AAPA 2018, Meeting Program Abstracts
April 11-14, 2018
Experimental studies suggest that the evolutionary success of C4 grasses in tropical environments is in part due to a greater physiological tolerance for low CO2 levels compared to C3 plants (i.e., trees and shrubs). Such studies predict that C4 biomass inversely tracks atmospheric pCO2 levels through geological time, which should be reflected in the structure of terrestrial ecosystems, including mammal communities.
We apply these conclusions to the Plio-Pleistocene fossil record of East Africa to better characterize environmental parameters as hominins evolved. We used mammalian species occurrence and dietary data from 96 eastern African fossil sites as a proxy for grassy versus woody habitats, and pCO2 data were derived from ice cores (back to 800 ka), and boron-based proxies for the older record.
We show a statistically significant inverse relationship between deviations in pCO2 and grazer abundances (r=-0.36, p=0.002) over the last ~4.5 Ma; before 3.4 Ma grazers comprise <20% of the total fauna when pCO2 is >250 ppm, and before 1 Ma they are <32% when pCO2 is <200 ppm. In contrast, browsers often exceed 30% of the total fauna before 3.4 Ma, but decline when pCO2 drops below 250 ppm.
Australopithecus appeared in eastern Africa when pCO2 was greater than 350 ppm, and Homo appeared at <300 ppm coincident with grassland expansion. Our findings have important implications for the role of paleoclimate in human evolution, as well as for the future stability of eastern Africa’s grassland habitats in the face of CO2 increases over the next century.