Ecosystem variability and early human habitats in eastern Africa

Abstract

The role of savannas during the course of early human evolution has been debated for nearly a century, in part because of difficulties in characterizing local ecosystems from fossil and sediment records. Here, we present high-resolution lipid biomarker and isotopic signatures for organic matter preserved in lake sediments at Olduvai Gorge during a key juncture in human evolution about 2.0 Ma—the emergence and dispersal of Homo erectus (sensu lato). Using published data for modern plants and soils, we construct a framework for ecological interpretations of stable carbon-isotope compositions (expressed as δ 13 C values) of lipid biomarkers from ancient plants. Within this framework, δ 13 C values for sedimentary leaf lipids and total organic carbon from Olduvai Gorge indicate recurrent ecosystem variations, where open C 4 grasslands abruptly transitioned to closed C 3 forests within several hundreds to thousands of years. Carbon-isotopic signatures correlate most strongly with Earth’s orbital geometry (precession), and tropical sea-surface temperatures are significant secondary predictors in partial regression analyses. The scale and pace of repeated ecosystem variations at Olduvai Gorge contrast with long-held views of directional or stepwise aridification and grassland expansion in eastern Africa during the early Pleistocene and provide a local perspective on environmental hypotheses of human evolution.

Water, plants, and early human habitats in eastern Africa

“…According to marine records, this juncture occurred during an interval of protracted grassland expansion across eastern Africa (1). In contrast, coeval terrestrial records from hominin archaeological sites such as Olduvai Gorge indicate recurrent fluctuations between open-grassland and closed-woodland ecosystems (5). Although such ecosystem fluctuations are commonly interpreted in relation to aridity, grassland expansion is sensitive to multiple factors (6), and proxy signals more closely linked to meteoric waters could strengthen interpretations.…”

“…We target subtropical and tropical taxa in clades most representative of plants in eastern Africa since the early Pleistocene (27,28). Leaves of subtropical and tropical plants commonly contain the leaf-lipid hentriacontane (nC 31 ) (29,30), and nC 31 is also abundant in lake sediments from Olduvai Gorge (5 4 graminoids is most negative (−146‰ ± 8‰, 95% confidence interval; n = 51). The median « 31/model value for C 3 herbs (−124‰ ± 10‰; n = 24) is more negative than for C 3 woody plants (−109‰ ± 8‰; n = 84).…”

Capturing climate variability during our ancestors' earliest days

“…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.…”