Out of Eden: The Peopling of the World (26 page)

In spite of the proximity of Toba to Perak, the Toba ash plume only grazed the Malay Peninsula. The occupants of Kota Tampan were the unlucky ones – others on the peninsula escaped. Some argue, on the basis of comparing skull morphologies, that the Semang aboriginal ‘Negrito’ hunter-gatherers, who still live in the same part of the dense northern Malaysian rainforest, are descendants of people like Perak Man. The continuity of the Kota Tampan culture as argued by Zuraina Majid provides a link back to the 74,000-year-old tools in the Toba ash. In the next chapter we shall look at the physical evidence that links the Semang to the first modern human explorers along the coast of the Indian Ocean.
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For a film documentary,
The Real Eve
(
Out of Eden
in the UK), with which this book is associated, Discovery Channel helped to fund a genetic survey of the aboriginal groups of the Malay Peninsula which I conducted in collaboration with English geneticist Martin Richards and some Malaysian scientists (see
Plate 16
). This survey was part of a much larger ongoing study of East Asian genetics. The mtDNA results were very exciting: three-quarters of the Semang group (i.e. the ‘Negrito’ types) have their own unique genetic Manju and Nasreen lines with very little admixture from elsewhere, which is consistent with the view that their ancestors may have arrived with the first beachcombers. Their two unique lines trace straight back to the Manju and Nasreen roots (the first two daughters of Eve outside Africa – see
Chapter 1
). Their Manju line is not shared with anyone else in Southeast Asia or East Asia (or anywhere else) and, although
it has suffered severe genetic drift through recent population decline, retains sufficient diversity to indicate an approximate age of 60,000 years. Their unique group on the Nasreen side, R21, comes from Rohani, Nasreen’s genetic daughter. This lack of any specific connection with any other Eurasian population is consistent with the idea that after arriving here so long ago, they have remained genetically isolated in the jungles of the Malay Peninsula.
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Another piece of evidence from the region may help place Anatomically Modern Humans in the Far East over 70,000 years ago. This is the famous southern Chinese Liujiang skeleton. Consisting of a well-preserved skull (see
Plate 19
) but few other bones, Liujiang was discovered in a small cave at Tongtianyan in the Guangxi Zhuang Autonomous Region in 1958 by people collecting fertilizer. There is no doubt that this person was anatomically modern, but from the start there has been controversy over its age. A uranium date of 67,000 years was reported, but has been questioned on the basis of its exact location in relation to dated geological strata. In December 2002, a Chinese group headed by geologist Shen Guanjun reported their reinvestigation of the stratigraphy of the cave and dating of the skull (extending to several neighbouring caves) and claim it should be placed in a time bracket between 70,000 and 130,000, and not less than 68,000, years ago. The skull was found in a so-called intrusive breccia, a secondary flow of debris containing jumbled material of different ages. From their paper in the prestigious
Journal of Human Evolution
, the lower date bracket of 68,000 years seems solid, since it comes from multiple date estimates of the flowstone above and covering the breccia. (A flowstone forms when flowing water deposits calcite down a wall or across a floor.) Their preferred dating of 111,000–139,000 years ago based on unstratified fragments of flowstone and calcite within the breccia seems to me more speculative. Whatever the outcome, the safer lower bracket for the presence of an anatomically modern human in the Far East
additionally tends to support the early exodus across the Red Sea before the Toba event of 71,000–74,000 years ago. This is because, to be in China by 68,000 years ago, their ancestors would still have had to expand across a quarter of the globe.
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Some are still convinced that Australian aboriginals represent an earlier migration out of Africa than that which gave rise to Europeans, Asians, and native Americans. Yet again our genetic trail tells us otherwise. Several studies of Australian maternal clans have shown that they all belong to our two unique non-African super-clans, Manju and Nasreen, and large studies of Y chromosomes show that male Australian lines all belong to the same Out-of-Africa Adam clan as other non-Africans, and belong to the Seth and Cain lines. The same pattern is seen with genetic markers not exclusively transmitted through one parent. In other words, the combined genetic evidence strongly suggests Australians are also descendants of that same single out-of-Africa migration. The logic of this approach, combined with the archaeological dates discussed above, places the modern human arrival in the Malay Peninsula before 74,000 years ago and Australia around 70,000 years ago. It is also consistent with the date of exit from Africa predicted on beach-combing grounds. As we saw in
Chapter 1
, the motivation for leaving the African continent may have been the failure of the beach harvest on the shores of the Red Sea caused by sharply rising salinity 85,000 years ago. My date estimates for the trek around the Indian Ocean en route from Africa suggest that the beachcombers could have taken as little as 10–6,000 years to eat their way down the coastline to Perak and another 10–4,000 years to reach Australia. Such a time requirement is fulfilled by the difference between leaving Africa around 85,000 years ago and arriving in Australia 65,000 years ago. The former date is consistent with a date of 83,000 years estimated for the African L3 cluster expansion using the molecular clock (see also
Chapter 1
).
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To gaze upon the face of the mother who launched a billion families
A question which has obsessed journalists and film-makers ever since
Newsweek
’s 1988 Mitochondrial Eve story is what she and her companions looked like. It is likely that they all looked very different from one another, more varied than any random selection of people from any non-African country might look today. They were, after all, the stock from which all non-Africans developed, and all the evidence suggests that the variation
within
individual groups decreases the farther away from Africa one goes, even today. Yet, it is also the case that differences between one group and another, between, say, Chinese, Native Americans, Polynesians, Melanesians, and Australians, increase the farther we move from Africa; so the total diversity
between
the so-called races might seem to increase with distance from Africa. Let me put this another way. My late and very dear mother-in-law, who was Chinese, once told me – and she wasn’t joking – that Europeans all looked the same to her. Europeans have said the same about the Chinese. There is a paradox here, which we need to resolve before we can continue to chart the genetic diaspora out of Africa.

