The Journey of Man: A Genetic Odyssey (24 page)
After 9000
BC
the eastern Mediterranean summer was becoming increasingly drier as the full effects of rising global temperatures kicked in. This reduced the yield of cereals, and (as with arid periods in the distant past) would have favoured mobility. However, the necessity of storing their gathered grain would have tied the Natufians to one
location. The pull of these two forces – reduced yields and a relatively immobile lifestyle – would, within a few hundred years, lead some Natufian settlements such as Jericho to try a new innovation: planting some of the gathered cereals (which are really seeds) in order to simplify the gathering process. Kenyon’s work at Jericho traced the development of the Neolithic, or New Stone Age, following this early innovation. Archaeologists and anthropologists continue to debate what happened after the first crops were cultivated – whether the need for a reliable water supply in order to grow crops led to the development of irrigation, which may have fostered water-rights issues and social hierarchies, and so on. What is clear is that the end of the ice age appears to have set in motion a series of events that were to culminate with the development of settled, agrarian communities within a thousand years. What archaeologist V. Gordon Childe called the ‘Neolithic Revolution’ had arrived.
The Neolithic marked a turning point for the human species. It was at this point that we stopped being entirely controlled by climate – as we were during our Palaeolithic wanderings – and began to assume control of our own destinies. By adopting agriculture, Neolithic humans initiated several developments that characterize modern civilization. The first is that of choice. The Natufians living at Jericho made a conscious decision not to wander for miles each day in order to gather their food. Rather, they decided to mould their environment to suit themselves, modifying the natural state of nature in order to favour human behaviour. While some hunter-gatherers practised forms of environmental control (the Australians, for instance, burned scrubland periodically in order to favour the grassland animals they hunted), the early agriculturalists of the Middle East, China and America were directly controlling the species in question. This gave them more choices as to where they could live, and would allow them to thrive in areas that had proved marginal for hunting and gathering.
The second development was that of greatly increased population density. One of the consequences of cultivating food and settling in
one place is that the necessity of not over-exploiting limited resources is relaxed – after all, if you want to have more children, you can simply plant and harvest more crops. While this does oversimplify the situation, it is true that settled, agrarian societies are more densely populated than those of hunter-gatherers. Coupled with the freedom to choose where to live, this can lead to very rapid population expansions, with agriculturalists spreading throughout a region. It is estimated by palaeodemographers, who study past population sizes using archaeological and anthropological methods, that the entire population of the globe was around 10 million at the time agriculture originated; by the dawn of the Industrial Age, around 1750, world population had risen to over 500 million. If Palaeolithic hunter-gatherer populations had taken over 50,000 years to increase from a few thousand individuals living in sub-Saharan Africa to a few million scattered around the globe, clearly the agriculturalists of the last ten millennia were making up for lost time.
The final new feature of the Neolithic revolution is that it demonstrates the importance of new technology to human migration. In much the same way as the central Asian steppe dwellers of 20,000 years ago used their technological superiority to occupy areas of Siberia that had been strictly off-limits to our hominid ancestors, so too did our more recent ancestors gain an adaptive advantage from technology. The first major technological development of the last 10,000 years was agriculture, and it would set in motion a massive acceleration of human social evolution. In fact, as we will see, it would be over 9,000 years before a similarly important series of developments would initiate another era of human evolutionary history. Clearly the development of agriculture was a pivotal event. If the Great Leap Forward had set the stage for the first Big Bang in human history, which led us to colonize the world, agriculture was to set in motion the second – one that would send our species hurtling into the modern age.
While agriculture played a critical role in the development of modern society, the genetic effects of agriculture were equally pronounced. While Upper Palaeolithic hunter-gatherers tended to maintain a relatively stable population size, except via the settlement of new territory, agrarian societies were able to expand massively without leaving home. As the first farming communities increased in size, their inhabitants gradually moved further afield in search of cultivable land. When they did so, they carried with them their genetic markers. One of the consequences of this is that we see the expansion of certain genetic lineages, giving us a glimpse of the origin and spread of agriculture. In the case of the Middle East, the genomes of today’s western Eurasians still retain a signal of those events at Jericho 10,000 years ago.
