The Evolution of Modern Humans: Where are we now?

by
Christopher B. Stringer
Natural History Museum, London

What follows is a version of the Millennium Distinguished Lecture presented on November 17, 2000 by Chris Stringer at the AAA meetings, San Francisco. The Distinguished Lecture was sponsored by the General Anthropology Division and the Biological Anthropology Section.
This article was published as: Stringer, C. 2001 The evolution of modern humans: where are we now? General Anthropology 7 (2): 1-5.

Today, I want to talk about the evolution of modern humans, and with so many recent discoveries, this is indeed an exciting time to be discussing the origins of our own species. There are two kinds of origin we must consider: one is the origin of the features that modern humans share, for example the shape of the skull, a relatively large brain, a chin, and a lightly built skeleton compared with our predecessors in the evolutionary record. But of course looking around the world today, there are also numerous differences between human populations: differences of colour, size, shape, and so on. We have to account for the origin of those differences as well, and bear in mind that the similarities may have originated over a different time scale than the distinctions between modern human populations.

As we learn more about early human evolution, we have realised how very complex it was. It seems to have been a radiation of distinct species and lineages over the last four million years, with the early part of the record restricted to Africa. At some stage, around two million years ago or soon after, humans left Africa for the first time and started to diversify. By this time, the species Homo erectus (early fossils of which some people call Homo ergaster) had evolved, with examples from east Africa dating close to 1.8 million years old, and others from Dmanisi in Georgia and Java perhaps nearly as old. The place of origin of these widespread populations is still uncertain, although most workers assume it was somewhere in Africa. This early human dispersal (sometimes called "Out of Africa I") was followed by evolutionary diversification over the next 1.5 million years.

Several different models have been developed to explain how OOAI relates to the evolution of modern humans. For example, the Multiregional Model posits that OOAI really marks the beginning of our global evolution, so all subsequent fossil samples represent evolving Homo sapiens. There is no single place where modern humans or their features originated: for example, a modern human character may have developed in Africa and then spread from there by gene flow, or it might have originated in Europe, China or Java, and spread from there. At the other extreme is the idea that I favour, the Recent African Origin or Replacement Model (sometimes just called "Out of Africa", or OOAII). This scheme posits that there was only one region where there was a complete evolutionary sequence from H. erectus/ergaster to modern humans, and that region was Africa. The evolutionary lineages that developed outside of Africa following OOAI did not give rise to modern humans, but were replaced as modern humans dispersed from their African homeland during the last 100,000 years. In this model, the differences, which characterise modern regional populations, have only developed very recently (mostly during the last 100,000 years or less), a key distinction from the Multiregional Model. A slightly different model, developed by Günter Bräuer, argues that the evolution of modern humans was primarily African, but there was some hybridisation with resident archaic populations (such as the Neanderthals) as modern humans dispersed outside of Africa. Fred Smith's Assimilation Model argues that, while modern human genes and characters primarily derive from Africa, they spread through a more complex and longer-term process of interbreeding so genes and morphology could change locally, rather than necessarily being subject to rapid replacement.

It's probably no surprise to anyone to say that I favour the Recent African Origin Model, so this is the one on which I will concentrate now. In my view there have been several dispersal events in human evolution over the last two million years, not just OOAI and II, and a particularly significant one occurred in the Middle Pleistocene of Africa and Europe, over 600,000 years ago, with the origin and spread of humans I classify as Homo heidelbergensis. I believe that this species subsequently underwent a gradual evolutionary split about 300,000 years ago, giving rise to Neanderthals, north of the Mediterranean, and to modern humans, to the south, in Africa. Meanwhile, further east, it appears that Homo erectus continued evolving in regions such as China and Java.

