Lone Survivors (39 page)

However, as I discuss in the last chapter, some of our “special features” and our differences from the Neanderthals might be there purely by chance, through the process of drift. And what if the Earth's climate had been subtly different over the last 200,000 years, or a volcano the size of Toba had gone off in Africa rather than Sumatra? What if those early modern humans had died out in Africa, and Neanderthals had managed to thrive in some part of southern Eurasia, building their population densities until they too fully—rather than partly—accomplished their own “Human Revolution”? Would they have eventually looked back at their success, at the benefits of the Eurasian environment, and the problems of surviving in Africa for their failed relatives, the ones with the weird foreheads? And would they have credited their big brains and occipital lobes, projecting noses, wide hips, ginger hair (sometimes), and the ancient use of black pigments for their evolutionary success, and used those to define a Neanderthal version of “modernity”? Even more remarkably, what if humanlike creatures had died out completely everywhere other than Flores, and the future of “humanity” then lay with the tiny Hobbit? Could it have survived and eventually spread from there to continue its own evolutionary story, and would its descendants have eventually pondered over an “Out of Flores” model?

We have traveled far and wide in this book, over millions of years and millions of square kilometers of the Earth's surface. Now it is time to take a hard and critical look at what we have discussed, to look at the past and future evolution of our species. Writing this book has been both a pleasure and a challenge for me, as I have had to revisit so many ideas of mine and those of colleagues. Along the way I hope that I have changed some of your perceptions of what it is to be human, just as I have changed mine.

9

The Past and Future Evolution of Our Species

As the study of modern human origins has blossomed over the last forty years, I have been privileged to be involved in several lines of accumulating evidence—fossil, chronological, archaeological, and genetic—that our species had a recent African origin. And yet, as has become obvious, there are many loose ends to be tied up in our early history, and, more than that, there are still many fundamental questions that remain unanswered. We have partial answers to the “when” and “where” of our origins, but still precious little about the “whys.” The basic reshaping of skull form to the “modern” pattern had occurred in Africa by 150,000 years ago, but the underlying factors remain largely unknown. Some may well have related to changes in the underlying brain shape, and others to quite different functional factors connected to our jaws, teeth, head balance, or vocal or respiratory tracts. However, there is an alternative and more mundane explanation for many of our “special” cranial features, which suggests that they were not really special at all—they were caused by genetic drift, essentially by chance.

I collaborated in research with two evolutionary anthropologists, Tim Weaver and Charles Roseman, and I can't do better than adapt a summary of our paper.

We used a variety of statistical tests from quantitative- and population-genetic theory to show that genetic drift can explain cranial differences between Neanderthals and modern humans. The tests were based on thirty-seven standard cranial measurements from about 2,500 modern humans from thirty populations, and twenty Neanderthal fossils. As a further test we compared our results with those for a genetic dataset consisting of 377 microsatellites typed from 1,056 modern humans from fifty-two populations. We concluded that rather than requiring special adaptive accounts, Neanderthal and modern human crania may simply represent two outcomes from a vast space of random evolutionary possibilities.

In fact the differences between modern human crania are fairly marked, despite our close genetic relationship, which is why forensic tests of regionality work so well, and as we saw, much of this could be the result of an accumulation of differences through drift, as modern humans spread out in small numbers. The variations we all show when compared with Neanderthals are even more marked, and as Erik Trinkaus pointed out, we diverged farther away from a potential ancestor like
heidelbergensis
than the Neanderthals did. Nevertheless, many of the contrasts may be the result of the same process writ large, the consequences of a geographic separation from our Neanderthal cousins about 400,000 years ago, after which we drifted—rather than were driven—apart. And it is possible, as Weaver suggested, that primates and earlier humans were much more constrained by natural selection to keep to a given skull form, whereas we (and perhaps, to an extent, Neanderthals) were able to escape those constraints through our cultural adaptations, allowing our cranial shapes to drift more randomly.

There does seem to be something different about modern human “cultures” when compared with those that came before. Ours seem to vary much more, and over much shorter periods, than those of the Middle Paleolithic, and even more so when measured against the limited variety of the Lower Paleolithic, which has been likened to 2 million years of boredom. There are probably many different reasons behind our cultural diversity; one is the sheer geographic range of modern humans and the variety of environments in which we live, and to which our ways of life are adapted. And yet the Upper Paleolithic people of Europe who lived in one region during one, albeit fluctuating, cold stage also showed great varieties and changes from one “culture” to another every few millennia. The anthropologists Robert Boyd and Peter Richerson argue that a key feature of
Homo sapiens
is our ability to imitate, and to learn from, each other—a trait that is there in the youngest of our children. And human societies give us the arena in which to rehearse, test, and modify what we learn through dialogue with our peers; as we saw, the denser the networks of connectivity and the wider the reach of those networks, the better. Boyd and Richerson also argue that with the rapid changes of environment suffered by humans in the past through climate change (or equally, I would add, the rapid dispersal of small numbers of modern humans to new and unfamiliar landscapes), there was no benefit in individuals separately trying to innovate their way to survival, and certainly no use waiting for evolutionary adaptations to come along—there simply would not be time.

Surprisingly, the best strategy for the average person in a variable environment might be to look around and rely on imitation, rather than individual learning. Some people might discover ways of coping with the changes, and, by imitation, this would provide a shortcut to success for the many. Moreover, the more copiers there were, the more chance that one of them might not copy accurately and by chance produce an accidental improvement on the original. If individual innovation was rare, progress would be slow—hence, perhaps, the 2 million years of boredom of the Lower Paleolithic. But if there were more innovators, the process could produce rapid cultural adaptations very quickly compared with individuals innovating in isolation, or with biological evolution. Thus through imitation and peer-group feedback, populations could adapt well beyond the abilities of an isolated genius, whose ideas might never get beyond his or her cave or might be lost through a sudden death. Boyd and Richerson contend that evolution tracked these developing tendencies for social learning. Thus selection would have favored minds that, rather than always attempting to invent something new (a risky and wasteful strategy overall), would have tended to conform to the majority behavior.

