A Troublesome Inheritance: Genes, Race and Human History Page 9
A clue to major population movements after the exodus from Africa is provided by human skin color, which evolved to be dark in equatorial latitudes and pale in northern ones. If one could look at the global population of 25,000 years ago, its differentiation might have been much simpler to trace. Agriculture hadn’t yet been invented, and population growth had not yet seriously upset the social structure of small hunter-gatherer groups. Anyone who could have flown around the globe would have seen dark-skinned people inhabiting its equatorial belt, pale-skinned people in its high northern latitudes and a smooth gradation of skin color between them.
What fractured this smooth pattern of association between skin color and latitude? By 25,000 years ago, the Pleistocene Ice Age was nearing its end but was by no means exhausted. The glaciers advanced south one more time, causing the extra cold period known as the Last Glacial Maximum. For the next 5,000 years or so, most of Europe and northern Siberia became uninhabitable. The light-skinned people living in northern latitudes did not wait for the glaciers to bury them. They moved south ahead of the advancing ice fields and as they did so they displaced and probably killed the darker-skinned people to the south of them. The southerners, after all, would hardly have welcomed invasion of their territory and would have defended it to the last. But the northerners would have had the advantage of being genetically and culturally adapted to living in the extreme cold that accompanied them south. Moving ahead of the glaciers, they would have experienced a cooling environment to their liking but arduous for the people whom they were able to displace.
In Europe the retreating northerners found refuges from the cold in Spain and southern France. When the glaciers retreated, starting around 20,000 years ago, both Europe and East Asia were repopulated by former northerners who had survived the Last Glacial Maximum in the southern refuges they had wrested from their previous inhabitants. In this way both Europe and East Asia came to be populated by people with pale skins, the descendants of those who had once lived in the high north.
Another two continents fell into the possession of the pale-skinned northerners around 15,000 years ago, when conditions became warm enough for people living in Siberia to inhabit Beringia, the now sunken landmass that once connected Siberia to Alaska. Perhaps as sea levels rose, some of the inhabitants of Beringia crossed over to Alaska. From there, once the ice sheets melted to open a corridor, they migrated southward to colonize the two continents of North and South America.
Also around 15,000 years ago, there began another process that marked a profound step in the evolution of human social structure—the emergence of the world’s first permanent settlements. These appeared independently in Europe, East Asia, Africa and the New World. For the previous 185,000 years, ever since modern humans first appeared in the archaeological record, they had lived as hunters and gatherers. Now, for the first time, people were able to settle down in permanent communities, construct shelters and accumulate property.
The decision to settle cannot have been in any way simple or a matter of pure volition, or it would have taken place many millennia previously. Most likely a shift in social behavior was required, a genetic change that reduced the level of aggressivity common in hunter-gatherer groups. The human fossil record shows that in the period prior to settlement, there had been a gradual thinning of the human skeleton, a process known to physical anthropologists as gracilization. Gracilization typically occurs in the skeletons of wild animal species as they become domesticated. It seems that humans underwent a similar lightening of their bone structure for the same reason—that they were becoming less aggressive. Like animals undergoing domestication, humans shed bone mass because extreme aggressivity no longer carried the same survival advantages, and the most bellicose members of a society were perhaps killed or ostracized. This profound change in social behavior was a necessary precursor to settling down in large communities and learning to get along with people who were not close relations.
The first of these settled societies was the Natufian culture of the Near East, which appears in the archaeological record some 15,000 years ago. Several thousand years after the first settlements, people found themselves inventing agriculture—somewhat inadvertently, because the process of harvesting wild grasses automatically selected for strains more suitable to agriculture. As the climate warmed toward the end of the Pleistocene Ice Age some 10,000 years ago, the incipient systems of agriculture took off, centered on wheat and barley in the Near East and on millet and then rice in China. With the new and more abundant sources of food, population started to increase, and the new farmers expanded their territories. Increased population enhanced social stratification and disparities of wealth within societies, and a brisker tempo of warfare among them. Human social behavior had to adapt to a succession of makeovers as settled tribes developed into chiefdoms, chiefdoms into archaic states and states into empires.
These population expansions vastly changed the pattern of human distribution around the globe. Linguists like to distinguish between what they call mosaic zones and spread zones. The most spectacular mosaic zone still in existence is that of New Guinea. The thickly forested territory is occupied by people who, when discovered by Europeans, were using Stone Age technology and embroiled in endemic warfare. The island’s population is separated by territory and by culture. Every 5 to 10 miles, a different language is spoken; the island is home to some 1,200 languages, one-fifth of the world’s total. Language is seen as a badge of identity and is deliberately made as different as possible from that of neighboring tribes. Until warfare was supressed by colonial administrators, most New Guineans could not safely travel beyond their native valley.
In contrast to New Guinea with its 1,200 different languages, the United States is a spread zone, because a single language has been spoken from one coast to another since English speakers conquered the original inhabitants with their mosaic zone of many different languages. Much the same kind of process has probably operated throughout the past 50,000 years in a cycle between mosaic zones and spread zones.
