Contrasting Patterns in Crop Domestication and Domestication Rates: Recent Archaeobotanical Insights from the Old World
Dorian Q Fuller*
Institute of Archaeology, University College London, 31–34 Gordon Square, London WC1H 0PY, UK
Received: 20 September 2006
Background: Archaeobotany, the study of plant remains from sites of ancient human activity, provides data for studying the initial evolution of domesticated plants. An important background to this is defining the domestication syndrome, those traits by which domesticated plants differ from wild relatives. These traits include features that have been selected under the conditions of cultivation. From archaeological remains the easiest traits to study are seed size and in cereal crops the loss of natural seed dispersal.
Scope: The rate at which these features evolved and the ordering in which they evolved can now be documented for a few crops of Asia and Africa. This paper explores this in einkorn wheat (Triticum monococcum) and barley (Hordeum vulgare) from the Near East, rice (Oryza sativa) from China, mung (Vigna radiata) and urd (Vigna mungo) beans from India, and pearl millet (Pennisetum glaucum) from west Africa. Brief reference is made to similar data on lentils (Lens culinaris), peas (Pisum sativum), soybean (Glycine max) and adzuki bean (Vigna angularis). Available quantitative data from archaeological finds are compiled to explore changes with domestication. The disjunction in cereals between seed size increase and dispersal is explored, and rates at which these features evolved are estimated from archaeobotanical data. Contrasts between crops, especially between cereals and pulses, are examined.
Conclusions: These data suggest that in domesticated grasses, changes in grain size and shape evolved prior to non-shattering ears or panicles. Initial grain size increases may have evolved during the first centuries of cultivation, within perhaps 500–1000 years. Non-shattering infructescences were much slower, becoming fixed about 1000–2000 years later. This suggests a need to reconsider the role of sickle harvesting in domestication. Pulses, by contrast, do not show evidence for seed size increase in relation to the earliest cultivation, and seed size increase may be delayed by 2000–4000 years. This implies that conditions that were sufficient to select for larger seed size in Poaceae were not sufficient in Fabaceae. It is proposed that animal-drawn ploughs (or ards) provided the selection pressure for larger seeds in legumes. This implies different thresholds of selective pressure, for example in relation to differing seed ontogenetics and underlying genetic architecture in these families. Pearl millet (Pennisetum glaucum) may show some similarities to the pulses in terms of a lag-time before truly larger-grained forms evolved.
As I recall, the lentils at Franchthi cave were just slightly bigger than wild lentils, suggesting that the 13,500 date for domestication could be push back possibly as much as 17,000 BP ( I’d say 15,000 BP was more likely). I’d seriously recommend reading this whole paper through if you are interested in the process of domestication of crops. It includes some Indian beans, rice, as well as African pearl millet, and is about the most comprehensive paper I’ve seen on the subject.
It also names the South Asian location of rice domestication as the middle of the Ganges valley, and has some very useful graphs showing the levels of domestication in various near Eastern sites. It suggests that an increase in seed size in cereals is evident for quite some time before a non shattering rachis is selected in. One interesting fact didn’t know was that millet was much more widely grown in China about 8,000 years ago than rice. One to read a couple of times through.
FIG. 1. An evolutionary model from foraging to agriculture, with archaeobotanical expectations indicated at the bottom (modified from Harris, 1989). The stages of pre-domestication cultivation are shaded. In this version, domestication is represented as a process of gradual frequency change, with an earlier, more rapid ‘semi-domestication’ and a later, slower fixation of full domestication. The gap in time elapsed between these two can be taken as a minimal estimate of domestication rate (d.r.).
FIG. 2. Map of south-west Asia, showing the locations of sites with archaeobotanical evidence that contributes to understanding the origins and spread of agriculture. Sites are differentiated on the basis of whether they provide evidence for pre-domestication cultivation, enlarged grains, mixed or predominantly domestic-type rachis data. Note that these sites represent a range of periods, and many sites have multiple phases of use, in which case the earliest phase with significant archaeobotanical data is represented. Shaded areas indicate the general distribution of wild progenitors (based on Zohary and Hopf, 2000, with some refinements from Willcox, 2005). It should be noted that wild emmer (Triticum dicoccoides) occurs over a sub-set of the wild barley zone, and mainly in the western part of the crescent.
Archaeological evidence indicates that the entire domestication syndrome did not suddenly appear when people began to cultivate plants. Rather, different aspects of the syndrome evolved in response to the new ecological conditions of early cultivation. What these data suggest is that in domesticated grasses, changes in grain size and shape (‘semi-domestication’) evolved prior to non-shattering ears or panicles (‘domestication’ sensu stricto). While initial grain size increases may have evolved during the first centuries of cultivation, within perhaps 500–1000 years, non-shattering was much slower, becoming fixed about 1000–2000 years subsequently. Pulses by contrast do not show evidence for seed size increase in relation to the earliest cultivation, but seed size increase may be delayed by 2000–4000 years. This implies that conditions that were sufficient to select for larger seed-size in Poaceae were not sufficient in Fabaceae. This implies different thresholds of selective pressure in relation to differing seed ontogenetics and underlying genetic architecture in these families. Pearl millet (Pennisetum glaucum) may show some similarities to the pulses in terms of a lag-time before truly larger-grained forms evolved. These results may aid in predicting when and where certain crop domestications are likely to have occurred based on counting backwards from the earliest known domestic finds. Thus, for example, we would predict that pearl millet cultivation began by 3200–2700 BC. These results also raise questions about taxonomically linked differences in evolution under the selection forces of cultivation.
