How close are we to the next mass extinction?

Around 250 million years ago our world was a very different place. Rather than the different continents we know today, there was only one giant land mass - Pangea - covered with plants and animals. But then something went horribly wrong. Over a few million years, more than 95 per cent of all species on the early earth were wiped out in an event known as the Permian mass extinction. So what caused it? One researcher who thinks he might know is Mark Sephton from Imperial College London and, as he explained to Kat Arney, this wasn't the first time that our world has come to the brink of disaster.