When the world outside Africa was first occupied, it would have crystallized, much like the New Guinea linguistic mosaic zone, into many thousands of territories, each occupied by a single tribe. With the passage of time, the language of each tribe would have become more unique and less like that of its neighbors, and its genetics too would have become more distinctive. In each small tribe, different alleles would have drifted up in frequency to fixation or down to extinction.
Why then isn’t the global human population far more varied than it is? Because most of these small tribes were destroyed or absorbed into larger tribes as spread zones, propelled by demographic expansion or conquest, rolled like a wave over vast areas of mosaic zone. In Europe, for instance, people bringing the new farming technology from Anatolia, the region now known as Turkey, created a vast spread zone as they overwhelmed the existing hunter-gatherer populations, in part by conquest, in part by intermarriage. An alternative hypothesis is that the spread zone was created by conquest, not the spread of agriculture, as warlike pastoralists from the Russian steppe burst out from their homeland and across Europe and India. In either case, the spread zone reflects the expansion of people who spoke an ancestral tongue, Indo-European, and whose descendants now speak the many languages of the Indo-European family, from Icelandic and Spanish to Iranian and Hindi.
In the Far East too, the rice farmers started to expand, killing the neighboring populations or absorbing them through sheer pressure of numbers. The rise of the Han Chinese to become the world’s largest population began just 10,000 years or so ago, this being the time when Mongoloid-type skulls first appear in the archaeological record. The demographic spread of the Han Chinese is still under way, with less numerous neighbors like Tibetans and Uigur Turks finding themselves steadily absorbed into the Han demographic imperium. In Africa, the Bantu expansion is another instance of a spread zone formed by an agriculturally driven population incr
ease. Many of today’s races and ethnic groups were probably once small tribes that expanded through population increase, followed by conquering and absorbing outnumbered peoples.
All these evolutionary and historical processes took place independently in each continental population, since there was little flow of people or genes among them. Many salient changes in social behavior—the transition to settled life, the increasing social complexity from village to empire—as well as the engulfment of smaller populations by larger ones, were parallel developments on each continent, although they took place on a different schedule. The first known settlements were in the Near East, followed by those of China, Africa and the Americas. The difference in timing probably depended on population. The denser the population on each continent, the greater the pressure for settlement and the emergence of larger social groups.
Because the genes underlying social behavior are for the most part unknown, the parallel and independent evolution of such genes in the various races cannot yet be demonstrated. But the parallel development of another trait, that of pale skin in East Asians and Europeans, as described below, can now be tracked at the level of the relevant genes.
A Three Way Split
The emigration from Africa marked the first known major division in the modern human population, between those who remained in Africa and those who left. After the split, the two populations no longer shared a common gene pool, being sharply separated by geography. Migrations back into Africa occurred later, but the numbers of people were far too small to remix the gene pool. Those outside Africa and those within continued to evolve but along different pathways as each adapted to its special set of circumstances.
The next major fork in the human family tree occurred between the populations that colonized the two major halves of the Eurasian continent. Migrants to the north became the ancestors of Caucasians in the west and of East Asians in the east. Caucasians include Europeans, Middle Easterners and the people of the Indian subcontinent. The term Caucasian is avoided by some anthropologists because Blumenbach, who invented the term, believed the inhabitants of the Caucasus were the world’s most beautiful people. But Blumenbach, as noted earlier, did not believe Caucasians were superior to other races. Because there is no other word to refer to this important grouping of populations, many geneticists use this term.13 The date of the split between East Asians and Caucasians is still uncertain but may have been as long as 30,000 years ago.
Both Caucasians and East Asians have light skin, an adaptation to living in high northern latitudes. The default state of primate skin is pale: chimpanzees, under their fur, have white skin (although their faces are dark because of heavy suntan). When our distant ancestors lost their fur, probably because bare skin allowed better sweating and heat control, they developed dark skin to protect a vital chemical known as folic acid from being destroyed by the strong ultraviolet light around the equator. The first modern humans who migrated to the northern latitudes of Europe and Asia were exposed to much less ultraviolet light—too little, in fact, to synthesize enough vitamin D, for which ultraviolet light is required. Natural selection therefore favored the development of pale skin among people living in high northern latitudes. Pale skin may also have been prized in sexual partners, in which case sexual selection, as well as the need to synthesize vitamin D, would have speeded the spread of the necessary alleles. Objectively speaking, pale skin is no more attractive than any other shade. If anything, it is probably less so, to judge by the existence of tanning salons. It could have been prized for arbitrary reasons or, given its association with vitamin D synthesis, because its owners had healthier children in extreme northern latitudes.
