The Journey of Man: A Genetic Odyssey

by Spencer Wells

  In Chapter 6 we learned that one descendant lineage of M9, defined by a marker known as M175, is widespread in east Asia. Based on its present distribution, this marker probably arose initially in northern China or Korea. Looking at the pattern of Y variation in modern Chinese populations, it is now clear that the first agriculturalists in China were descendants of M175. In fact over half of the entire male population of China have Y-chromosomes defined by a marker that shows evidence of a massive expansion in the past 10,000 years. M122, which first appeared on an M175 chromosome, is now widespread throughout east Asia. It is hardly found west of the great central Asian mountain ranges, and does not occur at all in the Middle East or Europe. This is the pattern we expect to see with a recent expansion, rather than an ancient event that typically leaves a more widespread trail.

  The genetic data shows that the development of rice agriculture in east Asia created a Wave of Advance. However, while the wave leaving the Fertile Crescent for Europe seems largely to have dissipated after inundating the Mediterranean, the one leaving China was to saturate the entirety of east Asia. Today, M122 – marking the descendants of the first Chinese rice agriculturalists – is found from Japan to Tahiti. A recent study by David Goldstein and his colleagues at University College London shows that microsatellite diversity on M122 chromosomes is very high in China and Taiwan, but drops significantly moving southward into peninsular Malaysia and Indonesia. This is precisely what would be expected from a population expansion originating in China within the past 10,000 years – and exactly parallels the archaeological evidence for the spread of rice agriculture. Together, M122 and a related Chinese haplotype (also a descendant of M175) defined by marker M119, account for nearly half of the Y-chromosomes in south-east Asia. In Europe, on the other hand, Neolithic immigrants account for only 20 per cent of the present Y diversity. In comparison to Europe, the Wave of Advance in east Asia appears to have been more of a tsunami.

  Double-edged scythe

  The massive population expansion made possible by the adoption of agriculture, whether it was through population growth of the originators themselves (as in east Asia) or the people who adopted agriculture from them (as in much of Europe), suggests that this innovation was nothing but good news. After all, if success is broadcast through excess, a massive increase in agricultural populations must have meant that life improved after the Neolithic transition. Recent evidence suggests that this may not have been the case.

  Early agriculturalists were taking on a new set of risks when they committed themselves to a settled existence. The most important was a decrease in the breadth of their resource base. By focusing cultivation on a few species, they were reducing their choices in the event of a climatic shift. Droughts, intense periods of cooling (such as the Dryas periods at the end of the last ice age) and shifts in watercourses were all very easy to deal with for Palaeolithic hunter-gatherers. Their response to any of these changes was to move into another area with better resources. The great migrations of Palaeolithic humans – those covered in Chapters 4 to 7 – were almost entirely determined by the climate. Once humans adopted agriculture, though, they were loath to move. This led to occasional famines, such as those seen today in many parts of the developing world. In the early days of agriculture, during the turbulent climatic conditions of the early postglacial period, famine episodes would have been even more likely.

  The second main worry for our Neolithic agriculturalists was the increase in disease. While hunter-gatherers may appear to have had a difficult life, relying as they did on apparently ‘primitive’ technology and the necessity of killing or gathering enough food to survive, in fact they were surprisingly healthy. While the incidence of broken bones and wounds is greater for Palaeolithic humans than for their sedentary Neolithic descendants, they do not appear to have died younger. In fact, the skeletal remains from early agricultural communities suggest that early agriculturalists may actually have had a shorter lifespan than their hunter-gatherer neighbours. This is thought to be due largely to an increase in disease.

