The Scars of Evolution Read online

  Thus a palaeontologist on an African dig may be obsessed by the problem of bipedalism and regard it as the last of the great unanswered questions. Crack this one, he feels, and we are home and dry. Meanwhile, in other parts of the world, other specialists are telling themselves that bipedalism must be pretty well understood by this time, and the really baffling characteristic still unexplained is the unusual nature of our skin – larynx – sex organs – sweat-glands – blood – adipose tissue – brain – tears – vocal communication – extended life span – or whatever is their own particular field of study.

  In short, the chief mystery does not lie in any one of these anomalies, not even the wonderful brain or the dexterous hands or the miracle of speech. It lies in the sheer number and variety of the ways in which we differ from our closest relatives in the animal kingdom. Some of these differences are major and some are minor; several appear arbitrary and pointless; others are embarrassing or inconvenient; and it has not proved easy to trace any logical connection between them.

  Something happened. Something must have happened to our own ancestors which did not happen to the ancestors of apes and gorillas. In the last analysis no one seriously disputes that. No one, for instance, expresses surprise that the anomalies were not more evenly shared, so that gorillas learned to speak, people walked on two legs, and chimpanzees became naked.

  Something changed in the environment, or in the mode of response to the environment, or in the genes; that change affected only one particular group among the ancestral anthropoids, and set them off on the long road to becoming human.

  The key question, then, is: ‘What happened?’ One possible response is: ‘Who cares what happened? Whatever it was, it was millions of years ago and it can’t possibly affect me.’ But that is not true. Whatever it was, it affects every one of us because it had far-reaching consequences on the way our bodies function – or, frequently, malfunction. Possibly 20 per cent of the ills that our flesh is heir to can be traced back to the critical period of transition from non-human to human.

  Theories about that turning point in our evolution cover a very wide spectrum, even if we omit the views of the Creationists who do not believe we evolved at all.

  On the wilder shores of speculation is the kind of scenario which involves the incursion of beings from outer space bringing the benefits of their own acquired wisdom. This genre is an offshoot of the myth of the evolutionary ladder. It does not merely assume that some form of life has arisen elsewhere in the universe, which, considering the size of the universe, is a very strong probability. It also implies that as soon as molecules combine into aminoacids they are channelled along predestinate grooves which must end in creatures endowed with powers of independent mobility, reasoning, and manipulation, able and eager to build space-ships and cruise the galaxies.

  That is infinitely less likely. It is also intellectually unsatisfying because it does not solve the problem of why human-like animals evolved; it merely relocates it in time and space. If we cannot work out how and why man evolved on our own familiar planet, it is not useful to refocus our enquiries onto how and why some alien analogue of ourselves emerged from an extraterrestrial precognitive life-form many light-years away.

  At the opposite end of the spectrum is the orthodox school of thought which contends that nothing remarkable happened. One population of ape-like anthropoids migrated from a forest habitat to more open ground. And that was it; that was allegedly enough to set in train the series of changes that made us what we are today.

  The strength of this idea lies in its near-inescapable logic, which runs as follows: (1) there are good grounds for believing that our very early ancestors lived in trees; (2) there are also grounds for believing that our less distant ancestors hunted big game on the plains of Africa; (3) therefore we know that our predecessors moved out from the trees to open country – that is, onto the African savannah.

  This reasoning is almost foolproof, but not quite. If you see a man at the north end of London Bridge and then see him at the south end, you may feel safe in declaring that he crossed the bridge. However, if your second sighting is five years after the first and you notice that in the interim he has acquired a deep tan, a UCLA T-shirt and an American accent, it crosses your mind that he may have made some kind of detour.

  The latter analogy is a closer parallel to what really happened. The earlier hominid fossils are of a creature utterly changed from an arboreal ape, walking upright on two legs. Previous to the appearance of these creatures there is a blank – a fossil gap which lasted for millions of years.

  One serious weakness in the savannah theory lies in the disproportion between the commonplace event it depicts and the spectacular consequences alleged to have flowed from it. Many primate species such as baboons and vervet monkeys and patas monkeys have left the trees to live on the savannah. They have lived there quite as long as our ancestors ever did, yet they show no signs of acquiring any human-like features such as hairlessness or bipedalism. They settled down in the new habitat with scarcely any physical modifications, and there is no good reason why a cousin of a chimpanzee could not have done the same. Then there would have been two forest apes and one savannah ape, but no people to write books about any of them.

  Despite these objections, the savannah theory continues to command the tentative allegiance of many of the best minds in the business. (It is only natural that the message getting through to most of the students is the allegiance rather than the tentativeness.) It is not that they are happy with it as it stands; there are far too many pieces that will not fit into the jigsaw. One subconscious factor may be a kind of curiosity fatigue, and a desire to work on different sectors of their field which yield answers rather more readily.

  A recent trend in evolutionary thinking attempts a new approach. It squarely confronts the three facts that are widely held to be self-evident: (1) we evolved on the savannah; (2) as Darwin revealed, evolution takes place through natural selection; (3) it is proving very difficult to demonstrate that our bodies are adapted by natural selection for life on the savannah.

