Lone Survivors

by Chris Stringer

  We still lack the amount of genetic data for African populations that we have for people from Europe and North America, but Africa is beginning to catch up. Charla Lambert and Sarah Tishkoff analyzed thousands of samples to reveal several deep and ancient population clusters, and, as we saw, Michael Hammer and his colleagues found evidence of archaic genes in three samples of moderns, but especially in West Africans. Now they have taken this work further by analyzing about half a million bits of genetic coding in samples of Mandenka (Senegal), Biaka pygmies (Central African Republic), and San (Namibia). They found strong evidence for split times of more than 100,000 years, predating the exodus from Africa, and they detected evidence of ancient admixture (with unknown “archaic” human groups) in both the Biaka and San. Philipp Gunz and his colleagues also recognized this from the great variability they found in late archaic/early modern African crania, as this edited extract of their conclusions shows.

  Our fossil AMH [anatomically modern human] data suggest that before there was isolation by distance [=drift] from Africa, there already existed (at least temporally) isolation by distance within Africa. Seemingly ancient contributions to the modern human gene pool have been explained by admixture with archaic forms of Homo, e.g. Neanderthals. Although we cannot rule out such admixture, the proposed ancestral population structure of early AMH suggests another underestimated possibility: the genetic exchange between subdivided populations of early AMH as a potential source for “ancient” contributions to the modern human gene pool. Any model consistent with our data requires a more dynamic scenario and a more complex population structure than the one implied by the classic Out of Africa model. Our findings on neurocranial shape diversity are consistent with the assumption that intra-African population expansions produced temporarily subdivided and isolated groups. Separated demes (population subdivisions) might have partly merged again, whereas others left Africa at different times and maybe using different routes, and still others probably also remigrated to Africa.

  However, there is still one significant loose end to be tied up, and that is the size of the human group that left Africa and founded the populations of the rest of the world. The lower genetic variation of peoples outside of Africa indicates that this population was only a small subsample of its parent group, and some calculations from mtDNA have suggested it could have been as low as a few hundred individuals. Yet it could not have been too sparse, or it would not have been able to contain the variation that, as I just argued, included archaic genes picked up in Africa.

  There are also some clues that the Out of Africa dispersal was more complicated than a single exodus. The possibility that there were two exits separated by a significant period of time is not consistent with the tight ancestral patterns of both mitochondrial and Y-DNA, unless the earlier dispersal (perhaps represented by samples like those from Skhul and Qafzeh) is now only represented in autosomal DNA, with the more ancient DNA from their mitochondria and Y-chromosomes being completely replaced during the later dispersal. But the possibility of two dispersals closer in time to each other seems to be supported by some intriguing interpretations of data from the X-chromosome, where the single X in males is inherited from the mother, whereas the two Xs in females are inherited one from each parent. Because of this sex-based asymmetry in the inheritance of X-chromosomes, it is possible to garner data on the relative sizes of ancestral populations of males and females. Given typical one-to-one pair bonding, the expectation would be that there were equal numbers of male and female forebears, whereas a pattern of polygamous mating could produce a surfeit of female ancestors. Some genetic data do suggest that a polygamous mating pattern of males either having harems or engaging in serial polygamy may have characterized our evolution in the longer term, but at the time of the Out of Africa exit something quite different may have happened.

  If there are equal numbers of males and females in a population, there will be three X-chromosomes for every four autosomal chromosomes, since, as we saw, males carry only one X. Genetic drift (the chance process of change in gene proportions that operates most markedly in small populations) should thus occur more strongly on the X-chromosome than on the other chromosomes, in the expected ratio of 4:3. Using a database of over 130,000 SNPs in the chromosomes of people from West Africa, Europe, and East Asia, the geneticists Alon Keinan, David Reich, and their colleagues calculated the actual amount of genetic drift that had occurred in the different populations. The West African sample (which should better represent the population structure within Africa before the exodus) met the expectations of approximately equal numbers of male and female ancestors. But the ratios in the non-African samples suggested accentuated drift in their X-chromosome DNA, indicating a smaller number of mothers than fathers around the time of the Out of Africa dispersal. Thus males must have dominated the population structure during or immediately after the exodus from Africa, probably across a time span of thousands of years.

  If Keinan and Reich were correct, something decidedly odd must have happened about 55,000 years ago in northeast Africa or the adjoining Levant or Arabia—but what? There are several possible explanations, but their favored one is that at least two sequential founding populations were involved in the African exodus. The first provided the female representation, and thus the ancestral mtDNA of non-Africans (haplogroup L3 or its first M and N descendants). But the males in that first wave were somehow outreproduced or even replaced by new males, in one or several succeeding waves of dispersal, thus increasing the total male representation compared with females, and by inference involving a replacement of the original Y-chromosomes. One possibility is that subsequent bands of men violently replaced the original males and then mated with their females; if so, I wonder whether this might mark the arrival of projectile points in the region (see chapter 6). Alternatively could the process have been more gradual, with the new men holding a reproductive advantage over the original males—perhaps the development of a high-status elite or the export of a powerful new religion with male shamans, who gained privileged sexual access to the women?

