I recently collaborated with the geneticists Phillip Endicott, Simon Ho, and Mait Metspalu to compare two existing calibrations for recent human evolution with newly calculated substitution rates that are not based on the ancient and somewhat uncertain 6-million-year separation time for chimps and humans. The new rates gave younger estimated ages for recent events in human evolution, but ones consistent with the latest fossil and archaeological data for the exit from Africa and for our arrival in Asia, Australia, Europe, and the Americas. “African Eve” would have lived about 135,000 rather than 200,000 years ago, the exit from Africa would have taken place about 55,000 years ago, and the arrival in the Americas at about 14,000 years ago. If they are correct, these new and younger dates for human mtDNA evolution necessitate rethinking the mtDNA time scale for several key events in our evolutionary history, implying a younger date for our divergence from Neanderthals, a separation of many millennia between the first modern human fossils in Africa and Eve, and they also cast doubt on ideas of an early exit from Africa toward China and Australia. I will return to these issues shortly, but it certainly seems that geneticists need to reconsider their reliance on the human–chimp divergence to calibrate much more recent events in human evolution.
Compared with mtDNA, the Y-chromosome—the source of data on male history—has been slower to make an impact on the reconstruction of modern human origins than its female-tracking mitochondrial equivalent. One of the major reasons for this is that the Y is actually rather small and boring in terms of its genes and DNA compared with other more enlightening parts of our genome. It is predominantly made up of less informative junk DNA, and only small parts of its genetic material are ever exchanged with the X-chromosome. Nevertheless, it has now been completely sequenced, and increasing refinements in analysis have meant that even this recalcitrant chromosome has yielded important data on recent human history.
The most recent detailed comparisons of the human and chimp Y-chromosomes by Jennifer Hughes and David Page showed that these two are surprisingly different, with the human Y retaining many more coding regions. Because it is inherited through males only, there is a theoretical “Adam” to represent the last common ancestor of all modern Y-chromosomes, and as with mtDNA there is so far no evidence of a more ancient surviving variant of Y that could have been inherited from archaic people like the Neanderthals. Until recently “Adam” was estimated to have lived about 80,000 years ago, much later than “Eve,” with the initial and deepest two branches of the Y evolutionary tree widespread in Africa, one common from Bushman to Sudanese populations, the other in central African “pygmy” tribes. But new analyses by the geneticist Fulvio Cruciani and colleagues have instead placed the common ancestor at about 142,000 years ago, most likely (based on present distributions) in central or northwestern Africa. As with mtDNA, populations outside of Africa have lower diversity, this time with a slightly younger common male ancestor some 40,000 years old. Y is also useful in tracking unusual demographic events involving males in recent human history, such as the dominance of one Y-chromosome type across much of central Asia with an antiquity of about 1,000 years—perhaps the legacy of Genghis Khan’s habit of impregnating large numbers of women in conquered populations, as well as the historically documented reproductive success of his known male descendants.
The use of autosomal DNA to study human population relationships has a long history, at least in terms of the study of the geographic distribution of its products such as blood groups, proteins, and enzymes. In the 1970s attempts were made to reconstruct the genetic history of humans by combining data on the frequency of many different genetic markers in populations from across the world. However, these often gave conflicting signals, sometimes relating the populations of Europe and Asia together, and sometimes indicating a closer relationship between Africa and Europe. One exception was the pioneering use of a genetic distance technique by the geneticists Masatoshi Nei and Arun Roychoudhury that allowed them to calculate that modern humans were closely related to each other but that Europeans and Asians had diverged about 55,000 years ago, while their ancestors had diverged from Africans about 115,000 years ago.
These estimates look crude now, and no one would suggest that these were real evolutionary “splits,” but the inferred relationships were in line with those determined by mtDNA and several other analyses a decade later. The arrival of techniques that used enzymes to chop the DNA into studiable segments (Restriction Fragment Length Polymorphisms) led to examination of the gene for betaglobin (which makes up part of our blood’s hemoglobin) in 1986, and to early support for the concept of an African origin and a subsequent Out of Africa dispersal. Since then, hundreds of studies of autosomal DNA have shown the same pattern: African populations have the greatest diversity, and people outside of Africa are essentially a subset of that variation. In one of the largest recent investigations of over 1,000 genetic markers in 113 African populations, it was shown that they could be classified into fourteen groups, closely matching known cultural and language affiliations. Populations such as central African “pygmies,” hunter-gatherers such as the Sandawe and Hadza of Tanzania, and the Khoisan of southern Africa shared ancestors about 40,000 years ago. What was also interesting was that the latter three populations all speak “click” languages, suggesting that this could have been an ancient shared aspect of their languages.
