Out of Eden: The Peopling of the World
Page 23
Another approach to the question of whether Mongoloids originated in the north or the south is to consider diversity. On general Darwinian principles, a new species (or, in this instance, new ethnic group expanding from a small founder population) should have one centre of origin, which should retain the greatest diversity of types (e.g. genetic) within that group. Now, whichever genetic system one uses, Southern Mongoloids always show far greater genetic diversity than do those in the north. Y-chromosome evidence has already been used to justify placing the Mongoloid homeland in the south.31 Although it is useful, we face two potential pitfalls if we take such a simplistic diversity approach. First, none of the gene lines or markers used has been clearly identified as ‘Mongoloid’: they are simply Mongoloid by implication because of where they have been found, so the argument is in danger of becoming partly circular if it is crudely applied. Second, North Asia, unlike tropical Southeast Asia, would have suffered depopulation, great extinction, and genetic drift during the last glaciation, so the low diversity would be expected in the north today and thus may not necessarily reflect the original picture.
An extreme hypothetical view of the effect of the last glaciation on Central and North Asia is that the entire Asian region extending north of the Himalayas was completely cleared of people, so that today’s populations are merely re-entrants from South China and Southeast Asia and from Europe. That would of course mean that we can never know the original genetic composition of Northern Asia. This view is not tenable. First, palaeoclimatological studies show that during the ice age extensive steppe tundra regions persisted in Central Asia which could have supported hunters. This is backed up by archaeological evidence for continuous occupation through the last glacial maximum (see Chapter 6). Dale Guthrie certainly argues against such a complete extinction in the south of the Mammoth Steppe heartland. Second, as we shall see, this picture fails to explain the deep genetic diversity of south-west Central Asia (Uzbekistan and Kirghistan), or the marked geographical difference in deep genetic branch lines between north and south Asia. In particular, as I shall show, there are a group of lines still shared uniquely between northern Europe, northern Asia and northern America that have no links at all with south China or Southeast Asia.32
Mapping the spread of the gene tree
As with the out-of-Africa story, the answer becomes clearer when we look at the Adam and Eve markers. The great advance that came with the mapping of the mtDNA and Y-chromosome trees was the ability to trace individual molecular branches and their twigs, like a vine, from one region to the next. The fancy name for this tracking game is phylogeography, but at its simplest it consists in following twigs back to their branches and connecting points on a map.
The rules are simple, although their application can be complex. To detect a migration from one region to another, we need to find the source branch type in both regions and a new unique twig in the target region that is not present in the homeland. For a homeland with multiple migrations in different directions, we need to identify common branches in the homeland that have different unique twigs in the other regions. Perhaps one of the best known and earliest examples of this approach was published by Italian geneticist Antonio Torroni and his colleagues in 1993. They identified four American founder mtDNA lines, which they labelled A to D. Each of these four bushy twigs, although showing unique new sprouts in America, could be traced back to the equivalent source branches, A, B, C, and D, in Asia (see Chapters 6 and 7).
We can apply this process to the much more difficult task of identifying the Mongoloid homeland in Asia. Taking East Asian mtDNA first, we find that at present there are nine well-described deep branches (haplogroups; see Figure 5.6), which have been designated A–G, X, and Z. With the exception of the sister groups C and Z, none of the nine groups can be derived from any other; they each arise more or less directly from the two Eurasian founder lines Manju or Nasreen. Several other minor groups (M7/M10, and N9) have recently been described, but their existence doesn’t alter the argument. (For details of the phylogeny of and sources for mtDNA haplogroup distributions in East Asia, see note 33.)
Figure 5.6 The Asian and Pacific mtDNA tree. Note: 1) no overlap in sub-branches of M and N between India and East Asia except for the M8/CZ clan which went to North Asia; 2) Oceania and Southeast Asia have unique local diversity associated with their aboriginal groups; 3) extensive overlap between lines in Southern and Northern Mongoloid groups, but the Southerners, including aboriginals, have a complete set while the North has fewer and more derived lines.
