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

Abstract

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.

Keywords:

• SUB-AMBIENT CO2
• ORIGIN OF AGRICULTURE
• YIELD
• CO2 LIMITATION HYPOTHESIS