Hello and welcome to episode 3 of a geochemical history of life on Earth on the BBC World Service.

I'm Justin Rolat and this is the series in which I retell the epic story of the love-hate relationship between our planet's chemistry and the evolution

of life.

It is a 4 billion year-long saga, replete with warnings for us humans about the folly of playing God with the

Earth's chemistry. How polluting her atmosphere with carbon dioxide and toying

with her climate could end in tragedy.

In this episode we leave behind the world of microbes and enter one filled with large

fantastical creatures from the first shellfish until the twilight of the dinosaurs.

It is a world shaped by cycles of fire and ice, of death and

rebirth, but most importantly by cycles of the key nutrients that sustain life, oxygen, phosphorus and carbon.

By mastering these cycles, life was able to flourish. But first I want to go back to a time just before the first animals and plants appeared

and life was still confined to the oceans

when a strange new organism emerged.

These aren't that.

That's somebody scrapes the moss off.

An organism that may have played havoc with all three of those nutrient cycles.

My name is Tim Lenton, I'm a professor of climate change and Earth system science at the University of Exeter in Devon in the UK.

I'm passionate about understanding the Earth as a living system that we appear to be busy messing up.

Exeter is, first of all, a really beautiful university on top of a hill.

We're looking down on the city now and the river X stretching in the valley ahead of us.

Yes I brought you out to the balcony but right in front of us we've got lichens colonizing the

concrete there's a white one here there's some dark ones there's some sort of

...