Of the nine better-known East Asian mtDNA branches, three (C, X, and Z; Figure 5.6) feature only north of an east–west line drawn below the Himalayas, while the other six (A, B, D, E, F, and G) are found in both Northern and Southern Mongoloids. At first these unique northern branches might appear to favour a Mongoloid homeland north of the Himalayas, but there is a simpler explanation. This is that C, with her closely related sister Z and the mysterious X group, were not part of early Mongoloid history and had intruded among Northern Mongoloids from somewhere else, such as Siberia or Central Asia. As we shall see, there is evidence to support this view. the distribution and tree structure of the other six branches appears to favour a genetic homeland south of the Himalayas, particularly for the oldest branches of B and F. So we appear to have several genetic sources among Southeast Asians that have also spread north. In contrast, there are several branches among Northern Asians which have no roots in Southeast Asia. In crude branch terms, therefore, the idea of a southern homeland with subsequent migration to the north would seem to be supported by the maternal genetic evidence, and would be consistent with Sundadonty-became-Sinodonty (see Chapter 5).
In order to focus on that possible secondary northern region of ‘further development on a theme’, we have to look for geographical areas possessing all the six mitochondrial branches – A, B, D, E, F, and G – that are common to both Northern and Southern Mongoloids. So, moving from west to east, we find Central Asia with five (lacking just G), Tibet with all six, Mongolia with five (lacking E), China with four (lacking E and G), and Korea with five (lacking E).34 Perhaps it is not too much of a coincidence that the countries with the highest proportion of lines generally associated with Mongoloid peoples – Central Asia, Tibet, Mongolia, and Korea – straddle Dale Guthrie’s 40th parallel of the Mammoth Steppe (see Figure 5.3).
On this basis, we could start to ask for how long those branches have been present around the 40th parallel. Toomas Kivisild has estimated the local age of four of these lines in Mongolia and they look fairly consistent: A at 35,500 years, B at 40,500 and 33,500 years (see below), D at 44,500 years, and F at 42,000 years. These dates are mostly younger than the overall ages of the same lines farther south in China. But again, they are consistent with dates for the start of the Central Asian colony, and after the Asian climatic amelioration that set in from 52,000 years ago (see Chapter 3), which could have allowed the early Asians to penetrate far into the Mammoth Steppe.35
By what routes, then, did modern humans first make their way up into this region? Returning to two of the oldest Southeast Asian lines, B and F (from Nasreen’s descendants), which appear to have spread north from Southeast Asia into Tibet, Mongolia, and northeast along the Pacific coast, we find that they stopped inland short of Siberia in Northern Asia.36 In this respect, the distribution of Groups B and F mirrors the ancient distribution of Sundadonty. Both B and F reach their deepest and greatest diversity among the aboriginal ethnic groups of Thailand, Vietnam, and Cambodia, thus pointing to Indo-China as their homeland.
Strong supporting evidence for Group F having originated in the south is provided by our newly identified ‘pre-F’ group (R9), which is common in the Aboriginal Malays of the Malay Peninsula (at the base of the Sundadont radiation – see Figures 5.6 and 5.7). This pre-F is also found in South China (Yunnan and Guangxi), Indo-China (Thailand and Vietnam), Sumatra, and the Andaman and Nicobar Islands, suggesting possibly the oldest link with t
he first beachcombers in Southeast Asia.37
Figure 5.7 First entry of mtDNA lines into Central Asia from the Indo-Pacific coastline. The entry routes mainly correspond with rivers shown in Figure 5.5 and along Silk Road branches once in Central Asia. With the exception of lines M8/C/Z which enter west of the Himalayas, all other lines originate from Southeast Asian beachcombers.
