How Many Friends Does One Person Need? (10 page)

But while there is something deeply engrossing about a good story well told, there is perhaps nothing quite so captivating as a story told around the campfire at night. We seem to be especially fond of story-telling at night, and there can hardly be a culture around the world in which this is not true. But why should darkness make stories seem so much more vivid?

It’s not enough to say that the evening round the campfire is the only time you have for relaxation – the day’s work is done, there is nothing more to do, so idle chatter can fill the time before bed. That’s not really a convincing explanation because, if there really was nothing useful to do, we could just as easily go to sleep as soon as it gets dark just like all the other sensible monkeys and apes. But we don’t: we stay up and chatter. What’s more, it’s a peculiarly social time, the time when we prefer to invite guests for dinner – even at the weekend when the days are presumably uncluttered by work and we could easily have invited them for breakfast, lunch or tea, it’s still dinner that we prefer. Of course, we can – and sometimes do – sit around the campfire of an evening doing useful chores like making or repairing clothing or hunting equipment. Yet we still tell stories while we do these things.

Perhaps it has more to do with psychology and the ambience. Perhaps story-tellers find it easier to play with our emotions in the dark, and we rise to that precisely because we get more of a kick out of it. Perhaps it is because many such stories are about mythical creatures, and daylight casts too much of a cold dose of reality on them to make them believable. Such stories may need the
[Page 83]
uncertainty of the dark, when we feel vulnerable because danger – whether natural predators or human muggers –can too easily get within our ‘escape distance’ (the distance at which we can still evade a predator once we have detected it). Perhaps it is just easier for a skilled story-teller to work on the audience’s emotions at night.
[Page 84]

We are, as Darwin reminded us in the
Descent of Man
, the product of a long evolutionary history. We still bear the scars of that evolutionary history today. Some of those scars – like our particular skin colour – are relicts of a surprisingly recent evolutionary history and date only to within the last few tens of thousands of years. Most of these are recent genetic mutations triggered by the great migrations out of Africa that resulted in modern humans peopling the whole planet. Others, however, are older, dating back to earlier species in the lineage that led to modern humans. One of these is the fact that, unlike all other primates, we give birth to very premature babies – one of the consequences of which seems to have been the need to persuade males to get involved in the business of child-rearing in a way that is very rare among mammals. And speaking of babies reminds me of milk – that special stuff invented by mammals on which to nourish their babies.

If, like me, you are of a certain age, you will remember that morning ritual at school when your exodus into the play-
[Page 85]
ground was delayed for precious minutes by the arrival of a little bottle... Love it or hate it, it was the moment when the milk turned up on your desk – just a shade short of ice pops in winter, curdled into something close to cheese in summer. Be grateful for a moment: most of us downed the stuff and occasionally even enjoyed it. But are you aware that, if you did down it with even a modest amount of pleasure, you are actually among the privileged few? Did you know that most of the people in the world cannot drink milk without becoming ill?

That’s not because they have some serious medical condition. It’s because we milk-drinkers are the aberrant ones. We all have a unique mutation that is only found in a small minority of modern humans – a mutation for the enzyme lactase that allows us to digest lactose, one of the main sugars in milk. Of course, all humans can digest milk as babies. But for most of the world’s population, the lactase gene that allows us all to do this is switched off at weaning; after that, milk, and milk products, become indigestible and consuming them can have the most unpleasant, even fatal, consequences.

It was only during the Second World War that we became aware of this. Milk had been such a central part of European culture that no one gave it a second thought.

It was, after all, very good for you – rich in proteins and energy, and loads of calcium for growing bones. So when the US government wanted to build up the health of their more deprived populations, someone had the obvious answer – milk, and lots of it. To everyone’s consternation, it had just the opposite effect in the black communities. Children started going down with diarrhoea and losing weight. More by luck than judgement, not many of them
[Page 86]
actually died – which, had this well-intentioned intervention gone on much longer, might easily have been the inevitable outcome.

