The Journey of Man: A Genetic Odyssey (12 page)
Because Africa is largely tropical, its climate depends less on the variation in solar intensity that produces seasons in higher latitudes. African weather patterns are largely determined by rainfall, with pronounced wet and dry seasons setting the tempo of life throughout the
continent. The famous migration of the wildebeest in Kenya and Tanzania, for example, is triggered by the onset of the dry season in June. But the seasons have not always occurred with the same intensity, producing a climate that, in the past, has sometimes been wetter and sometimes drier than that today. These long-term fluctuations have almost certainly affected the movements of animals – including humans.
Recent research by Robert Walter, an American geophysicist based in Mexico, suggests that a large-scale drying up of the African continent at the onset of the last ice age resulted in modern humans favouring coastal environments. This is because savannahs are unusual places. They are closely related to tropical forests in the chain of climatic relationships, and the two zones are interchangeable depending on the level of rainfall. In general, the areas of tropical Africa with more than three months of low rainfall are savannah, while those with fewer than three are forest. If there are substantially longer dry periods, the environment grades into steppe, and ultimately into desert as moisture becomes extremely scarce. While these regions are all found in particular locations in present-day Africa, their past extent has fluctuated. What Walter’s research suggests is that as Africa began to dry up, the savannahs of eastern Africa were replaced by steppe and desert, except in a narrow zone near the coast. It was in these coastal savannah environments that early humans would have congregated, exploiting food sources from the sea as well as those of the land animals living near by.
While the universality of this theory is uncertain, and it may turn out to be a minor sideline of human evolution, one thing is clear: there is incontrovertible proof that early humans were able to live off of the sea. Large middens, or garbage dumps, of shells from clams and oysters have been found in Eritrea, on the eastern Horn of Africa, dating from around 125,000 years ago. These middens also have human stone tools interspersed among them, showing that humans were living in the region and exploiting coastal resources. The presence of butchered remains of rhinoceros, elephant and other large mammals conjures up a prehistoric ‘surf ’n’ turf’ feast reminiscent of the massive platters of steak and shellfish served in American restaurants. It seems that our
distant ancestors had quite well-developed palates, even in those days of apparent hardship.
One of the most exciting details to emerge from Walter’s work is the fact that there appears to have been exchange with coastal dwellers thousands of kilometres away, who were exploiting the same types of resources in southern Africa. This is suggested by the similarities in tools found at the sites, coupled with their roughly contemporary dates. It seems that humans were able to migrate over long distances, relatively rapidly, by following the coast of eastern Africa.
Now for the big leap: if humans could migrate over long distances within a continent, using the same technologies and exploiting the same resources, why couldn’t they do the same between continents? The coastal route would be a sort of prehistoric superhighway, allowing a high degree of mobility without requiring the complex adaptations to new environments that would be necessary on an inland route. The resources exploited in Eritrea would be pretty much the same as those in coastal Arabia, or western India, or south-east Asia, or – wait for it – Australia. And because of the ease of movement afforded by the coast, the line of sandy highway circumnavigating the continents, this would allow relatively rapid migration. No mountain ranges or great deserts to cross, no need to develop new toolkits or protective clothing, and no drastic fluctuations in food availability. Overall, the coastal route seems infinitely preferable to anything further inland. There were only a couple of sections of open water that would have required a boat to cross. Most likely these boats would have been rather simple – probably a few logs lashed together – but we have no direct evidence, because wood disintegrates very quickly. Nevertheless, they did make it across.
It is clearly plausible that the early presence of humans in Australia, almost immediately after they left Africa, can be accounted for by migration along this coastal route – beachcombing along the southern coast of Asia. There are two remaining pieces of the puzzle to be evaluated, though – rather critical ones, in fact. If one of the early waves of migration out of Africa followed a coastal route, is there a telltale genetic pattern? It depends on the way in which the migration occurred, and what the migrants did along the way. We might expect to see a band of particular genetic markers along the coast,
differentiated from the populations living further inland. Or perhaps the signals have been homogenized among descendants of the coastal dwellers and the land migrants. The only way to find out is to examine populations from along the route and see what the genetic pattern is. The second critical piece of evidence is to be found in the pattern of archaeological remains along the route – are they consistent with such a journey?
Mitochondrial DNA and the Y-chromosome, as we saw before, display deeper lineages within Africa than outside. What does this really mean? If we imagine the genetic relationships among modern mitochondrial diversity as an actual tree – say a large oak – then the root and trunk, and the branches that are closest to the ground, are all found in Africans. These branches sprouted first, as the tree was growing, and they are therefore the oldest. This means that the tree started growing in Africa. As we move further up the trunk, branches start to appear that are found in non-Africans. These formed later. How far up do we have to go before we find the non-Africans? The answer is pretty high. If the tree started growing 150,000 years ago – the age of the root – then the non-African branches are much closer to the top, and do not pre-date 60,000 years. Most of human evolution has been spent in Africa, so it makes sense that there is greater diversity there. Most of the branches on the tree are found only in Africans.
