These events immediately followed a rapid 35% rise in atmospheric CO₂ at the end of the last glacial period. Was this a coincidence? Or were the two causally linked? Sage (1996) hypothesises that the jump in CO₂ was a necessary precursor for the origins of agriculture, causing significant increases in the yields of the wild ancestors of crop species, and making their cultivation and domestication a fruitful enterprise. This project is providing a crucial test for the hypothesis, through experimental investigation of the responses of C₄ crop progenitors to CO₂.

Early C₄ crops like maize, foxtail millet and sugarcane present a problem for the CO₂-limitation hypothesis, because they originated at the same time as C₃ crops such as wheat, barley and rice, but have a CO₂-concentrating mechanism that typically makes photosynthesis insensitive to CO₂. To address the apparent conflict between the CO₂-limitation hypothesis and C₄ plant physiology, this project is therefore developing the first comparative experiments investigating physiological and yield responses to sub-ambient CO₂ levels in the wild progenitors of C₃ and C₄ crops. Crucially it is utilizing newly developed technology for CO₂-free air production to test the hypothesis that rising CO₂ promotes yield in C₄ crop relatives via savings in water-use and improved nitrogen-use efficiency.

Sage R.F. (1996) Was atmospheric CO₂ during the Pleistocene a limiting factor for the origin of agriculture? Global Change Biology 1, 93-106.

Atmospheric CO₂ record from the Vostok ice core for the past 50,000 yrs, showing the dramatic rise after the last glacial period (18,000 yrs ago). Four early domestication events seem to have followed immediately.