Lone Survivors

by Chris Stringer

  As my work developed through the 1970s and early 1980s, I gravitated increasingly toward what Bill Howells in 1976 had dubbed the Garden of Eden (or Noah’s Ark) model. This was named not because Howells was any kind of biblical creationist, but because of the implication that all modern human variation had developed from a single center of origin. A lack of fossils from many parts of the world, together with inadequate dating for many of those we did have, meant that neither Howells nor I could specify where that center of origin might have been, although we thought we could exclude the European and Middle Eastern territories of the Neanderthals. We both believed that the distinctive shared features of modern humans, such as the high rounded skull, small brows, and chin, implied a recent common origin, as otherwise there would have been much greater differentiation over time. And I started to move away from the then widespread idea that fossils as different-looking as Broken Hill, the Neanderthals, and Cro-Magnon should all be classified with us as variants of our species, Homo sapiens. Initially I agreed with some other workers in differentiating “anatomically modern sapiens” (such as Skhul and Cro-Magnon) from “archaic sapiens” forms such as Neanderthals and Broken Hill. But during the 1980s I increasingly favored limiting the sapiens term to fossils closely resembling us. Moreover, along with a few other heretics, I started to argue that the Neanderthals should be returned to the status granted them by William King in 1864 as a distinct species, Homo neanderthalensis. I also suggested that the Broken Hill skull found in 1921 could be grouped with more primitive European forms (for example, the Heidelberg jaw discovery of 1907) as Homo heidelbergensis.

  As my views on our origins were developing toward a single-origin model, evidence began to accumulate that Africa was especially important in this story. The Omo Kibish find was joined by material from the sites of Border Cave and Klasies River Mouth Caves in South Africa. Moreover, new dating work hinted that Africa was not the backwater in cultural evolution that most considered it to be. Archaeologists such as Desmond Clark and Peter Beaumont argued that it might instead have been leading the way in the sophistication of its stone tools. By 1980 I was privately convinced that Africa was the main center of our evolution but, because of dating uncertainties, I could not rule out the Far East as also playing a role. It took another four years for me to take a strong “Out of Africa” stance publicly, as various lines of evidence started to fit together in my mind.

  However, further confusion was sown by the strong reemergence of Weidenreich’s Multiregional views in 1984. These were given a new lease on life by Milford Wolpoff (United States), Alan Thorne (Australia), and Wu Xinzhi (China). They distanced themselves from Coon’s views by returning to Weidenreich’s emphasis on the importance of gene flow between the geographic lines, considering the continuity in time and space between the various forms of Homo erectus and their regional descendants to be so complete that all of them should be classified with modern people as representing only one species: Homo sapiens. Thus in this model there was no real “origin” for the modern form of Homo sapiens. A feature like the chin might have evolved in a region such as Africa, and spread from there by interbreeding across the human range, followed by selection for it if it was an advantageous characteristic. Another feature such as our high forehead might have developed in, say, China, and then similarly spread from there through interbreeding. Thus modern humans could have inherited their “local” features through continuity with their ancient predecessors, while global characteristics were acquired via a network of interbreeding.

  But new developments in genetics research were about to have a huge impact. In 1982 I became aware of research work on a peculiar type of DNA that is found outside the nucleus of cells, in the mitochondria. These are little bodies that provide the energy for each cell, bodies that probably originated from a once-separate bacterium, which somehow survived being engulfed by a primitive cell. They then coevolved to confer mutual advantage and developed into the mitochondria that most organisms have throughout their cells. In humans, the DNA of a mother’s mitochondria is cloned in her egg when it becomes the first cell of her child, and little or no mitochondrial DNA from the father’s sperm seems to be incorporated at fertilization. This means that mitochondrial DNA (mtDNA) essentially tracks evolution through females only (mothers to daughters), since a son’s mtDNA will not be passed on to his children. This type of DNA mutates at a much faster rate than normal (nuclear) DNA, as we will discuss in chapter 7, allowing the study of short-term evolution. Early work on human mitochondria seemed promising, showing that our species apparently had low diversity and a recent origin, but the geographic patterns seemed unclear as to where that origin might be. By 1986 I had heard through the grapevine that startling new mtDNA results were on the way to publication, and a year later they appeared in the science journal Nature, shaking up arguments about recent human evolution in such a way that things would never be the same again. This seminal publication by Rebecca Cann, Mark Stoneking, and Allan Wilson put modern human origins on the front pages of newspapers, journals, and magazines for the first time.

  Milford Wolpoff, an architect of Multiregionalism, with a Homo erectus skull from Java.

