Exploring the impact of sub-ambient CO2 concentrations on Oxalis growth: implications for food sustainability among Pleistocene human foragers in the Greater Cape Floristic Region

Abstract

Humans inhabiting the glacial refugia of the Cape Floristic Region during the Middle Pleistocene are believed to have relied on underground storage organs (USOs) as a crucial carbohydrate source. While previous research has highlighted the nutritional potential of these plants for early human gatherers, such studies are typically conducted under modern climatic conditions and do not account for the reduced plant productivity and USO yields associated with the lower atmospheric COâ‚‚ concentrations characteristic of Pleistocene glacial periods. To evaluate the impact of COâ‚‚ availability on plant productivity, I cultivated two Oxalis species"”O. pes-caprae and O. punctata, both known to have been harvested by early foragers"”under COâ‚‚ concentrations ranging from 180 ppm to 400 ppm. Results showed that glacial-level COâ‚‚ concentrations significantly reduced bulb production in O. pes-caprae and photosynthetic rates in both species. Using these findings to model Pleistocene foraging behaviour, I estimate that human foragers would have needed to spend an additional 8 hours gathering food to meet their daily dietary requirements of 2000 calories, suggesting the need for alternative food sources to supplement their diet. These findings emphasise the importance of considering historical plant growth conditions, particularly atmospheric COâ‚‚ levels, when reconstructing past human diets. Failure to account for these factors may represent a significant gap in our understanding of human subsistence strategies during glacial periods.