Origins of Agriculture: an Ecological Perspective on Crop Domestication

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The proposed research aims to develop a new ecological model for crop domestication, integrating the roles of environmental change, plant traits, and human agency, under the constraints of the archaeological record. It addresses the idea that natural selection and human agency played a critical role at different stages in the emergence of agriculture, focusing on the interactions between plants, humans and environment during the period preceding fully agricultural societies. We propose a research programme with parallel experimental and archaeobotanical work packages that will consolidate the evidence necessary to develop and refine this new model. Our ecological model is formulated within the archaeologically documented framework of a gradual, widespread origin of agriculture, and is based on the proposition that different elements of the 'domestication syndrome' arose independently during different stages in the transition from gathering to farming. In particular it distinguishes the two archaeologically visible domestication traits: larger grain size and seed indehiscence, and sees these as consequences of different selective pressures operating at different stages in the domestication process. Our focus is on the former, and we argue that specialisation on a limited range of large-seeded species, and selection for larger seed size within these species, were both driven by an interaction between human diet choice and ecological processes. Our central thesis is that seed size correlates with a suite of functional traits which, through ecological processes, favour some species as crops over others and, through evolutionary processes, select for large-seeded genotypes of these crop species. We advance the model through the discussion of four hypothetical phases along the path towards greater sedentism and agriculture. Previous research has demonstrated that at least two of these phases can be recognized in the archaeobotanical record via the presence of non-food species representing either the wild communities from which seeds were gathered, or weed assemblages. Empirical archaeobotanical evidence, in the form of the plant spectrum, grain size and ecological conditions in different time periods will be used to assess the feasibility of the proposed model for explaining observed changes. The construction of an archaeobotanical database of pre-agricultural and early agricultural sites, and quantitative analysis of these records, will establish (a) the range of species likely to have been deliberately collected as food plants and provide the ecological context of gathering, (b) the extent, geographical locations, and date of the narrowing of the plant spectrum as the crop progenitor species came to prominence, and (c) the appearance of potential weed communities. Grain size measurements of both wild (crop progenitor and other grain species) and domesticated cereal and pulse crops will be analysed to establish when (in the case of crops) and whether (in the case of wild grains) seed size increased, and its timing in relation to changes in the associated non-food plant species assemblages, in particular in relation to changes in CO2 levels or climate during the period before potential weed assemblages are first recognized. At the same time, ecological experiments will assess the validity of the mechanisms of change proposed in the model by testing the underlying hypotheses concerning the differential responses of wild crop progenitors and other wild species (and of large and small-seeded genotypes of crop progenitors) to changing ecological conditions. Ecological experiments will determine the relationship of plant species and genotypes to increasing CO2 levels comparable to those occurring at the end of the last ice age, and to human-generated microenvironments with greater levels of disturbance and higher fertility.