The Science of Discworld II

by Terry Pratchett

  When we became more social, collecting into groups of a hundred or more, probably with agriculture, more stories appeared in our extelligence, to guide us. We had to have rules for behaviour, ways to deal with the infirm and the handicapped, ways to divert violence. In early and present-day tribal societies, everything that is not forbidden is mandatory. Stories point to difficult situations, like the Good Samaritan story in the New Testament; the Prodigal Son, too, is instructive by indirection, like Rumpelstiltskin. To drive that home, here is a tale from the Nigerian Hausa tribe, Blind Man’s Lantern.

  A young man is coming home late from seeing his girlfriend in the next village; it is very dark under a starry sky and the path back to his own village is not easy to follow. He sees a lantern bobbing towards him, but when it gets closer he sees that it is carried by the Blind Man of his own village.

  ‘Hey, Blind Man,’ he says. ‘You whose darkness is no darker than your noonday! What do you carry a lantern for?’

  ‘It is not for my need I carry this lantern,’ says the Blind Man. ‘It is to keep off you fools with eyes!’

  We, as a species, don’t only specialise in storytelling. Just as with the other specialities above, our species has a few more oddities. Probably the most odd characteristic that our elvish observer would note is our obsessive regard for children. We not only care for our own children, which is entirely to be expected biologically, but for other people’s children, too; indeed for other humankind’s children (we often find foreign-looking children more attractive than our own); indeed for the children of all land vertebrate species. We coo over lambs, fawns, newly hatched turtles, even tadpoles!

  Our sibling species, chimpanzees, are far more realistic. They too prefer baby animals. They prefer them for food, being more tender. (Humans also have a liking for lamb, calf, piglet, duckling … We can coo over them and eat them.) After the kind of warfare now well documented between chimpanzee groups, the victors will kill and eat the young of the vanquished. Male lions will kill the young of prides they take over, and eating the corpses is not unusual. Many mammalian females will eat their young if both are starving, and will frequently ‘re-process’ the first litter in this manner anyway.

  No, it’s very clear that we are the odd men out. Odd Men indeed. We do have mental circuits to delight in, and protect, our own babies, so that the later Mickey Mouse converged on the outline of a three-year-old toddler, as did E.T. It is no wonder that so many people paid his phone bill. But we also go daft about cuddly juveniles of far too many other kinds. Biologically, this is very odd.

  A by-product of our finding other species’ babies so attractive has clearly been the domestication of dogs, cats, goats, horses, elephants, hawks, chickens, cattle … These symbioses have given immense pleasure to billions of people, and to their animals, and have contributed greatly to our nutrition. Those who feel that we have exploited the animals unfairly should consider the alternatives for the animals, in the wild, where nearly all of them are eaten alive as babies, without even the benefit of a quick death.

  Agriculture can perhaps be attributed to our other propensity, storytelling: the seed becoming the plant has served as an image for so many new words and thoughts, metaphors, new understandings of nature. And the wealth generated by agriculture permitted people to afford princes and philosophers, peasants3 and popes. The cultural capital has grown as we have passed our knowledge on to succeeding generations. But it’s more comfortable to enjoy that culture if there are a couple of warehouses full of barley for beer, wheat in the fields and cows in the meadows.

  We have very recently made the whole business of symbiosis with plants and animals much more technical – those controversial ‘genetically modified organisms’ – and we have lost a lot by taking our animal helpers out of the system, especially dogs and horses, and replacing them with machinery.

  We could not have predicted what the animal and plant symbioses did for us, for our extelligence, and we don’t know what losing them will do. Events like that explode on us as the bicycle ride that is extelligence careers down this long technical hill, and can have totally unexpected effects.

  Yes, the Ford Model T made motoring affordable to many more people – but a socially much more important change was that it gave privacy in comfort for the first time, so that a large proportion of the next generation was sired on the upholstery of the car’s back seat. Similarly, the dog coming in as a symbiont meant that we could hunt more successfully. Then, as a guard dog, it meant that private farms could be protected, and there was help with rounding up the animals and keeping away predators, including human ones. Lapdogs probably affected our sexual courtesies, particularly in eighteenth-century France, and dog and cat shows have stirred our upper-middle classes with, in modern England, the lower aristocracy.

  Think for a moment about what we’ve done to dogs and cats. More than horses and cows, they grow up in our families. We play with them, like we play with our own children, and the play often involves our own children. As with our own children, this interaction begets minds in our pets. Even human children don’t do much good on the mind front if they don’t play. And Jack found, and showed Ian, that even invertebrates, bright invertebrates like mantis shrimps, can acquire minds if they’re involved in play activities. We described in Figments of Reality how this happens. Let’s just note here that we have uplifted4 our symbionts into the world of mind. Dogs worry about things, much more than wolves do. So dogs have at least some sense of themselves as a creature that lives in time, with some kind of awareness that it has a future as well as a present. Mind is catching.