  These are just speculations, of course, and Keinan and Reich’s scenario needs confirmation from further genetic analyses, but it will be interesting to see if new data emerge to support or disprove these conjectures. The possibility of multiple small bands of modern humans leaving Africa does increase the likelihood that there were enough people to carry some of those archaic African genes within them, in which case we do not always have to invoke the impact of hybridization with species outside of Africa to explain their presence. But equally, there clearly were at least occasional episodes of hybridization outside of Africa, with (in decreasing order of probability and increasing evolutionary distance) Neanderthals and other descendants of heidelbergensis (Denisovans?), surviving erectus in southeast Asia, and perhaps even (given the vagaries of human behavior) Hobbits in Flores!

  So where does this added complexity and evidence of interbreeding with Neanderthals and Denisovans leave my favored Recent African Origin model (RAO)? Has it been disproved, in favor of the Assimilation or Multiregional models, as some have claimed? I don’t think so, and to give a better perspective on this, I think we should revisit some of my first discussions about the early days of RAO, using a diagram (see p. 266). This compares different models of recent human evolution in terms of the extent of an African vs. non-African genetic contribution to present-day humans worldwide. On the left we have a pure Recent African Origin, with the total replacement of non-African genes, and at the other extreme we can envisage models that give Africa no place in the evolution of modern humans. (In chapters 1 and 3 I mention Clark Howell’s proposal for a Middle East center of origin, and Christy Turner’s for southeast Asia.) In between the extremes we can position “mostly Out of Africa” models like Günter Bräuer’s toward the left-hand side and classic Multiregionalism (which gives Africa no special role—see below) toward the right. Somewhere near the center is Fred Smith and Erik Trinkaus’s Assimilation
model. If archaic gene flow was the rule rather than the exception, there could have been as much as a 50 percent non-African genetic input; however, depending on the extent of archaic gene flow envisaged, the Assimilation model could approach either Günter’s Out of Africa + Hybridization model or classic Multiregionalism.

  Back in 1970, no scientists held the view that Africa was the evolutionary home of modern humans; the region was considered backward and largely irrelevant, with the pendulum of scientific opinion strongly swinging toward non-African, Neanderthal Phase, or Multiregional models. Twenty years later, the pendulum was starting to move in favor of RAO, as fossil evidence began to be increasingly reinforced by the clear signals of mitochondrial and Y-DNA. The pendulum swung even farther toward a pure RAO with growing fossil, archaeological, and genetic data, including the distinctiveness of the first Neanderthal DNA sequences recovered in the late 1990s.

  Now, the advent of huge amounts of autosomal DNA data, including the Neanderthal and Denisova genomes, has halted and even reversed that pendulum swing, away from an absolute RAO, and I would say we are looking at an RAO model that most resembles Bräuer’s early formulation (Out of Africa + Hybridization) or a version of the Assimilation model of Smith and Trinkaus. If the evidence for archaic assimilation remains modest and restricted to Africa and the dispersal phase of modern humans from Africa, constituting less than 10 percent of our genome, I think “mostly Out of Africa” is the appropriate designation—and, for me, that is still RAO. I would have been delighted with that level of support for an African origin for Homo sapiens during the fierce arguments of the 1990s, when opponents still claimed Africa had no special role, so I’m more than happy with it now. And of course RAO is not just about the African origins of our shared modern morphology and most of our genes; it is also about the origins of our shared patterns of modern behavior.

  Given that interbreeding seemingly did happen between modern and archaic humans, both in and out of Africa, does this mean that we should now abandon the different species names and lump all the fossils of the last million years or more as Homo sapiens, as some suggest? I think that if the hybridization events prove to have been widespread in time and space, we may well have to do that, but I don’t think we are at that point yet. There are still good scientific reasons to give populations that had long and (relatively) separate evolutionary histories different names—species or otherwise. As we saw, we can measure the amount of morphological variation in primate species today, and then compare it with the differences between, say, Homo erectus and heidelbergensis skulls, or between those of Neanderthals and moderns. This shows they are distinct enough to be classed as different species, whether or not they meet the biological species criterion of no interbreeding (a standard that numerous recognized primate species today do not achieve).

  The “pendulum” indicating where we evolved has regularly shifted over the last century.

  But if we nevertheless proceed to merge even Neanderthals and modern humans, we end up with a Homo sapiens characterized by, for example, a high and rounded skull, and a long and low skull; by no continuous brow ridge, and a strong continuous brow ridge; by a well-developed chin even in infants, and no chin; by no suprainiac fossa in adults, and a suprainiac fossa in adults; by an inner ear of modern shape, and an inner ear of Neanderthal shape; by a narrow pelvis with a short thick superior pubic ramus, and a wide pelvis with a long thin superior pubic ramus—and so on. The disparate nature of Homo sapiens would become even more extreme if we started to add in the features of species like heidelbergensis, antecessor, and erectus.

