Neogene biomarker record of vegetation change in eastern Africa

Abstract

The evolution of C4 grassland ecosystems in eastern Africa has been intensely studied because of the potential influence of vegetation on mammalian evolution, including that of our own lineage, hominins. Although a handful of sparse vegetation records exists from middle and early Miocene terrestrial fossil sites, there is no comprehensive record of vegetation through the Neogene. Here we present a vegetation record spanning the Neogene and Quaternary Periods that documents the appearance and subsequent expansion of C4 grasslands in eastern Africa. Carbon isotope ratios from terrestrial plant wax biomarkers deposited in marine sediments indicate constant C3 vegetation from ∼24 Ma to 10 Ma, when C4 grasses first appeared. From this time forward, C4 vegetation increases monotonically to present, with a coherent signal between marine core sites located in the Somali Basin and the Red Sea. The response of mammalian herbivores to the appearance of C4 grasses at 10 Ma is immediate, as evidenced from existing records of mammalian diets from isotopic analyses of tooth enamel. The expansion of C4 vegetation in eastern Africa is broadly mirrored by increasing proportions of C4-based foods in hominin diets, beginning at 3.8 Ma in Australopithecus and, slightly later, Kenyanthropus This continues into the late Pleistocene in Paranthropus, whereas Homo maintains a flexible diet. The biomarker vegetation record suggests the increase in open, C4 grassland ecosystems over the last 10 Ma may have operated as a selection pressure for traits and behaviors in Homo such as bipedalism, flexible diets, and complex social structure.

Keywords: carbon isotope; hominin; leaf wax; mammalian evolution; molecular distribution.

Fig. 1.

Map of eastern Africa showing DSDP core Sites 228, 232, 235, and 241 sampled for this study. Previously studied Site 231 is also shown. Yellow triangles indicate hominid fossil localities from 10 Ma to 1 Ma. (Inset) The two hominin sites are Toros-Menalla (Chad, central Africa) and the Chiwondo Beds in the Karonga Basin of northern Malawi. Both sites may fall within the plant wax source regions for cores used in this study. Base map is from GeoMapApp (, www.geomapapp.org).

Fig. S1.

Age−depth models for DSDP cores are based on calcareous nannofossil biohorizons (61, 62) and volcanic ashes in the cores (3), where each filled circle represents an age−depth tie point. Age uncertainties for tie points are, on average, less than 0.02 My. Biomarker samples are shown with open red symbols.

Fig. S2.

Gas chromatograms of the saturated aliphatic fractions showing FID detector intensity vs. time. Plant-derived n-alkanes used in this study include the C₂₇ to C₃₅ homologs. The asterisk (*) indicates the internal standard peak (5α-androstane). (A) Sample KU354 (Site 241, 9.65 Ma) is representative of most samples analyzed, with characteristic odd-over-even preference in long-chain alkanes. (B) Sample KU358 (Site 241, 23.4 Ma) is characterized by a relatively large UCM from 10 min to 35 min. Plant-derived n-alkanes elute after the UCM and, although comparatively small, still exhibit characteristic odd-over-even preference and concentrations sufficient to make carbon isotope measurements.

Fig. 2.

Carbon isotope values of n-C₃₁ alkanes indicate C₃ vegetation from 24 Ma to 10 Ma. After 10 Ma, the increase in δ¹³C values reflects the increasing proportion of C₄ vegetation on the landscape. The near-linear increase in samples from the Somali Basin (DSDP Sites 235 and 241) and Red Sea (Site 228), suggest a regionally coherent expansion of C₄ vegetation. Percent C₄ is calculated from endmembers compiled in ref. . Large variability in endmember values leads to uncertainties in percent C₄ of 15–21% (Dataset S5). Uncertainty of measured δ¹³C values, however, is much smaller (∼ ±0.10‰).

Fig. S3.

