Farmers, Evolution, Property Rights
Stephen Shennan on pioneer genotypes surfing the waves of advance
Stephen Shennan wrote:
My aim in this book is to take an evolutionary perspective on understanding the interactions between population, subsistence and socio- cultural traditions that resulted in the origin of cereal agriculture in south-west Asia and its subsequent spread westwards into Europe. The book’s central argument can be summarised in two claims.
Farming originated because broadening their diet breadth led people to increasing sedentism through growing dependence on plant resources that were dense and sustainable; as a result they had more children and more of them survived.
Farming spread because it enabled people to be reproductively successful by colonising new territories that had low- density forager populations, so long as they kept passing on the knowledge, practices, and the crops and animals themselves, to their children.
SYNTHESIS
The examples described above show that different combinations of static/increasing fertility and increasing/decreasing child survival can result in increased reproductive success and population growth for sedentary ways of life, including cultivation, compared with mobile foraging, because of reduced metabolic loads and/or decreased risk and improved weaning possibilities.
Moreover, improved infant survival and greater reproductive success can still be consistent with poorer health and lower life expectancies for the population as a whole. It is important to note though that population growth depends on the sedentary adaptations and cultivation having a higher sustainable carrying capacity – supporting more people per unit area – than mobile alternatives, which is not the case everywhere. Even then, however, high fertility will eventually be matched by high mortality, sooner rather than later given the implications of even low rates of increase, as we have seen. This situation can be mitigated if promising dispersal opportunities are available, as with the European colonisation of North America: growth can continue but it is now spatially expansive rather than locally intensive. Moreover, promising dispersal opportunities also introduce a further consideration, not immediately obvious, that tends to favour life histories that increase fertility: demographic competition for expansion. In terms of the competition for reproductive success, a strategy that postpones reproduction when an expansion opportunity exists will be less fi t than one that takes immediate advantage.
Finally , the ‘variance compensation hypothesis’ … is also likely to be relevant to the transition … in that the costs of having extra children may be higher for nonsedentary hunter- gatherers given the requirement for mobility, so they may err towards the low side of a notional ideal number of children in the face of environmental unpredictability. Agriculturalists on the other hand, may tend to err on the high side, not just because of the increased unpredictability of child survival as a result of increased disease risk but also because children become useful in farming at a young age … [It has been shown, references] that males in hunter- gatherer societies do not start producing more than they consume until the age of 20. In agricultural societies older children actually subsidise the investment in younger ones.
As we will see, traditional archaeological accounts of the link between the origin and spread of farming and population emphasised the idea of population pressure and were puzzled by the evidence that adopting farming seemed to be associated with a decline in human health, as well as increasing amounts of labour: it was seen effectively as expulsion from the Garden of Eden of the ‘original affluent society ’ (Marshall Sahlins reference). It should be clear now from what has been said that to think in terms of human well-being as a goal in this context is mistaken. While levels of well-being at population equilibrium may vary in different populations depending on local circumstances, what matters is the short-sighted evolutionary process of achieving reproductive success in the circumstances prevailing … The ethnographic and historical evidence indicates that the large-scale adoption of sedentism and farming created a Malthusian population pull as a result of the increased reproductive success associated with the increased energy availability and improved recruitment that ensued, while high population growth is by no means incompatible with poor nutrition and health.
Why [for example] did some Agta become sedentary and start farming given that the reproductive advantages only appear after the changes have occurred? … The long- term consequences of farming depended on the long-term transmission of farming practices from generation to generation and on this continuing to provide a survival and reproductive advantage compared with alternatives. Under standard natural selection, specific versions of genes that provide a selective advantage over others will be passed on from generation to generation and will spread through a population because they improve survival and reproductive success in some way. On the basis of the ethnographic and historical evidence we have seen we can suggest that agriculture represented a … strategy that was under natural selection. It was transmitted from generation to generation and spread because it improved survival and reproductive success in comparison with other possibilities , but it depended on the continued transmission of agricultural knowledge and domesticated plant and animal resources. That knowledge could be transmitted from farmers to non-farmers but it could only continue to provide a selective advantage if it was passed on by parents to their children. On average, the grandchildren of anyone who failed to pass on the knowledge and resources would be fewer in number than those of individuals who did. It follows from this that, as well as providing a measure of economic growth at a given point, the population growth rate associated with farming as a … transmitted subsistence strategy is also a measure of its fitness, and one that we can get from the archaeological record.
