To What Do We Owe the Origin of Agriculture?

In a thought-provoking research report published in Environmental Archaeology, Cunniff et al. (2010) write that "the origin of agriculture in numerous independent regions soon after the last glacial period [Byrne, 1987; Zeder, 2006; Purugganan and Fuller, 2009] points to a global limitation for domestication," which began to be relaxed at approximately the same time (about 10,000 years ago) everywhere. An intriguing hypothesis that has been put forth to explain this observation -- that of Sage (1995) -- suggests, in the words of Cunnif et al., that "the post-glacial rise in atmospheric CO₂ concentration from 180 to 270 ppm increased the productivity of wild crop progenitors," thereby enabling humans of that era to abandon their hunter/gatherer lifestyle and become stay-at-home custodians of much smaller -- but ever more productive -- parcels of land devoted to a mix of crops that they learned to successfully cultivate. However, as they continue, "the inclusion of C₄ crops among the earliest domesticates challenges this hypothesis, because these species possess a carbon-concentrating mechanism that is expected to offset CO₂ limitation," so that the increase in CO₂ would have had but a minimal impact on their degree of productivity.

In light of this seemingly half-right/half-wrong hypothesis, the four UK researchers conducted a set of experiments designed to further test it by studying the physiological and yield responses to glacial and post-glacial atmospheric CO₂ concentrations of modern representatives of the wild progenitors of C₃ and C₄ cereals. This they did in the laboratory within controlled environment chambers maintained at atmospheric CO₂ concentrations of either 180 or 280 ppm, finding that "both the C₃ and C₄ species responded positively to rising CO₂ from the glacial to post-glacial level," with vegetative biomass near-doubling and yield rising by 50% in the C₃ species, which is consistent with the findings of Polley et al. (1992) and Dippery et al. (1995), and with a 10-15% yield enhancement under well-watered conditions in the C₄ species, consistent with the findings of Polley et al. (1996), Ziska and Bunce (1997) and Wand et al. (1999). In addition, in the case of the C₄ species, which typically are found in semi-arid environments, they discovered that the transpiration-reducing effect of atmospheric CO₂ enrichment had a considerably larger positive impact on plant growth and yield. And they write that "this stronger response of photosynthesis to soil drying in the C₄ species could represent an important route for rising CO₂ to stimulate biomass accumulation in a semi-arid environment," citing the analysis of Cunniff (2009).

In concluding the report of their work, Cunniff et al. say that the data described in their paper "provide experimental support for Sage's CO₂ limitation hypothesis, showing that atmospheric conditions of the last glacial period would have placed direct and indirect limitations on the productivity of crop progenitors." Although they state that the hypothesis does not answer the question of why the domestication of crops occurred, they state that "it does offer evidence-based explanations of why agriculture did not begin earlier."

Additional References
Byrne, R. 1987. Climate change and the origins of agriculture. In: Manzanilla, L. (Ed.). Studies in the Neolithic and Urban Revolutions: The V. Gordon Childe Colloquium, Mexico, 1986. (British Archaeological Reports International Series 349), Archaeopress, Oxford, UK, pp. 21-34.

Cunniff, J. 2009. The Roles of Atmospheric Carbon Dioxide and Plant Ecological Traits in the Origin of Agriculture. PhD Thesis, University of Sheffield, United Kingdom.

Dippery, J.K., Tissue, D.T., Thomas, R.B. and Strain, B.R. 1995. Effects of low and elevated CO₂ on C₃ and C₄ annuals. I. Growth and biomass allocation. Oecologia 101: 13-20.

Polley, H.W., Johnson, H.B. and Mayeux, H.S. 1992. Growth and gas-exchange of oats (Avena sativa) and wild mustard (Brassica kaber) at sub-ambient CO₂ concentrations. International Journal of Plant Sciences 153: 453-461.

Polley, H.W., Johnson, H.B., Mayeux, H.S., Brown, D.A. and White, J.W.C. 1996. Leaf and plant water use efficiency of C₄ species grown at glacial to elevated CO₂ concentrations. International Journal of Plant Sciences 157: 164-170.

Purugganan, M.D. and Fuller, D.Q. 2009. The nature of selection during plant domestication. Nature 457: 843-848.

Sage, R.F. 1995. Was low atmospheric CO₂ during the Pleistocene a limiting factor for the origin of agriculture? Global Change Biology 1: 93-106.

Ward, J.K., Tissue, D.T., Thomas, R.B. and Strain, B.R. 1999. Comparative responses of model C₃ and C₄ plants to drought in low and elevated CO₂. Global Change Biology 5: 857-867.

Zeder, M.A. 2006. Central questions in the domestication of plants and animals. Evolutionary Anthropology 15: 105-117.

Ziska, L.H. and Bunce, J.A. 1997. Influence of increasing carbon dioxide concentration on the photosynthetic and growth stimulation of selected C-4 crops and weeds. Photosynthesis Research 54: 199-208.