Aridity and hominin environments

The shores of Lake Turkana, in Kenya, are dry and inhospitable, with grasses as the dominant plant type. It hasn't always been that way. Over the last four million years, the Omo-Turkana basin has seen a range of climates and ecosystems, and has also seen significant steps in human evolution.

 

Scientists previously thought that long-term drying of the climate contributed to the growth of grasslands in the area and the rise of large herbivores, which in turn may have shaped how humans developed. It's tough to prove that hypothesis, however, because of the difficulty of reconstructing four million years of climate data.

Researchers from the University of Utah have found a better way.

By analyzing isotopes of oxygen preserved in herbivore teeth and tusks, they can quantify the aridity of the region and compare it to indicators of plant type and herbivore diet.

The results show that, unexpectedly, no long-term drying trend was associated with the expansion of grasses and grazing herbivores. Instead, variability in climate events, such as rainfall timing, and interactions between plants and animals may have had more influence on our ancestors' environment.

This shows that the expansion of grasslands isn't solely due to drought, but more complex climate factors are at work, both for modern Africans now and ancient Africans in the Pleistocene.1

Aridity and hominin environments

Scott A. Blumenthal, Naomi E. Levin, Francis H. Brown, Jean-Philip Brugal, Kendra L. Chritz, John M. Harris, Glynis E. Jehle, and Thure E. Cerling

Edited by James O'Connell, University of Utah, Salt Lake City, UT, and approved May 25, 2017 (received for review January 11, 2017)

Significance: Oxygen isotopes in modern and fossil mammals can provide information on climate. In this study, we provide a new record of aridity experienced by early hominins in Africa. We show that past climates were similar to the climate in eastern Africa today, and that early hominins experienced highly variable climates over time. Unexpectedly, our findings suggest that the long-term expansion of grasses and grazing herbivores since the Pliocene, a major ecological transformation thought to drive aspects of hominin evolution, was not coincident with aridification in northern Kenya. This finding raises the possibility that some aspects of hominin environmental variability might have been uncoupled from aridity, and may instead be related to other factors, such as rainfall seasonality or ecological interactions among plants and mammals.

Abstract:  Aridification is often considered a major driver of long-term ecological change and hominin evolution in eastern Africa during the Plio-Pleistocene; however, this hypothesis remains inadequately tested owing to difficulties in reconstructing terrestrial paleoclimate. We present a revised aridity index for quantifying water deficit (WD) in terrestrial environments using tooth enamel δ18O values, and use this approach to address paleoaridity over the past 4.4 million years in eastern Africa. We find no long-term trend in WD, consistent with other terrestrial climate indicators in the Omo-Turkana Basin, and no relationship between paleoaridity and herbivore paleodiet structure among fossil collections meeting the criteria for WD estimation. Thus, we suggest that changes in the abundance of C4 grass and grazing herbivores in eastern Africa during the Pliocene and Pleistocene may have been decoupled from aridity. As in modern African ecosystems, other factors, such as rainfall seasonality or ecological interactions among plants and mammals, may be important for understanding the evolution of C4 grass- and grazer-dominated biomes.

oxygen isotopes,terrestrial paleoclimate, human evolution, mammals, Africa