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. 2013 Jan 22;110(4):1167-74.
doi: 10.1073/pnas.1206276110. Epub 2012 Dec 24.

Ecosystem Variability and Early Human Habitats in Eastern Africa

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Free PMC article

Ecosystem Variability and Early Human Habitats in Eastern Africa

Clayton R Magill et al. Proc Natl Acad Sci U S A. .
Free PMC article

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.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Present day precipitation patterns during long rains (A; March to May) and short rains (B; October to December) (75). Bold horizontal lines mark the ITCZ. The IOC (B; dashed line) is prominent during short rains, and its eastward displacement correlates with higher seasonal precipitation (66). Targets show Olduvai Gorge’s location (●, ODP site 662; ○, ODP site 722). (C) Depositional environments at Olduvai Gorge during the early Pleistocene about 2.0–1.8 Ma. Expanded and contracted lake levels are reconstructed based on detailed stratigraphic correlations (11, 12, 43).
Fig. 2.
Fig. 2.
Histograms and box-and-whisker plots for ɛSOM/31 from published δ13CSOM and δ13C31 values from subtropical and tropical regions (SI Appendix). Published data are binned at 0.5‰ intervals (counts) based on site averages. Values for δ13CSOM (dark gray) and δ13C31 (light gray) are plotted relative to δ13Cleaf. Values of ɛSOM/31 equal about 9‰ in both C3 and C4 ecosystems. Individual boxes contain interquartile ranges (IQRs). Bold vertical lines within boxes mark median values. ○, mean values. Horizontal whiskers mark minimum and maximum values, except values outside of 1.5 IQR (●). Notch half-width values indicate confidence in differentiating median values.
Fig. 3.
Fig. 3.
Stable carbon-isotope compositions for SOM (δ13CSOM; bottom x axis) and nC31 (δ13C31; top x axis) relative to woody plant cover (A), ecosystem (B), and plant community composition (C). The published relationship between woody plant cover and δ13CSOM is described by the following function, where −13 ≤ δ13CSOM ≥ −31‰ (10): formula image. Here, we relate δ13CSOM to δ13C31 using a value of 9‰ for ɛSOM/31. The relationship between woody plant cover and δ13C31 is, thus, described by the following function, where −22 ≤ δ13CSOM ≥ −40‰): formula image. Ecosystem definitions adhere to African plant communities according to UNESCO terminology (40).
Fig. 4.
Fig. 4.
Biomarker and isotopic signatures for organic matter preserved in lake sediments at Olduvai Gorge. (A) Leaf lipid δ13C values for nC31 (δ13C31). (B) TOC δ13C values (δ13CTOC). (C) TOC percentages (%TOC). (D) Diagrammatic depiction of relative lake level changes at Olduvai Gorge based on lake margin lithostratigraphy (43). (E) Ratios of algal lipids (nC17 + nC19) relative to algal and terrestrial plant lipids (nC17 + nC19 + nC29 + nC31). Higher values reflect relatively increased algal inputs (31). (F) Ratios of pristane (Pr) to phytane (Ph). In lake sediments, values above two generally reflect a dominance of terrestrial plant inputs (31). (G) Ratios of macrophytic lipids (nC23 + nC25) relative to macrophytic and terrestrial plant lipids (nC23 + nC25 + nC29 + nC31; that is, Paq). Higher values reflect relatively increased macrophytic inputs (50). (H) Values of δ13C for the macrophytic lipid nC25 (δ13C25). Horizontal gray bands highlight periods of time characterized by δ13C31 values higher than about −28‰ (i.e., open grassland and wooded grassland ecosystems).
Fig. 5.
Fig. 5.
Time series for global, regional, and local proxy indicators during the early Pleistocene. (A) Global benthic oxygen-isotopic composite (19). Gray lines connect original data, and the bold black line shows a five-point smoothing. (B and C) Alkenone-derived SST estimates for the western Indian Ocean (SST722) and eastern Atlantic Ocean (SST662), respectively (–20). Gray lines connect original data, and bold black lines show five-point smoothing. (D) Calculated orbital precession (ωp). Values for ωp equal the product of calculated eccentricity (e) and the sine function of the longitude of the perihelion (ω). (E) Leaf lipid δ13C values for nC31 (δ13C31; ○ connected by bold black lines) and TOC δ13C values (δ13CTOC; gray lines). F, forest; G, grassland; W, bush/shrubland and woodland; WG, wooded grassland. Ecosystem definitions adhere to African plant communities according to UNESCO terminology (40) based on δ13C31 values (Fig. 4). Vertical gray bands highlight periods of time characterized by δ13C31 values higher than about −28‰ (i.e., open grassland and wooded grassland ecosystems). Hominin taxonomic grades and fossil occurrences are illustrated using information in the work by Wood (76); skulls denote taxonomic grades that have been identified at Olduvai Gorge.

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