How Many Friends Does One Person Need? (2 page)

Few traits are ever that simple in biology. But taking a gene’s-eye view in which the benefits of a trait are costed out in terms of the impact they have on how often a particular gene is represented in the next generation brings us closer to Darwin’s original conception of the theory of evolution by natural selection. More importantly, perhaps, it moved us away from the naïve genes-determine-all-behaviour view that has so often bedevilled thinking in this area to one in which an individual’s freely made decisions on how to behave, free of any direct genetic input, could still be understood in a Darwinian framework. The following decades saw a veritable explosion of research.

We learned so much in so short a space of time. Looking back, it is difficult now to convey the excitement of the time. So much of what was then novel is now accepted as fact.

Charles Darwin did not, of course, invent the theory of evolution. It had already had a long history within European biology dating back at least a century before
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young Charles was even a twinkle in his mother’s eye. In fact, his own polymath of a grandfather, Erasmus Darwin, had himself made a seminal contribution to promoting the idea of evolution in one of his own best sellers. If anyone deserves the credit for inventing the theory of evolution it should probably be the great eighteenth-century French biologists – Cuvier, Buffon, Lamarck, among others. But they had been locked into a medieval mindset that had its origins in the views of Aristotle and Plato, filtered through the intellectual spectacles of the Church Fathers, a seminal group of medieval Christian theologians who established the core tenets of modern Christian theology.

Building on the thinking of their Greek predecessors, they saw evolution as progressive, with each species inexorably climbing slowly but surely up the ‘Great Chain of Being’ from primitive life forms to join the angels just below God, who, at least as far as they were concerned, inevitably stood at the pinnacle of it all.

The publication of Darwin’s book
On the Origin of
Species
in 1859 set aside the old
scala natura
, or Great Chain of Being, that had been the linchpin of evolutionary thinking ever since Plato. Darwin set in train a new way of thinking about the natural world, a world whose history is driven by the demands of successful biological reproduction. In the process, of course, he upset quite a few apple carts, not least because his new vision of evolution challenged Victorian beliefs about the established order. Not only were Englishmen not the high point of evolution, but there wasn’t that much room at the top for God either.

Darwin’s great genius was to recognise that natural selection is the engine that drives evolution. In doing so,
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he dragged the theory of evolution out of the medieval doldrums into the modern world. He provided a mechanism that could explain how life on earth could have evolved without need for a creator. And it was a mechanism that, at the same time, could explain how and why a species might have evolved particular traits, traits that enabled individual animals to reproduce more successfully.

As with all scientific ideas, Darwin’s theory underwent extensive development in the decades after the publication of the
Origin
. He expanded his ideas on natural selection to include sexual selection (selection for traits that enhance attractiveness to prospective mates). He applied his ideas to the nascent discipline of psychology – commenting at length on topics such as music, language, emotions and physical attractiveness – and even finally the evolution of Man.

Nor did his theory come to a halt with his death in 1882. It continued to be developed by those who came after him. We know so much more now than Darwin himself ever did, but the core of modern evolutionary theory and its many intellectual derivatives still lies firmly in Darwin’s elegantly simple idea: organisms behave in ways that tend to enhance the frequencies with which the genes they carry are passed on to future generations.

It was into this heady atmosphere that I was thrust as a young researcher in the 1970s. We were galvanised and excited by the opportunities on offer, by the heady mix of new Darwinian theories whose strong predictions could guide our research and give us new questions we could ask that no one had thought of asking before. Looking
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back on three decades or so of this research is to realise what a privileged generation we had been. We witnessed a genuine scientific revolution as it happened. Our ways of thinking were changed for ever, just as the Victorians had had their worldview changed by Darwin. New conceptions of how animals behaved and evolved emerged that challenged our long-held assumptions about how the world was. A decade or so later, we began to apply these same ideas to human behaviour.

In the chapters that follow, I try to convey some of that excitement. Much of the research I will talk about is my own, or was done by members of my research group. But some of it will draw, somewhat idiosyncrat-ically no doubt, on research by others that bears on the topics that have driven my own research over the past decade – why humans behave as they do, what it is to be human.

So, let me now invite you to explore with me those parts of you that, in the words of the advertisement, even the most proverbially exotic beers can never reach – how many friends you have, whether you have your father’s brain or your mother’s, whether morning sickness might actually be good for you (or, at least, for your baby), why Barack Obama’s victory in the 2008 US presidential campaign was a foregone conclusion, why Shakespeare really was a genius, what Gaelic has to do with frankincense, and why we laugh. In the process, we’ll examine the role of religion in human evolution, the fact that most of us have unexpectedly famous ancestors, and the reason why men and women never seem able to see eye to eye about colours. I’ll couch all this in terms of evolution and Darwin’s great
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insights, something that will make us ponder the very bases of science itself. But let’s begin with the very core of what makes us human... our big brains.
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Of all the traits that natural selection has managed to evolve for us, our brains are surely the most valuable. Brains are the greatest evolutionary invention of all time. They were designed to free us from the worst of the evolutionary grind to which the rest of brute nature is subjected by allowing us to fine-tune our behaviour to circumstances. We can consider the options, weigh up the pros and cons, worry about the implications of behaving one way or the other, and then choose what seems like the most sensible thing to do. Thus it is that we rise above brute nature – a paragon of evolution. Or, at least, so it seems. In reality, brains are more complex than you might think. Yet, they are not quite as flexible and omniscient as we would like them to be. And we owe a good deal more of our brains to the vagaries of evolutionary history than we might wish.