  For a century or so this diabolic triangle has been tackled by battering away at proposition number three, and saying it is only a matter of time before we understand. The new thinking has elected to batter away at number two. Perhaps, it suggests, we are wasting our time looking for an adaptationist solution; perhaps there simply isn’t one. Perhaps evolution does not take place only through natural selection; there may be other forces at work which Darwin failed to identify.

  That might be one way of cutting the Gordian knot. All that remains is to find out what those forces are, and how they might complement or even negate the laws of natural selection, allowing it to be suspended or overridden or outflanked; that would enable chance to take a hand in the game, so the argument runs.

  Suppose a genetic mutation arose by chance … Suppose it happened in a small, isolated population where it could quickly become established … Suppose it was not an ordinary run-of-the-mill gene (affecting, say, eye colour) which mutated, but a master gene which triggers or controls the operation of other genes, or affects relative rates of growth in different parts of the body … In the Burgess Shale, high in the Canadian Rockies, fossils were found of tiny invertebrate creatures living 530 million years ago, so weird and varied that the accepted laws of Darwinian natural selection seem scarcely to have begun to operate on them. Suppose some other more random factor dictated their designs, and suppose that 530 million years later that other factor momentarily resurfaced and produced arbitrary changes in the ground plan of one particular African primate … Or suppose that natural selection only dictates certain key areas of genetic inheritance, while other areas which have escaped our attention are left to chance because they are too marginal to affect survival. Suppose some convergence or concatenation of such changes happened to … Suppose … or maybe … The suppositions have aroused a great deal of interest, but so far none of them has crystallised into a convincin
g explanation of the emergence of man. Arguably, the law of natural selection, like the law of England, does not concern itself with trifles, and small physical variations may arise within any species (like the exact number of stripes on a zebra) which do not affect its chances of survival one way or the other. But the differences between men and apes are not of that order. It is conceivable that by genetic chance a gorilla might be born hairless, just as on very rare occasions a human baby is born hairy. But hairlessness would not become established in a band of gorillas – however small and isolated – because the nude individuals would be the least likely to survive and procreate in the type of environment where they live. Chance proposes; natural selection disposes.

  When Einstein was asked to express in a few words how the idea of relativity came to him he replied, ‘I ignored an axiom.’

  It is time to tackle the problem of the diabolic triangle by ignoring an axiom. Eliminate proposition number one, that man evolved on the savannah. Postulate, just for the sake of argument, that the crucial early stage of humanoid evolution did not take place on the savannah.

  We are now confronted with a brand new question: Where did it take place?

  2

  Fossil-Hunters

  ‘Practically all palaeontological discoveries can be described as bones of contention.’

  John Napier

  There are two ways of trying to reconstruct the evolutionary history of a living species. One way is to examine extant specimens, studying their behaviour when they are alive and their anatomy when they are dead. The other way is to hunt for the fossilised bones of their distant ancestors.

  These approaches are complementary, each telling us things that the other cannot. The fossil-hunters’ contribution will be considered first, because they loom largest in the public mind as the evolutionary experts. Whenever a journalist needs to consult an expert on human origins, or a television producer wants to make a programme on the subject, these are the people they approach.

  There are several good reasons for this. Hunting for pre-human fossils calls for a rare combination of physical and mental abilities, such as courage, stamina, determination and a fair amount of luck, so those who succeed in it tend to be energetic, eloquent and extrovert. Frequently their discoveries make news, and almost as frequently the claims they make about their discoveries result in noisy disputations – which further endears them to the media.

  They have great expertise in the comparative skeletal anatomy of humans, apes and the prehistoric hominids whose bones they unearth. Because the skeleton is the only part of the body which fossilises, their knowledge of the non-skeletal organs is much hazier, and when questioned on matters outside their own province they have occasionally been known to give silly answers.

  By contrast, anatomical research was long regarded as the plodding and non-glamorous end of evolutionary investigation. Ernst Mayr, Harvard’s most authoritative analyst of the processes of biological evolution, once pointed out: ‘Ideas based on a study of comparative anatomy have in no case been refuted by subsequent discoveries in the fossil record.’ Despite this, anatomists can produce nothing which makes quite the same impact on the public imagination as the empty eye sockets of an ancestral skull.

  The most informative and enlightened account of the fossil-hunters’ search for human origins is Roger Lewin’s Bones of Contention, published in 1987. The story begins early in this century, when the race was on to discover the ‘missing link’ between man and the common ancestor he shared with the apes. With the whole world to choose from, there was considerable uncertainty about where to start digging.

  Darwin had predicted that the birthplace of our species would be found in Africa, but for a long time there was a peculiar reluctance, even among Darwinians, to follow up that lead. They were not comfortable with it; they were seeking the origins of the lords of creation, and that conjured up a specific image in their minds. We cannot afford to be patronising about this. Even today, when an illustrator is asked to draw a progressive line of creatures beginning with an ape, growing steadily more erect and intelligent, and ending with a human being, you can be fairly certain that the human being at the end of the line will be male, adult, and white.

