The expansion of grassland ecosystems in Africa in relation to mammalian evolution and the origin of the genus Homo
Introduction
The expansion of grasslands in Africa has been linked to several key events in human evolution. A broad set of ideas, generally referred to as the savanna hypothesis, indicates that humans emerged as a consequence of the spread of savanna grasslands in Africa. According to this long-held hypothesis, adaptation to the open savanna led to the divergence of humans from our nearest living relatives, the apes, who remained tied to the dense forests of central Africa Dart, 1925, Robinson, 1954, Jolly, 1970, Klein and Edgar, 2002). In Dart's (1925) words, it was the “vast open country with occasional wooded belts and a relative scarcity of water, together with a fierce and bitter mammalian competition” that provided the ecological context for the divergence of humans from the apes. Variations on this theme abound in anthropology textbooks (e.g., Boyd and Silk, 1997). However, in the last decade or so, new evidence and interpretations show that early hominins may have inhabited rather closed, forested environments Andrews, 1989, Andrews, 1995, Rayner et al., 1993, WoldeGabriel et al., 1994, Clarke and Tobias, 1995, Berger and Tobias, 1996. This evidence also suggests that although grasslands may have been present in Africa since earlier times, they did not become a prominent element of the landscape until the late Pliocene or early Pleistocene.
An idea related to the savanna hypothesis posits that the expansion of savanna grasslands is tied not only to important events in human evolution, but also to major pulses of faunal turnover in many mammal taxa over wide geographical scales. This idea, postulated by Vrba, 1985, Vrba, 1988, Vrba, 1995 links global climatic change to faunal turnover and the emergence of the hominin genera Homo and Paranthropus. There is much evidence that supports the hypothesis of broad climatic changes in the late Pliocene Shackleton et al., 1984, Kennett, 1995), with these changes expressed by increasing aridity in Africa Burckle, 1995, deMenocal, 1995, Dupont and Leroy, 1995). The emergence of the genus Homo in the Pliocene of East Africa does appear to be broadly correlated in time with the advent of major global and regional climatic changes Brain, 1981, Vrba, 1985, Vrba, 1988, Stanley, 1992, but the turnover pulse hypothesis in relation to human evolution remains controversial Hill, 1987, Kimbel, 1995, White, 1995, Behrensmeyer et al., 1997, Feibel, 1997, Feibel, 1999.
An altogether different idea about the influence of climate on human evolution posits that it was the increasing variability of Plio-Pleistocene climates and environments that selected for the adaptations that characterize the hominin clade. This increasing variability, rather than the trend toward more open (i.e., grassland) and arid conditions in Africa, caused selection pressures that led to new coping mechanisms for hominins, including the reliance on technology and culture Potts, 1996a, Potts, 1996b, Potts, 1998a, Potts, 1998b. In this study, we consider these hypotheses in light of a new analysis of fossil mammals from the northern Turkana Basin of Kenya and Ethiopia (Fig. 1). Thus, we provide new analyses of turnover, grassland-adapted taxa, and faunal variability through the Plio-Pleistocene, but first, we briefly review the evidence of grassland expansion in East Africa.
What is the evidence for the earliest savanna grasslands in East Africa? Here we use the term grassland, or savanna grassland, for tropical ecosystems dominated by C4 grasses Sarmiento, 1984, Cerling, 1992, not edaphic, or wet grasslands characterized by waterlogged conditions. Grasslands may have been present in Africa since Miocene times, but the extent and distribution of grassland ecosystems during that epoch remain unclear. Analysis of paleosol micromorphology and fossil pollen from the Middle Miocene site of Fort Ternan in Kenya indicates that grassy areas, and perhaps more widespread grasslands, occurred in East Africa as far back as 14 Ma Bonnefille, 1984, Retallack et al., 1990, Retallack, 1992. If so, these were probably cool growing season (C3) grasses, because paleosol and dental enamel carbon isotopes from the same locality document a dominantly C3 ecosystem (Cerling et al., 1991). Paleosol carbonates from the Baringo Basin of Kenya show little evidence of strong trends toward C4-dominated ecosystems during the Miocene to Pleistocene (Kingston et al., 1994). Instead, the Baringo Basin pedogenic carbonate analyses support a view of a persistent mosaic of habitats that included grasslands and wooded or bushy vegetation. Macrobotanical remains from 12.6 Ma Ngorora Formation deposits, also in the Baringo Basin, show that tropical rainforests with West African affinities were, at least at times, an important component of East African ecosystems (Jacobs and Kabuye, 1987). Thus, it appears that C4 grasslands existed in the East African Miocene, but the available evidence indicates that closed, wooded, bush-dominated, or forested environments with some C3 grasslands were dominant throughout this time interval.