Evidence has accumulated from the fossil and archaeological record during the last few years that these two lineages (Neanderthals and modern humans) may have encountered each other in areas of overlap such as the Middle East (e.g. Israel) about 100,000 years ago, and Europe about 35,000 years ago, yet in my view these lineages show scant evidence of intermixture. So I will now discuss a little more of the history of these lineages. The Neanderthals were the people I began studying for my doctoral research some thirty years ago. At that time, I concentrated on comparing, through measurements, the cranial shape of Neanderthals and their successors, the early modern Cro-Magnons, in Europe. It was true then, and still is today, that Europe has the best, best-studied and best-dated evidence of human evolution over the last 500,000 years, but we should not assume that Europe was necessarily representative of, or central to, developments in the evolution of modern humans. Nevertheless, in historical terms, before significant evidence emerged from regions such as Africa, Neanderthals were sometimes pushed into the position of primitive "missing links" between apes and humans, portrayed with prehensile toes, bent knees, long arms and a stooping gait. At the other extreme was the view of the anthropologist Carleton Coon, who memorably remarked that a cleaned-up and soberly dressed Neanderthal would hardly merit a second glance on the New York subway. (I sometimes remark that this may be telling us as much about the denizens of New York as it does about the Neanderthals!). I think the truth about Neanderthals is actually somewhere between these extreme views. While they were certainly not primitive sub-humans (they were actually highly evolved in aspects of their morphology and behaviour), the Neanderthals were definitely distinct from living people, different enough to be regarded as a distinct species, H. neanderthalensis, in my view. So for me, what is especially fascinating about the Neanderthals is that they were every bit as human as we are, yet they were different. What we share with them is a measure of what it means to be fully human, what differentiates us is what it means to be a Neanderthal, or a modern human (H. sapiens).

About thirty five thousand years ago, there was a remarkable juxtaposition in Europe of late Neanderthals and early modern humans (Cro-Magnons). Our main method of physical dating, radiocarbon, does not give us great accuracy during this critical period of time, but is sufficient to suggest that the two populations were broadly contemporaneous within regions, even if we cannot yet establish that they co-existed in the same valleys or even in adjoining sites. This juxtaposition is also marked by what some archaeologists see as a major technological and behavioural revolution, sometimes called "The Human Revolution", one that occurred around the time modern humans arrived in Europe. Certainly the period of the Upper Palaeolithic was a time of much greater technological, artistic, and ritual complexity than what had come before, for example in the preponderance of specialized blade tools, and the appearance of art, symbolism and human burials accompanied by rich adornments of bone, antler, shells or mammoth ivory beads. In this context, it has been argued that a child buried over 25,000 years ago at Lagar Velho in Portugal, associated with red ochre and pendants, is a member of a mixed Neanderthal - Cro-Magnon "hybrid" population. In particular, Erik Trinkaus has suggested that the skeleton shows mosaic features such as a rather modern skull combined with a cold-adapted, robust build like that of Neanderthals. I remain sceptical of these particular claims, while awaiting detailed publication, but I do not rule out the possibility that there could have been some hybridisation between Neanderthals and modern humans, even if they were distinct species (see my separate comment "What happened to the Neanderthals?).

In my view, there is good anatomical evidence that Neanderthals and modern humans are more different from each other than today's human populations are different from each other, and that is as true when we compare Neanderthals and their approximate contemporaries, the Cro-Magnons, as it is when comparing the skeletons of Neanderthals and recent humans. Part of my doctoral and later research has attempted to quantify the differences between Neanderthals and modern humans, such as the Cro-Magnons, using multivariate analyses of skull measurements to show differences in size and shape. As an example, a typical result gave the following shape distances from the early Cro-Magnons (including here the Mladec; specimens that some researchers believe show Neanderthal features): Skhul-Qafzeh early moderns 11; African "archaic sapiens" (such as Jebel Irhoud, Ngaloba) 33; Asian Neanderthals 51; European Neanderthals 58. So despite their geological and geographic proximity, in this analysis the Neanderthals are the most different in cranial shape from the Cro-Magnons. By comparison, the range of average shape distances of five indigenous modern samples (from Norway, Santa Cruz, South Africa, Australia and Japan) from the Cro-Magnons in this analysis was 5-8. Recently, mitochondrial DNA from three Neanderthal fossils has been sequenced, confirming that Neanderthals and modern humans represent deep and separate evolutionary lineages, which probably began to diverge in the Middle Pleistocene, a finding independent of, but consistent with, the fossil record.

