes are unclear.

The Permian extinction is also known as 'The Great Dying'. It occurred 245 million years ago when 95 per cent of all species became extinct. The causes of this are thought to be volcanic activity, asteroid impact, or severe global warming, which reduced oxygen levels in the oceans to virtually nil.

The Triassic extinction is also thought to have been partly prompted by volcanic activity and occurred about 208 million years ago. It accounted for the disappearance of around half of the species on the planet, and opened the door for the era of the dinosaurs.

The Cretaceous extinction was the last of its kind until relatively modern times and accounted for the disappearance of the dinosaurs. It is thought to have been caused by a huge asteroid, some 180km in diameter, that hit the earth around 65 million years ago in what is now southern Mexico.

'It's believed that we're experiencing another mass extinction right now: the Holocene extinction event. Although this can't be put down to a single cause such as an asteroid strike or an ocean of molten lava, there's no denying its severity.'

The grim truth is that modern-day extinctions are largely attributable to the increase in the number of human beings on the planet. There's certainly no imminent threat of extinction for us but the struggle to feed and house everyone has seen resources and land diverted from other species to ourselves, causing a huge knock-on effect for the biodiversity of the planet.

What is under threat?
The problem extends far beyond the species that we're used to seeing campaigns about, such as gazelles, whales or bears. There are also millions of species we haven't yet managed to catalogue that may also be under threat - many of them microscopic in size.

These smaller species are often responsible for such important tasks as pollinating our crops, filtering our water and recycling our waste, and we may not realise their importance to us and the danger they may be in, until it's too late.

What can be done to help?
We have an advantage over the species that lived through the previous five great extinctions because we're very aware of what's going on around us today. We have the ability to do something about the situation by devising and implementing conservation schemes.

However, ensuring we don't destroy the habitats of other life forms in order to sustain our own rapid development is going to be a very tough assignment. Only history will truly be able to assess whether we did everything we could to help combat this sixth great wave of mass extinction.