It is all a question of how the diversity is split up. What appears to be a paradox is actually the result of multiple unequal group splits followed by genetic drift in action. It can be explained thus: as the explorers split into more and more branches, the original genetic variation was parcelled into different groups which today live scattered around Eurasia and America. Over the past 50,000 years or so, members of these individual splinter groups in the ex-African diaspora have drifted to become more like one another than like members of other groups. The splinter groups have diverged from one another. While I shall argue later that many of these changes were true evolutionary adaptations to new environments, some of them were just down to the random and unequal way the cake was cut.

How is this? The answer lies in the same processes that operated
when that first small group left Africa. Imagine – as we did in an earlier chapter – our stock of human diversity as a bag of marbles. Let us say that in Africa 75,000 years ago there were 10,000 marbles in the bag. When the exodus crossed the Red Sea, that diversity was reduced to 250 marbles in the founding colony. That reduction is known as a ‘founder effect’. When that first colony had been living as an isolated colony for several thousand years somewhere in south Arabia, a number of marbles had failed to reproduce themselves and others became the dominant types. The bag of genetic variation became even smaller. As discussed in
Chapter 1
, that is genetic drift.

As the colony moved on there were unequal splits. When the group size became too large for the beach to sustain it, smaller groups, ‘pioneers’, broke off and ventured inland, travelling up rivers. Those smaller groups carried even fewer of the original genetic marbles that left Africa. They were undergoing further founder effects and drift.

Now we have to ask which was the tree and which was the stem. Were the beachcombers always in the majority, and the river explorers the minority, or did the main trunk veer inland at some stage? Which group kept most of the marbles? Such a question might seem academic, but if our journalists want to gaze upon the faces of a modern group which is as close as possible to the original exodus, this is the question we must answer: which modern non-African population has retained most varieties of the original ancestral African marbles?