Carbon isotope data from n-C₂₉, n-C₃₁, n-C₃₃, and n-C₃₅ alkanes from all four DSDP Sites (228, 232, 235, and 241). Sites are indicated by symbol, and n-alkanes are indicated by color.

Fig. S4.

Carbon isotope data from n-C₂₉, n-C₃₁, n-C₃₃, and n-C₃₅ alkanes from the Somali Basin DSDP Sites 235 and 241. From 24 Ma to 10 Ma, all δ¹³C values correspond to C₃ ecosystems. At 10 Ma, all δ¹³C values begin to shift toward more positive values, most pronounced in the +3.1‰ shift in the n-C₃₅ alkane, signaling the onset of expansion of C₄ vegetation. Also shown are the n-alkanoic acid δ¹³C values from DSDP Site 231 in the Gulf of Aden (7). These samples also indicate the onset of C4 expansion at ∼10 Ma, although the trend and absolute values are different from the Somali Basin sites.

Fig. S5.

Molecular distributions of long-chain alkanes from Somali Basin samples divided into two groups, pre- and post-10 Ma. Fractional abundances are normalized to the most abundant homolog (C₃₁) for both groups. The fractional increases in C₃₃ and C₃₅ and corresponding decrease in C₂₉ in post-10-Ma samples with respect to pre-10 Ma supports the isotopic evidence for C₄ grasses and the δ¹³C of the C₃₅ homolog as a first indicator for the onset of C₄ vegetation expansion. In pre-10-Ma samples, the second most abundant homolog is C₂₉ (75% of C₃₁) followed by C₃₃ (50%). C₃₅ abundance is only 8% of C₃₁. In post-10-Ma samples, C₃₃ is the second most abundant homolog (70% of C₃₁), C₂₉ is slightly less abundant (67%), and C₃₅ increases to 23% of C₃₁.

Fig. S6.

Normalized molecular distributions for all samples. For concentration and additional sample information, see Dataset S4.

Fig. 3.

Multiproxy carbon isotope data from eastern Africa illustrate the onset of the expansion of C₄ vegetation at 10 Ma and the dietary response of proboscideans. (A) Carbon isotope data from Somali Basin cores show a 3.1‰ increase in the n-C₃₅ alkane at 10 Ma marking the onset of expansion of C₄ vegetation. Carbon isotope values from nC₃₁- and nC₃₅-alkanes increase from 10 Ma to present. (B) Tooth enamel δ¹³C data from east and central African proboscideans (from many of the same locations as hominids in Fig 1) have C₃-dominated diets (<−8‰) from 17 Ma to 10 Ma, after which there is an immediate increase in δ¹³C at 9.9 Ma from the inclusion of minor amounts of C₄ vegetation into their diet. Data are compiled from refs. and . Other mammalian lineages that had C₄ vegetation in their diets by ∼9.6 Ma include equids, bovids, and rhinocerotids. (C) Pedogenic carbonate δ¹³C data from eastern African fossil localities (see ref. for sites) represent local vegetation conditions. Gray circles are individual data points; histograms draw from 2-My bins from 19 Ma to 10 Ma, 1-My bins from 10 Ma to 6 Ma, and 0.5-My bins from 6 Ma to 1 Ma. Bins with less than five data points are shown in gray. Data are compiled from refs. and . Vertical dashed gray lines at −8‰ in B and C indicate the boundary between C₃ and mixed C₃−C₄ values. (D) Schematic of selected Neogene large-bodied primate taxonomy from eastern Africa. Probable and confirmed taxa of the subtribe homonini (7 Ma to present) occur after the onset of C₄ grassland expansion at 10 Ma. Taxa and age ranges are compiled from refs. , , and –.

Fig. S7.

Linear regressions of δ¹³C versus sample age from 10 Ma to present for (A) Sites 235 and 241, (B) the stack of Somali Basin sites, and (C) all sites except for 232, which has been excluded from all Africa data.