The formation of the Earth occurred some 4.6 billion years ago but it was only in the last 570 million years that the first familiar life forms began to evolve. These were arthropods and were followed 40 million years later by the first fish. The first land plants began to appear around 475 million years ago, with the first forests appearing 90 million years later

Dinosaurs began to evolve around 225 million years ago and effectively ruled the world before suddenly becoming extinct 160 million years later. Our species, Homo sapiens, have only inhabited the Earth for the last 200 thousand years.

Most people will know of the mass extinction that saw the end of the dinosaurs, but many will not realise that this was actually the fifth such event in the Earth's history.

The first occurred around 440 million years ago at the end of the Ordovician when a period of relatively severe and rapid global cooling caused such a pronounced change in marine life that 25% of families were lost.

The second came around 370 million years ago near the end of the Devonian Period. Again this was possibly the result of climate change and this time 19% of families were lost.

Around 245 million years ago at the end of the Permian period came the third major extinction when 54% of families were lost. Various theories exist as to why this should have occurred. It has long been felt that it was the result of climate change brought about by tectonic plate movement, but recent evidence suggests some form of extraterrestrial impact.

The fourth major extinction came at the end of the Triassic Period around 210 years ago. This was shortly after dinosaurs and mammals had first evolved. 23% of families were lost at this time and speculation continues as to the cause.

So we come to the fifth major extinction that occurred 65 million years ago. Speculation continues among scientific circles as to the exact reason for this. The general consensus is that it was the result of a catastrophic collision between the Earth and one or more extraterrestrial bodies such as a comet, but other scientists believe that it was caused by a great volcanic event. In either case it is thought that debris blotted out the sun's rays, causing disruption to the world's climate and ecosystem.

Virtually no large land animals survived, plants were greatly affected and tropical marine life was decimated.

These events illustrate the vulnerability of the world and there is concern that we are now in the middle of the its sixth mass extinction. The difference between this one and the previous five is that this one is the result of human activity.

Some scientists maintain that this latest mass extinction began when the first modern humans began to disperse to different parts of the world around 100,000 years ago, but the rot really set in when they turned their attention to agriculture around 10,000 years ago. It was agriculture that brought about the most profound ecological change since life on Earth first began. Humans no longer had to interact with other species in order to survive and were able to manipulate them for their own use.

As long ago as 1993 Harvard biologist E.O.Wilson was estimating that the Earth was losing around 30,000 species per year, which equated to about one species every 20 minutes.

Thousands of creatures are now endangered and appear on the so-called Red List of Endangered Species. A survey revealed that at least 1,141 of the world's 5,487 mammals, including marine mammals, are facing extinction and at least half are in decline. One in three amphibians and one in five reptiles is fighting for survival.

Loss of habitat and degradation by agriculture and deforestation affects 40% of the world's mammals. Over harvesting is wiping out larger mammals. This is a frightening sign of what is happening to the ecosystems of the land where they live.

Many creatures have been lost for ever as a result of human action in what shows every sign of being the world's sixth mass extinction. There is still time to reverse this trend. Swift international action is necessary if we are not to wipe out many of our closest relatives.