The domestic crops and animals themselves can be regarded as part of a transmitted environment that maintained the selective conditions favouring the farming way of life in a process of ‘niche construction’, not only by altering physical environments but also by changing the nature of social institutions such as property rights that would have affected the payoffs of different economic and social strategies.
On the other hand, if farmers found themselves in a situation where conditions for farming were less favourable, perhaps as a result of climate change, then the selective advantage might disappear, with various possible consequences. People might return to foraging, though this would almost certainly result in a drop in local population and, in many cases, an increase in mobility. However, if the social norms associated with farming practices were very powerful this might not happen and migration to seek more favourable conditions could take place. It certainly cannot be assumed that a technical innovation overcoming the problems created by the new conditions would automatically occur …
THE SOCIAL CONTEXT [PROPERTY RIGHTS]
While we can see … striking patterns and acknowledge that in broad terms … symbols, ideas and ritual practices had some role in shaping sedentary community life and addressing the challenges it posed, it is much harder to specify how they might have worked. However, one concrete challenge that has been widely acknowledged is that posed by the potential link between agriculture and the privatisation of property. How is it possible to get from the sharing rights generally prevalent among mobile hunter-gatherers to the private property rights over land, herds and produce that are seen extensively among farmers and herders? Such new conventions have to be agreed by a whole community and for a customary practice as deep-rooted and important as sharing, agreement on a change may be difficult to achieve. Thus [it has been] argued that farming could not have developed in the context of generalised sharing rights, because there would have been no incentive to undertake the work required for cultivation. This required a way of ensuring that people had a valid claim to the results of their work. Simulations of their model of the process showed that farming could only take off if private property rights co-evolved with it, and that this depended on the resources of farmers being excludable and defensible; conversely, such rights were not viable where subsistence depended on mobile or thinly distributed wild resources whose day-to-day exploitation was subject to high levels of uncertainty.
Further exploration of [the property rights] model … showed that the emergence of farming was more probable in a population structured into separate groups, with small (but not too small) group sizes, a very low level of behavioural experimentation, and the presence of ‘farming-friendly’ property rights. However, we should not assume that the transition was a sudden one … Thus , the broad spectrum economies of the Late Epipalaeolithic with their increasing populations and lower mobility had probably already created conditions favourable to private property rights associated with farming.
One real archaeological context in which the issue of property rights can be seen is in the development of storage, which becomes increasingly apparent from the PPNA [Pre-Pottery Neolithic A] … The site of Dhra’, in the Jordan valley in the southern Levant, has revealed … purpose-built communal granaries … the earliest dating to c.11,300 BP, a time when cereals were beginning to be cultivated. These roughly round structures had a suspended floor to enable air to circulate and to protect the contents from rodents and they were located in spaces between other buildings that contain evidence for plant processing and were presumably residential. [One author] proposes that from 10,500 BP … storage patterns began to change, with a move to storage within houses, followed by evidence for dedicated storage rooms within houses a thousand years later. These developments are taken to indicate changing property institutions and a move to household-based ownership. [Some authors] have also argued that at Çatalhöyük the storage of crops was privatised, with storage bins within individual houses.
The theoretical arguments made above about the conflict inherent in changing property rights and the evidence for the importance of storage and some degree of privatisation of property at the household level (at least of the product of farming if not of the land on which crops were grown for which we have no evidence one way or the other), lead us to expect evidence of conflict … and growing inequality … In fact, evidence for conflict is striking in terms of its lack so far. Evidence of social inequality is more equivocal. There are no indications of marked social stratification in terms of vast differences in the wealth and monumentality of burials or the size and elaboration of houses but there are signs of more limited social differentiation ….
… In any case, it seems that the impact of such factors led to growing pressure for independent household economic decision-making, resulting in a widespread pattern of dispersal. In other words, households that started to make their own production, storage and distribution decisions, and perhaps claimed household landownership, did better and were emulated as a result. This seems to be well seen in the increasingly dispersed late phase at Çatalhöyük and reflected in the changed activity patterns there identified [by various authors].