  Infectious diseases do not arise spontaneously as a by-product of a settled lifestyle, but rather from exposure to disease-causing organisms in such a way that transmission occurs from one infected individual to another. Most diseases can exist only in large populations, where a threshold number of people remain infected, allowing the disease to remain in the population. These are so-called endemic diseases, such as smallpox or typhoid. A population of several hundred thousand is necessary to maintain the disease – otherwise it is lost because not enough people remain susceptible to infection. Populations of this size only arose after the development of agriculture. Other diseases can be introduced from an outside source, such as an animal. While humans had contact with animals as hunter-gatherers, the sort of prolonged, close contact that encourages the spread of disease occurred only after the domestication of animals in the Neolithic. Measles, for instance, is closely related to rinderpest, a disease of cattle. It is likely that the domestication of livestock around 10,000 years ago introduced this disease into Neolithic populations. Historian William McNeill has suggested that many of the plagues described in the Bible may have had their origin in the outbreaks of epidemic disease during the early days of the agricultural transition in Eurasia.

  The final negative aspect of a sedentary lifestyle was the growing stratification of society. In general, hunter-gatherers are remarkably egalitarian, having few social divisions. Typically, taking modern-day populations such as that of the San or Australian Aborigines as a model, there is a group leader who sits in judgement over some aspects of group life, but no formalized set of social divisions such as the ones that exist in settled societies. Perhaps because there is simply less to fight for (in terms of accumulated wealth), large-scale warfare is rare in hunter-gatherer societies – although inter-group battles do occur. The massive growth in population during the Neolithic created conditions in which some form of social stratification was inevitable. Once this occurred, the seizure of power and the growth of empires was not far behind, which led to war on a scale that had never been seen in the Palaeolithic. And while warfare was bad enough on its own, it also had a knock-on effect on other aspects of Neolithic life. The high mortality associated with large-scale warfare was probably exacerbated by the spread of disease and the destruction of cropland during the hostilities, leading to a vicious chain reaction of mortality.

  Given all of the negative aspects of the Neolithic revolution, why did our ancestors still embrace their new lifestyle? Not everyone did, in fact – small pockets of hunter-gatherers existed in almost every region of the world until quite recently. Their reasons for maintaining an ancient lifestyle probably had something to do with the environment (for instance the San and the Australian Aborigines live in marginal, arid environments that are difficult for agriculture), as well as a conscious decision to remain hunter-gatherers. For the rest of the world’s population, though, there was no turning back. It is possible that the shift in thinking that allowed humans to accept agriculture, in spite of all its negative aspects, could have occurred in a few generations. Once the collective memory of hunting and gathering was replaced by one involving food production, it would have been virtually unthinkable to return to the old ways. Ask yourself if you would be prepared to make weapons and hunt for your dinner – most of us would probably say no.

  Babbling

  The onset of the Neolithic established many of the regional patterns of cultural diversity we see in the modern world. Expanding waves of agricultural migrants in east Asia spread rice cultivation to Indonesia and beyond, and today their descendants still carry the genetic traces of this event. As we saw earlier, the first inhabitants of south-east Asia may have been more similar to today’s Andamanese or Semang Negritos. It is likely that most of these groups were engulfed by the wave of expanding rice-growers, their culture subsumed into the agricultural mainstream. Similarly, hunter-gatherer groups in Europe, the Americas and Africa
all gave up their Palaeolithic lifestyle in favour of the new way of feeding themselves. But culture is defined by far more than eating – it encompasses social traditions, clothing and tool-making styles, means of transport and thousands of other things. And one the most important aspects of culture is language.

  Most American visitors to Britain soon notice the huge number of regional accents. If London is the first stop, then the Cockney accent will be one of the first ones they encounter. Even if they’ve been practising their Dick van Dyke equivalent (‘Cor blimey, Mary Poppins!’), it is sometimes difficult for them to believe that the same language is being spoken. My English wife finds it similarly perplexing to talk to some of my American friends from the South. George Bernard Shaw was right when he noted that the Americans and British are two people separated by a common language – and he wasn’t even taking into account local variation within each country. Accents are familiar examples of language variation, and the difficulty we may have in understanding them reveals an insight into the process of language change. Languages are not uniform entities, in spite of the efforts of the Académie Française to impose order on the rowdy French populace. As with any aspect of culture, there is a great deal of variation from place to place. But does the apparent chaos of linguistic diversity reveal anything about the spread of human cultures?