While autosomal DNA studies have repeatedly confirmed the low diversity of most gene systems in non-Africans, they have also thrown an intriguing light on the pattern of dispersal of modern humans from their ancestral homeland. Just as non-African DNA variation can be seen as a subset of African variation and was originally sampled from it, so, as modern humans dispersed, that pattern seems to have repeated itself over and over again. The front line of expanding moderns from Africa was evidently small in number, and thus these pioneer groups radiating out from southwest Asia themselves only represented a small part of their parent population, with consequent lower DNA diversity. As a relic of that process today, DNA diversity steadily declines with overland distance from Africa, reaching its lowest points in faraway regions such as Arctic Europe, the Americas, Polynesia, and Australasia—and a matching pattern can even be found in the DNA history of Helicobacter pylori, a bacterium that infects most of us and can cause peptic ulcers!
Just as intriguingly, this pattern of decreasing diversity from Africa can be picked up in the measurements of skulls of populations from different parts of the world, suggesting that most of the regional differences between crania that are utilized by forensic programs were generated by drift rather than natural selection. I say most because there is evidence that certain populations like the Siberian Buryats and Greenland Eskimo underwent head and face shape selection under the impact of extreme cold—being large-headed and flat-faced seems to be advantageous under such conditions. But they represent exceptions to the general rule. Such decreasing diversity in both genes and morphology provides a challenge to the assimilationist idea that the expanding moderns mixed everywhere with remaining populations of archaics such as the Neanderthals and descendants of Homo erectus in the Far East. If that were so, we would expect to see repeated reversals of the decline in diversity where such different kinds of humans had significant input into modern human variation, and this has not been observed so far—with one very important exception. The geneticist Jeffrey Long and his colleagues recently reported a hot spot of increased diversity in the southeast Asian islands of Melanesia, and this was a clue to significant local complications in the Out of Africa dispersal, as we will see shortly.
One of the most difficult aspects for some people to accept, if we evolved very recently in Africa, is why we all look so different. As I said more than twenty years ago, “we are all Africans under the skin,” and yet what lies in and on the skin seems to distinguish us from each other so markedly. Humans come in many different sizes, shapes, and colors and differ in the form of their eyes, hair, nose, and lips. These “racial” or,
better, regional or geographic differences are immediately apparent, and thus some people assume they must be highly significant genetically. Yet if we had a recent African origin, these differences must have evolved after we became the modern human species and started to spread out from our place of origin. Thus we evolved our shared species-specific features—our high and rounded skull, small brow ridges, small retracted faces, chins, and so on—first in Africa. Then, on that shared modern template, the regional features were superimposed. But what led to those additions? Here there are several different ideas, and two in particular stand out: climatic adaptation through natural selection, and sexual (in humans, also cultural) selection. Surprisingly, despite Darwin’s (and Wallace’s) emphasis on natural selection as the predominant agent of evolutionary change, when Darwin came to publish The Descent of Man in 1871, it was the second part of the title—and Selection in Relation to Sex—that dominated his thoughts on the evolution of “racial” characters in humans.
Map showing the spread of early modern groups as traced using mitochondrial DNA (numbers refer to thousands-of-years-ago). The routes are notional, not precise.
We have now seen that the external characteristic differences between the races of man cannot be accounted for in a satisfactory manner by the direct action of the conditions of life, nor by the effects of the continued use of parts, nor through the principle of correlation. We are therefore led to inquire whether slight individual differences, to which man is eminently liable, may not have been preserved and augmented during a long series of generations through natural selection. But here we are at once met by the objection that beneficial variations alone can be thus preserved; and as far as we are enabled to judge … none of the differences between the races of man are of any direct or special service to him …
We have thus far been baffled in all our attempts to account for the differences between the races of man; but there remains one important agency, namely Sexual Selection, which appears to have acted powerfully on man, as on many other animals … it can be shewn that it would be an inexplicable fact if man had not been modified by this agency, which appears to have acted powerfully on innumerable animals. It can further be shewn that the differences between the races of man, as in colour, hairiness, form of features, &c., are of a kind which might have been expected to come under the influence of sexual selection …
For my own part I conclude that of all the causes which have led to the differences in external appearance between the races of man, and to a certain extent between man and the lower animals, sexual selection has been the most efficient.
While I think Darwin was right to question natural selection as the factor behind features like thick or thin lips, and the distinctive eye form of many oriental populations, we saw in chapter 4 that Allen’s and Bergmann’s “rules” of climatic adaptation seem to affect body-shape variation in humans from different regions. It seems likely that nose shape and skin color have been shaped by natural selection—in the former case via differences in the local temperature and humidity of the air, and in the latter case through the strength of sunlight, particularly in ultraviolet (UV) wavelengths. The theory behind skin pigmentation differences is that they have evolved as a balance between the need for the skin to receive enough sunlight to allow essential vitamin D to be synthesized under our skin, and the need to protect our skin from an excess of UV, which can damage folic acid levels (vital during pregnancy) and skin cells, leading to cancers. Studies are complicated because humans have recently become much more mobile, thus confusing some correlations of pigmentation with UV levels that may have existed previously.