EVOLUTIONARY PATTERNS PROCESSES
Although the particular long-term history that this book has described is unique, we can recognise within it the action of a number of general evolutionary processes operating at different timescales. The origin of farming in south-west Asia should be seen in terms of the interaction between payoffs to hunter-gatherers at the day-to-day scale and those at the generational scale. As long as higher day-to-day payoffs could be obtained by focussing on the exploitation of low-density mobile animal resources there was no possibility of regional population increasing beyond a low level, because the carrying capacity [MGH: ‘supporting more people per unit area’] of this mode of subsistence was low; increased fertility would not be rewarded with successful recruitment to the next generation. If such [hunter-gatherer] resources decline, for whatever reason, then the rate at which people encounter them will drop and people will broaden their subsistence activities to include resources with lower return rates that are encountered more frequently and thus increase return rates overall. If the resources that are then introduced are both dense and sustainable then people will become more sedentary. In these circumstances the previous population ceiling will be lifted; more children can be born and raised successfully and population will increase up to the limit of the new resources.
In the case of the Late Pleistocene Fertile Crescent, spatial distributions of resources were constantly changing as a result of climatic fluctuations and the evidence suggests that more sedentary subsistence systems came and went in response. However, in the 1500 or so years before the beginning of the Holocene … the northern and southern Levant were sufficiently stable and benign that populations relying on dense and sustainable locally available plant resources could increase to the unprecedented level observed in the radiocarbon population proxy.
At this point the beginning of the Holocene introduced exceptionally good conditions for plant growth in terms of both temperature and precipitation. It may have been the ensuing expansion of woodland that led people to cultivate to ensure continued access to sustainable grass/ cereal crops, which themselves were changing their distribution in response to the new conditions. This raised the sustainable carrying capacity at which births and deaths were in balance still further, especially when the mutualistic relationship of cultivation between plants and people led to high-yielding domesticated forms. Nevertheless, fluctuations in local limits continued to occur, no doubt at least partly for climatic reasons.
Already at the beginning of the Holocene, when cultivation had barely begun, we see people trying to come to terms with the consequences of the previous 1500 years of increasing population density and more sedentary ways of life. One response was the colonisation of Cyprus [MGH: Chapter 5 examines the westward expansion of farming, including “human groups deliberately seeking out new territories at a very early date”]; another was the appearance of new forms of ritual, symbolism, monumentality and settlement …
These innovations continued through the Pre-Pottery Neolithic and spread from one region to another through links evidenced, for example, by the movement of obsidian. In adaptive radiations, when a new environment or energy source opens up, it is unclear what its possibilities are and which ones are viable, so all sorts of different possibilities may be tried in a process of ‘breadth-first’ search; this seems to be what is evidenced in the extraordinary social and symbolic developments during the Pre-Pottery Neolithic. It is only later that the less successful ones are winnowed out.
In contrast to new technologies or subsistence practices, where the relative success of different options can emerge relatively quickly, it may take much longer to establish whether some social arrangements are more successful than others, and for whom. One likely arena of change in these unprecedented situations is around property rights and the potential for social inequality.
When hunting mobile animals is the main focus of subsistence, fluctuations in day-to-day returns as well as over different stages of the lifespan mean that sharing is the best means of insurance for evening things out.
However, once returns start to depend more on the effort put in, then they are increasingly restricted to immediate dependents, while the fact that members of a community are effectively stuck in one place means that they are all vulnerable to the same natural disasters.
The evidence for increased crop storage within houses is probably a reflection of such changes in property rights and the relative autonomy of individual households in this respect. However, it seems likely that household autonomy would have been in tension with the growth of the so-called ‘megasites’, settlements with populations in the low thousands at the top end , some of which, such as Çatalhöyük, lasted for a millennium. These must have had special institutional arrangements to keep them together, all the more so given the lack of evidence for warfare, which is generally one of the main reasons for settlement nucleation and the overcoming of centrifugal social forces.
In contrast to the innovative developments of the Pre-Pottery Neolithic, the story of the expansion of farming into and across Europe is one of conservatism. It represents the success of one version of the many social, cultural and economic variants that had flourished in south-west Asia. That version was based on small groups of autonomous households lacking much of the symbolic elaboration of the core area and with a rather stereotyped subsistence pattern based on the integration of crops and animals.
This new niche was highly portable, based as it was on annual cereal crops that were harvested every year and could be re-sown somewhere different, as well as on mobile domestic animals. It turned out to be remarkably successful, in that it created possibilities for population growth and expansion that enabled its possessors to take advantage of the existence of vast areas that were very well-suited to the new subsistence system but only very thinly occupied by hunter-gatherers, because of the low carrying capacity that their terrestrial wild resources afforded.