  Language similarities had been recognized since Classical times, particularly among such well-studied European examples as Latin, French, Spanish and Greek. By the eighteenth century scholars had begun to take a broader view, focusing on the languages of Asia, Africa and the Americas. For instance Janos Sajnovics, in his obscure 1770 treatise ‘Demonstration that the language of the Hungarians and Lapps is the same’, arrived at the conclusion given in the title. We now know that both Hungarian and Lapp belong to something known as the Uralic language family, uniting them with more obscure languages such as Khanty, Nenets and Nganasan. Sajnovics, though, was unaware of these more distant relationships. And while he, like many other scholars, recognized the similarities uniting different languages, he crucially failed to explain how they had arisen.

  An explanation for the similarities among members of a language family arrived a few years after the Sajnovics study. In a 1786 address to the Royal Asiatic Society, Sir William Jones – then a judge in India – noted that Sanskrit (the religious language of Hinduism) bore a closer resemblance to Greek and Latin ‘both in the roots of verbs and in the forms of grammar, than could possibly have been produced by accident’. So much so, he concluded, that they must ‘have sprung from a common source’. It was this last statement that was to be his most lasting contribution, since it implied a mechanism for the generation of linguistic diversity. Languages change over time, he was saying, and if there are enough deep similarities among a group of languages, then they must have had a common ancestor in the past and subsequently diverged from each other. It was an evolutionary explanation for linguistic diversity, presaging Darwin by over sixty years.

  The languages that Jones described all belong to what became known as the Indo-European language family, after the geographic locations of the languages. There are 140 separate languages in the family, ranging from those belonging to the Celtic branch, spoken in the extreme north-western parts of Europe (Gaelic and Breton are two examples), to Sinhalese, spoken in Sri Lanka. English is a member of the Germanic branch of Indo-European, although its complicated history has left it with many words borrowed from French. Clearly, this is a widespread and diverse collection of languages.

  Today the hypothesis that Jones advanced – that all of the Indo-European languages trace their descent from a common ancestor – is widely accepted by linguists. In fact it is one of the few language families to have received universal acceptance. The implication of his model, known as the genetic model of language classification, is that at some point in the past there was a group of people who spoke an ancestral form of Indo-European, which later evolved into the languages we see today. Like our soup recipes, additions and modifications of ingredients have produced local linguistic varieties, which eventually became distinct languages. The parallels with DNA evolution seem obvious. But is it possible to learn anything about language diversity – and to understand the present distribution of the world’s languages – from the study of genetics?

  The subject of language change has always been a key interest of Luca Cavalli-Sforza, particularly its overlap with genetic patterns. Instead of drawing vague comparisons between genetic and linguistic diversity, in 1988 he decided to test the hypothesis directly – much like Dick Lewontin had done with the genetic data from different races. He and his colleagues examined genetic data from forty-two worldwide populations and drew a tree of their relationships based on minimizing the differences in marker frequencies between them. The tree that resulted – in effect, a genealogical tree of the populations – corresponded very well with known linguistic relationships. So, for instance, speakers of Indo-European languages tended to group together in the genetic tree, as did speakers of Bantu languages in Africa. There were obvious inconsistencies, such as the deep split between northern and southern Chinese (almost certainly resulting from the pattern of early migrations discussed in Chapter 6), but overall the genetic and linguistic groups seemed to be very similar to each other. This suggested that genetic data could be used to study the origin and dispersal of languages.

  There were two caveats made by Cavalli-Sforza and his colleagues in their study. The first is that the genetic markers they were studying did not cause the pattern of linguistic diversity – there was no Bantu gene that forced its hapless carriers to speak those languages. Rather, similar genetic markers reflected the common history of the speakers of that language, as markers of descent. The second caveat is that in many cases relationships suggested by genes and languages disagreed, showing that the correspondence wasn’t absolute. The reasons for this might be language replacement, in which people learn to speak a new language without a corresponding influx of outside genes, or gene replacement, in which there was a significant influx of genes but the language stayed the same. The first explains the difference between northern and southern Han Chinese, while the second may explain the close genetic similarity between linguistically unrelated groups, such as Na-Dene-speaking Native Americans and neighbouring Amerind speakers. Thus, genes were often markers of linguistic relationships, except when they weren’t. Either way, the genetic data should help to shed light on language relationships, by illuminating the way in which languages have spread.