Archaeologists had long known that agriculture spread from its origin in the Middle Eastern ‘Fertile Crescent’ to Europe over the course of several thousand years. The earliest evidence is found in the Balkans, and it appears later and later as you move to the north-west. It is only relatively recently that ancient Britons left behind their hunter-gatherer lifestyle, several thousand years after their cousins in Jericho had done the same. Crucially, it is exactly those plant species initially cultivated in the Fertile Crescent that make their appearance in the advancing wave of agriculture as it moved into Europe. It seems that the European hunter-gatherer lifestyle was replaced by the new Middle Eastern invention.
In the 1970s Luca Cavalli-Sforza, along with fellow geneticists Alberto Piazza and Paolo Menozzi, initiated a study of the genetic effects of agriculture. The question that they asked was about the way in which agriculture had spread. In particular, they wanted to know if the migration of agriculture into Europe marked the migration of people, or simply the spread of a sexy new cultural development – the MTV of its era. In effect, they were asking a question about the genetic composition of modern Europeans. Was there evidence for an expansion of certain genetic markers out of the Middle East, or did modern Europeans appear to be relatively free of Neolithic markers?
At the time the study was done, the only data available was that on
the ‘classical’ markers we learned about in
Chapter 2
– blood groups and other cell-surface protein markers that served as convenient polymorphisms but gave little information about their underlying DNA sequence changes. The analysis of these markers led Cavalli-Sforza and his colleagues to conclude that there had been a mass migration of genes out of the Middle East, and the genetic pattern was very similar to that observed for the first appearance of agriculture: the genetic signal decreased regularly as you moved from south-east to north-west across Europe. The methods of analysis used in this study limited what the researchers were able to infer, since it wasn’t possible to obtain an accurate date for this migration, but their findings did corroborate the theory that agriculture had spread with farmers as their population had expanded, rather than as a purely cultural phenomenon that ‘migrated’ as Palaeolithic Europeans learned farming skills.
Cavalli-Sforza’s results became accepted wisdom, leading to what they called the ‘Wave of Advance’ model for the diffusion of agriculture. The assumption made by many (although not Cavalli-Sforza and his colleagues) was that the majority of the European gene pool was Neolithic in origin, since it was the most pronounced genetic pattern in Europe (although Cavalli-Sforza’s later work showed that it still accounted for less than a third of the genetic variation). Many anthropologists remained sceptical, but it was to be over twenty years before the model received a serious re-evaluation. This came in the late 1990s with the detailed analysis of mtDNA lineages in Europe and south-west Asia by Martin Richards and his colleagues at Oxford University. In a series of scientific papers they analysed mtDNA lineages from a selection of populations across Europe and the Middle East, carefully dating the lineages using the absolute methods similar to those we learned about earlier. This allowed them to evaluate the relative contributions of various migrations to the European gene pool. Their results suggested that, rather than having a significant genetic effect on the population of Europe, the expansion of agriculture involved very few Middle Eastern migrants. Most of the lineages in Europeans seem to have been present since the time of the Upper Palaeolithic, between 20,000 and 40,000 years ago.
One of the objections to the Richards study, raised by Cavalli-Sforza and others, was that mtDNA actually provided very little resolution
between European populations. It was difficult, for instance, to distinguish between eastern and western Europeans with mtDNA data alone – they have very similar patterns of mtDNA markers. Nonetheless, the mtDNA result was suggestive. What was needed was to look at the male lineage, with its greater inherent resolution, in order to see if it showed the same pattern.
This was finally done in 2000, when Ornella Semino and her colleagues (among them Cavalli-Sforza) analysed the Y-chromosomes of over 1,000 men from Europe and the Middle East, looking specifically for evidence of the agricultural expansion. What they found was that lineages defined by Neolithic Middle Eastern markers are found in a minority of modern Europeans. In fact, the results from the Y agree almost perfectly with the mtDNA data, suggesting that 80 per cent of the European gene pool traces back to other waves of migration, primarily during the Palaeolithic. In western Europeans, this Palaeolithic component is largely defined by our friend from the last chapter M173, which links Europe back to central Asia. Only 20 per cent of European Y-chromosomes – defined by more recent markers, particularly one known as M172 – derive from Neolithic Middle Eastern immigrants. In effect, modern Europeans are largely genetically Cro-Magnon on both their maternal and paternal sides.