  About 150 types of mtDNA from around the world were investigated, and their variation was determined. Then a computer program was used to connect all the present-day types in an evolutionary tree, with the most economical pattern of evolutionary change (mutations), reconstructing hypothetical ancestors for the living types. In turn, the program connected those ancestors to each other, until a single hypothetical ancestor for all the modern types was created. The distribution of the ancestors implied that the single common ancestor must have lived in Africa, and the number of mutations that had accumulated from the time of the common ancestor suggested that this evolutionary process had taken about 200,000 years. This, then, was the birth of the now-famous Mitochondrial Eve, or “lucky mother,” since the common mitochondrial ancestor must necessarily have been a female. These results seemed to provide strong evidence for a Recent African Origin view for modern humans, since the research suggested that a relatively recent expansion from Africa had occurred, replacing any ancient populations living elsewhere, along with their mtDNA lineages. However, the work was soon heavily criticized. It was shown that the kind of computer program used could actually produce many thousands of trees which were all more or less as economical as the published one, and not all of these alternative trees were rooted in Africa. Moreover, other researchers criticized the calibration of the time when Mitochondrial Eve lived, while yet others questioned the constitution of the modern samples analyzed (for example, many of the “African” samples were actually from African Americans). As a result, multiregionalists were, for a while at least, able to reject these mtDNA results as irrelevant or misleading, arguing that fossil evidence (and their interpretation of it) remained the only valid approach to reconstructing recent human evolution.

  However, the results strongly supported the Recent African Origin view that people like Günter Bräuer (from Hamburg) and I had been developing from the fossils. Günter was less inclined to view Homo sapiens as a newly evolved species, and more inclined to think that hybridization had occurred with people like the Neanderthals, following the dispersal from Africa, but we both welcomed the new mtDNA data. For me, it gave greater confidence that even where the fossil evidence was patchier or more ambiguous, such as the Far East and Australasia, the story of replacement that I had read from the European record probably applied there too.

  In 1987 the archaeologist Paul Mellars and I co-organized an international conference in Cambridge where recent fossil and archaeological results were compared with the new DNA data, and the discussions were electric at times, as experts got to grips with the rapidly changing landscape of recent human evolution. A year later, taking the conference discussions and DNA analyses fully on board, I wrote a review of that emerging picture for the journal Science, with my Natural History Museum colleague Peter Andrew
s. We laid out the two contrasting models of Multiregionalism versus Recent African Origin and what would be expected from the fossil, archaeological, and genetic data if either model was an accurate representation of recent human evolution. (I actually prefer to use the term Recent African Origin [RAO], despite the popularity of Out of Africa, because we know from more ancient fossils that there were earlier human dispersals from Africa. Hence some people distinguish them as Out of Africa 1, Out of Africa 2, et cetera, although we don’t actually know how many there were—and no doubt there were some “into Africa” events as well!)

  Overall, we showed that RAO was best supported, although we recognized that the archaeological record in general and the fossil records of several regions in particular were still not adequate to test the models properly. I was shocked, though, by some of the vitriolic reactions to that paper. Both in the anonymous reviews that some other scientists sent to the journal before publication, and in letters and media comments afterward, scorn was poured on our views and interpretations, a scorn that seemed to extend to personal abuse at times. Relations became strained with a number of scientists, some of whom were people I certainly counted as my friends. Cordiality was eventually restored in most cases, but for a few people, what was seen as an extreme position, in league with the heresy of Mitochondrial Eve, was not easily forgiven or forgotten.

  Two of the architects of the Recent African Origin model, Günter Bräuer (left) and Chris Stringer, pictured in the 1980s.

  As more fossil and, particularly, genetic data emerged to support a recent African origin, what we can term the classic RAO model was developed by a number of researchers, including me, working separately or in collaboration. By the turn of the millennium, this had become the dominant view. Fleshing it out with the consensus view for earlier human evolution, the classic RAO model argued for an African origin of two human species—Homo erectus and Homo sapiens—and perhaps also of Homo heidelbergensis between them (in my view, though, the derivation of heidelbergensis is still unclear). Having evolved from something like the earlier species Homo habilis in Africa nearly 2 million years ago (Ma), Homo erectus dispersed from Africa about 1.7 Ma, in the event commonly known as Out of Africa 1. The species spread to the tropical and subtropical regions of eastern and southeastern Asia, where it may have lingered on, evolved into other forms, or died out. About 1.5 Ma, African erectus developed more advanced stone tools called handaxes, but these did not spread far from Africa until they turned up rather suddenly with the descendant species Homo heidelbergensis in places like southern Europe, and then in Britain, 500,000 to 600,000 years ago.

  So my view was that H. heidelbergensis subsequently underwent an evolutionary split around 300,000 to 400,000 years ago: it began to develop into the Neanderthals in western Eurasia, while the line in Africa had evolved into the ancestors of modern humans by about 130,000 years ago. The origin of modern Homo sapiens must have been a relatively recent and restricted one in Africa, based on marked similarities between recent humans in both body form and DNA, and it may have been quite rapid, in one small favored area such as East Africa. Some modern humans dispersed to the Middle East (Israel) about 100,000 years ago, and they had perhaps moved on as far as Australia by about 60,000 years. However, Homo sapiens did not enter Europe until about 35,000 years ago, following the rapid development of more advanced Later Stone Age tools and complex behaviors by African moderns about 50,000 years ago. Such progress finally allowed the moderns to spread into Europe, where, as Cro-Magnons making Upper Paleolithic tools, they quickly took on and replaced the Neanderthals through their superior technology and adaptations. Bear this narrative in mind, as I will revisit it at various times later in this book.