Our brains are massively expensive, consuming about twenty per cent of our total energy intake even though they only account for about two per cent of our total body
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weight. That’s a massive cost to bear, so brains really need to be spectacularly useful if they are going to be worth the cost. The consensus, at least for the primate family, is that we have our big brains to enable us to cope with the complexities of our social world. However, that story has recently acquired an interesting new twist as a result of studies on birds and other groups of mammals that my colleague Susanne Shultz and I have done. It seems that it is pairbonding that is the real drain on the brain. So let me ask: have you been struggling yet again with your partner’s foibles? If you find relationships really hard work, then it seems you are in very good company. Among the birds and mammals in general, the species with the biggest brains relative to body size are precisely those that mate monogamously. Those that live in large anonymous flocks or herds and mate promiscuously have much smaller brains.

The birds make it especially clear that the real issue is strong, resilient, long-lasting pairbonds. Birds that mate monogamously come in two quite different kinds. There are those, like many common garden birds such as robins and tits, that choose a new mate each breeding season.

But there are many others, such as many birds of prey, the owls and most of the crow and parrot families, that mate for life. It is this second group which have the biggest brains of all among the birds, far bigger than those that are seasonally monogamous, and this is true even when we control for differences in lifestyle, diet, and body size.

Among mammals, monogamy is much rarer (only about five per cent of mammals mate monogamously), but here too those that do so – including the many species of the dog/wolf/fox family, and antelope like the little klip-
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springer and the diminutive dikdik – have bigger brains than those that live in larger social groups where mating is promiscuous.

Biologists probably wouldn’t get so excited about having a big brain, were it not for the fact that brain tissue is extremely expensive to grow and maintain – only your heart, liver and guts are more expensive. Evolving a bigger brain is thus no idle matter in evolutionary terms.

And, given what brains do, this suggests that something about pairbonded relationships is significantly more taxing than life in the large anonymous flocks of shorebirds or the herds of deer and plains antelope. So what makes monogamous pairbonds so cognitively demanding?

One likely reason is that lifelong monogamy carries enormous risks. A poor choice of mate – one who is infertile, a lazy parent or prone to infidelity – risks jeopardising your contribution to the species’ gene pool. Since, biologically speaking, that is what life is all about, it is not difficult to see that there are enormous evolutionary advantages to paying the cost of having a brain big enough to enable you to recognise the signs of a bad prospect when you see one. That way, you get to avoid a whole lot of trouble, and do better for yourself in the evolutionary stakes.

But there is another aspect to monogamy that might be just as important, and that’s your ability to co-ordinate your behaviour with that of your mate. Consider the case of the average songbird in your garden. The business of mate choice is over, the female has laid her eggs, and now comes the tough bit – the long job of sitting on the nest while the eggs incubate, and the feeding of the fledglings that follow. Now, were it the case that one or other of the
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pair spent the whole of its day down at the avian equivalent of the pub, its mate would soon end up with the invidious choice between abandoning the eggs to cooling and predation so that it can feed, or staying on the nest and starving. For a small bird that has to eat its own body weight in food each day just to stay alive, this is no mean issue. In short, you need a mate that is smart enough to figure out what your needs are, and when it should return and take over its share of the nesting duties.

So perhaps it’s the need to be able to factor your mate’s perspective in to your own that is so cognitively demanding. Our own experiences would tell us that keeping a relationship on course through the years is a very delicate business, requiring a lot of fancy footwork to anticipate and see off at the pass all those potential sources of disagreement. Or, when they come from left field and we don’t see them until they hit us, it’s being able to see how to mend the fences and restore the equilibrium once again.

So as you struggle to figure out why your spouse has behaved so badly yet again, console yourself with the thought that evolution has blessed you with one of its crowning glories – a brain capable of figuring out how to get the best out of a bad job. After that, it’s all plain sailing. Even the humble birds on your garden table can sort that one out.

Think about it: you have two parents, who each provide you with one set of genes, a complete set for everything about you. But you aren’t just a fifty–fifty mixture of each of them. In most traits, you tend to resemble one or the
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other, so that by and large you end up as a kind of mosaic – your mother’s nose, your father’s chin, perhaps even your grandfather’s hair through some quirk of a throw-back to earlier generations. All this is pretty well understood, thanks mainly to the pioneering efforts in the 1850s of that indefatigable scientist-monk, Gregor Mendel, the founding father of modern genetics.

Now, one might expect that you would be a random mosaic of bits inherited from your two parents, and that these would vary between individuals – half the population would inherit a particular trait from their fathers, and the rest would inherit it from their mothers. It seems not. Instead, it turns out that some bits are always inherited from your mother and other bits always inherited from your father. The genes seem to know where they have come from, and which of them should switch themselves off (be ‘silent’ in the technical jargon).

The surprise is what happens in your brain. In an experimental study of natural genetic deficits in rats, Barry Keverne and his colleagues at Cambridge University found that animals with no maternal chromosomes lacked a fully developed neocortex, whereas those with no paternal chromosomes lacked a fully developed limbic system. This process whereby one set of genes is always ‘silenced’ is known as ‘genomic imprinting’. Although the mechanisms involved are not yet fully understood, it seems that, in effect, individual genes ‘know’ whether they were paternal or maternal genes.

This finding gels rather neatly with another recent study. Rob Barton from Durham University and his colleagues have shown that, across the broad range of primate species, the size of a species’ neocortex correlates best
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with the number of females in the group, whereas the size of the limbic system (part of the emotional response mechanism) correlates better with the number of males in the group. Since the number of females that a species can sustain in a typical group mainly reflects the females’ social skills, this makes sense because the neocortex is related to social skills. On the other hand, in most primate species, male–male relationships are based more on competition for dominance rank (which is what allows males to be successful in the mating game), and this understandably has much more to do with males’ willingness to fight.