Knowing whether Group B originally spread west from China into Mongolia along the Silk Road or moved north from Burma through Tibet would tell us the route of that first colonization, which was perhaps as long ago as 50,000 years ago. There are clues. Toomas Kivisild estimated the ages of the two common subgroups of B in Mongolia as 40,500 (B1) and 33,500 (B2) years. This all suggests that although Mongolia and parts of western Central Asia may have received both B types during the Palaeolithic, the older of the two could have come up through Tibet, while the younger could have moved west into Central Asia from the Pacific coast, where it now predominates.38
We also have the three partly related Manju lines D, E, and G, whose distribution in East Asia increasing from south to north mirrors that of Sinodonty. The genetic evidence suggests that they came from the south. There is still the puzzle of where those other ‘intrusive’ founders of the Mongoloid Mammoth Steppe colony (C, X, and Z) came from in the first place, and which of the three possible corridors into the Mammoth Steppe they took. These three North Eurasian lines are very uncommon south of the Himalayas. The sister branches C and Z reach their highest rates in Siberia and Northeast Asia, and are hardly found farther south at all except in India, Mongolia, Central Asia, and Tibet. They stretch in a broad east–west continuum across the Asian Steppe, achieving significant rates in Central Asia and even as far west as Turkey. Group C even got to America via Alaska, yet their northerly distribution in East Asia suggests they could not have moved up north from Southeast Asia via China. Rather, they are more likely to have arrived from farther west in Asia, along with the eastern spread of the Upper Palaeolithic technology that appeared in Kara Bom in the Russian Altai 43,000 years ago. They belong to the Manju group, and their ultimate ancestors appear to come from Pakistan or India, having moved up the valley of the Indus via Kashmir or Afghanistan round the western end of the Himalayas (see Figures 5.5 and 5.6). This all suggests that they moved into the Asian Steppe between 40,000 and 50,000 years ago from west Central Asia (see Figure 5.7). Consistent with the concept that C and Z spread east across the steppe with Upper Palaeolithic technology, Toomas Kivisild has estimated the age of C in Mongolia at 42,000 years.39
The last of the northern lines, X, is found only among Europeans and Native Americans, with a single report from southern Siberia, but the link between the Old and New Worlds is up to 30,000 years old. This suggests that X must originally have spread right across Central and Northeast Asia with the mammoth-hunting culture, but she was all but extinguished in Asia during the last ice age, surviving only at the extremes of her distribution in America and Europe (see Chapter 7).40
The complicated mitochondrial picture described above suggests that Mongoloids derive primarily from the south, while Central Asian peoples came mainly from a West Asian source, but combined with additional East and Southeast Asian sources in Central and Northeast Asia. This genetic evidence then supports the geographical theory of a three-pincer colonization of Central Asia from the Indo-Pacific coast about 40,000–50,000 years ago. The dissection of the various admixed genetic contributions to Northeast Asia clarifies and is consistent with the concept of an ultimately Southeast Asian origin of Mongoloid peoples.
Adam’s story
Now, having looked at the mtDNA evidence, when we study the same Asian story in the Y-chromosome tree the picture of several inputs to Central Asia takes on more contrast. Adam’s tree is in many respects a scientific newcomer. As yet, it lacks the vast number of unique twigs sprouting from Eve’s tree of life, and the Y-chromosome branch dates are still very unreliable. So why should I keep turning to Adam, merely to back up the same story told much better by Eve? Because where the Y chromosome comes into its own is in clearly marking regional patterns. We saw a good example of this with the peopling of Europe (see Chapter 3), where different paternal clans have greater regional clarity within Europe than do Eve’s maternal clans. The latter, although regionally structured, seem more blurred in their distribution. The same is true of East Asia. The Adam and Eve trees both suggest that Central Asians and Americans have Northwest, East, and Southeast Asian origins, but the separation of these three strands for the founding peoples of the Mammoth Steppe is much clearer in the Adam tree. To understand this better, we need to remind ourselves briefly of the three main out-of-Africa Adam founding lines: Abel (YAP or M145), Cain (or RPS4Y), and Seth (or M89).41 (See Figure 5.8.)
