To What Do We Owe the Origin of Agriculture?
Cunniff, J., Charles, M., Jones, G. and Osborne, C.P. 2010. Was low atmospheric CO2 a limiting factor in the origin of agriculture? Environmental Archaeology 15: 113-123.
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."
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.