by Steve Jones
These early modern humnns had arrived in Israel, in the caves ol t)al/(.-li.ind Skhul, by a hundred thousand years.u;<>. Cro-Magnon man, the first modern European (who lived, like a sensible man, in the south of France) was there by forty thousand years before the present day. As in Africa, their antiquated relatives, the Neanderthals, held out for a time; and lasted in southern Spain for ten thousand years after the arrival of the newcomers.
This account of history is the 'out of Africa' model believed by most evolutionists. Some feel that humans emerged more or less at the same time over the whole world so that today's Chinese evolved from an ancient Chinese ancestor and Africans from a predecessor in their own land. The idea that the same species can evolve simultaneously in different places flies in the face of theories of the genetics of speciation. Some fossils might, perhaps, support the idea of local evolution. One, from the Han River in China, resembles Homo erectus, but has a flattened face which, to its discoverers, looks rather like that of a modern Chinese. Those in favour of local evolution make much of the 'shovel incisors' in fossil jaws from Asia. The teeth are scooped out at the back, as are those of some of today's Chinese. In some places in Europe, as well, a third of people have shovel incisors, so that this is not a forceful argument. So few fragments have been preserved that it seems that, too often, history is in the eye of the beholder. Africa was the centre in which most primates originated and there is no reason to suppose that humans came from anywhere else.
Another fossil controversy is the question of evolution by creeps or by jerks. Darwin felt that the origin of species was gradual and continuous. The past was no more than the present writ large. Because of the vast time available the enormous transformations which took place through the history of life could be explained by the slow and almost imperceptible changes that influences it today. His was a leisurely and Victorian view of the way the world worked; one of gradual and almost inevitable movement.
The opposing view (the theory of 'punctuated equilibrium' as it is known in its latest guise) has a more twentieth-century flavour. It sees evolution as boredom mitigated by panic. New species appear as the result of a sudden burst of revolutionary change. Between these historical disasters, life is tranquil. Punctuationists suggest that the origin of species has little to do with what happens to a species once it has originated and that the process of evolution today cannot tell us much about what went on in the past.
The greatest strength of this theory is its ability to annoy Darwinists. Hundreds of scientific papers have been written in support of or against the idea. One important problem is that of time-scale. What might appear an instant to a geologist can seem an eternity to a biologist. A 'jerk' between one species and its successor may encompass tens of thousands of years; nothing in terms of geological epochs, but more than enough generations to allow gradual evolution to make fundamental changes. Those opposed to creeping advance point out, quite fairly, that most species do not change at all through their evolutionary career, which is not what Darwin would have expected.
Whatever the merits of each doctrine, the human fossil record has so many gaps that there is just not enough information to tell whether humans evolved suddenly or slowly. The remains are so sparse that it is quite possible that relics of the lineage that led to the peoples of today are as yet undiscovered. Fossils are the best of all evidence that we did evolve, but cannot tell us much about how that evolution took place. They do show that the attributes which make us what we are arose bit by bit, seen first in a remote ancestor and reaching completion {if indeed they have) only in the past hundred thousand years or so. No single primate awoke out1 morning ro find itself human.
Tilt* central problem in using the dead to recreate history is that we can never be sure that any fossil left a descendant. Our extinct predecessors are just that: extinct. This makes it difficult to work out their relationships to each other and to ourselves.
Another window has opened onto the past. Every modern gene descends from times long gone. The connections between humans and primates are preserved in the DNA of living animals. Darwin himself saw better ways to reconstruct history than to depend on the frozen accidents of fossilisation. His own claims about our predecessors used indirect evidence (such as a comparison of the anatomy of humans with that of apes). Nowadays this evidence is much more complete and has painted a new portrait of our ancestors.
Molecular biology is just anatomy writ small, plus an enormous research grant. To a geneticist, everyone is a living fossil and contains the heritage of his or her predecessors. Genes recreate the ancient world. 'Man' — Darwin said — 'still bears in his bodily frame the indelible stamp of his lowly origin.'; or, as W. S. Gilbert put it: 'Darwinin-ian man, though well-behaved, is really just a monkey shaved.' Genetics allow us to search for who that shaved monkey may have been.
Bones show that humans are more related to apes than to monkeys and that their closest kin lie among chimps, gorillas and orang-utans. Anatomists once assumed that Homo sapiens must be quite distinct. Often, it was contrasted with these "great apes1. We differ from them in obvious ways — brain size and hairiness, for example, and in other talents. Most people are righthanded and the patterns of breakage of stone tools suggest that our ancestors were the same. Individual chimps and gorillas may use one hand rather than the other, but about half the animals prefer left and half right. The human brain, too, is asymmetrical and it is more than a coincidence that speech and language are coded for on only one side.