Cain was the line that travelled rapidly south-east along the Indian Ocean coast to become the first male founder in Eastern Indonesia, Australia, and New Guinea. The beachcombers did not confine themselves to the Antipodes but moved on around the Indo-Pacific coast to Japan and Korea, where they formed the earliest colonies there. Somewhere along the Palaeolithic coast-ride, most likely towards the end of the trail in Japan, Cain had a unique progenitor Asian son whose descendants spread throughout Northeast Asia, then west along the Silk Road into Mongolia and Central Asia. They also eventually went east into the Americas.42
We saw in Chapter 4 that Abel’s western sons contributed respectively to the peopling of Europe, the Middle East, and Africa, but there was another, uniquely East Asian branch of Abel’s line. Just like Cain, this Asian Abel followed rapidly round the coast road, south-east to Indonesia, and then north to Taiwan and Japan, but no farther north than Korea. Asian Abel’s descendants cannot now be found en route in India. This may be because they travelled rapidly, but it could be that their Indian representatives were wiped out in India by the ash cloud from the great Toba volcanic eruption of 74,000 years ago (see Chapter 4). However, there is now a high Abel frequency in Tibet, suggesting that some of his ancestors were survivors of Toba from eastern India who took the river route up from Southeast Asia onto the high Asian plateau, from where they also spread into Mongolia and the Russian Altai.43
Figure 5.8 The Asian Y-chromosome tree. Note the almost complete separation between South Asia and Southeast Asians, except for overlap in tribals from West Bengal and shared root lines among all relict groups; also, unlike mtDNA, the near total separation of Northern vs. Southern Mongoloids, with latter dominated by Ho and former sharing lines extensively with South Asia and west Central Asia, particularly Polo derivatives.
So, the first two male beachcombing founder clans used both of the eastern corridors to enter the Central Asian Steppe (Figure 5.9). Their final numerical contribution to world colonization was tiny compared with the third Adam branch, Seth, whose dominant seed spread to every corner of the non-African world and whose offspring eventually used all three routes to enter Central Asia. A recent survey showed Seth’s line accounting for 93 per cent of 12,127 Asians and Pacific islanders, with the descendants of Cain and Abel making up the rest. It is important to appreciate that, like the daughters of Out-of-Africa Eve, Nasreen and Manju, Seth travelled the coast road from Africa in company with and at the same time as his brothers Cain and Abel. He did not come out at a later time or by another route, as several geneticists have suggested. The evidence for this may be seen in the simultaneous presence of descendants of each of the three brothers, including specific Seth types, in relict beachcomber populations around the Indian Ocean, including those in Southeast Asia (see Figure 5.9).44
It is Seth’s trail that confirms the alternative use of the third route – the northern trail into Central Asia round the west of the Himalayas (along with the more southerly beachcomber trails to Indo-China and round the eastern edge of the Himalayas). We can see where he started. Two-thirds of the world deep-branch diversity of this major founder Eurasian line is found in India. Seth represents a quarter of all Indian Y chromosomes, and his sons account f
or most of the rest.45 Another 10 per cent of the Seth node is found in Central Asia and, correspondingly, all four of his first-generation sons have significant Central Asian representatives, again suggesting the potential for direct spread north at the same time as the North Eurasian mitochondrial groups C, X, and Z around 40,000 years ago (see pp. 234–5).
We have seen that three of Seth’s sons were responsible for the colonization of much of India, Europe, the Middle East, and Mediterranean lands.46 In this part of the story, however, we are not so much concerned with Seth and his three West Eurasian sons as with his fourth genetic son, Krishna, and the latter’s role in fathering the bulk of Eurasians not to mention nearly all Native Americans.
Krishna accounts for about 40 per cent of Y-chromosome types discovered so far. His wide distribution in Europe, Asia, the Pacific, and the Americas suggests that he was born in India very soon after the initial out-of-Africa dispersal. In spite of his early birth and the wide dissemination of his line, Krishna’s immediate sons have very discrete geographical distributions. Several are local to Pakistan and India, with some minor spread north to the Levant and Central Asia; another is found only in Melanesia (New Guinea and the surrounding islands); yet another (TAT) is exclusive to Central Asia and north-eastern Europe.47