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The Neolithic Transition and the Genetics of Populations in Europe
By Albert J. Ammerman, Luigi Luca Cavalli-Sforza
PRINCETON UNIVERSITY PRESSCopyright © 1984 Princeton University Press
All rights reserved.
Human evolution is now thought to extend back well over two million years, but it is only during the last ten thousand years that food production has emerged as the main way by which humans provide for their subsistence. Previously, people met their food needs by means of hunting and gathering — that is, through the exploitation of seasonally available wild animals and plants. Agriculture, therefore, is a recent development when viewed in terms of the full course of human evolution. Associated with the shift to farming as a way of life are changes in technology, demography, and social organization. In Europe, this transformation did not occur as a sudden event but involved processes that required many generations to be worked out. Thus, rather than thinking in terms of a "neolithic revolution," as did the prehistorian V. Gordon Childe, we prefer to speak of a neolithic transition. In trying to account for the spread of early farming in Europe, we began to realize that the neolithic transition may have a major bearing on patterns of genetic variation observed among human populations in Europe. The interpretation of such patterns has long represented a challenge to population geneticists. There is the intriguing possibility that two seemingly unrelated research problems, one concerned with cultural development in the remote past and the other with the genetic structure of living populations, are closely linked.
At first glance, it would appear to be surprising for two researchers such as the authors, coming from fields as different from one another as prehistoric archaeology and population genetics, to find themselves working together. When we first began our collaboration in 1970, it was far from clear where the study would lead, and much of our initial effort was spent in trying to bridge the differences in our backgrounds. As various lines of investigation have unfolded over the last twelve years, it has been our experience that basic questions have become more clearly defined, and there is a better sense of how answers can be obtained. The aim of this book is to bring together the various studies that we have undertaken and to attempt a synthesis of the work that has been done in collaboration.
Very few populations in the world today live on the basis of hunting and gathering; almost all of the some five billion people composing the world's current population are sustained by foods derived from agriculture. If we could go back to 10,000 B.C., we would find that almost all populations had subsistence economies based solely on hunting and gathering (see figure 1.1). Moreover, rough estimates would place the world's total population at that time on the order of ten million people. Our ideas about hunting and gathering as a way of life are drawn primarily from ethnographic studies of surviving hunter-gatherer groups, such as the bushmen and pygmies of Africa and the aborigines of Australia. One obvious limitation to such studies from a broad, comparative point of view is that hunter-gatherers tend to survive only in more extreme environmental settings or in those not suited to the practice of agriculture. Allowing for this possible bias, some common features of hunters and gatherers can be recognized, such as their small population sizes and their low population densities. Most hunter-gatherers exploit a wide variety of plant and animal foods and do not experience food shortages. Contrary to the view once held that hunter-gatherers lead a harsh life dominated by an endless quest for food, they appear able to meet their subsistence needs in most cases through the expenditure of only modest amounts of effort. It is common for hunters and gatherers to live in small camps that range in size from 15 to 40 people. Such a group or band, as anthropologists would refer to it, often follows a seasonal pattern of movement, occupying a series of different camps during the course of a year. The light and easily transported material culture that such groups usually possess would seem to be well adapted to a nomadic life style. One of the striking features of hunting and gathering, especially in contrast with modern ways of life, is the inherent ecological balance that this way of life entails. As anthropologists have come to take a more positive view of hunters and gatherers, the explanation of the origins of agriculture has become a more complex matter. In the next chapter, instead of regarding agriculture as an innovation providing a solution to recurrent food shortages among hunters and gatherers, we shall ask: Why did the shift to food production occur at all, and why did it take place in certain contexts and not others?