  In search of a homeland

  If we accept that William Jones was right, and that all Indo-European languages descend from a common source, then the implication is that there must have been, at some point in the past, a single group of people who spoke this ancestral form of Indo-European. The search for the identity of the first Indo-Europeans, and their geographic location, has been one of the main areas of archaeological and linguistic investigation over the past 200 years. It has become a sort of quest – although like all good quests, it is in some ways rather quixotic. The attempt to disentangle the web of conflicting evidence surrounding the location of the Indo-European ‘homeland’ illustrates a particularly exciting new application of genetics to our understanding of human history.

  Gordon Childe, who coined the term ‘Neolithic revolution’, proposed in the 1920s that the Indo-European homeland should be identified with a culture originating north of the Black Sea that had distinctive ‘corded’ pottery – marks that resembled impressions left by cord or twine. The theory was revived by archaeologist Marija Gimbutas in a series of articles published in the 1970s. Gimbutas argued that the remains left by nomadic horsemen of the southern Russian steppes, dating from around 6,000 years ago, mark the earliest signs of a culture that can be identified as proto-Indo-European (PIE), which included Childe’s Corded Ware people. The Kurgan culture, as she called it, left enormous burial mounds (known as kurgans) that are still dotted across the entirety of t
he Eurasian steppe, from Ukraine to Mongolia and south to Afghanistan. The golden treasure hoards recovered from kurgan excavations in the twentieth century confirmed the existence of a people who were known to Herodotus as the Scythians – fearsome horsemen of the Asian grasslands, and previously thought by many scholars to be mythical.

  The evidence that the Kurgan people spoke PIE is based on an analysis of words common throughout Indo-European languages. If a word can be shown to derive from the same root, then it is likely (although not certain) that it was inherited from the common ancestor. For instance, the English word ox has cognates in the Sanskrit uksan and the Tocharian (an early Indo-European language spoken in western China) okso. Similarly, many words for animals and plants are common throughout the Indo-European languages, as are those for tools and weapons. Perhaps most interestingly, there is a rich vocabulary for horses and wheeled vehicles in common among all the languages, suggesting that the PIE speakers had domesticated the horse as a draft animal. Coupled with the archaeological remains showing that the horse was domesticated in the southern Russian steppes, this pointed toward the kurgan-builders being the PIE people.

  But while the evidence in favour of the Kurgan people being early Indo-Europeans was compelling, there was no archaeological evidence for the spread of their culture into western Europe. Their culture, dominated by horses, was ideal for the steppes, but it was not well suited to European forests and mountains. It was difficult to see why the steppe horsemen would have been able to conquer Europe and impose their language upon its inhabitants. For this reason, Colin Renfrew proposed in his 1987 book Archaeology and Language that the Kurgan culture did not mark the origins of Indo-European, but rather a later, eastern extension of it. Renfrew suggested that PIE had been a Middle Eastern language, originally spoken 9,000 years ago, which spread with the agricultural Wave of Advance into Europe. He identified Anatolia as the Indo-European homeland, on the basis of it being roughly central in the modern distribution of Indo-European languages, and also the home of several extinct examples. The hypothesis he advanced is that the early farmers carried their language – PIE – with them as they expanded their population, and thus the linguistic inundation of Europe should have involved a genetic wave as well. It was a bold suggestion, which had little initial support from the linguistics community. As we have seen, the Wave of Advance actually contributed little to the gene pool of modern Europeans, and its influence seems to have been largely limited to the Mediterranean region. The Indo-European speakers living in Ireland, for instance, have virtually no Neolithic Y-chromosome markers, while Greeks have a substantial Neolithic component. What this suggests is that, if farming spread Indo-European languages throughout Europe, it must have done so largely without the actual spread of farmers – thereby reducing the strength of Renfrew’s argument.