Lone Survivors (30 page)

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 P
ř
edmostí 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.

Having looked at the genetic variations between us and our closest living relatives, the chimpanzees, and then at variation within our own species, we now turn to the tremendous breakthroughs that have been made in studies of the DNA of our close extinct relatives the Neanderthals. Twenty years ago, the idea that useful genetic data could be recovered from Neanderthal fossils to compare with our own sounded like science fiction, given the huge problems of extracting minute traces of DNA from ancient bones that had suffered from the effects of degradation, water, temperature changes, and soil acids for many millennia. Even if it was preserved (which seemed unlikely), it would be too difficult to find, too difficult to recover in large enough quantities to study, and too problematic to distinguish from all the other contaminating DNA that would also be there.

However, the field of ancient DNA did get off the ground in the early 1980s, with the sequencing of part of the mtDNA genome of the quagga, a recently extinct close relative of the zebra, whose skins survived in museum collections. And in 1984 a technique called the
polymerase chain reaction
(
PCR
) was discovered, which enabled researchers to produce millions of copies of specific DNA sequences in just a few hours. With this and improved recovery techniques and comparative DNA databases, it started to become possible to recognize and distinguish ancient DNA, where it survived in sufficiently large and well-preserved quantities. So in 1997 the recovery of the first Neanderthal mtDNA from the most famous representative of the group—the 1856 Neander Valley skeleton—caused a sensation. I was lucky enough to be asked to talk about the research at the press conference in London where Svante Pääbo announced the results, and I remember getting so carried away that I hailed it as an achievement comparable with landing someone on Mars! But for paleoanthropology it was a remarkable breakthrough, although things have moved on so fast in the last decade that over twenty Neanderthal fossils have now yielded this genetic material.

Because our cells generally contain hundreds or thousands of copies of the mtDNA genome, compared with the single set of autosomal DNA contained in each nucleus, and because the mtDNA genome was completely known by 1981, mtDNA was specifically targeted in early research on ancient DNA. But by 2006, using particularly well-preserved Neanderthal fossils and massive improvements in analytical techniques and computing power to recover and recognize small ancient DNA fragments, two international teams of scientists reconstructed the first large-scale genetic maps of the Neanderthal autosomal genome. Two fossil sites have proved particularly valuable in Neanderthal genome work, and in both cases their human remains may have resulted from cannibalism; in fact, there is speculation that defleshing the bones may even have helped the preservation of ancient DNA by heading off some of the proximate causes of DNA decay. One is the cave site of Vindija in Croatia, where small fragments of leg bones have by far and away the best preservation of Neanderthal DNA found so far, and the other is El Sidrón in Spain, which we discussed in chapter 4, and where great attention has been paid to recovering fossils with the minimum possibility of contamination by recent DNA.