Masters of the Planet

by Ian Tattersall

  The earliest modern human fossil known from outside Africa, this skull was found at the Israeli site of Jebel Qafzeh. Known as Qafzeh 9, it is anatomically a standard modern human. However, other hominid fossils from the same site do not have typically modern skull anatomy, and all the Qafzeh hominids are associated with a Mousterian industry similar to made by Neanderthals in the same region. Drawing by Don McGranaghan.

  The Qafzeh hominids are often spoken of in the same breath as those from the rock shelter of Mugharet-es-Skhl, a burial locality a few dozen miles away on the western slope of Mount Carmel, overlooking the Mediterranean. Excavations at Skhl produced the remains of ten adults and juveniles, probably around 100 thousand years old. These fossils present a more uniform physical aspect than those from Qafzeh, but they are no less odd for that. Like modern humans they have high, rounded cranial vaults that held impressively large brains of some 1,450 to 1,590 cc; but, unlike ours, the fairly heavily built faces of the Skhl hominids were unretracted, jutting proudly afore the vault and topped by a transverse bar of bone instead of a vertical forehead. The scientists who described these fossils before World War II were perplexed by this curious morphology, to such an extent that although they wrote a very large volume on the subject, what they actually concluded, if anything, about the identity of the Skhl hominids remained spectacularly obscure.

  Cranium V from the site of Skhl, in Israel. Now thought to be well over 100 thousand years old, the Skhl fossils have for long been assigned to Homo sapiens, but they are actually quite distinctive in morphology. Drawing by Don McGranaghan.

  One possibility, of course, was that they represented a hybrid population between moderns and Neanderthals. Geographically this would make sense, for the Skhl locality is only a few minutes’ easy stroll from the cave of Tabn, long occupied by Neanderthals. Indeed, Neanderthals seem to have been in residence there close to the time that the Skhl burials were made, although there is no independent reason to believe that they were around at exactly the same moment. Biologically, though, the story is rather different: Neanderthals and moderns were built on a fundamentally different plan; and although we really have no idea at all what a modern/Neanderthal hybrid should look like, we do know that hybrids tend to exhibit traits of both parental populations. And this is certainly not what we are seeing at Skhl.

  As a result of this equivocal anatomy, paleoanthropologists have typically yielded in recent years to the temptation to avoid the vexing issue of who these hominids actually were, by brushing them under that “archaic Homo sapiens” rug. But this is really evading the issue, and there are other possibilities. One is, of course, that the Qafzeh/Skhl hominids represent an entirely distinct lineage of hominids of which we otherwise have no record. But a more intriguing possibility is that their curious anatomy was the result not of interbreeding between modern Homo sapiens and Neanderthals, but of interbreeding between moderns and the descendants of the North African Aterians exemplified by the Jebel Irhoud and Dar-es-Soltan fossils. We have no idea what we ought to expect from a hybrid between these populations; but somehow this pairing looks like a much more plausible combination, and with pretty recent common African origins the two were presumably very closely related. The prevailing environmental conditions also were right, as was the timing. It is quite possible that, during the wet period around 120 thousand years ago, Aterians spread eastward across North Africa and then turned north to cross a relatively hospitable Sinai Peninsula into the Levant, while at the same time sub-Saharan Africans were moving directly north along the Nile Corridor, dog-legging thence up into Israel. At some point, the two kinds of hominid would have encountered each other, and might have successfully interbred despite their physical differences. Certainly, they were a lot less differentiated from each other than the moderns were from Neanderthals. Precisely how Mousterian methods of stoneworking came to be adopted in the new land remains obscure, though they were not greatly different in concept from either ancestral toolmaking manner, and various North African stone tool assemblages have at one time or another been identified as “Mousterian.” As DNA technology improves, perhaps a way will become available of testing the many complex scenarios that potentially arise from this particular possibility of intermixing.

  Meanwhile, the presence of a couple of thoroughly anatomically modern Homo sapiens at Jebel Qafzeh would, on the face of it, reinforce the notion of a mixed—or rather, newly mixing—population, while at the same time demonstrating a local association between anatomically modern hominids and the Mousterian. Whoever the Qafzeh moderns were, they were not demonstrably behaving in any significantly different manner from Neanderthals. The same thing goes for Skhl, where the lithic context is also definitely Mousterian. But at Skhl the picture may be complicated a bit by the recently reported presence both of pigments and of shells that were apparently pierced for stringing. We will come back to this in a moment; meanwhile, what is clear is that the early foray of anatomical moderns outside of Africa, whatever exact form it took, was ultimately unsuccessful. By 60 thousand years ago Neanderthals seem to have been back in charge of the Levant, and we see no more evidence of Homo sapiens in the region until much later, by which time our species had contrived to establish the cognitive and technological superiority that it had evidently lacked earlier.

