Now I know that later on when we get to an urban society, you do have harems and you do have these other developments because then men were in a position where they could introduce artificial powers. You could have dungeons and you could have police and all these other things which were developments that couldn't happen in a small tribe.

And... but even then, even in modern urban society, you'll find that the majority of people still form pair bonds. Yes, they break up. Yes, they're unsuccessful. Yes, they collapse under the pressures of modern life, but falling in love is a deep-seated biological imperative of our species and developing an attachment to a mate and also to a close friend. You can... you can have a very strong bond of love between two men who have helped one another throughout their lives which has nothing to do with sex. Or two... or between two women. And of course there is parental love where our babies are so helpless that if they didn't have special stimuli, the shape of the head and the roundness of the body and the crying, all these ancient signals which trigger a maternal – massive maternal response – the human female isn't aware of just how massive her response to a baby is. And we did some tests on this.

We took a lot of young... we did this for a film we made... we took a lot of young mothers and we lined them up. And they'd all just had babies recently. It was quite elaborate to arrange this but we arranged it all. So we had this row of young mothers and each one had a baby. And all the babies were given to nurses and we stood them in a row and then we would bring one baby out and we would carry that baby and the mothers would all be blindfolded. We've got a row of blindfolded young mothers and we'd carry a baby and we'd carry it along and we'd just carry it beneath their noses and said, just sniff as we go past. And blindfolded young mothers could identify their own babies by their body fragrance, I'll call it that. And when we'd take a baby past all these mothers and we'd say, right, now, I want the mother of this baby to raise her hand, and one mother would put her hand up and we said, right, take your blindfold off, and she'd take the blindfold off – it's my baby! And she didn't know... she didn't know that she could identify her own baby by its body fragrance and distinguish it from any other baby. There are all these factors at work.

And there's another thing too that... in a tribe, you see, all the babies were sleeping together in the settlement, and in the night they'd cry. Now if they... because they were all together in the settlement, if every time a baby cried all the mothers woke up, they wouldn't get any sleep. And the way that evolution took care of this – very interesting – in a tribal village like that, the mothers could identify... even when the mother was asleep, she could identify the crying of her own baby and would only wake up if her baby cried; she wouldn't wake up if somebody else's baby cried. Now, because today almost always there's just one baby in a house, this ability to identify your own baby's crying, you don't realise you've got it. A mother doesn't know that this is the case. But again, we did some tests. And if you put several babies together, the mother will only wake up when her baby cries and doesn't wake up when somebody else's baby cries. So she can identify her own baby's crying even in her sleep.

Now there I've just mentioned two qualities: body fragrance and crying. But there are a lot of other things going on which mothers aren't aware of and which mean that they have this massive, massive emotional attachment to their baby and which ensures its protection and its care. And the... I mean, I've just... you know, I've just mentioned a few of them. Babies are the same. Babies can identify their own mothers' bodies, just simply from the fragrance of their mothers' bodies. It's an incredibly complex biological relationship between mother and baby. And this is maternal love. And it's far more wonderful than people realise.