Thus my early research suggested to me that Europe was not the place to look for the evolutionary origins of modern humans and I turned my interests to the Middle East, where there were also Neanderthal and modern human fossils. When I began that research, it was believed the human succession paralleled that of Europe, with Neanderthal fossils such as Shanidar and Amud evolving into, or giving way to, early moderns such as Skhul and Qafzeh perhaps 40,000 years ago. However, the application of physical dating techniques to artefacts and fauna from the sites instead suggests an intriguing ebb and flow of Neanderthal and early moderns populations, with the latter present in Israel about 100,000 years ago, perhaps accompanied by some Neanderthals, Neanderthals present about 60,000 years ago, and then early moderns again from about 40,000 years.

The search for the origin of those very early moderns in Israel eventually leads back to Africa, in my view. The same dating techniques applied to the Israeli fossils have also revolutionised our view of recent African evolution. When I was a student, the Singa cranial vault from the Sudan was dated at about 20,000 years old, while the Florisbad partial cranium from South Africa was believed to be about 45,000 years old. As such, they were sometimes used to suggest that African human evolution was retarded compared with that of Europe or western Asia. Now those same fossils are believed to be at least 130,000 and 250,000 years old, respectively, and they tell a very different story. Just as we may be able to track a gradual transition from H. heidelbergensis to H. neanderthalensis in Europe from about 300,000 years ago, through fossils such as those from Atapuerca, Ehringsdorf and Krapina, the African material may similarly document the gradual emergence of modern humans through fossils such as Florisbad, Ngaloba and Singa over a comparable time period. Of course this is not to imply that all of these fossils necessarily form a neat, single sequence of descent – we have learnt that human evolution was more a series of complex evolutionary radiations than an inexorable progression. But the presence of fossils with mosaic morphologies certainly suggests that neither the Neanderthal nor modern human anatomies evolved rapidly and punctuationally as a complete package.

The African behavioural record for this time period (the Middle Stone Age, equivalent to the Mousterian of Europe and western Asia) now has a time scale as extended as that of the human fossils, and it is unfortunate that many of its rich and varied industries have no associated human fossils. Nevertheless, it is apparent that some modern human behaviours such as the production of composite tools, the systematic exploitation of marine resources, and the symbolic use of red ochre may also go back deep into the African past. Although some workers have argued that modern anatomy may have evolved before modern human behaviour, I now think that both may have emerged more gradually during the course of the Middle Stone Age. Exactly when and where the first modern humans appeared is still uncertain, although eastern Africa has suggestive evidence more than 150,000 years old. The adaptations of Middle Stone Age humans to coastal living may also provide a clue to the routes by which early modern people first left Africa. Although an exit route via the Sinai Peninsula into western Asia has usually been favoured, it is possible that coastal routes were used to expand around the Red Sea and Arabia, into southern Asia. At times of low sea level, this could have led these early modern humans as far as Indonesia. If they had also developed watercraft to increase their foraging ranges or cross river mouths, Australia and New Guinea would then have been within reach, and there is new evidence that this major colonisation was achieved at least 60,000 years ago. Dated artefacts had already suggested that humans were present in northern Australia at least 55,000 years ago, but in 1999 the Mungo 3 burial from southeastern Australia was redated to about 62,000 years. Such an astonishing age estimate implies that this burial of a modern human, associated with red ochre, was carried out far from Europe and long before any other comparable examples. However, recent claims that this skeleton has also produced ancient DNA, distinct from that of other humans, require very careful evaluation, given the problems experienced with contamination in many younger skeletons excavated from Europe.

If modern humans dispersed through southern Asia, when did they arrive in regions such as China? Unfortunately at the moment, the Chinese record has a significant hiatus between late archaic fossils dated up to about 100,000 years ago, and early modern fossils reliably dated only as far back as about 30,000 years. So we are unable to reconstruct events as well as we can in Europe or western Asia, and can only speculate about possible replacement events or hybridisation. Nevertheless from various analyses, including my own, it appears that there is also a large morphological gap between the latest archaic and the earliest modern fossils in China, and in terms of the first appearance of recent regional characteristics in the skeleton, these are apparently not clearly present in China until about 10,000 years ago. This finding is in line with studies of the earliest moderns from Europe, western Asia, Africa and the Americas and suggests that either we have not yet correctly identified the true ancestors of today's regional populations, or their distinctive characteristics were still evolving even 10,000 years ago. This latter view is supported by recent genetic analyses, which suggest that "racial" features are coded by only a few genes, and could have evolved quite recently and rapidly. In my view, natural selection, sexual selection, drift and founder effect could have created and developed these differences during dispersals from Africa over the last 50,000 years, particularly following the periods of severe cold and aridity at the end of the last Ice Age 10-30,000 years ago.

