4＋7=____

    There might be as many as 10 million species of complex life on this planet today —— a huge number. But add up all of the complex species that ever lived and some biologists think the grand total would be about five billion. The estimate leads to an astonishing conclusion: a staggering 99% of species are not around any more. They have been driven to extinction.

    More species are joining the ranks of the extinct every year. Many scientists believe we are living through an episode of remarkably rapid extinction, on a scale that has been seen only five times in the last half a billion years.

    They call this current episode the sixth mass extinction —— a large, global decline in a wide variety of species over a relatively short period of time. And they tend to agree that humans are the main cause.

    Over-hunting, overfishing, and human-driven habitat loss are pushing many species to the brink. In fact, we have changed the planet so much that some geologists are now suggesting that we have entered a new phase in Earth's history; an epoch they call the ＂Anthropocene＂. By 2100, it is expected that humans will have caused the extinction of up to half of the world's current species.

    Because we are living through this extinction, it is relatively easy for us to study the driving forces behind it. But how do we determine what caused other mass die-offs that happened long ago? To do so we have to look at what archaeologists, palaeontologists, geologists and other scientists have concluded from the evidence they have gathered.

    The trouble is, those scientists do not always agree with one another —— even about the most recent extinction event. As well as the five-or six- mass extinctions, there have also been many smaller extinctions.

    One of these mini extinction events happened towards the end of the Pleistocene, a few tens of thousands of years ago. It is sometimes called the ＂megafaunal＂ extinction because many of the species it claimed were particularly large animals, weighing more than 97lb (44kg). However, its cause remains a debate amongst scientists.