  Altogether then, in the light of the frustratingly little we know, it seems reasonable to see the initial excursion of anatomical Homo sapiens out of Africa and into the neighboring Levant as the fortuitous product of circumstance, facilitated or even spurred by a benevolent change in climate. Later on, the new people beat a retreat back into their native continent (or more probably died out on the spot), quite likely driven by a climatic deterioration, for we know that conditions became extremely arid during that cold snap about 60 thousand years ago. This drying event severely hit those original Aterian populations in the Sahara, too; and by around 40 thousand years ago this culture was limited to a few lingering outposts along the Mediterranean coast. But whatever the ultimate identity and fate of the Aterians, and despite those intriguing fossils at Qafzeh and those tantalizing hints from Skhl, we have no evidence that Homo sapiens managed to launch a widely successful invasion of Eurasia until much later in time.

  THE MOLECULAR EVIDENCE

  The notion of an initial ancient foray out of Africa by behaviorally archaic Homo sapiens, who later found themselves confined again to their natal continent, fits well with the conclusions of molecular anthropologists. Extensive comparisons of DNA data sets obtained from living human populations from all over the world suggest an origin of our species somewhere in the African continent (most likely somewhere in its eastern or southwestern regions). That founding group subsequently expanded south, north, and west to populate the rest of its home continent, and ultimately Eurasia and the world. With this spread came population expansion and local diversification; and within the African continent at least 14 distinct modern lineages descended from the ancestral population have been identified, each with its own variants. This represents a degree of genetic diversity which, compared to data from the rest of the world, suggests on its own that humans have been evolving in Africa for longer than they have been elsewhere. But to foreclose any argument, all of the major genetic lineages found in other parts of the world are best interpreted as diversified subsets of the variety found in Africa, again pointing toward an African origin for our species. Interestingly, molecular researchers have found that their conclusions are also broadly supported by linguistic and cultural divisions, despite the fact that cultural innovations (because they can be transmitted laterally within the same generation) are subject to weaker constraints than those controlling the spread of biological innovations.

  Another set of molecular studies has concluded not only that the founding population was African, but that it was very small. It turns out that, for all the structured DNA variety we find in human populations, this variety is not very impressive when we compare it to what we see in other species, e
ven close relatives. A single population of chimpanzees in West Africa, for example, is said to show more diversity in its mtDNA than the entire human species does today. This can mean one of two things, or both: that our species itself has a recent origin, hence has not had a very long time in which to diversify; or that the founding population was very small. In the event, both of these factors appear to have played a role. Homo sapiens seems to have separated from its (now extinct) closest relative only about a tenth as long ago as the two surviving chimpanzee species appear to have split. And, although we don’t know what extinct relatives the chimpanzees might have had, it’s clear that by general mammalian standards Homo sapiens is a very young species. But that’s not all. Close analysis of the way in which human DNA variants are distributed today also reveals a pattern strongly suggesting that the ancient human population passed through one or more bottlenecks, or severe contractions, over the course of the late Pleistocene. The most significant of these bottlenecks plausibly occurred around the time at which both archaeological and paleontological indicators imply that people who were both anatomically and intellectually modern first left Africa, ultimately to populate the world.

  The dates and duration of the bottleneck vary a bit depending on which of the available data you are looking at; but broadly this event appears to have taken place at some time between 75 thousand and 60 thousand years ago. I include the earlier date mainly because of one scenario that points to a hugely dramatic environmental cause as the main culprit: the explosive eruption of the Indonesian volcano Mount Toba. Around 73.5 thousand years ago, Toba was blasted apart by what was certainly one of the largest and most violent volcanic eruptions in recent geological history. This event devastated the local area, and millions of tons of fine ash were thrown into the air in a cloud that possibly lasted for years, blocking incoming sunlight and causing a “volcanic winter” that affected regions throughout the Old World. It’s also been argued that, in combination with the effects of a subsequent drop in world temperatures at the beginning of MIS 4 about 71 thousand years ago, this winterizing effect would have contributed to a dramatic decrease in hominid populations, including that of the nascent Homo sapiens in Africa. And while many doubt that Toba’s antics, destructive as they doubtless were, would have had effects quite as far-flung as this scenario implies, what is almost certain is that the cold MIS 4 (about 71 to 60 thousand years ago) took its toll on hominid populations everywhere in the Old World.

  In Africa the onset of this harsh spell ushered in the extended period of drought that expelled the Aterians from the Sahara, and there’s no doubt that it severely afflicted other hominid populations too. As we’ve seen, the kind of environmental disruption caused by this climatic deterioration is just the kind of thing that promotes response in small, fragmented populations. And it is more than plausible that one local African population of Homo sapiens, emerging from this environmental trial as fully symbolic, went on to populate the world. For the first stirrings of the symbolic spirit were already visible well before the stresses of MIS 4 took hold.

