Woody cover and hominin environments in the past 6million years

Journal name:
Nature
Volume:
476,
Pages:
51–56
Date published:
DOI:
doi:10.1038/nature10306
Received
Accepted
Published online

Abstract

The role of African savannahs in the evolution of early hominins has been debated for nearly a century. Resolution of this issue has been hindered by difficulty in quantifying the fraction of woody cover in the fossil record. Here we show that the fraction of woody cover in tropical ecosystems can be quantified using stable carbon isotopes in soils. Furthermore, we use fossil soils from hominin sites in the Awash and Omo-Turkana basins in eastern Africa to reconstruct the fraction of woody cover since the Late Miocene epoch (about 7 million years ago). 13C/12C ratio data from 1,300 palaeosols at or adjacent to hominin sites dating to at least 6million years ago show that woody cover was predominantly less than ~40% at most sites. These data point to the prevalence of open environments at the majority of hominin fossil sites in eastern Africa over the past 6million years.

At a glance

Figures

  1. Correlation of [dgr]13C between gap and canopy samples for 76 tropical soils used in this study.
    Figure 1: Correlation of δ13C between gap and canopy samples for 76 tropical soils used in this study.

    Best-fit line is using the major axis regression where δ13Ccanopy = 0.79×δ13Cgap −6.4; r2 = 0.89.

  2. Woody cover and soil [dgr]13C for 76 tropical soils used in this study.
    Figure 2: Woody cover and soil δ13C for 76 tropical soils used in this study.

    Canopy-weighted δ13C values from Supplementary Table 1 have been corrected for the Suess effect (to 1,750)43, 44, assuming a residence time for carbon in soils to be 10years22. Ordinary linear regression was carried out on arcsine square-root-transformed values of fractional woody cover ( )45; the dashed line is the OLR function δ13C = −9.02 −14.49 (r2 = 0.77). SOM, soil organic matter.

  3. Surface soil temperatures from soil temperature profiles.
    Figure 3: Surface soil temperatures from soil temperature profiles.

    Calculated maximum daily soil-surface temperatures for a 12-month interval for forest, woodland and grassland sites in the Meru National Park region, Kenya (see Supplementary Tables 1 and 2).

  4. Estimated fraction of woody cover based on >1,300 published analyses of palaeosols from eastern African hominin sites from 6[thinsp]Myr ago to present.
    Figure 4: Estimated fraction of woody cover based on >1,300 published analyses of palaeosols from eastern African hominin sites from 6Myr ago to present25, 26, 29, 30, 31, 46, 47, 48, 49, 50.

    Vegetation classification is from ref. 23. Top: cumulative frequency of palaeosol values related to fraction of woody cover. Bottom: histogram of palaeosol values related to fraction of woody cover.

  5. Map with modern soil sites and hominin-fossil bearing localities in the East African Rift System overlain on GTOPO30 digital elevation model.
    Figure 5: Map with modern soil sites and hominin-fossil bearing localities in the East African Rift System overlain on GTOPO30 digital elevation model.

    Red circles, modern soil sites; yellow triangles, hominin-fossil-bearing localities.

  6. Composite record of palaeosol stable isotopic composition from the Awash Valley, Ethiopia (left) and Omo-Turkana Basin, Kenya and Ethiopia (right).
    Figure 6: Composite record of palaeosol stable isotopic composition from the Awash Valley, Ethiopia (left) and Omo-Turkana Basin, Kenya and Ethiopia (right).

    A hominin phylogram is shown at the centre (adapted from ref. 32). Stable isotope data are presented as normalized probability density functions of predicted woody cover determined for palaeosols in a series of temporal bins defined for each basin. Temporal bins are divided based on marker tephra in each sedimentary basin (see Supplementary Tables 3 and 4). The number of pedogenic carbonate analyses (nc) and of organic matter analyses (nom) are indicated for each temporal bin; the median value of woody cover for all data from each temporal bin is shown with a narrow white bar. Data from the Awash Valley include relatively continuous records from co-adjacent research areas at Hadar, Gona and Dikika, along with data from the Middle Awash region (Aduma, Daka, Aramis and Western Margin). Because the Shungura formation of the Omo River Valley preserves different environments from the remainder of the basin36, data from this region are shown with probability density functions distinct from the remainder of the record (Koobi Fora and Nachukui formations, East and West Turkana). The number of analyses from the Omo Valley (nO) and the Lower Turkana Basin (nT) are indicated. Hominin species ranges are spread according to their age distribution (vertical axis) and roughly corresponding to their anatomical features (horizontal axis)32. Major archaeological innovations of early stone tool development are also indicated: Oldowan technology, first stone tools, 2.6 Myr ago; Acheulean technology, 1.7Myr ago.

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Author information

Affiliations

  1. University of Utah, Salt Lake City, Utah 84112, USA

    • Thure E. Cerling,
    • William Mace,
    • Anthony N. Macharia &
    • Christopher H. Remien
  2. University of South Florida, Tampa, Florida 33620, USA

    • Jonathan G. Wynn
  3. Kenya Wildlife Service, PO Box 40241-00100 Nairobi, Kenya

    • Samuel A. Andanje &
    • David Kimutai Korir
  4. James Cook University, PO Box 6811, Cairns QLD 4870, Queensland, Australia

    • Michael I. Bird
  5. Johns Hopkins University, Baltimore, Maryland 21218, USA

    • Naomi E. Levin
  6. University of Arizona, Tucson, Arizona 85721, USA

    • Jay Quade

Contributions

S.A.A., M.I.B., T.E.C., D.K.K., N.E.L., W.M., J.Q. and C.H.R. designed the modern soil surveys. T.E.C., M.I.B., A.N.M., W.M. and J.G.W. evaluated the amount of woody cover. C.H.R. and W.M. analysed the soil temperature data. M.I.B., N.E.L., A.N.M. and J.G.W. analysed modern soils. N.E.L., J.Q. and J.G.W. contributed new palaeosol data. T.E.C. and J.G.W. wrote the paper with input from all authors.

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The authors declare no competing financial interests.

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Supplementary information

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  1. Supplementary Information (1.3M)

    This file contains Supplementary Figures 1-2 with legends, Supplementary Methods Supplementary Tables 1-6 and additional references.

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