Pale skin evolved independently in the Caucasian and East Asian populations, showing that the two populations have remained substantially separate since their split. This is known to be the case because pale skin in Caucasians is caused by a largely different set of genes than those that cause pale skin in East Asians. The independent but parallel evolution of pale skin in the two halves of the Eurasian continent came about because each was exposed to the same stress—the need to protect vitamin D synthesis in northern latitudes. But natural selection can work only with whatever alleles—the different versions of a gene—are present in a population. Evidently different alleles for making pale skin were available in the Caucasian and East Asian populations. This is not so surprising. Making, packaging and distributing the granules of pigment that give skin its color is a complex process, and there are many ways it can be tweaked so as to yield a particular outcome.
Among Africans, dark skin is maintained by the gene known as MC1R. A single version of this gene is found throughout Africa, whereas at least 30 alleles, all different from the African allele, are found among Europeans, and other variants are special to East Asians. It seems that any mutations or changes in the African allele of MC1R lead to lighter skin, which is harmful in the African context. Carriers of such an allele in Africa have no or fewer children, and the variant versions of the MC1R gene that keep cropping up because of mutation are constantly eliminated by purifying selection.14
Europeans have pale skin in part because purifying selection on their MC1R gene has been relaxed. But this is not the only reason. They have several alleles that promote pale skin. One is an allele of the gene known as SLC24A5. The SLC24A5 gene specifies a large protein—a chain of amino acid units—in which the 111th unit is the amino acid known as alanine. This is the ancestral form of the gene and is the allele found in almost all Africans and East Asians. Almost all Europeans have an allele in which there is a critical difference in the triplet of three consecutive DNA bases, known as a codon, that specifies the 111th amino acid unit. Different codons determine the 20 kinds of amino acid unit of which proteins are composed. In the case of the SLC24A5 gene, the 111th codon in the ancestral allele is the triplet of bases ACA, which specifies the amino acid alanine. In Europeans, the first A in the triplet has mutated to a G, giving the sequence GCA, and this codon specifies the amino acid known as threonine. This single switch of amino acids alters the function of the protein.
Almost all Europeans have two copies (one from each parent) of the threonine-denoting, skin-lightening allele of the SLC24A5 gene. Africans have two copies of the alanine allele that darkens the skin. African Americans and African Caribbeans who have one copy of each allele have intermediate hues of skin.15
East Asians have skin that can be just as pale as that of Europeans. But East Asians carry the ancestral dark skin form of SLC24A5. Natural selection has found other routes by which to lighten the skin of East Asians.
Several other differences are already known between East Asians and Europeans, testifying to the ancient split between the populations. One is the greater thickness of East Asian hair. Africans and Europeans, who have thin hair shafts, carry the same version of a gene called EDAR. A different allele is widespread in East Asians, occurring in 93% of Han Chinese, about 70% of people in Japan and Thailand, and in 60 to 90% of Native Americans. In the 370th codon of the gene, a T has mutated to C, so that the amino acid coded for is alanine instead of valine.16 Because of the switch of valine (V) to alanine (A) at the 370th codon, the allele is called EDAR-V370A.
East Asians who carry the EDAR-V370A allele also have thick and lustrous hair. But correlation is not proof, so how can one be certain that EDAR-V370A is indeed the cause of East Asians’ thick hair shafts? Researchers who wished to prove this point recently generated a strain of mice whose EDAR gene was converted to the East Asian form. They found the mice had thicker hair, proving that the allele is the cause of thick hair in East Asians, but also noticed two other interesting changes.17
First, the mice had more eccrine sweat glands than usual in their foot pads. Sweat glands come in two versions, eccrine glands, which secrete water so as to cool the body by evaporation, and apocrine glands, which secrete proteins and hormones. Checking in a Chinese population, the researchers found that th
e EDAR-V370A causes people too to carry significantly more eccrine glands, a fact that had been previously unknown.
The mice also had smaller breasts than usual, indicating that the EDAR-V370A allele is probably the reason why East Asian women tend to have smaller breasts than African and European women.
A fourth probable effect of EDAR-V370A is that it causes the characteristic dentition of East Asians, whose front teeth look shovel shaped when seen from the back. The mice were less useful in elucidating this effect because their teeth are so different from human teeth.
It may seem surprising that a single gene can have so many profound effects. EDAR has great influence on the body because it is switched on early in embryonic development and helps shape organs such as the skin, teeth, hair and breasts.
The fact that the EDAR-V370A allele has so many effects in East Asians raises the intriguing question of which particular effect was the target of the natural selection that made the allele so common. One possibility is that thick hair and small breasts were much admired by Asian men, or equally that thick hair in either sex was attractive to the other. In either case, these traits would have been acting as agents of sexual selection, a particularly potent form of natural selection.
Another possibility is that the sweat glands were the driving force behind the rise of EDAR-V370A. East Asians are usually assumed to have evolved in a cold climate because of certain traits, such as narrow nostrils and a fold of fat over the eyelid, which seem helpful in conserving body heat. But researchers have calculated that the EDAR variant emerged some 35,000 years ago, at which time central China was hot and humid.
A third possibility is that many or all of the effects of EDAR-V370A were advantageous at one time or another, and that natural selection favored each in turn. Effects of less obvious advantage, such as the shaping of the teeth, were dragged along in the wake of the traits found favorable by natural selection.