Nevertheless, there seems to be clear negative evidence for the protective benefits of dark pigmentation in the prevalence of folic acid destruction and skin cancers in lightly pigmented people of European origin who moved to high UV regions such as South Africa and Australia. And, demonstrating the opposite process in highly pigmented peoples, African and southern Asian peoples moving to northern regions such as Scotland and Canada have a greater risk of vitamin D deficiency (and thus of the disease of rickets), which is exacerbated if they also go out less and cover their bodies more when they do go out.
These data imply that our original (African) ancestral skin color was indeed darkly pigmented, and that selection favored lighter skins as modern humans spread to regions where UV levels were low and the diet was not providing enough vitamin D. In fact the favored mutations that produce lighter skin in Europeans are young (on some estimates one of the most important genetic changes only occurred about 11,000 years ago), and several (but not all) are different from those that have evolved recently in north Asians. But this is not to say that natural selection is the only factor at work in human skin color, or nose shape for that matter, since sexual/cultural selection could also have played a part. An example of this comes from blue eye color, which is common in northern Europe. The mutation responsible for this has probably occurred many times by chance in human evolution but has not generally been favored. However, the European version seems to be young—less than 20,000 years old—so we can imagine it originating in a Cro-Magnon population somewhere in Europe. Lightly pigmented eyes are certainly disadvantageous in conditions of strong sunlight, but in Europe the unusual nature of the light color might have led to it being favored as an attractive and only mildly disadvantageous variant, which then proliferated through sexual/cultural selection. This variant is down to only one tiny segment of DNA, which shows how small genetic changes can produce striking differences in appearance.
“Racial” features have largely evolved more recently, through quite small changes in our DNA, but they have a strong impact on us because they affect what we notice when we meet people for the first time: their color, facial appearance, and hair. Because of their importance in signaling, I have no doubt that such traits could have been selected for sexual/cultural reasons through differing norms of attractiveness or to enhance group identity. But also at work as modern humans dispersed quite rapidly from Africa in relatively small numbers would have been the effects of drift and founder effect. The former process is the result of random events; once populations stop exchanging genes, they may “drift” apart purely by chance. The latter process is the result of chance too, but in this case a small and perhaps atypical group may go on to found a much larger population, which will reflect their idiosyncratic genetic makeup rather than that of the original. These phenomena may have combined, as moderns spread rapidly, to produce something called surfing, after the popular water sport: particular gene combinations that were rare can end up as very common if they are lucky enough to “ride” on the expanding population wave, and hence proliferate in the new daughter populations—and this certainly seems to explain some distinctive gene frequencies outside of Africa.
These genetic complexities show why old “racial” categories such as “Negroid,” “Caucasoid,” “Australoid,” and “Mongoloid” have largely been abandoned by science, because they are not meaningful descriptors of levels of biological variation. Additionally, all of us are to a greater or lesser extent “mixed” in our origins, since each of our genes will have its own separate history, and they will not all tell the same story of origin. Hence the golfer Tiger Woods reacted to being hailed as a model for blacks in America (a rather tarnished one, now) by saying he was actually Cablinasian, as in Caucasian-Black-[American] Indian-Asian, reflecting his multiple lines of descent. As we said, African populations probably contain as much genetic variation as the rest of the world put together, and the boundaries between these categories are often fuzzy in reality. This is not to say that many populations cannot be distinguished at a general level by the prevalence of common inherited features, and this is also reflected in traits like cranial and facial shape, which is why forensic scientists can often confidently place a skull back into its parent population through study and measurement. But in line with the expectations of a recent African origin, if we try those forensic tests that are
based on modern patterns of regional variation on early modern skulls more than 20,000 years old, the results are invariably confused. Hence when I tested the 30,000-year-old Předmostí skulls from the Czech Republic, they came out as “African,” while one of the Upper Cave Skulls from Zhoukoudian, China, appeared “Australian.” This does not imply a close relationship to those modern populations, but rather that a kind of regionality existed then that was different from the pattern we have today.
The hoary subject of apparent differences in brain quality and IQ between regional populations is not something that is going to go away any time soon. In this respect, things have not changed much since I was threatened with legal action over what was written about the subject in one of my previous books, African Exodus. I don’t intend to say much more about this controversial subject here, except to acknowledge that some cognitive differences could, of course, have evolved over the last 50,000 years (for example, see the discussion of the microcephalin gene later in this chapter), just as they have in physical features. But if so, I would expect a large and genetically varied region like Africa to show a high level of such differences, rather than the supposed uniformly low IQ values that some studies report. Additionally, as other research has shown, IQ tests only measure some aspects of “intelligence,” and environmental differences in nurture, nutrition, and health make a strong contribution to the results too.
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