Water, plants, and early human habitats in eastern Africa

Abstract

Water and its influence on plants likely exerted strong adaptive pressures in human evolution. Understanding relationships among water, plants, and early humans is limited both by incomplete terrestrial records of environmental change and by indirect proxy data for water availability. Here we present a continuous record of stable hydrogen-isotope compositions (expressed as δD values) for lipid biomarkers preserved in lake sediments from an early Pleistocene archaeological site in eastern Africa-Olduvai Gorge. We convert sedimentary leaf-and algal-lipid δD values into estimates for ancient source-water δD values by accounting for biochemical, physiological, and environmental influences on isotopic fractionation via published water-lipid enrichment factors for living plants, algae, and recent sediments. Reconstructed precipitation and lakewater δD values, respectively, are consistent with modern isotopic hydrology and reveal that dramatic fluctuations in water availability accompanied ecosystem changes. Drier conditions, indicated by less negative δD values, occur in association with stable carbon-isotopic evidence for open, C 4 -dominated grassland ecosystems. Wetter conditions, indicated by lower δD values, are associated with expanded woody cover across the ancient landscape. Estimates for ancient precipitation amounts, based on reconstructed precipitation δD values, range between approximately 250 and 700 mm·y −1 and are consistent with modern precipitation data for eastern Africa. We conclude that freshwater availability exerted a substantial influence on eastern African ecosystems and, by extension, was central to early human proliferation during periods of rapid climate change.paleohydrology | plant waxes | carbon isotopes

Capturing climate variability during our ancestors' earliest days

“…Given the large magnitude and variation of water delivery to the region, the authors argue that hominins living on this landscape likely relied on freshwater springs. Notably, many hominin fossils sites in Olduvai Gorge and other regions are associated with freshwater oases (20), which is consistent with the authors' model (2).…”

“…This approach has been used to separate the variation in hydrogen isotope values as a result of plant community composition or hydrologic cycle changes (19). In their record, Magill et al find relatively minor variation in the hydrogen isotope ratio (−148‰ to −132‰), which is surprising considering their carbon isotope values suggest plant community turnover associated with major changes in hydrology (2). To account for the attenuating effects of plant type on hydrogen isotope values, they model a landscape hydrogen fractionation by using their estimates of the plant community composition.…”

“…In their two contributions, Magill et al (1,2) use terrestrial biomarkers to provide insights into plant community change and hydrologic dynamics during a key period of hominin evolution in East Africa. The authors use a continuous section of lake sediments from the Olduvai basin and capture an integrated regional ecosystem signal at high temporal resolution.…”

“…In PNAS, Magill et al (1,2) present records of terrestrial habitat and hydroclimate variation and pacing from Olduvai Gorge sediments during a key interval of our ancestors' divergence and dispersal approximately 2.0 Ma. Although there is general consensus that ecosystem variability shaped resource and landscape distributions on the East African landscape, causing changes in early Homo foraging strategies and diet (3)(4)(5), the temporal linkages between climate and environmental changes and hominin evolution remain far more contentious (6).…”

“…Constraining time is much easier in marine sediments given their continuous sedimentation; however, deep marine cores are far removed from the Olduvai ecosystem and cannot be directly tied to the Olduvai sediments. In their two contributions, Magill et al (1,2) extract climate archives from ancient Lake Olduvai sediments. By using lake sediments in the basin where early hominins were living, Magill et al capture an integrated ecosystem signal and take advantage of the best qualities of terrestrial and marine records: local environmental and climate information and high temporal constraints given continuous sediment deposition.…”