At archaeological sites such as Tell Aswad in the Near East, which dates to the eighth millennium B.C. (see Table 2.1), we first begin to see positive evidence for the cultivation of cereal crops. Since domesticated plants and animals are required for food production, the processes of domestication may have started at an even earlier date. The domestication of a plant or animal involves human interference in the reproduction of a population, leading to changes over time in its morphology and behavior. Traits considered to be favorable for purposes of food production, such as a more uniform time of ripening in domesticated strains of wheat, are obtained by means of artificial selection. Various aspects of plant and animal domestication and current explanations for the origins of agriculture in southwest Asia will be examined in Chapter 2. Well-developed systems of food production based on the cultivation of wheat and barley and the herding of sheep, goats, pigs, and cattle are documented by 6000 B.C. at archaeological sites that have a geographic distribution from Iran to Greece. With the exception of Greece, early farming sites are not observed in other parts of Europe at this date. Over the next two thousand years, agriculture as a way of life was established in most areas of Europe. The middle chapters of this book are devoted to the basic question of how this transition occurred. The early farming cultures that are found in different regions of Europe are introduced in Chapter 3. It is worth noting here that wheat and barley were not domesticated locally within various regions of Europe but were introduced into Europe from the Near East. Insight into how this transfer occurred can be gained, as we shall see in Chapter 4, by measuring the rate of spread of early farming in Europe. This was the initial question that we explored at the beginning of our collaboration. The slow, regular pattern of the spread as a whole suggested a model for explaining the diffusion of early farming that is developed in Chapter 5. The wave of advance model, as it is called, regards population growth and local migratory activity as setting up an expanding population wave that advances at a steady radial rate. The model has major implications for the patterns of genetic variation that would be established among populations in Europe, if such a population expansion did occur.
At the conceptual level, it is worth distinguishing two modes of explanation for the spread of early farming in Europe. The first of these involves cultural diffusion, by which cereals and farming techniques are passed from one local group to the next without geographic displacement of groups; the second is what we have called demic diffusion, where the spread is due to the movement of farmers themselves. These two modes of explanation are not necessarily mutually exclusive: the real question may well be to evaluate their relative importance in different regions of Europe.
As mentioned in the preface, archaeologists have become more cautious in their use of diffusionary explanations during the last twenty years, in part as a reaction against the abuses of diffusionism in the earlier literature. One of the main problems was that the distribution of a given object or trait was often considered to explain in itself what was happening; no real explanation of the process or processes involved was actually put forward. In addition, diffusion seemed to imply that cultural change derives in large measure from external factors. More recent literature places increasing emphasis on explaining cultural change in terms of endogenous factors or developments taking place within a society or population. Yet, although the reaction against diffusionism has been a positive one in many respects, there has also been some failure to realize that diffusionary processes can occur in some cases as a result of events essentially at the local level. Moreover, adopting a regional perspective, as is frequently done in the more recent literature, opens the temptation to regard the region itself as an actor in a process, when it may only be the spatial framework or stage setting for the play. The actors are more likely to be people or cultures, and their drama as it is acted out over time may not always stay within the confines of their original theater.
As indicated above, the hypothesis of a demic expansion of early farming in Europe has major genetic implications. In Chapter 6 we turn to the subject of human genetics and the development of methods for characterizing genetic patterns among European populations. During the last forty years considerable strides have been made in understanding how the forces of evolution operate at the population level. At the same time, a vast body of data on gene frequencies has accumulated for human populations in Europe. The interpretation of maps of gene frequencies has nevertheless long remained a challenge to geneticists. When we look at maps of individual genes, such as those belonging to the Rh and ABO blood group systems, we usually see quite different patterns of variation from one map to the next. What are the factors that produce these differences? To what extent can we recognize common patterns for different genes? In order to answer the latter question, new methods for analyzing many different genes all at the same time had to be introduced, and considerable effort was spent to find methods appropriate for the generation of synthetic gene maps. Once such maps were obtained, they showed an interesting relationship with patterns expected under the demic hypothesis for the spread of early farming in Europe. From the viewpoint of the geneticist, order began to emerge from what had been a chaotic and uninterpretable accumulation of genetic facts. The underlying patterns were apparently connected with major cultural developments in the past that had left a deep imprint on the genetic structure of populations, and the patterns persisted during the course of those populations' subsequent histories.
