Aridity and hominin environments

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.

Constraints on Fe‐Oxide Formation in Monsoonal Vertisols of Pliocene Kenya Using Rock Magnetism and Spectroscopy

“…Several hypotheses involve wetter conditions in the Pliocene followed by increased aridity beginning at (~2.6 Ma) the start of the Pleistocene (Cerling et al, ; deMenocal, ). However, evidence from clumped isotope temperatures of pedogenic carbonates as well as tooth enamel δ 18 O values of fossil mammals fail to recognize such a changeover in the Plio‐Pleistocene Turkana Basin (Blumenthal et al, ; Passey et al, ).…”

“…Ecometric analysis of the fossil mammals by Fortelius et al () resolved about 500–1,000 mm/year at 4.0–3.0 Ma. At ~3.3 Ma, Blumenthal et al () interpreted submesic/subxeric conditions from oxygen isotopes of fossil mammal teeth. Whether the present‐day aridity of Turkana is something very recent or related to the perceived progressive drying of Plio‐Pleistocene East Africa remains to be determined (Polissar et al, ).…”

Brain expansion in early hominins predicts carnivore extinctions in East Africa

“…We retrieved information on evolutionary changes in hominin brain size (White et al ; Seymour et al ) and proxies for climate and vegetation. As potential climatic predictors, we used mean annual temperature (Fortelius et al ), mean annual precipitation (Fortelius et al ), percentage forest cover (Cerling et al ) and annual water deficit (Blumenthal et al ). All the environmental predictors are estimated based on the sites from which the carnivore fossils originated.…”

“…(e) Fraction of forest cover (Cerling et al 2011). (f) Annual water deficit (Blumenthal et al 2017). For (c-f) The raw data are depicted as circles with horizontal bars showing the dating uncertainties around each measurement.…”

Does Oxygen Stable Isotope Composition in Primates Vary as a Function of Vertical Stratification or Folivorous Behaviour?

“…Stable oxygen isotope analysis has emerged as a useful tool for detecting ecological partitioning in mammalian communities. The reasons are twofold: first, variation in the isotopic composition of an assemblage stems from differences in forag-Folia Primatol 2020;91:219-227 220 Fannin/McGraw DOI: 10.1159/000502417 ing behaviour [Clementz and Koch, 2001;Cerling et al, 2004;Crowley, 2014;Crowley et al, 2015] and water flux [Bryant and Froelich, 1995;Kohn et al, 1996;Levin et al, 2006;Podlesak et al, 2008;Blumenthal et al, 2017], and, second, this variation is preserved in the fossil record, allowing palaeoecological interpretations of diet and environmental conditions [Koch, 2007;Nelson, 2007Nelson, , 2013White et al, 2009;Patnaik et al, 2014]. A primate species will evince high δ 18 O values when its body water is derived mainly from evaporated plant tissues [Crowley, 2012;Nelson, 2013;Carter and Bradbury, 2016] and/or because it is sensitive to evaporative fractionation from sweating or panting during thermoregulation [Kohn et al, 1996;Sponheimer and Lee-Thorp, 1999;Crowley et al, 2015].…”