Masters of the Planet (33 page)

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/Skh
l
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 Skh
l, where the lithic context is also definitely Mousterian. But at Skh
l 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 Skh
l, 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, even 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.