As one way of evaluating whether the approach we adopted to the analysis of gene maps is an appropriate one, simulation studies were undertaken that involved populations in Europe experiencing first genetic differentiation and then, with the introduction of agriculture in southwest Asia, a demic expansion. This work, described in Chapter 7, provides support for the methods employed in generating the synthetic gene maps. From a heuristic point of view, the construction and operation of a simulation model also provide a means of exploring the internal logic of a process. In the present case, the simulation studies are particularly useful in drawing attention to the important role played by interactions between farming and hunter-gatherer populations during the spread of early farming and in showing how genetic change at the population level can occur in the context of cultural events.CHAPTER 2
THE ORIGINS OF AGRICULTURE
The shift from hunting and gathering as a way of life to a reliance upon food production represents one of the major transformations in the course of human evolution. Although the origins of agriculture have long been a subject for inquiry, it is only during the last fifty years that archaeological investigations have begun to provide direct lines of evidence. For example, we now have a reasonably clear idea about when and where domesticated forms of cereals such as wheat and barley made their appearance in southwest Asia. At the same time, a fair amount of evidence is now available on the cultural context in which these early crops were domesticated. It is of equal interest, as we shall see later in this chapter, to consider how early agriculture developed as an economic system.
Familiarity with our own agricultural and industrial ways of life makes it difficult for us to appreciate the scope of the transformation brought about by food production or to imagine what life would be like without agriculture. Studies of contemporary groups of hunters and gatherers such as the bushmen and pygmies in Africa and the aborigines of Australia provide some insight into this way of life. As mentioned in Chapter 1, hunter-gatherers usually live in small bands that shift their camps at various times during the course of a year. For food, they exploit a wide range of seasonally available wild plants and animals. Recent studies indicate that, rather than having to expend a good deal of time and energy in fulfilling their dietary needs, hunters and gatherers in most cases can sustain themselves with only a modest work effort and that they actually have a fair amount of leisure time. Food is not commonly stored for future use but is usually consumed within a few days of its collection. Low population densities, which, except among those exploiting fish, seldom reach the level of one person per square kilometer, appear to be a common feature of this way of life. As mentioned before, a rough estimate of ten million people has been put forward for the total world population of hunters and gatherers at 10,000 B.C., the period just prior to the origins of agriculture. This figure compares with a current world population of some five billion people.
Concentration on the production of one or only a few crops seems to be a regular feature of most of the farming systems practiced in the world today. Three cereal crops — wheat, rice, and maize — account for a combined annual production of almost one billion metric tons, providing the bulk of the food consumed by the world's current population. Properties shared by these three cereals are their comparatively high caloric values as foods and their high yields as crops. Even in cases where subsistence forms of farming are still practiced, yields are usually well over half a ton per hectare of land. Such yields are by no means a recent development; estimates on the order of one ton per hectare have been made for wheat yields in Mesopotamia during Sumerian times as well as for ancient Greece and Rome. In situations where more modern or intensive techniques of farming are employed, yields of several tons per hectare can be obtained. One implication of such yields is that a substantial proportion of a person's annual food requirements can be met today through the cultivation of only a fraction of a hectare of land. Another is that high local population densities can readily be sustained in this way. As a consequence of agriculture, new relationships have been established between the land, its use as a resource, and human populations.
Over the last one hundred years, scientists from a wide range of disciplines — agronomy, genetics, geography, and anthropology, to name only a few — have contributed to the study of the origins of agriculture. In order to gain some sense of historical development, it is worth recalling the contributions of a few of the main workers, starting with the Swiss botanist Alphonse de Candolle. In his treatise on the Origins of Cultivated Plants (1884), de Candolle attempted to locate the region of origin of some two hundred cultivated plants by drawing upon information from a wide variety of sources. Considering the rather limited nature of the evidence at the time, especially that available from dated deposits at archaeological sites, he performed an admirable, if highly speculative, intellectual exercise.
Excerpted from The Neolithic Transition and the Genetics of Populations in Europe by Albert J. Ammerman, Luigi Luca Cavalli-Sforza. Copyright © 1984 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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