  The merging of sapiens and erectus has been regularly proposed by champions of Multiregionalism, and if our modern genes do come from more than one region, why hasn’t the Multiregional theory been proved correct, as its remaining supporters are now asserting? To deal with that claim, I think it is worth reminding ourselves what classic Multiregionalism actually proposed. Here is a quotation from a paper written in 1994 by Milford Wolpoff and four other prominent advocates of the model at that time.

  The evolutionary patterns of three different regions show that the earliest “modern” humans are not Africans and do not have the complex of features that characterize the Africans of that time or any other … There is no evidence of specific admixture with Africans at any time, let alone replacement by them … There is indisputable evidence for the continuity of distinct unique combinations of skeletal features in different regions, connecting the earliest human populations with recent and living peoples.

  This model gave Africa no special place in our evolution and claimed specific connections in individual features between Homo erectus fossils more than a million years old in each region and humans in the same regions today. Hopefully the evidence I have presented in this book shows that these particular views have been pretty comprehensively shown to be false.

  But still, if we do have significant components of archaic genes in our makeup, why have these not featured strongly in genetic analyses until now? I mentioned there were hints of them from time to time, but the reality is that if our archaic component represents only about 5 percent of our genome, then ninety-five times out of a hundred this would not show up in individual genetic marker studies, just as it did not register in our mtDNA and Y-chromosomes. It has taken much larger genome sweeps, and comparisons with actual archaic genomes, to show up the signs of ancient hybridization.

  The big picture is that we are predominantly of recent African origin, so is there a special reason for this? Overall, I think that the preeminence of Africa in the story of modern human origins was a question of its larger geographic and human population size, which gave greater opportunities for morphological and behavioral variations, and for innovations to develop and be conserved, rather than the result of a special evolutionary pathway. “Modernity” was not a package that had a unique African origin in one time, place, and population, but was a composite whose elements appeared at different times and places and were then gradually assembled to assume the form we recognize today. But if that was so, is the assembly over, and has the evolution of modern humans finally ended? Are we the finished product and in control of our destiny, or are many of the same processes that operated in our past continuing to affect us now and in the future? The scientist and author Stephen Jay Gould’s view was clear: “There’s been no biological change in humans in 40,000 or 50,000 years. Everything we call culture and civilization we’ve built with the same body and brain.”

  When I give public lectures, I am invariably asked where evolution will lead us, and what humans will look like in the future, and equally invariably I try to avoid answering such tricky questions. I have, though, taken a different view in public from Gould and my geneticist friend Steve Jones over whether human evolution is over. Jones suggests that modern culture and its benefits like medical care removed the power of natural selection to affect humans, since virtually everyone now reaches reproductive age. I disagree because, first, changes in our genome are occurring all the time, whether we can detect them or not; some calculations suggest that each of us could have about fifty new mutations compared with our parents’ DNA. Second, life in the developed world has its own differential costs in terms of reproduction and health, with the general availability of contraception, but also of junk food, alcohol, and drugs. Third, and even more significant, at least a quarter of the world’s population is still denied the benefits of decent health care and the necessities of healthy living conditions and diets. Thus selection is operating strongly on those billions of people, and I cannot see that stopping any time soon. From my perspective, evolution is certainly still working away on Homo sapiens, and there is even evidence that its effects have accelerated rather than diminished over the last 10,000 years, as we will see below.

  Science fiction images of humans of the future often show us with huge brains but, as we saw, big brains are not necessarily the best brains—witness the extinct Neanderthals—and if anything our brains
have actually shrunk in size over the last 20,000 years. In practical terms, unless the process of birth is bypassed, our brain size is already at the limit at which the female pelvis can cope with delivery. Then there is the sheer cost in energy of running a big brain, and the evidence that larger brains are not necessarily as efficient at some tasks. And anyway, so much of our memorizing and thinking is done externally now—in the brains of other people or in the processors of our computers. All of these factors could be responsible for our shrinking brains, as well as more mundane factors like an overall reduction of body size compared with our Paleolithic ancestors.

  More realistically for our future evolution, there is the prospect of genetic engineering, which is already happening on a small scale. Genetic counseling is available to advise potential parents about harmful DNA mutations that could be passed on to their children, and to give them the choice about whether to proceed. As this becomes more common and wider in its reach, future gene pools will be affected. Even more ambitiously, gene therapy could be applied to a faulty organ in the body, and germ-line therapy could plant a permanent change in the genome of an unborn baby. There are formidable ethical questions to be addressed here, not to mention the scientific ones. For example, we know that the actions of genes are often interrelated, and that a single gene may perform more than one function. So great care would be needed to ensure that the targeted change in DNA achieved only what was intended. And the social consequences of even giving people the simple choice of a male or a female child are enormous, let alone providing opportunities to enhance that child’s beauty, talents, or intelligence.