  Fossil-hunters began digging in Europe, and their hopes were high. The first of the Neanderthal fossils had already been discovered there within Darwin’s lifetime. Neanderthals failed to qualify as the missing link because no one could believe we were descended from such ugly brutes, with prognathous faces and villainously low foreheads – even though their brain-cases were slightly larger in capacity than our own. But they reinforced the idea that Europe was good fossil-hunting territory.

  Thus, when some practical joker planted the cranium of a modern man and the jaw of an orang-utan in Sussex in 1912, the spuriousness of this missing link – ‘the Piltdown Man’ – was not finally established until forty years later. The victims of the hoax, as one of them later confessed, were only too ready to believe that ‘the first man was an Englishman’. In the ‘20s a counterbid was made on behalf of the United States. A tooth dug up in 1922 was hailed by H. F. Osborn of the American Museum of Natural History as a Pliocene relic of ‘Nebraska Man’. (It was later identified as the tooth of a peccary, a species of Pig)

  By the 1920s the prevalent belief was that man had emerged in Asia. The first hint of this had come in the 1890s, when Eugène Dubois dug up, in Java, the first specimen of the species now known as Homo erectus. Asia was acceptable where Africa was not, and there was no lack of theoretical arguments to buttress the belief in our Asian origin.

  It was reasoned that the enervating climate of tropical Africa was the reason why the African apes had been too languid to scale the evolutionary heights; our own ancestors must have been the product of some more bracing clime. And Asia was, after all, the site of very old civilisations, which suggested that man had been around there longer, and gained a head start. So the experts came down firmly on the side of Asia, and the question was regarded as settled.

  The man who brought Africa back into the picture was Raymond Dart. He was an Australian who had studied in London, and he applied for a Chair of Anatomy in a South African University as a way of furthering his career. He did not go there with any presentiment that it was good fossil-hunting country. His mentors had assured him that it was not, and he saw no reason to doubt them.

  Yet within two years, in January 1925, he announced that he had made an important discovery. He had been given a small fossilised skull found by limestone workers, plus an endocast of the skull – limestone that had solidified inside the cranium, producing an exact replica of the size and shape of the brain. It later became famous as the ‘Taung baby’; the teeth attested that it had died young.

  The face was ape-like, but Dart detected some features which struck him as more human than ape-like. One of these concerned the hole in the base of the skull (foramen magnum) which indicates the angle at which the skull was set on top of the spine. From its position Dart deduced that the creature belonged to a species which had walked upright. He was excited by it and acted more precipitately than scientific protocol dictates. He sent a letter to Nature (published in February 1925) which constituted, in effect, a claim to have found the missing link.

  With the exception of a Scottish doctor called Robert Broom, also working in South Africa, nobody believed Dart. He was denounced and derided for not recognising the skull of a young chimpanzee when he saw one, for not realising the brain was far too small for any ancestor of humans, for imagining the missing link could possibly turn up in South Africa. He was even attacked for classical illiteracy in the name he had chosen for his specimen – Australopithecus. (‘Australopithecus’ means ‘Southern Ape’, but the word comes half from Greek and half from Latin.) Sir Arthur Keith, the dominant figure at the time in this branch of science, examined a plaster cast of the Taung skull and gave his opinion on Dart’s interpretation of it. ‘These claims,’ he declared, ‘are preposterous.’

>   Dart’s ideas began to sink into oblivion. New books on human evolution made no mention of his South African discovery. In 1931 he submitted to the Royal Society a monograph intended as the definitive document on the skull and its significance. The Royal Society rejected it, and it has never been published.

  Eleven years after the Taung discovery, Broom found another Australopithecine, this time an adult. Yet it was not until eleven years later still, in 1947, that the tide of professional opinion began to turn in Dart’s favour. A spate of recantations followed, led by W. le Gros Clark and crowned by Sir Arthur Keith’s succinct admission: ‘Professor Dart was right and I was wrong.’

  On his ninety-second birthday Dart reminisced: ‘I knew people wouldn’t believe me. I wasn’t in a hurry.’ Anyone putting forward an idea which strikes the scientific establishment as ‘preposterous’ cannot expect to make converts overnight, and 22 years struck Dart as being about par for the course.

  From that point on, hunters for the missing link abandoned their dreams of visiting China and focused on Africa instead. In the 1960s and 1970s a series of remarkable discoveries were made – by Louis and Mary Leakey in Tanganyika, by their son Richard in Kenya, and by Don Johanson and Tim White in Ethiopia. There was often fierce disagreement over where the various fossils fitted into the family tree of man’s ancestry, or whether they had any place in it at all.

  But one general trend appeared to emerge. The hunt was moving further and further north, and the fossils were getting older. The possibility arose that South Africa was not man’s original birthplace after all. Taung’s tribe of ‘southern apes’ may have been the descendants of a species of northern apes which migrated south along the Rift Valley.