A great many new facts about human evolution have been found in recent years, all of them strengthening the belief that our ancestors were rather like apes. Indeed it becomes more and more likely that there were no fully human creatures on earth until as recently as forty thousand years ago. It is a great pity that these new discoveries have been written up in a sensational manner by some anthropologists. The title of this book, The Myths of Human Evolution, panders to the idea that there is some doubt about whether evolution has occurred at all, which is of course just what creationists are saying so vociferously in the United States. Their clamour, and the political and educational issues raised, make the subject into News, and one can’t help suspecting that the authors of the book (or their publishers) mean to exploit this interest with their title.
In fact both authors are reputable scientists who have no doubts that evolution has occurred. The question they are raising is whether it goes on ‘gradually’ or by ‘sudden jumps’, with long periods in which species remain unchanged. The ‘myth’ they wish to attack is that evolution proceeds by slow adaptive change. This was of course what Darwin himself proposed and is probably still the view of most biologists today. The authors call it a ‘myth’ because its truth is ‘taken for granted’. They believe that it grew up because ‘Darwin got his notions of how evolution works from general ideas of change around him, and he characterised evolution in these terms as a technique to convince his peers that species are not the indelible fixed sort of entities biologists had long considered them to be.’ I wonder whether this is really a correct account of how Darwin worked. Other ‘myths’ are also attacked in the early part of the book. ‘Reductionism’, which is ‘explaining all human behaviour in terms of biological principles (themselves reduced to genetics)’, seems to be one of the culprits, and is due to ‘physics envy’, generated by the American public’s belief that the physicist is the paragon of what a scientist ought to be. How this affects evolution theory is not clear, and it is apparently the fault of sociobiologists who are ‘reductionists’.
Another ‘myth’ is that evolution has been ‘progressive’. This idea may have been inherited, as the authors suggest, from the Victorian concept of social evolution: but surely it is now possible to agree that in the 3000 million years of evolution from the earliest known bacteria to mammals and flowering plants there has been a change that can be called an advance. The higher animals and plants are more complex systems than their ancestors. They have devices that enable life to continue in conditions far different from the sea in which it first arose. In this sense ‘progress’ is not a myth: it has occurred. But many organisms remain simple still – the problem is to find out why some have changed and some have not.
This takes us back to the main thesis of the book, which is that new species are formed not by gradual adaptation but by sudden jumps. Whether or not this is correct depends on what is meant by the words ‘sudden’ and ‘gradual’. A species consists of a group of individuals who are fertile among themselves but cannot breed with members of another species. Obviously a whole new group cannot appear overnight: it must arise by the modification of a previous group of individuals over a period of time, presumably at least some years. The question is how many individuals and how many years? How long does it take to make a new species? Geology tells us that the times involved are very great compared to individual lives and one has to be very careful about the words one uses to describe these evolutionary processes. The authors’ versions of rapid change vary between the establishment 600 million years ago of ‘all the basic groups of animal life [in] only about 15 million years’ (which, incidentally, is not true) and ‘the rapid growth of computers [as] a function of successive waves of electronic inventiveness’! These are dangers one faces in trying to find ‘Patterns in Evolution’ and ‘Patterns in History’ – to cite some of their chapter headings. Their general thesis should not, however, be obscured by such exaggerations. They have very interesting things to say about the origin of species.
Unfortunately the above definition of a species is very hard to apply even to existing organisms and can’t be used at all for animals or plants that are extinct. It is not possible to discover whether two groups living thousands of years ago could interbreed. So the paleontologists who study fossils have no sound criterion by which to decide when they have a new species. This makes the question of the origin of species very difficult, since the best evidence for evolutionary change comes from the sequence of fossils in layers of rocks. Eldredge and Tattersall point out that paleontologists may find nearly identical specimens separated by as much as five million years. Indeed some creatures, such as lung fishes, have hardly changed in 200 million years. This conservatism among some types is one of the puzzles of evolution, but it does not alter the fact that there are many other cases where the fossils seem to show quite rapid change and those who find them can’t decide where to draw the line between species. The teeth of elephants, for instance, can be shown to have changed ‘gradually’ throughout the last two million years. At least four stages can be recognised within one species as it changes into another. Elephants’ teeth are very hard and a great many specimens have been found. Human fossil skulls are much less common.
Of course there is very good evidence that some changes are going on all the time. The populations of moths in industrial areas have become darker in colour within the last century because in this way they are better protected from attack by birds. Eldredge and Tattersall dismiss such examples as temporary changes within a species and discount the idea that further changes of this kind would lead to reproductive isolation. If the events that happen in a few human lifetimes cannot show the origin of species it is likely that we will never be able to prove it. In fact there are some good cases of rapid evolution in recent times. For instance, seven new species of fish have appeared in Lake Nabugabob since it became isolated from Lake Victoria only five thousand years ago. Strangely, the authors do not quote this example, which seems to support their view that new species arise in the isolation provided by new habitats.