Evolution would have particularly favored a tendency to imitate high-status individuals in the group, or successful peers, and we can see how this could have led to group identities being symbolically displayed, and to the growth of fads and fashions, useful or not.
Style
is used both by archaeologists to label artifacts in past cultures and by people in the fashion industry today, and it may well explain a lot of the puzzling variations that rapidly come and go in the Upper Paleolithic record. Boyd and Richerson also looked at recent hunter-gatherer groups and how they have managed to occupy such an astonishing range of habitats. When we look at such groups in, say, the rain forests of Brazil or New Guinea, what is astonishing is their diversity there too, and the way they are often split into groups with mutually unintelligible languages, distinct traditions, and symbolically differentiated identities. Imitation-based cultures give these groups the capacity to behave somewhat differently from each other, and therefore the potential to track and exploit separate parts of their habitat—in a sense, to behave like different species in the way that they partition their use of the environment.

So this is the other side of cultural evolution, and there must always have been a tension in modern humans between the need to maintain and build on innovations, which, as we saw, requires high densities and connections between people (fusion), and the tendency of populations to go their own way and become increasingly culturally subdivided (fission). These alternative trajectories would have different genetic consequences too, given the need to maintain a healthy gene pool through partner exchanges with the neighbors (unless these were achieved through raids and kidnaps, which do seem to be surprisingly common, considering the “peaceful” image we may have for hunter-gatherers).

My fellow Neanderthal researcher Jean-Jacques Hublin worked with the evolutionary anthropologist Luke Premo to model the effect of culture in directing gene flow in humans—modern ones, certainly, but perhaps also the Neanderthals. As we saw in chapter 7, one of the striking things about both Neanderthals and
Homo sapiens
is their low genetic diversity compared with our primate relatives, and the implication that this must derive from low population numbers (bottlenecks) in our respective evolutionary pasts. But Hublin and Premo's modeling suggested that long-term culturally channeled migration—that is, between population subdivisions sharing cultural values—would have acted to suppress the effective population size of Pleistocene humans, just as bottlenecks do. We already saw that some modern human populations in Africa such as the Bushman and central African “pygmies” have deep and seemingly separate roots back to the time of the Middle Stone Age for some genetic markers, and this suggests that 100,000 years ago Africa may have comprised a collection of separate subgroups who predominantly exchanged genes internally, rather than across a single continuous population—perhaps because of geographic isolation, but perhaps also because they were subdivided by cultural distinctions and hence genetically relatively “inbred.” This is something to which I will return later in this chapter, after further consideration of hybridization between early
Homo sapiens
and other human species such as the Neanderthals and
Homo erectus
.

As discussed in chapter 1, my view of
Homo sapiens
and the Neanderthals as representing distinct species has never led me to say that interbreeding between them was impossible. My old friend Erik Trinkaus is convinced that this would actually have been the norm, rather than the exception, as modern humans spread from Africa; as he succinctly puts it, “sex happens.” He sees widespread evidence for it from quirky anatomical features in early modern fossils ranging from Portugal to China which, according to him, most likely derived from such intermixture. While certainly not ruling this out everywhere, I see the evidence differently and consider that many of these features were probably part of the normal variation of early
Homo sapiens
, rather than necessarily emanating from interbreeding with our archaic relatives.

Hybridization is also known as
reticulation
—the netlike pattern produced by gene flow between evolutionarily separate lineages—and in mammals it is apparent at several levels of the taxonomic hierarchy. Most often it occurs between closely related species (for example, monkeys within the widespread genus
Cercopithecus
), but sometimes it can occur between different genera (such as African and Indian elephants), and occasionally between much more distantly related forms (for example, through artificial insemination, between the Asian camel and the South American guanaco). Sometimes hybridization has only been detected through genetic studies of seemingly “pure” species, revealing that they had exchanged genes at some time in their evolutionary past, and its impact on the populations concerned may range from damaging (for example, where the hybrids are deformed or infertile) through to advantageous (for example, the rarer outcome of “hybrid vigor,” or heterosis). As these examples show, they undermine, or at least modify, the concept of a biological species as genetically self-contained and watertight from extraneous gene flow, and they emphasize the fluidity of species concepts, which are, after all, humanly created approximations of reality in the natural world.

Many studies have examined the human genome for signs of interbreeding with our archaic relatives, and as we saw in chapter 7 this has even been investigated through the DNA of our head lice. Up to now, the big picture, from our autosomal, mitochondrial, and Y-chromosome DNA, has generally lacked signs of introgression from other human species, although scientists such as John Relethford, Vinayak Eswaran, Henry Harpending, and Alan Templeton have argued that indications were indeed there. Short branches in our gene trees, particularly in Y-DNA and mtDNA, pointed to a simple, recent African origin, and simulations from mtDNA data of the level of possible Neanderthal and Cro-Magnon admixture suggested that it was either zero or very close to zero. However, despite the fact that mtDNA and Y-DNA provide such clear genealogical signals, they constitute only about 1 percent of our total DNA, and signs of hybridization were clearly lurking in the rest of our genome. Inconsistencies and exceptions to the simple patterns shown in Y-DNA and mtDNA now suggest a more complex evolutionary history for our species. This is an area of exciting current research, particularly with the arrival of Neanderthal and Denisovan genomes for comparison with the growing number of modern ones from all over the world.