It was going to be a climactic moment, but by the time we actually got there, I could barely see because of an allergic reaction.

In the introductory programme, I wanted to introduce the notion of fossils.

I wanted to demonstrate that we were going back in time.

And the way you go back in time on this Earth is that you go deeper and deeper into the rocks, to the earliest, which were the first to be deposited.

The Colorado River, aided by wind and rain, has cut a gigantic section through the sandstones and limestones of Arizona.

So the idea was that we would be filmed going down deeper into half a mile or so into the Earth's crust in the Grand Canyon.

And for that we went on donkeys.

But I discovered, once weve started, that I was actually allergic to the dust that comes from donkeys' fur.

And by the time we got to the bottom, my eyes were almost closed.

And here I am about two thirds of the way down, say about 3500ft below the rim.

And Chris Parsons, who was the director, obviously wanted to end it with a piece in which I was going to say "Well, here we are, at the bottom of all these rocks, so these are the earliest".

It's going to be a climactic moment.

But by the time we actually got there, I could barely see because of an allergic reaction.

The rocks here are getting on for 2000 million years old.

And so we had to end this great climactic moment.

Couldn't do it in the close up.

So we did it in a rather distant shot, which any filmmaker would say, why on earth is he going... If there was ever a place for a close up where he could climax moment, this is it.

They've made a mess again.

For many years, it was thought that all rocks of this great age were without any fossils.

Why was this?

Was it because they were so unimaginably old that they had had all traces of life crushed from them?

Or did life really begin with creatures as big as a worm?