Lucky Chris Stringer, to have spent the last forty years immersed in new discoveries about the origin of our species. I don’t suppose that when he began his PhD research in 1970, setting off on a tour around the museums of Europe to measure as many skulls as he could get his hands on, he imagined that today he would need to be as conversant with mutations on the Y chromosome as he is with the shape of the pubic ramus (part of the pelvis). Or that we would now be able to establish the diet of our remote ancestors by analysing the chemistry of their bones, or to examine the structure of the minute inner ear bones of fossil skulls by means of computerised tomography. Or that it would now be possible to use luminescence techniques to date single grains of sand. Stringer’s mobile phone has more processing power than the multi-room University of Bristol computer he used in the 1970s to undertake one of the first applications of multivariate statistics on human fossils – something we would now expect undergraduates to do.
Stringer played a pivotal role in the formulation of a ‘Recent African Origin’ model for Homo sapiens – the species to which everyone alive today belongs. By suggesting that we had a single and recent origin in Africa, he was opposing the 1980s trend towards a multiregional model for human origins, which had originally been proposed by Franz Weidenreich in the 1930s. Weidenreich suggested that each population of Homo erectus around the world had evolved into its own variant of Homo sapiens, with its own distinctive racial characteristics. Both the Recent African Origin and the multiregional model come in several variants, but the basic contrast has framed research into human origins throughout the last three decades. That research has been diverse: new fossil discoveries and archaeological excavations, new dating methods, new procedures for the extraction of ancient DNA and so on. Stringer’s account is made all the more impressive by his having been involved in so many groundbreaking projects of every type – archaeological, fossil and genetic.
The major players in Stringer’s story are Homo erectus, the first human to disperse out of Africa sometime after two million years ago; Homo heidelbergensis, a descendant of Homo erectus and most probably the common ancestor of the Neanderthals and ourselves; Homo neanderthalensis, found in Europe and South-West Asia, which appears so biologically similar and yet so culturally different from ourselves; and finally Homo sapiens. He charts the advances in our understanding of the fossil, archaeological and genetic evidence in turn, before bringing all the evidence together in the most up to date synthesis available. Genetics – both ancient and modern – has made by far the most dramatic advances since I was last closely involved with the subject a decade ago, but there have also been impressive developments in the dating and analysis of fossils. In contrast, the chapters dealing primarily with the archaeological evidence, and focusing on what it can tell us about language, thought and symbolism, have a familiar air: there is only a limited quantity of new data and although the terminology is new, the arguments are not. It is striking that very little has been learned from the analysis of stone artefacts, once by far the most abundant source of evidence for past behaviour, over the last thirty years.
The study of both modern and ancient DNA, by contrast, is opening avenues into the past which until recently were unimaginable. Twenty-five years ago Rebecca Cann and her colleagues published a seminal paper in Nature that analysed the diversity of mitochondrial DNA (MtDNA) among living people around the world. MtDNA is the component of our DNA that is passed on from the mother alone and has an especially high level of mutation. Cann and her colleagues concluded that Homo sapiens most likely had a single origin in Africa around 200,000 years ago, so coinciding with, and confirming, Stringer’s reinterpretation of the fossil record.
The analysis of the genetic diversity of living people now encompasses the analysis not only of MtDNA, but also of autosomal DNA – which makes up the chromosomes contained in the nuclei of our body cells – and the male Y chromosome, described by Stringer as being small, boring and full of junk (junk DNA, that is). This confirms the findings from MtDNA, and my impression is that we are about to see an avalanche of studies that will enable further leaps in our understanding. Genetics won’t replace the analysis of fossil and archaeological remains, but it has already transformed the way we interpret them. It has also yielded some fascinating detail; it appears, for example, that blue eyes were sexually selected in Europe around the peak of the last Ice Age, 20,000 years ago.
The study of modern genetic diversity has been complemented by analysis of ancient DNA, extracted from fossils. The results can be just as exciting. One must interpret them cautiously because of the small number and size of the samples and the risks of contamination by modern human DNA, but samples have now been extracted from more than twenty Neanderthal specimens, confirming that they probably shared a common ancestor with us between 350,000 and 500,000 years ago. There is also strong genetic evidence for some degree of interbreeding between modern humans and Neanderthals: Stringer argues that ‘if you are European, Asian or New Guinean you probably have a bit of Neanderthal in your make-up.’ As with the study of genetic diversity, there are fascinating details too. Samples of Neanderthal DNA from skeletal remains in Spain and Italy indicate that southern European Neanderthals would have had pale skin and red hair. The analysis of one group of Neanderthal fossils, which appear to have come from a single family, indicates that the males shared identical MtDNA while the females did not; the likely explanation is patrilocality – females left their natal groups to join other bands.
Some of the most startling surprises in the last forty years have come from new discoveries. No one, for example, had expected to find such ‘primitive’ looking Homo erectus fossils at Dmanisi in Georgia in 1991; it had been assumed that only larger-brained and large-bodied hominids could have dispersed from Africa. But that surprise was far surpassed by the discovery on the Indonesian island of Flores in 2004 of fossils from a small-brained hominid with rather weird limbs that was still in existence a mere 18,000 years ago. No persuasive explanation has yet been offered for this find, regrettably nicknamed ‘the hobbit’. And how remarkable that seventy years after the discovery of Lascaux’s painted cave, and after all the archaeological research, caving and tourism in the area since then, a new painted gallery was found not far away, at Cussac, in 2000.