  Usually we think of the domestication of the dog as a selection process driven by human intentions. The process may have started accidentally, perhaps with a tribe bringing up a wolf pup that the kids had brought into the cave, but at a relatively early stage it became a deliberate training programme. Proto-dogs were selected for obedience to their master and for useful skills such as hunting. As time passed, obedience evolved into devotion, and the modern dog arrived on the scene.

  However, there is an attractive alternative theory: the dogs selected us. It was the dogs that trained the humans. In this scenario, humans that were willing to allow wolf pups into the cave, and had the ability to train them, were rewarded by the dogs, for example by a willingness to assist in the hunt. Those humans that performed best at these tasks found it easier to acquire new pups and train new generations. The selection on the human side would have been cultural rather than genetic, because there hasn’t been enough time for genetic influences to make a lot of difference directly. However, there may well have been selection on the genetic level for having enough intelligence to appreciate how useful a trained wolf could be, or having the generalised teaching skills, such as persistence, to carry out successful training. At any rate, the tribe as a whole benefited from those few individuals who could train proto-dogs, so that the selective pressure in favour of generalised dog-training genes must have been slight.

  This isn’t one of those either-or choices: we are not obliged to accept one theory to the exclusion of the other. And this is a point we should make very firmly, for this and many other theories: things happen, all over the place and apparently in some confusion, and afterwards mankind goes and chops it all up into ‘stories’. We need to do it like that that, but occasionally we should stand back and realise what it is we are doing.

  In the case of the dogs, there is in all likelihood a fair amount of truth in both theories, and what happened was a complicit co-evolution of dogs and people. As dogs became more obedient and easier to train, people became more willing to train them; as people became more willing to consider possessing a dog, the dogs became more adept at playing along and making themselves useful.

  The situation is, perhaps, more clear-cut with cats. Here it was very much a case of the cats being in the driving-seat. Rudyard Kipling’s Just-So Story of ‘The Cat That Walks By Itself’ is too naïve an acceptance of the impression tha
t cats set out to give – that they do things their own way and merely tolerate those people who play along – but in most cases you can’t train a cat. Very few cats are willing to perform tricks, whereas many dogs visibly enjoy performing for human pleasure. To the ancient Egyptians, cats were tiny gods on Earth, personified in the cat-goddess Bastet. Bastet was originally worshipped around Bubastis, in the Nile delta, and she had a lion’s head; later, this transmuted (transmogrified?) into a cat’s head. Bastet-worship then spread to Memphis, where she became conflated with Sakhmet, a local lion-headed goddess. Bastet was a generalised goddess of things that were especially important to women, such as fertility and safe childbirth. Cats were also worshipped, as godling avatars of Bastet, and they were often mummified because of their religious significance. There was a sort of dog-god, the jackal-headed Anubis, but the difference was that he had a substantial ‘hands-on’ job: he was the god of embalming, and his role was to assist (or impede) the passage of the dead through the underworld. Anubis judged whether the dead were worthy of the afterlife. The only duties that the cat-godlings had were to allow humans to worship them.

  Nothing new there, then.

  Even today, cats assiduously give the impression of being independent; they seldom come when called, and are liable to depart at a moment’s notice for reasons that are never very clear. All cat-owners know, however, that this impression is superficial: their cats need attention, and know it. But this need shows up indirectly. For example, Ian’s cat ‘Ms Garfield’ usually comes out to the front of the house to greet the arrival of the family car, but her pleasure at the car’s appearance is heavily disguised as a strident harangue: ‘Where the bloody hell have you lot been?’ After absences on holiday or overseas, family members find that whenever they are in the garden, the cat coincidentally is in the same part of the garden – but either asleep or apparently just passing through. It looks as though house-cats are slowly losing the domestication battle, but putting up a strong fight. Feral cats are another matter, and real working cats like farm cats are often genuinely independent. These days, though, many farm cats are treated much like house-cats. At any rate, there are some good research projects still to be carried out on the co-evolution of ancient humans and their livestock and pets.

  In another instance of this co-evolution, the horse made chivalry a possible culture (hence the name, of course: compare the French ‘cheval’) and enabled the Mongols to achieve one of the largest, best-controlled empires in human history. Under the Khans it was said that a virgin could walk unmolested from Seville to Hang Chou. Only in the twentieth century was that again achievable, with luck and possibly a harder search for the virgin. The Spanish took horses to America, where humans had killed off several equine species some 13,000 years before,5 and changed the lives of all the North American Indian tribes – and the cowboys, of course. And, a little later, Hollywood.

  The horse did wonders for the genetics of humans, too. Just as they say that the invention of the bicycle saved East Anglia from an incest implosion, so the people that had come out of Africa were only a tiny part of early Homo sapiens’s genetic diversity. All recent studies of the DNA genetics of human populations agree that the genetic diversity outside Africa is only a tiny fraction of the diversity that is still found on that continent. Those who left, to go as far as Australia or China, to Western Europe or via the high Arctic to America, are less diverse in total than many small indigenous African peoples. With the arrival of the horse, it became possible for traders to carry goods – and gene alleles – for very long distances, very effectively. So the out-of-Africa humans have inherited a relatively small part of the African gene-pool: they are genetically impoverished, but well stirred.