  To complete the picture of the emergence of the human species and its occupation of the world, molecular anthropologists have been able to draw in the routes by which humans colonized the globe by studying the distributions of various DNA markers in diverse populations. Allowing for the different data sets used (e.g., mtDNA, Y-chromosomes, and various nuclear DNA markers), they have been able to do this with remarkable precision, and to a very fine level of historical detail. The fact that males turn out to have had different migration histories from females complicates things; and although it is actually quite understandable given the typical differences between the human sexes in social and economic roles, it nevertheless confuses the histories of populations as wholes. When it comes to Homo sapiens, it seems, nothing is simple. Despite all the complications, though, the various molecular scenarios fit reasonably well with what we know of the fossil record, sketchy as it is.

  Apart from those early and unsophisticated émigrés in Israel, we do not have any clear Homo sapiens fossils from anywhere outside Africa earlier than the molecular evidence suggests we should find them. A fragmentary mandible some 100 thousand years old from the cave of Zhirendong in southern China has recently been touted as that of a Homo sapiens; but its features actually group it most plainly with the endemic “Peking Man” Homo erectus, rather than with any potential early modern invaders. Broadly, the molecules indicate that in the period following about 60 thousand years ago, as the rigors of MIS 4 were giving way to the kinder conditions of MIS 3, the bearers of several African DNA lineages left the parent continent. The first principal migration was via Asia Minor into India, whence a coastal route was followed into southeast Asia. All this happened quickly: as we know from archaeological evidence, humans were in Australia by at least 50 thousand years ago. This is all the more remarkable because the first Australians must have crossed at least 50 miles of open ocean to reach their new home: a feat that would have required not only boats—or at least, sophisticated rafts—but excellent navigational skills as well.

  Meanwhile, one branch of the migrants continued down into southeast Asia, and another went north to colonize China and Mongolia, eventually doubling back into Central Asia. Migrants of African origin reached Europe, presumably via Asia Minor, by about 40 thousand years ago; and even as climatic conditions were descending to the trough of the last glacial period some 21 thousand years ago, modern people had ventured as far as northern Siberia, above the Arctic Circle. The extraordinary extent of this culturally enabled achievement, involving as it did survival in some of the most difficult conditions the world has to offer, is emphasized by the fact that the supposedly cold-adapted Neanderthals had tended to shun such environments by hundreds of miles.

  The process that led to the modern human takeover of the Old World—and later of the New World and the Pacific—was not, of course, one of deliberate expeditioneering. Humans almost certainly expanded their ranges largely by simple demographic spread, as populations grew and budded off new groups into new territories. Of course, as local conditions fluctuated, the process would neither have been regular nor inexorable. Tiny populations were certainly washed back and forth by constant climatic and demographic vicissitudes, with numerous local expansions and extinctions. But this didn’t mean that on balance human spread could not occur rapidly: if a human population expanded its range by only ten miles in a generation, this would add up to more than 1,500 miles in a mere 2,500 years, something quite feasible on the time-scale involved. Whatever the details, though, just in itself population growth on this scale implies that there was something different about these new migrants: namely, an unprecedented ability to intensify their exploitation of the environments around them. This fed back into growing populations, and consequently into further geographical expansion.

  This demographic difference is also implicit in the fact that the new Homo sapiens was not moving into territory that was virgin for hominids: related species almost certainly already resided in many if not most of the regions into which it expanded. And the larger pattern their encounters took is clear. When behaviorally modern humans moved into Europe, the behaviorally archaic Neanderthals yielded. When they moved into southern Asia it was Homo erectus, which flourished equally late in its last southeast Asian island redoubt, that promptly disappeared. The same went, a little later in time, for those unfortunate Hobbits of Flores; and probably also in poorly documented Africa for any other hominids that may have survived the rigors of MIS 4. There was clearly something special about the new invaders. From the very beginning of hominid history, the world had typically supported several different kinds of hominid at one time—sometimes several of them on the very same landscape. In striking contrast, once behaviorally modern humans had emerged from Africa the world rapidly became a hominid monoculture. This is surely telling us something very important about ourselves: thoughtlessly or otherwise, we are not only entirely intolerant of competitio
n, but uniquely equipped to express and impose that intolerance. It’s something we might do well to bear in mind as we continue energetically persecuting our closest surviving relatives into extinction.

  THIRTEEN

  THE ORIGIN OF SYMBOLIC BEHAVIOR

  Our ancestors made an almost unimaginable transition from a non-symbolic, nonlinguistic way of processing and communicating information about the world to the symbolic and linguistic condition we enjoy today. It is a qualitative leap in cognitive state unparalleled in history. Indeed, as I’ve said, the only reason we have for believing that such a leap could ever have been made, is that it was made. And it seems to have been made well after the acquisition by our species of its distinctive modern biological form.