The beauty of the genetic data is that it gives us a clear, stepwise progression out of Africa into Eurasia and the Americas. The diversity we find around the world is divided into discrete, although related, units, defined by markers – the descendants of ancient mutation events. By mapping these markers on to the map of the world, we can infer details of past migrations. Following the order in which the mutations occurred, and estimating the date and any demographic details (such as population crashes or expansions), we can gain an insight into the details of the journey. And the first piece of evidence comes from one man in particular, who had a rather important, random mutation on his Y-chromosome between 31,000 and 79,000 years ago. He has
been named, rather prosaically, M168. More evocatively, he could be seen as the Eurasian Adam – the great … great-grandfather of every non-African man alive today. The journeys taken by his sons and grandsons defined the subsequent course of human history.
It is perhaps surprising that the clearest evidence for the route followed by our ancestors on their journey out of Africa comes from the Y-chromosome – surely men tend to ‘sow their oats’, causing the widespread dispersal of regional genetic signals? Oddly enough, no – and the rapid loss of ancient soup recipes on the male lineage (which we used to explain Adam’s recent date) means that men living in a particular area tend to share a recent common ancestor, providing us with clear ‘fingerprints’ of particular geographic regions. What this means is that the Y gives us the clearest evidence for the journeys followed by early humans. It is literally a ‘journey of man’, but it is the best tool we have for inferring the details of the trip. It is obviously important to examine the female lineage to see if it follows the same pattern – to make sure the fish stays with the bicycle, so to speak – but the Y-chromosome does provide us with the cleanest distillation of human migrational history.
As we look more carefully at the arrangement of branches on the mitochondrial tree, we find that there is a similar pattern – all of the non-African mitochondrial branches descend from a particular branch of the tree trunk, implying that our M168 Adam was paired with an Eve. Thankfully, this Eurasian Eve lived around 50–60,000 years ago, suggesting that she and Eurasian Adam could have met. She is called by the (again) rather mundane name L3, and her daughters accompanied the sons of M168 on their journey to populate the world.
Based on the distribution of the descendants of M168 and L3 in Africa today, it is likely that they both lived in north-east Africa, in the region of present-day Ethiopia and Sudan. Like all men alive today, M168 shared deeper roots with his African cousins. His lineage is a major branch leading off the human family tree, with his descendant ‘terminal branches’ found in the DNA of all of today’s Eurasians, but he connects them back through M168 to our species’ African root. In our tree metaphor, each marker that we study defines a node on the tree – a point where a branch splits into two smaller branches. If we had no markers apart from M168 and L3, our trees would be fairly sparse,
comprising a root (Adam and Eve) and one split on the tree, defined by M168 or L3, on the branch leading out of Africa, and another branch remaining in Africa. Luckily, the tree is packed with dense foliage, defining a pattern of growth that traces the map of our journey.
Intriguingly, on both the mitochondrial and Y branches, there is another split, immediately after M168 and L3, dividing the Eurasian branch structure into distinct clusters – two in the case of mtDNA, and three in the case of the Y
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. For both the Y-chromosome and mtDNA, one cluster is more common than the other(s), accounting for around 60 per cent of the non-African branches (or lineages) in the case of mitochondrial DNA, and more than 90 per cent in the case of the Y. In other words, the majority of non-Africans alive today have mtDNA and Y-chromosomes belonging to the more numerous clusters – people living all over the world, in places as disparate as Europe, India and South America. The rarer lineages, though, are found only in Asia, Australasia and the Americas. It is these rare lineages that constitute the majority of the mitochondrial and Y types in the Australian Aborigines.
Our rare mitochondrial cluster is given the name M – like the head of MI6 in James Bond movies. In biblical terms, Eve begat L3, and L3 begat M. According to recent research by Lluis Quintana-Murci, a Catalan researcher working in Paris, the distribution of the M cluster is indicative of an early migration out of Africa, which proceeded along the coast of south Asia, ultimately reaching south-east Asia and Australia. M is virtually absent from the Middle East, and is not found at all in Europe, but it constitutes 20 per cent or more of the mitochondrial types in India, and close to 100 per cent of those in Australia. Quintana-Murci estimates its age to be 50–60,000 years, and from its distribution it seems that people who carried the M lineage never made it into the interior parts of the Middle East. The most
likely explanation is that the ‘M people’ left Africa very early on, carrying their distinctive genetic signature across the south of the continent along the coastal highway.
Figure 3 MtDNA genealogy, showing the split into M and non-M lineages outside Africa.
And what about the Y? Is there a male counterpart to our M mitochondrial lineage? Luckily, the answer is yes. Again assuming biblical style, Adam begat M168, and M168 begat M130. M130 appears to have accompanied mitochondrial M on her coastal journey, and the present-day distribution of his descendants provides us with an insight into the nature of the trip. Like the M mitochondrial lineages, M130 Y-chromosomes are limited to Asia and America, but the dynamics of lineage extinction that we see for the Y have left a much more striking pattern than the one seen for their mitochondrial counterparts. M130’s descendants are virtually unknown west of the Caspian Sea, but they comprise a substantial proportion of the men living in Australasia. M130 is only found at low frequency in the Indian subcontinent – 5 per cent or less. But as we move further east,
the frequency increases: 10 per cent of Malaysian, 15 per cent of New Guinean and 60 per cent of Australian aboriginal men trace their ancestry directly to M130. There is a quirkily high frequency of M130 in north-east Asia, particularly in Mongolia and eastern Siberia, which suggests a later migration that we will revisit in
Chapter 7
. For the purposes of our Australian story, though, M130 provides us with a clear fingerprint of the coastal migration out of Africa.
