Was low atmospheric CO2 a limiting factor in the origin of agriculture?

The origin of agriculture in numerous independent regions soon after the last glacial period points to a global limitation for domestication. One hypothesis proposes that the post-glacial rise in atmospheric CO2 concentration from 180 to 270 ppm increased the productivity of wild crop progenitors, thereby removing a barrier to domestication. However, 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. We used an experimental approach to test this aspect of the hypothesis, finding that an increase in CO2 from glacial to post-glacial levels caused significant gains in vegetative biomass in wild modern representatives of C3 and C4 cereals. Investigation into the underlying mechanisms showed photosynthesis to be limited by CO2 at glacial levels in both types. More significantly, for the C4 species, transpiration rates were reduced, leading to indirect benefits for photosynthesis when water was limited. Finally, higher CO2 levels stimulated yield by 50% in C3 species and 10–15% in C4 species. The data provide experimental support for the CO2-limitation hypothesis, showing that atmospheric conditions of the last glaciation would have placed direct and indirect restrictions on the productivity of crop progenitors.