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.

The Early Miocene Critical Zone at Karungu, Western Kenya: An Equatorial, Open Habitat with Few Primate Remains

“…Their data offer a large-scale picture of vegetation types in East Africa, but necessarily sample externally-drained continental basins. However, the Ngira locality was potentially an endorheic basin, given the rapid lake level fluctuations documented in the stratigraphy , and therefore is very unlikely to have contributed to the record of Uno et al (2016). This finding underscores the fact that, while a major turnover from C 3 to C 4 vegetation occurred in the late Miocene, much work remains for documenting landscape heterogeneity earlier in the geologic history of East Africa.…”

“…Stable isotope analyses of compoundspecific biomarkers (Uno et al, 2016), pedogenic carbonate (Cerling, 1992), and fossil tooth enamel (Cerling et al, 1998) suggest that C 4 vegetation was not common across East Africa until ca.10 Ma, with a gradual rise of C 4 -dominated grasslands starting in the late Miocene at ca. 8 Ma.…”

“…Uno et al (2016) present a compound-specific isotope record from cores taken from the Somali Basin, Gulf of Aden, and Red Sea. Their data offer a large-scale picture of vegetation types in East Africa, but necessarily sample externally-drained continental basins.…”

“…8-10 Ma (Cerling et al, 1998;Jacobs et al, 1999Jacobs et al, , 2010Uno et al, 2016), implying that unchanging habitats allowed biotic factors to drive evolution of early hominoid lineages through most of the early and middle Miocene. However, a few studies have indicated the presence of open canopy habitats based on environmental interpretations from paleosols and stable isotopes from early Miocene strata at Karungu and Rusinga Island in western Kenya (Bestland and Krull, 1999;Forbes et al, 2004;Driese et al, 2016).…”

Mio-Pliocene aridity in the south-central Andes associated with Southern Hemisphere cold periods

“…Much of our knowledge of latest Miocene terrestrial climate is derived from plant fossils, hydrocarbon biomarkers, and the isotopic composition of paleosoils and fossil teeth, features that are rare or poorly preserved in most sedimentary sections (25,26). Likewise, most terrestrial sediments are deposited in association with active mountain belts, creating the additional challenge of deconvolving regional climatic signals from local tectonic-orographic signals.…”