As to the success of modern humans, why is it that we are here and not the Neanderthals? Some researchers believe it was simply that modern humans had superior brains, more complex language and better organisational skills than people such as the Neanderthals, and so moderns simply took over from the Neanderthals wherever they overlapped. But it is difficult to draw reliable behavioural inferences from the archaeological and palaeontological record, and we should resist denigrating the abilities of the Neanderthals and people like them, who survived in and adapted to a range of challenging environments in western Eurasia through the last 250,000 years. It is also evident that earlier members of the Neanderthal and modern lineages may have overlapped in western Asia about 100,000 years ago, and one could argue that, if anything, the reverse replacement occurred. Furthermore, the apparent replacement phase in Europe can now be seen to be more protracted and complex than a simple and rapid take-over by the Cro-Magnons.

In my view, modern human capabilities continued to develop through the last 100,000 years, and it may not have been until the invention of Upper Palaeolithic technologies that modern humans could successfully begin to challenge the Neanderthals in their core territories. And at that very time, Ice Age climates were starting a period of astonishing instability, with fluctuations from relative warmth to extreme cold and back again, occurring every few thousand years. In Europe, these changes were linked with major reversals of North Atlantic ocean circulation, which could freeze or thaw the Atlantic in less than a decade. So in the lifetime of a Neanderthal or Cro-Magnon individual, all the climates, plants and animals with which they were familiar could have been swept away and replaced by a different suite of climates, plants and animals. In previous times, the European Neanderthals probably responded by going extinct locally, surviving in refugia (such as southern France, or the Mediterranean coast), and recovering their range and numbers when the climate improved. However, about 35,000 years ago, new people accompanied them for perhaps the first time, and during these highly stressful climatic fluctuations, the populations best able to cope with rapid change would have been favoured. If the Cro-Magnons had advantages such as larger social networks and more effective clothing and dwellings, these factors could eventually have led to the demise of the Neanderthals. In my opinion there was nothing inevitable about the success of modern humans, or the extinction of the Neanderthals, and a different combination of circumstances could have produced a completely different outcome. Indeed, if we are prepared to grant the Neanderthals the same evolutionary potential as modern humans, I might now be a Neanderthal instead, trying to explain to an audience of Neanderthals why "we" are still here, and what happened to those strange-looking people who used to live in Africa!

To finish on a more serious note, there are still shadows hanging over debates about the origin of modern humans. These shadows can be seen in the writing of many anthropologists of the past, for example in Coon's unfortunate comparison of what he called the "Alpha and Omega" of living humans in his book The Origin of Races. All of the proponents of the current models of modern human origins that I discussed earlier would certainly unite in condemning such a portrayal, and yet I find myself having to acknowledge that the Recent African Origin Model I helped to develop is being used to support the idea that present-day "races" can be arranged in a hierarchy of evolutionary advancement with "Blacks" as the most primitive, "Whites" in between, and "Orientals" as the most evolved. In my view this misuse of science should not make us shy away from investigating the differences between modern human populations, and how they evolved – indeed, if mainstream workers do not take up the challenge of this research, the field could be left to those with extreme political agendas. From my perspective, Africa was our genetic, physical and behavioural homeland, and Africa today may well contain as much genetic diversity as the rest of the World put together. I hope that the coming century will see research in modern human origins build on the tremendous advances of the last twenty years to further confirm that all of us are indeed "Africans under the skin".

This page is part of the Fossil Hominids FAQ at the talk.origins Archive.

The Evolution of Modern Humans: Where are we now?

byChristopher B. StringerNatural History Museum, London

by
Christopher B. Stringer
Natural History Museum, London