WAS LOW ATMOSPHERIC CO2 DURING THE PLEISTOCENE A LIMITING FACTOR FOR THE ORIGIN OF AGRICULTURE

Agriculture originated independently in many distinct regions at appro ximately the same time in human history. This synchrony in agricultura l origins indicates that a global factor may have controlled the timin g of the transition from foraging to food-producing economies. The glo bal factor may have been a rise in atmospheric CO:! from below 200 to near 270 mu mol mol(-1) which occurred between 15,000 and 12,000 years ago. Atmospheric CO2 directly affects photosynthesis and plant produc tivity, with the largest proportional responses occurring below the cu rrent level of 350 mu mol mol(-1). In the late Pleistocene, CO2 levels near 200 mu mol mol(-1) may have been too low to support the level of productivity required for successful establishment of agriculture. Re cent studies demonstrate that atmospheric CO2 increase from 200 to 270 mu mol mol(-1) stimulates photosynthesis and biomass productivity of C-3 plants by 25% to 50%, and greatly increases the performance of C-3 plants relative to weedy C-4 competitors. Rising CO2 also stimulates biological nitrogen fixation and enhances the capacity of plants to ob tain limiting resources such as water and mineral nutrients. These res ults indicate that increases in productivity following the late Pleist ocene rise in CO2 may have been substantial enough to have affected hu man subsistence patterns in ways that promoted the development of agri culture. Increasing CO2 may have simply removed a productivity barrier to successful domestication and cultivation of plants. Through effect s on ecosystem productivity rising CO2 may also have been a catalyst f or agricultural origins by promoting population growth, sedentism, and novel social relationships that in turn led to domestication and cult ivation of preferred plant resources.