  At the end of the twentieth century there was, for some years, a belief that Homo sapiens was a polyphyletic species. This word means that different groups of Homo sapiens evolved from different groups of Homo erectus in different places. This, it was thought, might account for the racial differences, especially differences in skin pigmentation, that seemed to fit geography pretty well. From DNA studies, we now know this theory can’t be true. On the contrary, there was a bottleneck in our population as we came out of Africa – humanity was reduced to rather small numbers – and all of us living today, all of the out-of-Africa ‘races’, were extracted from that small population. All the Homo erectus died out. The evidence so far looks as if there was only one exodus, of a minimum of some 100,000 people. We were all there in potentia in that tiny population, Japanese and Eskimos and Norsemen and Sioux and Beaker people and Mandarin Chinese; Indians and Jews and Irishmen. In the same way, all the current kinds of dog were ‘present’ in the original domesticated wolf (assuming it was indeed a wolf) – that is, they were in the wolf’s space of the adjacent possible – and we’ve pulled out Saint Bernards and chihuahuas and labradors and King Charles spaniels and poodles from that local region of organism-space.

  There was, about thirty years ago, a brief fashion for the concept of ‘mitochondrial Eve’, and many media reports seem to have picked up the idea that there was just one woman, a veritable Eve, in that ancestral bottleneck. This is nonsense, but the media reports were written up to encourage the belief. The real story, as always, was a little more complicated, and it goes like this. There are mitochondria in the cells of people, indeed of most animals and plants. These are the billions-of-generations, descendants of symbiotic bacteria, and they still have some of their ancient DNA heredity, called mitochondrial DNA. Mitochondria from the mother go into the embryo’s cells, but those from father do not: they die, or go only into the placenta. In any event, mitochondrial inheritance is very nearly all maternal. The mitochondrial DNA accumulates mutations over time, with important genes changing less (presumably because the resulting babies, if any, were defective) and some DNA sequences changing quite quickly. That enables us to judge how far back it is to the common ancestor of any pair of women, from the accumulated differences in several DNA sequences. Surprisingly, nearly all such pairs from very different women converge on to a single consensus sequence, about 70,000 years ago.

  A single woman, the ancestor of us all.

  Eve?

  Well, that was the story that the media latched on to, and you can see why. However, it doesn’t hang together. The occurrence of just one mitochondrial DNA sequence doesn’t mean that there was just one woman with that sequence, or that she was the ancestress of all the other women whose DNA was sequenced. Evidence based on the current diversity of various genes shows that there were at least 50,000 women in the human population 70,000 years ago, and many of them will have had that particular DNA sequence, or one that cannot be distinguished from it with the evidence remaining today. The lineages of the women who did not have that sequence continued for some time, but eventually died out: their ‘branch’ of the human family tree doesn’t reach all the way to the present day. We can’t be certain why those lineages died out, but in mathematical models such effects are commonplace. Perhaps the women carrying sequences like today’s sole survivor were more ‘fit’, or they simply came to outnumber the others by chance. It is even possible that the choice of the contemporary women to test was in some way biased, and that more than one mitochondrial DNA sequence is actually present in today’s women.

  How do we know that there were at least 100,000 humans 70,000 years ago, and not, as in the stories, just two 6,000 years ago? Many (about 30 per cent) of the genes in the cell nucleus have several versions in today’s human population. Like most ‘wild’ populations (not bred in the laboratory or for dog shows), each individual human has two versions of about 10 per cent of his or her genes, different versions received from father and mother in sperm and egg. Humans have roughly 30,000 genes, of which about 3,000 will be represented by two versions in the average person. For some genes, notably those of the immune system that give each of us a very specific lock-and-key individuality, making us susceptible to some ailments but resistant to others, there are hundreds o
f versions of each gene (of four important ones, anyway). The (common) chimpanzee has a set of these immune variants that is very like the human: in one list of 65 variants of one immune gene, only two were not exactly the same. We don’t know about the DNA of enough bonobos yet to see if the story is the same for them, but the smart money says that it will be, possibly even more so. The gorilla set seems to be a little different again (but only about thirty gorillas have been tested).

  At any rate, all of these immune gene variants had to come out of Africa in that ‘bottleneck’ population that produced all the ex-African human populations. It is unreasonable to suppose that each individual inherited different versions of each variable gene from their parents: some will have carried only one version, the same from both parents, and no one can have carried more than two. The humans that came out of Africa have about 500 immune variants, at least, in common with, chimpanzees, out of about 750 possibilities. The humans who stayed in Africa have more: they weren’t subject to the bottleneck. There are many other genes where several ancient versions (ancient because they’re common to us, chimpanzees, perhaps gorillas, maybe other species) have come through; 100,000 people is a reasonable minimum to carry all those. If you want to be critical and get that number down a bit, you could argue that a.few variants from African populations may have been mixed in later, for example via slavery to the US, or to Mediterranean peoples and then via Phoenician sailors to the rest of us. Still, the evidence does not point to an Adam and an Eve, unless they came with a lot of servants, slaves or concubines.