In the Pliocene and Pleistocene, soil carbonate evidence from the Turkana Basin indicates that the interval from 4 to 1.8 Ma was dominated by C3 plants with some C4 components, and that C4 plants began to dominate the landscape at 1.8 Ma (Cerling et al., 1988). New evidence from the Gona deposits in the lower Afar Depression of Ethiopia shows a shift from woodlands and grassy woodlands in the early Pliocene to more open environments by the late Pliocene (Levin, 2002). This indicates regional variability between different parts of the East African Rift System in the extent of open grasslands by the late Pliocene. Paleosol carbonates from the Turkana and Olduvai–Laetoli (Tanzania) areas show a peak of C4-dominated vegetation at about 1.7 Ma, followed by lower values between 1.6 and 1.4 Ma, and another peak at about 1.2 Ma (Cerling, 1992). Thus, the paleosol carbonate record indicates that Pliocene ecosystems in East Africa were dominated by C3 vegetation, and that the Pleistocene spread of C4 grasslands occurred at different times in different regions.
Palynological records from the East African Pliocene are discontinuous and subject to significant taphonomic biases but provide a broad picture of environmental change in the region. Records from the Turkana Basin indicate closed and humid environments at about 4 Ma, and again at about 3.4 Ma, followed by an increase in grasslands at about 2.5 Ma Bonnefille and Letouzey, 1976, Bonnefille and Dechamps, 1983, Bonnefille, 1995. An analysis of nine pollen samples from Olduvai Bed I and lower Bed II (between 1.9 and 1.7 Ma) shows that there was a trend toward increasing aridity culminating at 1.77 Ma, but oscillating conditions thereafter (Bonnefille, 1995).
Against this background, we examine another line of paleoenvironmental evidence derived from the mammalian fauna of East Africa. One of the most complete records of mammalian evolution in Africa comes from the Turkana Basin of northern Kenya and southern Ethiopia (Fig. 1A). The well-dated sediments of the Omo Group deposits of this area span much of the Pliocene and early Pleistocene, and provide a unique window into environmental and faunal changes during a time when key events in human evolution were taking place Butzer, 1976, de Heinzelin, 1983, Howell et al., 1987, Harris et al., 1988, Feibel et al., 1989, Feibel et al., 1991, Brown and Feibel, 1991, Brown, 1994, Brown, 1995.
Section snippets
Materials and methods
Our analysis takes a multi-layered approach to faunal change in the East African Plio-Pleistocene: it ranges from a broad view of faunal turnover in the Turkana Basin to finely resolved, locality-based analysis of faunal associations in the lower Omo valley. The analysis of faunal turnover, i.e., first and last appearance data (FADs and LADs), presented here is based on the Mursi, Usno, Shungura, Koobi Fora, and Nachukui Formations, all situated in the northern Turkana Basin. Data from the
Grassland indicators
As a proxy for the extent of grassland environments in the paleo-Omo ecosystem, we use the abundance of individuals belonging to mammalian species or clades that show clear adaptations to grassland environments. Grassland adaptation has been determined both through direct functional analysis of teeth (hypsodonty) and skeletal morphology (cursorial adaptation) as well as known adaptations to grassland habitats among members of a clade with modern representatives, such as the tribe Alcelaphini
Results and analyses
The interval from 4 to 1 Ma shows major shifts in the abundance of the main families of mammals from the Omo sequence (Table 2 and Fig. 2A–B). Fig. 2A shows that suids underwent a significant decline in abundance between the middle and late Pliocene but had a resurgence in the early Pleistocene beginning at about 1.75 Ma. Bovids, the most common family of fossil mammals at most Plio-Pleistocene African localities, experienced an increase in abundance during the late Pliocene, especially between
Discussion
Our analysis has shown that in the interval from 4 to 1 Ma there were profound faunal changes in the Turkana Basin. The most important of these changes include significant shifts in the abundance of the most common families of mammals, episodes of high faunal turnover, and an increase in the number and abundance of species that show adaptations to grassland ecosystems. Hominins, an integral element of Plio-Pleistocene East African faunas, fit into these patterns of change in complex ways that
Conclusions
Our results show that there were profound faunal and environmental changes in the Turkana Basin during the interval from 4 to 1 Ma. The most important of these changes include significant shifts in the abundance of the most common families of mammals, episodes of high faunal turnover, and an increase in the number and abundance of species that show adaptations to grassland ecosystems. The Paranthropus clade originated at about 2.7 Ma, following an episode of major faunal change in the Omo at
Acknowledgements
We thank Caroline Strömberg for inviting us to participate in the symposium “The Evolution of Grass-Dominated Ecosystems During the Tertiary” held during the 2001 North American Paleontological Convention in Berkeley, California. We give deep thanks to Clark Howell, leader of the American contingent of the International Omo Research Expedition, and to the many field workers who contributed their efforts to the success of the Omo project. For helpful and thoughtful comments and suggestions on
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