Nuclear genetic markers: a bigger bag of marbles to look for older markers

To look at this question of diversity further, we cannot rely on our mitochondrial and Y-chromosome genetic markers. As far as they are concerned, only one original African marble from each single parent line ultimately survived to mother and father the rest of the world. As I have said, that is how we can argue there was only one exodus
(see
Chapter 1
). There is no diversity in one line. No, if we want to compare ancestral African genetic diversity with the rest of the world, we need genetic marker types that survived the founder effect and genetic drift of the exodus in greater variety and larger numbers than one line each. So we have to look at the rest of our genetic make-up that resides in the nucleus of our cells and is not solely linked to one sex or the other.

For clarity’s sake, my marbles analogy was grossly oversimplified. When we look at the surprisingly small proportion of our DNA that actually does anything useful, we find that for each gene there may be from one to a dozen slightly different versions. These versions can usually be linked together in a family gene tree. The base of each gene tree and its lower branches are usually predominantly African, whereas later branches and occasionally some of the earlier ones are found in different frequencies outside Africa. In addition, we can look at the non-functioning parts of our DNA that also progressively acquire extra packets of meaningless information which can, nonetheless, be utilized by geneticists as tracer junk mail. These extra packets may accumulate to the point where they constitute as much as 10 per cent of our entire DNA. The absence of specific extra packets in an individual generally means that they have retained the ancestral or African type.
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When we look at which populations have preserved most of our original African genetic heritage, we come back to Australia and New Guinea. These two landmasses together make up the huge, remote, and partially submerged Sahul Continental shelf and have retained more original African gene types than any other part of the non-African world.
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Today, however, New Guineans and Australians do not look alike (see
Plate 17
), and superficially the only population in the region who look at all like modern Africans are Melanesians, with their very dark skin and tightly frizzy hair. Even then, New Guinean highlanders, who are regarded as the ‘least diluted’ Melanesians from the original Pleistocene populations, are
much more robust-featured (heavy-boned with strong supra-orbital ridges – see
Chapter 5
) than present-day Africans. But this begs the question as to what Africans looked like at the time of the modern exodus around 80,000 years ago. Almost certainly they did not look exactly the same as the gracile (fine-boned) Bantus, Nilotics, Pygmies, or !Kung look today. The fossil Qafzeh and Skhul skulls, dated to between 90,000 and 120,000 years ago, are much more robust than those of modern African populations, suggesting that Africans may have changed more than New Guinean highlanders in this respect. Having lived for a number of years both in New Guinea and Africa, it is my guess that the reconstruction of one of the Qafzeh skulls commissioned for our documentary would today pass unnoticed in New Guinea, but might be harder to place in Africa (see
Plate 18
).

Genetic traces of the exodus among relict populations nearer to Africa

What of other peoples who may be genetically closer Africans as a result of their position on the old beachcombing trail – what can comparing the frequencies of retention of ancestral gene types in African and non-African peoples tell us? Two regions stand out. The closest (along with Australians and New Guineans) are those in Pakistan and the southern Arabian peninsula, followed by Europeans and Indians. Trailing a long way behind, with the least retention of African diversity and the most drift, are the peoples of East Asia and the Americas.
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The root position of Pakistanis and southern Arabians in retaining ancient African genetic diversity is certainly what we would expect from our proposed southern route out of Africa, and there are other pieces of evidence to support this. Along the south coast of Arabia are the isolated Hadramaut peoples, described by some as Australoid. Their maternal genetic make-up includes 40 per cent of
African genetic lines; but although some of these markers could be related to the founding Out-of-Africa Eve, the majority of such lines have arrived from Africa more recently. Farther along the Indian Ocean coast the peninsular Indian populations also group genetically closer to the African root than do more easterly Asian peoples. Indian ethnic groups, both caste and tribal, were included in a large study of nuclear autosomal (non-sex-linked) markers. They were found to retain a higher rate of the African ancestral types than do Europeans and other Asian groups.
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