It is hard ro measure how much genetic divergence a difference in hairiness or handedness might represent and such comparisons are not much use as a test of the biological gap between apes and humans. DNA docs a better job. Men, apes and monkeys share many genes. We vary both in the way we taste the world, and in how we see it. In suinc monkeys, many of the males are red-green colour-blind. Chimps have A and O blood groups, while gorillas are all blood group B. About a thousand distinct stained bands can be seen in the human chromosome set. Every one is also found in chimps. The main change is nor in the amount of chromosomal material but in its order. Many of the bands have been reshuffled and two chromosomes are fused together in the line that led to humans. We have forty-six chromosomes in each cell, while chimps and gorillas have forty-eight. Pope John Paul in 1996 referred to an 'ontological discontinuity' between humans and apes. Perhaps that moment was marked by the fusion of ape chromosomes 1 and 2., with the human spirit coded somewhere on its amalgamated equivalent.
At the DNA level, too, (and forgetting issues of ontology) there has been little physical change. In one of the haemoglobin pseudogenes, the 'rusting hulk' of a gene (a structure that accumulates many mutations as it has no function), humans are about 1.7 percent distant from both chimps and gorillas, 3.5 per cent from orang-utan and 7.9 per cent from rhesus monkeys.
To sort out man's place in nature we need to combine information from as many genes as possible. DNA hybridisation, crude as it is, does just that. The method depends on the extraordinary toughness of the DNA molecule and its overwhelming desire for togetherness; for each strand to pair with a sequence which marches its own.
When a double helix is heated up it separates into two individual strands, each of which bears a matching set of the four bases. As the liquid cools, the strands come together, A with T and G with C, to reconstitute the original paired structure. If DNA from two different species is treated in this way, some of the single strands from each one form a hybrid molecule that contains one strand from either species. The more related they are, the more similar their DNA and the tighter the fit. If the strands are very similar, they stay together even at high temperatures but, if they share fewer sequences, are less stable. The temperature at which the hybrid melts hence estimates how alike the two DNAs must be and gives a measure of their relatedness. DNA hybridisation has already sorted out some thorny problems of classification. Thus, it shows that the nearest relatives of New World vultures are storks, rather than the vultures of the Old W
orld.
The results from primates are remarkable. Humans and chimps share 98.4 per cent of their DNA, rather more than either does with the gorilla. The orang-utan is not so related and New World monkeys even less so. Any idea that humans are on a lofty genetic pinnacle is quite wrong. For a series of functional genes, the DNA sharing between ourselves and chimps is as much as 99.5 per cent, with the gorilla further out on the same limb. A taxonomist from Mars armed with a DNA hybridisation machine would classify humans, gorillas and chimpanzees as members of the same closely-related biological family — indeed, he might count all three as so similar as to share the Latin name Homo, once seen as unique to ourselves.
Humans and chimps are not, needless to say, just minor variants on the same theme. Evolution involves more than overall change in DNA. The Hawaiian islands have more species of fruit fly than anywhere else in the world, with a vast diversity of form. One looks like a hammerhead shark, with huge protuberances on either side of its head. DNA hybridisation shows that this wild evolutionary euphoria is accompanied by almost no change in the genetic material.
Brains and behaviour are what separate humans from any other animal. Since the split from chimps, the brain has added about a thousand cells a year. The human brain is five times larger than would be expected for a typical primate of the same size. The genes involved lose their importance in a measure of average genetic difference; and the brain itself produces a whole set of intellectual and cultural attributes that appear once a crucial level or intelligence has been reached and are not coded for by genes at all.
Somewhere in that brain, or what it is thinking, is what makes us different. Although it shares most of its DNA with humans, no chimp can speak. Some claim that they can manipulate symbols in a primitive 'language' (and trained parrots can do almost as well). The attempt to show that apes might talk is one of the great blind alleys of behavioural research. Samuel Butler's comment on a Victorian attempt to teach a dog sign language makes the point: 'If I was his dog, and he taught me, the first thing I should tell him is that he is a damned fool!' To make too much of the shared DNA of chimps and humans is to be in danger of the same foolishness. Humans, uniquely, are what they think.
Whatever their limitations, shared genes say a lot about history. The biological differences between humans and their relatives come from the mutations that have taken place since the primates began to diverge. They can be used to guess at when the human family separated from the others: the more differences, the longer ago the split. If such generic accidents happen at a regular rate, they can even be used as a 'molecular clock', which uses changes in genes to infer when two stocks last shared a common ancestor.
Molecular clocks depend on several assumptions, some of which may be justified. First, mutations must happen at a constant pace as the generations succeed. In addition, they should not damage their carriers (and, as most of those used are in parts of the DNA that do not contain any meaningful instructions, perhaps they do not). DNA errors accumulate over the years. Some are lost because, by chance, those who carry them do not reproduce, but are replaced as new ones come along. The genetic make-up of any line hence changes with time. The transformation of the inherited message is a clue as to when two species began to diverge. To date the split, there must be evidence from fossils (or from other sources such as the date of appearance of a barrier such as a mountain range) about when two extant members of the group last shared a common ancestor. To compare their genes sets the rate at which the clock ticks and makes it possible to work out the date of separation of others whose ancestors left no fossils.