Reconsidering sickles and cereal domestication
There has been a tendency to assume that harvesting with a sickle was the selective force that led to domestication, i.e. non-shattering (as discussed above). The archaeological evidence, however, does not support this in any documented case. In China, as discussed already, rice grains begin to plump and increase in size but domestication is indicated by the shift towards predominantly mature-grained harvests (and inferred non-shattering), during the fifth millennium BC, and by approx. 4000 BC. In this region there are no clear archaeological sickles until after 3500 BC, the Later Songze period (approx. 3500 BC), after which they become widespread in the Liangzhu culture (3300–2200 BC). These sickles may be a cultural borrowing from millet cultivators in central China, where such tools were in use since at least 5000 BC (cf. Chang, 1986). Even in central and northern China, the earliest sickles occur at sites that already have millet cultivation, and earliest documented domestic millets from Xinglonggou (near Chifeng, China), before 6000 BC (Zhao, 2005), come from a culture without sickles. In China, sickles consistently represent a technology development after domesticated plants are fully established.
In the Near East sickles were in use prior to agriculture and must now be argued to be transferred to agriculture relatively late, after domestication. Preserved sickles, and more commonly lithic sickle blades, are known from Natufian contexts (13 000–10 500 BC), in a period for which there is no evidence for domesticates, and non-shattering domesticates continued to be absent through the PPNA (through 8800 BC) (see Fig. 3). Microscopic studies of ‘sickle gloss’ have been used to suggest they were cereal-harvesting (Unger-Hamilton, 1989; Anderson, 1992), but we cannot rule out harvesting of sedges (Cyperaceae) and reeds (Phragmites) as materials for basketry or thatching. As suggested by Sauer (1958), the early Natufian sickles were prototype saws, designed for raw material gathering rather than seed collecting. As indicated by the archaeobotanical evidence reviewed above, the rate of evolution of tough rachis einkorn and barley is far too slow to be accounted for by a model of strong selective pressure that would be expected if sickling was carried out regularly, as modelled by Hillman and Davies (1990). Thus, it would appear that early cultivators continued to employ the time-efficient harvesting methods associated with hunter-gatherers. Once cultivated, and populations had noticeably large proportions (majorities) of non-shattering types, then the transfer of the sickle technology to agriculture may have been seen as an obvious enhancement. In evolutionary terms the sickle is thus an ‘exaptation’ (sensu Gould and Vrba, 1982), in that it developed for some other purpose, and was later transferred to crop-harvesting of already domesticated crops.
I would propose alternative explanations for the selection of domesticated-type crops that can account for the slow creep towards domestication. As others have noted, the harvesting of cereals when green, i.e. immature, regardless of technique, will not select for domesticated types (Hillman and Davies, 1990; Willcox, 1999). Harvesting green, however, may not provide full returns from a given stand of crops, as additional seeds (including late tillers) may form and approach maturation subsequent to the harvest. For the early farmers, who have invested significant labour into a restricted unit of land, it becomes important to maximize returns from that unit of land (as noted by Hillman and Davies, 1990: 69; Bar-Yosef, 1998). This may encourage multiple episodes of harvest. Later harvestings, whether by plucking or beating, will encounter domesticated genotypes in a higher frequency than earlier harvests. If, as an aspect of random variation, some farming households choose to store the late harvest as seed for sowing the following year, those fields so sown will start an increase in the domesticated type. Other households, however, may store for sowing their earlier harvests. Therefore, taken at the level of a human community, or on a regional scale, there might be only a very small proportion of sown crop that had some selection for the domesticated type. Such a model might therefore account for significantly longer periods involved in the fixation of non-shattering types in cultivated populations. By contrast, every farmer and every sown population would be under selective pressure to germinate rapidly, leading to seed size increase and loss of germination inhibitors. Similarly, natural selection for dispersal aids such as awns will be uniformly reduced. Thus, we should expect these ‘semi-domestication’ traits to evolve more rapidly.
Domestication as an interdisciplinary study of evolution
Domestication in plants is not one thing, nor has it been one uniform process. While there are recurrent parallels, due to the same selective pressures of cultivation, different domestication traits have evolved at different rates and these have varied markedly across families, such as between cereals and legumes. Further archaeobotanical research will help to pin down the actual rates at which different domesticates evolved, and needs to be expanded to address a larger range of species. The archaeological record also provides insights into what people are doing during this evolutionary process in terms of their technologies and ecological adaptations. Understanding past domestications is an exciting area of interdisciplinary investigation, between archaeologists and plant scientists, which may offer insights relevant to future directions in the evolution of crops under human manipulation.