Did the post-glacial increase in atmospheric CO2 level provide a selective productivity advantage to crop progenitor species?
The Neolithic transition from hunter-gather subsistence to settled populations employing agricultural cultivation of plants and animal husbandry marks a profound step for human societies and the beginning of the Anthropocene. Prior to this transition, deglaciation after the last ice age released a large amount of carbon dioxide (CO2) into the earth’s atmosphere. It is hypothesized that this increase of CO2 and the simultaneous elevation of average temperatures removed environmental constraints on plant productivity, allowing a surplus in human food production. During earlier hunter-gatherer stages, humans exploited a large variety of different species suitable for human nutrition. However, only a minor subset of the species range initially used were taken into agricultural cultivation. This small subset of species was to become the progenitors of founder crops as well as modern crop species. In the work presented here, we investigated the effects of post-glacial atmospheric CO2 increase from 180ppm to 270ppm on the selection of crop progenitor species out of the broad and diverse pool of wild grass species collected and gathered in the pre-cultivation era.
Our results suggest that, under low CO2 levels, productivity differences between progenitors and wild grasses were weaker than under higher levels of CO2. However, with increasing [CO2], reproductive allocation in progenitor species became less dependent on the trade-off with vegetative growth. Therefore, with rising CO2, crop progenitors could translate the positive differential in productivity to a greater extent into increased reproductive output and consequently higher yields. The resulting ecological advantage could have promoted the competitive fitness and hence abundance of progenitors within natural communities. We hypothesise the higher productivity gain of progenitor species in response to increased CO2 would have been amplified in situations where the production potential of the habitat was further increased by human activities e.g. optimizing growing conditions by removal of competitors and fertilisation during cultivation.