  If RAO is the most accurate model, regional (“racial”) variation only developed during and after the dispersal from Africa, so any seeming continuity of regional features between Homo erectus and present counterparts in the same regions outside of Africa must have been as a result of parallel evolution or coincidence, rather than of genes passed down from archaic predecessors, as argued in the Multiregional model. Like that model, RAO argued that Homo erectus evolved into new forms of humans in inhabited regions outside of Africa, but in RAO these non-African lineages eventually became extinct, without evolving into modern humans. Some, such as the Neanderthals, must have been replaced by the spread of modern humans into their regions, and hence the RAO model not only is popularly known as Out of Africa but is sometimes also known as the Replacement model.

  As RAO gathered support and influence, it increasingly made an impact on the views of people like the American anthropologists Fred Smith and Erik Trinkaus, who believed in continuity outside of Africa but were not classic multiregionalists. Instead, they advanced what has become known as the Assimilation model, which can be seen as a moderate position between the extremes of RAO and what I have dubbed classic Multiregionalism: one where Africa dominated as the source of modern features, but where these were taken up more gradually by people outside of the continent, through a blending of populations. Modern features thus diffused out of Africa rather than being imposed through the invasion and dominance of dispersing moderns, and early moderns outside of Africa could therefore be expected to show features of the “natives” with whom they were mixing. And while the various models of human evolution were adjusting themselves to the post-mtDNA landscape, the genetic work itself was undergoing reevaluations.

  I already mentioned the heavy criticisms of the 1987 “Eve” paper, from the point of view of the samples used, the methods of analysis, the rate of evolution, and the strong conclusions drawn. The team involved in the original work acknowledged that there were deficiencies, and, over the next few years, they set out to address the problems in a series of further analyses that served only to reinforce their conclusions, as we shall see in chapter 7. But as we shall also observe, most workers now agree that mtDNA, while very useful, is only one small part of the genetic evidence we need to reconstruct our evolutionary origins.

  For the rest of this book, I will mainly be discussing three other human species along with our own: Homo erectus, H. heidelbergensis, and H. neanderthalensis. So how do we recognize distinct human species in the fossil record and our own ancestors? Well, that is not a straightforward question, and specialists will give differing answers. (For example, as I explained earlier, multiregionalists often regard Homo sapiens as the only human species on Earth during the last million years, so species like Homo erectus and Homo heidelbergensis have no real meaning for them.) But for me, there are features in the skeleton that, taken together, can diagnose distinct human species in the past, and that similarly characterize our species today. Because of variations in time and space, these features are rarely absolute, but in combination I think they can distinguish separate evolutionary lines that we can call species, based on their skeletal structure.

  For our own species, Homo sapiens (modern humans), these features include: a large brain volume; neurocranial globularity (the curvature and doming of the bones of the braincase, and its increased height); in rear view a braincase that is wider at the top and narrower at the base; a higher and more evenly arched temporal bone at the side of the braincase; decreased height of the face and its tucking-in under the braincase; a small and divided brow ridge; a narrow area of bone between the eye sockets; increased projection of the middle of the face and nose; a bony chin on the lower jaw, present even in infants; simplification and shrinkage of tooth crowns; a lightly built tympanic bone (this contains the ear bones); a short pubic ramus that is nearly circular in cross section (this is a bone at the front of our pelvis); no iliac pillar (this is a near-vertical ridge of bone reinforcing the pelvis, above the hip socket); and femora (thighbones) that are oval in cross section and thickened most at the front and back.

  In contrast, for Homo erectus, the human species that had appeared in Africa and Asia more than 1.5 million years ago, the characteristics included a small average brain volume; a relativel
y long and low braincase, narrow across the top but broad across the base; a lower and more triangular temporal bone; an angled occipital bone at the back of the skull, with a strong torus (ridge of bone) across it; bony ridges that reinforce the frontal and parietal bones of the braincase; a thick tympanic bone; a strong and continuous supraorbital torus (brow ridge); a strong postorbital constriction (the skull is pinched in behind the brow ridge when viewed from above); a wide area of bone between the eye sockets; a face that juts out from the braincase; a flatter and elongated superior pubic ramus; an iliac pillar; and femora that are rounded and evenly thickened in cross section.

  Homo erectus seems primitive in many respects by the standards of later humans, but it represented a benchmark of change to the human condition in many aspects of its skeleton: a brain size beyond any ape or australopithecine, a human face with projecting nasal bones, small teeth, a humanlike posture for the skull, and a body frame of human rather than apelike proportions. The evolutionary biologists Dennis Bramble and Daniel Lieberman believe that erectus had made a fundamental transition to life in the open, first scavenging and then hunting over long distances. We are unique among primates in our capacity for endurance running, which may first have evolved to allow humans to get to carcasses for scavenging ahead of the competition. And people like the San today are able to gradually wear down their prey through persistent pursuit: ungulates, for example, can run much faster than humans over short distances but completely exhaust themselves over long distances, at which point they are easy to dispatch. Features of the erectus (and later human) skeleton in body shape, legs, ankles and feet, head balance and stability, and our reliance on sweating to thermoregulate could all be relics of an early adaptation to sustained running, according to Bramble and Lieberman.