Puzzled by this, the scientists set to work to figure out what had gone wrong. Eventually, it transpired that the ability to digest fresh milk post-weaning is a peculiarity of Caucasian peoples (and particularly people of northern European extraction at that), plus a few cattle-keeping people along the southern fringes of the Sahara. Almost everyone else in the world avoids milk like the plague – or at best consumes milk only in highly processed forms like yoghurts or cheese, or better still by cooking it to death first.

Which is also why sending powdered milk to Africa during famines is probably not the smartest thing to do. Doling out large quantities in such situations is the best way of making a bad situation worse. It can place the lives of babies, already weakened by famine, in even greater peril.

The answer, it turns out, has to do with the fact that – as northerners know only too well – the sun gets steadily weaker as you head into higher latitudes. The problem is that human skin synthesises vitamin D as a part of a reaction to ultraviolet (UV) light, and this is the only way we can acquire this essential vitamin naturally. Calcium is involved in this process, so being able to consume lots of extra calcium helps the body to synthesise vitamin D more effectively in the watery sunlight of the north. Having light-coloured skin helps enormously because it allows more UV light to penetrate the surface. The dark skins of
[Page 87]
the tropics – caused by a layer of dense melanin cells under the skin surface – are designed explicitly to reduce the amount of harmful UV light that would otherwise penetrate. More of this in a moment.

Tolerance of lactose involves just a single gene mutation – not in the form of a novel gene as such, but rather as a fault in the mechanism that would normally switch off the gene that codes for lactase, which would normally happen at weaning. So the genetic change required was very modest. But the genetic change on its own was not enough: it also required a cultural change to encourage the keeping of dairy animals and an enthusiasm for drinking calcium-rich milk.

Northern latitudes also have another problem that is not encountered in the tropics – they are much more seasonal. In the tropics, the growing season often extends virtually throughout the year, and several successive crops can often be sown and reaped each year. As you go further north and the climate gets more seasonal, the growing season becomes so short that it imposes long stretches of the year when things can be pretty tough. Having milk to fall back on means that you don’t have to slaughter your entire herd to survive the winter. Domestic animals become a walking larder.

And this might explain why lactose tolerance also occurs among the milk-drinking, cattle-keeping people like the Fulani who live in the Sahel, the arid zone along the southern border of the Sahara. This is an area that has always been, and very much still is, prone to famine. How useful to be able to resort to a renewable form of food-on-the-hoof when times get bad!
[Page 88]

Mention of skin colour raises that hoary old question of why people who live in the tropics tend to be darker-skinned than those who live at higher latitudes. I mentioned that this has something to do with keeping harmful sunrays out of the skin. A recent study by Nina Jablonski and George Chaplin of the California Academy of Sciences has done much to clarify this.

They were able to show that seventy-seven per cent of the variation in skin colour in northern-hemisphere peoples, and seventy per cent of that in southern-hemisphere peoples, correlates with the level of ultraviolet radiation (UVR) – the component of sunlight that is so damaging to skin cells and, as light-skinned folk are now so often warned when heading for the beach, a major cause of skin cancer. UVR levels decline as you go progressively further north or south of the equator because the curvature of the earth means that, with the sun positioned more or less above the equator, there is more air mass for the sun’s rays to pass through. Since sunlight is absorbed by the air, less UV radiation reaches the earth’s surface as you get nearer to the poles.

However, UVR levels do not correlate perfectly with latitude. High-altitude areas that lie at mid-latitudes, like Tibet and the Andes plateau of South America, have high UV levels because there is less air mass above them to absorb the harmful rays. Similarly, local cloud cover has an effect because water vapour in the atmosphere helps to filter out UVR. The Atacama desert in Chile and the deserts of the southwestern USA and the Horn of Africa have unexpectedly high UVR levels for their latitude
[Page 89]
because they are so dry and, unlike the lucky folk in the British Isles, lack a nice layer of clouds above them.