While they have had relatively little impact so far on understandings of the processes involved in the origins of agriculture, recent studies of whole-genome ancient DNA data using new methods have had a major effect on understanding the European spread. These studies have settled decades of inconclusive debate by showing that farming spread across Europe as a result of the spread of successive generations of descendants of farmers originally from south-east Anatolia. In other words, farmers passed on farming knowledge, practices and the resources themselves to their children, through the generations, in a process of vertical transmission that enabled them to successfully colonise new areas. On the edge of the spread, with endless dispersal opportunities, increased fertility would have been an option that gave a fitness advantage to those that made use of it.
The tight linkage of people, niche and culture by vertical transmission over several thousand years would have created possibilities of gene-culture coevolution that we still know very little about … Such transmission also means that all aspects of culture and practice, as well as the adaptive environment, were being transmitted together, representing a coherent … core, not separate packages with different histories, tightly connected by common descent to the genes. Some of these practices and resources were those which gave these people their selective advantage, especially the domesticated plants and animals and the knowledge to turn them into food while ensuring their continuing reproduction. We still need to know more about the factors affecting the transmission of such features as monumental burial traditions or the LBK house, but particular social institutions and practices were probably also key to the success of these expansions.
Others would simply have been ‘hitchhiking’, carried along with the expanding population, for example the language they spoke.
In fact , there is remarkably little evidence of foragers adopting farming as a result of cultural diffusion … This in itself is telling us that it would not have been simple to adopt. It was a way of life that had been put together and then modified for local circumstances over thousands of years. … This point is confirmed by the evidence …
… It is worth noting that there is even less evidence of farmers becoming foragers: this is not simply a matter of more or less hunting but commitment to a completely different set of values and priorities. Paradoxically, it has been those most in favour of cultural diffusion explanations of the spread of farming who think that people can easily change their culture on the basis of an assessment of costs and benefits of different ways of life.
Rather, foragers were incorporated, albeit at low rates, into farming communities. Recent work is beginning to tell us in some cases when this incorporation might have occurred and the extent to which it was a result of major pulses at certain times or a gradual process over longer periods. How low or high these rates of incorporation were remains unclear. If they were occurring at the farming frontier they could have been extremely low. The process of ‘surfing on the wave of advance’ [described earlier on p. 88 as a pattern in which the few individuals at the front of the wave are likely to be the most successful, so “pioneer genotypes are continually transmitted forward and surf along with the wave”], means that those versions of genes present in a population at the point where it is expanding have a high probability of reaching significant frequencies purely by chance, because the number of individuals there is small but they are likely to be successful. Thus the proportion of forager genetic admixture in a population may give an exaggerated picture …
… Once a particular set of knowledge and practices has been established as something that works, it becomes the basis for decisions that are made in the future, because it defines the range of options that can be envisaged. For example, once a number of criteria have been established for what represents a good place to establish a new settlement, other possibilities will be ignored. It is in keeping with this idea that … contexts in which innovation does occur are cases where different traditions meet …
… The fact that farming expansion stalls for centuries in certain places … may be an indication of the difficulty of making innovations in a world of mainly vertical transmission [generational genes and knowledge]. Environmental factors, including adverse climate conditions and lack of success in growing cereals in more northerly latitudes until genetic modifications had taken place, may well be relevant … but ‘technological lock-in’ that made it impossible for people to imagine doing things in different ways from what they had been doing may also be a factor … What we can certainly conclude is that, despite being on the edge of new areas to colonise, there were periods when people were not prepared to take the risk.
[The] much-reviled ‘wave of advance’ [i.e. surfing] model … remains a useful framework for thinking about and modelling the factors behind the range expansion of farming and farmers … because of the pauses that have just been discussed. When expansion does occur it is very fast, a point that has long been recognised by the introduction of the concept of ‘leapfrog migration’, but …the new genetic evidence now excludes the alternative possibility that the speed could be accounted for by the diffusion of farming to foragers. Given that we now know it was migration we need a principled way of explaining the rapid spread episodes. During these episodes movement occurs long before local farming landscapes fill to their carrying capacity …
[MGH: the 2 remaining pages in the book sum up the ‘boom-bust’ process]
The Source:
Stephen Shennan, The first farmers of Europe: an evolutionary perspective, Cambridge 2018 [pp. 1, 9-12, 50-54, 207-214]
Evolutions of social order from the earliest humans to the present day and future machine age.