Surprises have also come from the application of new methods of analysis to skulls and other finds from many years ago. The Florisbad skull from South Africa was discovered in 1932; it had a modern looking face but a rather primitive brow ridge. It was assumed to be 40,000 years old, but in 1996 Stringer and his colleagues made one of the first applications of Electron Spin Resonance (ESR) dating to a tiny fragment of enamel from the one surviving molar tooth. It was found to be 260,000 years old, and its potential role in human evolution revolutionised at a stroke. In contrast, the Broken Hill Skull from Zambia, discovered in 1921 and one of the best preserved human fossils, was assumed to be at least half a million years old on the basis of the enormous brow ridges glowering over its eye sockets. But recent ESR dating by Stringer and colleagues has shown that it is probably less than 300,000 years old, possibly contemporary with the Florisbad skull and other modern looking fossils. So either there was a very rapid evolutionary phase in Africa, rather than the gradual evolution that had been assumed, or there was a great deal more variation in its human populations than previously suspected.
Palaeoanthropologists not only have to live with the risk that new discoveries will overturn their most cherished theories, but also with immense frustrations. Given that the fossil record is so sparse, it is unfortunate (a very mild way of putting it) that the rich collection of Homo erectus fossils recovered from Zhoukoudian near Beijing between 1929 and 1936 was lost in 1941 following the Japanese occupation. Imagine what could have been learned from that collection by the application of new techniques. More recently, a crucial fossil was lost in Lebanon: the partial skeleton of a child, thought by Stringer to be one of the earliest modern humans. And then there are fossils which are recorded only in the notebooks of early excavators: an account of the 1932 dig at Tabun Cave in Israel refers to the skeleton of a foetus tucked in next to that of a woman. The latter was recovered and is now in the Natural History Museum in London (it is one of the best preserved Neanderthal specimens), but there has never been any trace of the foetus. That the relative rates of foetal and child development are potentially critical for understanding the cultural differences between Homo sapiens and Neanderthals makes the loss especially frustrating.
Mysteries remain, and the fossil and archaeological evidence continues to turn up new ones from time to time. Who, for instance, were the Denisovans? This human population has been identified by the analysis of DNA extracted from a 40,000-year-old finger bone and molar tooth found in Denisova Cave in Siberia. The expectation was that the DNA would assign the bones to either Homo sapiens or Homo neanderthalensis, but instead it indicated a completely new human species living in eastern Asia, a derivative from Homo heidelbergensis well over half a million years ago. Even more mysterious, the distinctive Denisovan DNA has been found in a population of living humans: not East Asians, as might have been expected, but Melanesian inhabitants of places such as New Guinea. The Melanesians also seem to have a distinctive number of Neanderthal genes. Stringer bravely suggests who the Denisovans may have been and how the Melanesians came to have such a mixed genetic heritage. But the reality is that these are unsolved mysteries.
Here is another one: how can ‘modern behaviour’, as represented by innovative stone tools and symbolic artefacts, appear in South Africa 72,000 years ago and then again 65,000 years ago, only to disappear again in each case after a few thousand years, returning our ancestors to the cultural tedium of the Middle Stone Age? Is the explanation for this really that there weren’t enough people who knew about these innovations to transmit them from generation to generation, and from group to group? I think not. Why is the anatomy of the Neanderthal inner ear different from that of modern humans, ours being closer to the evolutionarily more distant Homo erectus? If we are getting into what Stringer calls ‘the biology of odd quirks’, why do our eyes have white sclera while those of our closest living relatives, the great apes, are entirely dark? And why is the human penis not only twice as thick as that of any great ape but also has a much more bulbous end?
Towards the end of the book, after we have learned how many of us today carry Neanderthal and possibly Denisovan genes, and how the concept ‘species’ itself has been challenged by the finding that species of monkeys that had been thought to be distinct appear to have exchanged genes, the ambiguity of the book’s title becomes clear. It refers to the origin of Homo sapiens, of course, but it also implies what the book makes explicit, which is that all the human ‘species’ – Homo erectus, heidelbergensis, neanderthalensis, sapiens and others – are artificial constructs. They are ‘our species’ in the sense that we have created them and possess what is left of them, but they can only ever be ‘approximate reflections of real world complexities’, Stringer writes, quoting his mentor, the anthropologist Clifford Jolly.
There have always been arguments as to how many species there actually are: some would like to lump together all the fossils from the last million years as Homo sapiens; others would merge the Neanderthals with modern humans to form a single species, while excluding Homo heidelbergensis and Homoerectus. Species defined by the morphology of their fossilised remains may map onto some historical reality, or they may not – they are all academic constructs. Imagine that we only had the fossil record of Homo sapiens to analyse. How many species would we construct? Modern humans do after all show marked differences in the shape of their crania. At one time this was thought to be a racial characteristic, but the differences are now understood to be a consequence of genetic drift as humans spread out in small numbers. Neanderthals too show internal variation and Stringer argues that we should assume that this variation included behavioural and physiological characteristics.
He concludes by asking which model of modern human origins is the ‘right’ one. He plumps for one that is ‘mostly out of Africa’. This acknowledges some degree of interbreeding between Homo sapiens and archaic species, and Stringer suggests that this happened in Africa before the global diaspora of Homo sapiens, which may have begun as recently as 55,000 years ago. He also stresses that we are continuing to evolve. The emergence of settled farming communities produced new selective pressures, influencing our diets and extending our lifespans past reproductive age, presumably because of the immense social benefits gained in this way. Indeed, changes in individual DNA sequences suggest that human evolution has accelerated over the last 10,000 years. We are evolving a hundred times faster than we were when we split from the lineage of chimpanzees around six million years ago.