And I was just about to start talking.

When I felt a hand come on my head.

And she twisted my head so she could look straight in my eyes.

After months of planning, we traveled to Rwanda for one of the last shoots of the series.

We had no idea what we were about to witness.

There's one ape, however, that spends nearly all its time on the ground.

It lives here, 10,000ft up, on the flanks of the volcanoes of Central Africa, on the borders of Rwanda and Zaire.

The gorilla.

You don't stare at a gorilla.

That's a challenging thing to do.

So you keep your head down and you make these belch vocalisations [imitates gorilla] all the time.

And that's the sort of conversational acknowledgment that you're in their presence.

It seems really very unfair that man should have chosen the gorilla to symbolise all that is aggressive and violent.

When that's the one thing that the gorilla is not, and that we are.

And John said, "Go over there, and when you get to them, start talking about the thumb and the forefinger and how it's important to grip things."

"Right you are, John," I said.

Next thing we know is that her two youngsters come out and actually sit on him.

There was a moment when you could barely only saw the top of David's head, literally.

And my jaw dropped.

I mean, everyone's jaw dropped.

Didn't expect this at all.

I honestly don't know how long it was.

I suspect it was about ten minutes or even a quarter of an hour.

I was simply transported.

I mean, you just didn't encounter at the time, this extraordinary acceptance.

And I was just about to start talking about the opposition of the thumb and the forefinger when I felt a hand come up on my head.

And it was this adult female, and she twisted my head so she could look straight in my eyes And looked inside my mouth and put a finger in my mouth, And then made this belch vocalisation.

So I did my best to respond.

And you saw her look into one of his eyes and then into his other eye.

And I thought, my God, his head's going to come off and we haven't finished the series yet.

Which was a very uncharitable thing to think.

And I left, crawled back through the undergrowth and I said, "That's one of the most extraordinary moments of my life, was it wonderful?"

He said, "well, we got to got a bit."

I said "only a bit?"

He said, "well, yes, because I'm waiting for you, I didn't want to run out of film when you're in the middle of explaining about the thumb and the forefinger."

Extraordinary, really.

I mean, it was one of the most privileged moments of my life, really.