Linguists use the same logic. As words are passed from parents to children errors creep in. Sometimes the changes are scarcely noticeable. In Shakespeare's As You Like It the court jester makes a speech which causes great amusement: looking at a clock he says 'Thus we may see how the world wags; 'tis but an hour ago since it was nine; and after one hour more 'twill be eleven; and so, from hour to hour, we ripe and ripe; and then, from hour to hour, we rot and rot; and thereby hangs a tale.' Just why this should be so funny is lost on modern audiences; unless they realise that in Shakespeare's time the word 'hour' sounded almost the same as the word 'whore'. Such errors can be used to date manuscripts. They were copied by hand, often by people who had little idea what they meant. As copy followed copy, more and more mistakes crept in. The number of inaccuracies gives a good idea as to when any version of an original was in fact written.
Such changes are small, but can make a big difference. Languages as distinct as Bengali and English;ire related. They owe their existence to the accumulation of tiny changes to a common ancestor spoken long ago. Take the word for kings and queens. In Sanskrit, this was raj* in Latin, rex, in Old Irish ri, in French rot, in Spanish rey and in English, royal. Different transmission errors took place in the pathway to each language. If we know the date of the split (as we do from the literary fossils known as books) we can make a linguistic clock. In Europe, this ticks at a rate which means that two languages share about eighty per cent of their words a thousand years after they divide. The language timer is an imperfect one: some words hardly change while others shift more quickly. Nevertheless, it can be used to trace the origin of modern tongues although their ancestral speakers are long dead.
The idea of a molecular clock driven by mutation is elegant and simple. As usual, the more we learn the worse it gets. It speeds up and slows down, and ticks at different rates for different genes. A similar confusion led the nineteenth-century Linguistic Society of Paris to ban discussion of the origin of languages. There have been some spectacular failures by molecular clockmakers to see the biological wood for the evolutionary trees. Even so, they have had some triumphs and these can be used to date human evolution. Several cautions are called for. The primate fossil record is so patchy that it is difficult to find firm dates with which to set the clock. Fossils and genes suggest that the primates began to diverge about sixty million years ago, the line to baboons had split off by twenty-five to thirty million years ago and that to orang-utans by twelve to sixteen million years before the present. They say nothing about iIk- date of the splir between humans, chimps and gorillas. A molecular clock based on the genes of our relatives suggests that this division took place five to seven million years before the present, with the divergence of the gorilla line before that of those to chimps and to humans.
The last common ancestor of chimps and humans hence lived about three hundred thousand human generations ago. Homo sapiens is a recent arrival even in the history of the primates, let alone in the four thousand million-year pedigree of life itself. The molecular clock suggests that the Philippines tarsier (an unremarkable small brown monkey) split from the western tarsier (which is almost identical in appearance) at about the same time as our divergence from chimps, showing just how quickly our ancestors must have evolved.
When — or how — the attributes which separate humans so absolutely from any other creature first appeared is not a question for biology. Perhaps the best we can do is to agree with Keats that we are all 'twixt ape and Plato' and leave it to individual preference just where on that long road we place ourselves.
Chapter Nine TIME AND CHANCE
The Good Book points out, in Ecclesiastes, that 'The race is not to the swift, nor the battle to the strong. hut time and chance happeneth to them all.' Evolution is all about change with time, but how things evolve is often a matter of chance. The nature of inheritance means that random events are bound to direct our genes as the generations succeed one another. As a result, much of the human condition is shaped by accident.
The importance of chance in evolution was noticed by the English cleric Thomas Malthus. He became interested in the history of the burghers of Berne and followed the records of their names over several centuries. To his surprise, many of the surnames present at the beginning of the period had gone by the end, although the number of burghers stayed about the same. Francis Galton showed why.
A s
urname is rather like a gene; it passes from father to son. Every generation there is a chance that a particular father does not have a male child. Perhaps he has just daughters (who lose their names on marriage) or no children at all. His name is then lost from his family line. If he has no children the name will go at once. It is also more likely to become extinct if he has a smafl family, as a limited brood will quite often consist of daughters alone. If, in a closed community like that of the Bernese bourgeoisie, this process goes on for long enough, more and more names will disappear as the years pass. Given enough time, just one surname will survive. Everyone will carry the same inherited message (at least as far as their name is concerned). In addition, the community will be more inbred than before, as rhc only mates available will be people who share the same name, all of whom descend from a common ancestor.
Just the same happens to genes. Perhaps, among the mediaeval Bernese bourgeoisie, there was a rare gene — an unusual blood group, for example. Because Berne was a small town, few people carried it. If none passed it on (because they had no children, or because by chance the gene did not get into the sperm or egg that made the next generation) then it was lost. On the other hand, the carriers might, again by chance, have had more children than the rest, in which case the variant became more common. In either case, its frequency altered (which means that the population evolved) in a manner that depends only on the accidents of time.