A further factor is that geologists are beginning to find evidence that some climatic changes may be very rapid indeed. About ninety thousand years ago the climate changed perhaps in as little as a hundred years from warmer than it is today to fully glacial conditions. This has been shown by study both of cores taken from Greenland ice and deposits from the floor of the Gulf of Mexico, and confirmed by stalagmites in caves in France. The authors do not cite this case either, but such evidence supports their hypothesis that new types may have first appeared in small populations surviving in isolation after some almost cataclysmic happening.
The second half of their book is concerned with showing how something similar may have occurred in the course of human evolution. Unfortunately the fossil material available is still so scanty that they do not succeed in convincingly establishing their case. However, they provide a very readable and detailed account of the sequence of discoveries of human fossils and of the anthropologists who have found them. The story of human evolution is inevitably, if rather sadly tied up with the history of the controversies that followed the various finds. We are given all of them here (even the faked Piltdown Man) without too much criticism of the ‘myths’ that influenced the views of the finders. A recent sequence in the drama was the discovery in 1974 and 1975 by Johanson and his colleagues at Hadar in Ethiopia of the nearly complete skeletons of the ‘First Family’, Lucy and her 13 relatives, who lived about three and a half million years ago and undoubtedly walked on two legs, though they had ape-sized brains. This was followed in 1977 by Mary Leakey’s finding of footprints of similar individuals preserved in volcanic ash from about the same age, at Laetoli near the Olduvai Gorge which she and her late husband Louis and son Richard have so fully investigated.
Evidence has been appearing so rapidly in the last two decades that it is courageous of our authors to try to put it all together as a thesis about the rate of human evolution. They believe that ‘by looking in detail at the hominid fossil record, we have seen that the idea of slow, gradual progression is not borne out.’ But in the course of arriving at this judgment they allow themselves some dangerous and even contradictory statements. Thus Johanson’s treatment of the assembly of fossils at Hadar is first said to be ‘not unchallenged’ but later ‘most reasonably interpreted’. Can we then place any great weight on the belief that ‘this small, bipedal, non-tool-making hominid was relatively small-brained, and as far as can be told persisted unchanged throughout a span of about a million years’? Their thesis rests largely on the idea that species remain unchanged for very long periods, but the fossils vary so much that their evidence often seems to show exactly the opposite. Thus one of their long-lived species is the famous Java and Pekin Man: ‘Homo erectus shows virtually no change, local and geographical variations are at least as striking as differences between older and younger members of the lineage.’ This is almost like saying that difference is the evidence for sameness. The conclusion about persistence may be correct but is not yet proved by the evidence.
The other plank of the anti-gradualistic thesis is that new types appear ‘suddenly’. Here again the evidence from fossil hominids is insufficient for the conclusion. The species Australopithecus boisei ‘springs forth fully-fledged’ and Homo erectus turns up ‘unanticipated in the eastern African record at about 1.6 million years ago’. Since new fossils and new dates are being found every year it is hard to accept these ‘sudden’ appearances. Only last December the anthropologist Michael Day wrote an article in Nature entitled ‘Lucy jilted’, showing that the date suggested for her may be wrong by 400,000 years. The authors are serious scientists who would probably admit these difficulties, but like other anthropologists before them they find it hard to choose the appropriate language. Expressions such as ‘fully-fledged’ may make for easy reading but they are unfortunate when used about a period of millions of years and evidence from a few damaged fossils. The question of our ancestry is an emotional subject. When we look back two million years we are thinking about our own great-grandparents about eighty thousand times removed. It is not easy even now to realise that they were our direct ancestors, even though that is an awful lot of grandparents, and brain size has doubled in the meantime.
Eldredge and Tattersall have something to say about the latest stages of evolution, too. They believe that fully modern Homo sapiens also appeared ‘suddenly’, thirty-five thousand years ago, and dismiss such fossils as that from Swanscombe in Kent as ‘post-erectus’, although their brains were of fully human size much earlier. Neanderthal man ‘disappeared abruptly in Europe around 35,000 years ago, to be replaced by men of fully modern aspect’. They think the Neanderthals couldn’t have been absorbed by interbreeding because ‘the pattern of abrupt displacement is too regular and too short-term to allow in situ evolution by any mechanism whatever.’ These changes may indeed have taken place as they suggest but they seem to be using their general belief to decide uncertain questions. The evidence is not sufficient to show in detail what really happened over those long periods so very long ago. What is certain is that no known fossil skulls older than half a million years have a brain size and other characteristics close to those of modern man. We can be sure that we have evolved from non-human ancestors, but we know only roughly how long the process took. Eldredge and Tattersall believe that it was episodic rather than a process of continuous slow adaptation, but they cannot pinpoint when the ‘jumps’ occurred, or say how fast they were. Nevertheless it is useful to have the question raised. Perhaps one day there will be enough fossil evidence to answer it.