Jablonski and Chaplin argue that the evolutionary origins of this variation in skin colour actually have less to do with skin cancer than with a trade-off between the competing benefits associated with two different vitamins. One is the extent to which sunlight breaks down vitamin B (folic acid). The melanistic cells that produce dark skin tones (and suntans in pale-skinned Europeans) protect vitamin B in the skin from sunlight. Like all primates, we don’t synthesise vitamin B, but instead have to acquire it by eating the meat of animals that do. Protection against excessive sunlight thus helps to reduce the need to worry about the lack of vitamin B in our diet.

The converse, however, holds with vitamin D, a vitamin that is important in calcium absorption (and hence, strong bones). We can synthesise vitamin D for ourselves, thanks to a reaction between sunlight and skin cells. However, when light levels are low, as they are at high latitudes, people with dark skins cannot synthesise enough vitamin D. Albino African children in South Africa, for example, require less dietary vitamin D supplement than children with normal dark African skin colour. Hence, lighter skin colours have evolved in more northerly populations. (Since there isn’t much land surface outside the tropics in the southern hemisphere, there isn’t a native white southern race like there is in the north. However, it is far enough south that the ancestral inhabitants of southern Africa – the San Bushmen – have a lighter-coloured, more coppery skin than the dark-skinned Zulus who arrived in southern Africa only a few hundred years ago.)
[Page 90]

One surprising observation that supports this explanation is the fact that women and babies typically have lighter skins than adult men in all human races, including among Africans. Women have a particular need for calcium and vitamin D during pregnancy and lactation –in traditional societies, after all, women spend much of their adult lives in one or the other of these two states. Having a higher capacity to synthesise vitamin D is thus beneficial for the women.

Despite the neatness of this explanation, we are left with several puzzles – why is the relationship between skin colour and latitude/UVR quite a bit stronger in northern-hemisphere peoples than it is in southern-hemisphere peoples? And why, given the importance of vitamins, is the relationship not perfect?

As it turns out, the answer to both questions has to do with a combination of history and culture. The biologist and polymath Jared Diamond has pointed out that many populations whose skin colour is ‘out of place’ are peoples whose ancestors undertook lengthy migrations within recent historical times. Thus, the dark skins of the Bantu peoples of southern Africa reflect the fact that their ancestors arrived in southern Africa from a west African homeland near the equator only within the last few hundred years. Similarly, the rather light skins of many southeast Asians (Filipinos, Cambodians, Vietnamese) reflect the fact that their ancestors migrated from a homeland in southern China only about two thousand years ago. All the descendants of the original inhabitants of these countries (often collectively known as ‘hill tribes’ and ‘negri-tos’) have much darker skin colours.

One illuminating exception is provided by the Eskimo,
[Page 91]
who have somewhat darker skins than we would expect for a people who live so far to the north. The explanation is that they rely heavily on marine mammals like seals and polar bears as a source of food. These species have livers that are especially rich in vitamin D, and liver is much favoured as a food by Eskimo peoples. Since this helps take care of their vitamin D problem, it allows the interests of vitamin B to take precedence and select for darker-coloured skin – which is why the Eskimo have their classic coppery-coloured skin.

For most of us, skin colour is a function of where our recent ancestors have lived. Even so, the pace of change can be astonishingly fast in evolutionary terms. The ancestors of modern Europeans have occupied the more northerly parts of Europe only since the end of the last Ice Age, a mere ten thousand years ago. The blondness of Scandinavians probably has a very short history.

Babies have their own appeal, and never more so than to their doting parents and grandparents. It’s probably just as well, since human babies are born wildly premature. In mammals as a whole, the length of gestation is dictated by the size of the brain. It seems that brain tissue can only be laid down at a set rate, so if you want to grow a big brain, you can only do it by growing your brain for longer. Species with large brains thus typically have long gestation periods. In effect, it is the babies who decide when they are ready to be born – a theory in biology that has come to be known as ‘the baby in the driving seat’.