This is not to say that the advent of agriculture had no effect on Europe – far from it. There is clear genetic evidence for a significant European population expansion after the end of the last ice age, almost certainly aided by the onset of food production. Evidence for this comes in the form of a recent analysis by David Reich and his colleagues at Massachusetts Institute of Technology. They studied markers from many independent regions of the genome and found a pattern of variation suggesting that the European population underwent a substantial decrease in population size between 30,000 and 15,000 years ago, as Europe was moving into the depths of the last ice age. This was then followed by a population expansion from the few survivors after the end of the last ice, producing the relative dearth of variation seen in Europe today. In other words, the human population had been through what is known as a bottleneck – a reduction in size followed by a period of growth. Patterns of mtDNA variation also support this model of postglacial population growth. Archaeological evidence
suggests that the Palaeolithic population of Europe was confined to Iberia, southern Italy and the Balkans during the period of most extensive glaciation around 16,000 years ago, and that human populations then expanded northward during the postglacial period. Agriculture almost certainly contributed to the end of this population expansion, because it allowed much higher population densities.
How do we reconcile the pattern seen for the Y-chromosome and mtDNA, of a Palaeolithic European population relatively unaffected by Neolithic immigration, with the Wave of Advance? The pattern seen by Cavalli-Sforza and his colleagues clearly exists, but they were studying large-scale patterns across the entirety of Europe and the Middle East. The agricultural expansion was simply one population movement into Europe – there is clear archaeological evidence for several others. As their later analysis showed, it still accounted for a minority of the genetic variation in Europe. Furthermore, because the Wave of Advance had no estimate of age, the Neolithic component could have been confounded with Palaeolithic immigration from the Middle East. Finally, since central Asian populations were not included in their analysis (there was no data available at the time their study was conducted), it is possible that the pattern reflects a general trend of migration from Asia to Europe during the Upper Palaeolithic. After all, if we simply had Y-chromosome data from the Middle East and Europe, we would infer that M89-bearing populations had migrated into Europe via the Balkans, giving rise to M173 in Europe. It is only because we know that M173 arose on an M45-containing lineage that we trace the Upper Palaeolithic settlement of Europe back to central Asia.
The Y data actually provides a partial solution to the apparent conundrum. It seems that southern European populations experienced a greater influx of Neolithic farmers from the Middle East, carrying lineages such as M172, than did northern Europeans. One possible scenario is that farming spread first around the Mediterranean, with Neolithic Middle Eastern immigrants favouring its climate, similar to that of the Levant. Only later did indigenous Palaeolithic Europeans take up agriculture in the interior, gradually spreading the culture – but only a small percentage of the genes – of the Neolithic throughout. The Cro-Magnons of northern Europe appear to have made a
conscious decision to leave behind the Palaeolithic for a new Middle Eastern lifestyle with a small minority of Middle Eastern immigrants.
While the complexity of Neolithic spread in Europe makes a simple interpretation of the genetic data difficult, the situation in the other major centre of Asian domestication is a bit clearer. The pattern of settlement and intense exploitation of a few plant species that characterized the Middle East was seen at around the same time in China. Northern Chinese sites such as Banpo and Zhangzhai in Shaanxi province show early evidence of millet agriculture, around 7000
BC
. Millet, a cereal crop like wheat, seems to have been domesticated around the Yellow River, spreading from there to the rest of northern China. The remains at Pengtoushan, on the Yangtze River in central China, indicate that rice was being cultivated there independently around the same time. At both sites pottery was used for storing grain, and the people lived in carefully constructed clay houses, suggesting that the Neolithic lifestyle was well developed, even at this early date. Agriculture soon spread throughout China, with rice dominating in the south, where the wet, humid conditions favoured this grain. Rice agriculture spread down the Yangtze, and was widespread in southern China by 5000
BC
, perhaps helped by a second, independent domestication along the south coast. By 3500
BC
it was being cultivated in Taiwan, and by 2000
BC
in Borneo and Sumatra. It reached the rest of the Indonesian archipelago by 1500
BC
. Overall, the archaeological evidence suggests that rice agriculture spread from an origin in central-southern China to the islands of south-east Asia in the space of roughly 3,000 years – timing similar to that of the agricultural expansion into Europe. However, unlike in Europe, there is a very strong genetic signal of this expansion, suggesting that it was people, and not merely the culture, that moved.
