The Early Miocene of Africa

During the Eocene, forest vegetation, increased rainfall, and hotter conditions spread from the equator to the poles. The earth's ecology was relatively homogeneous. With the Eocene/Oligocene transition, however, ecological stability started to break down, and the world was thrown into the "big chill" (Prothero, 1994). By 25 million years ago, the position and shape of the continents where beginning to look something like they do today, though North and South America remained separated, the Himalaya and Tibetan plateau had yet to develop, and Africa was still an island continent. At the Oligocene/Miocene transition, however, we see the return of warmer climatic conditions. Even so, from the earliest Miocene, the rainforest belt, which had covered most of Africa, had been breaking up into a number of distinct ecological niches. Instead of the homogeneous tropical cover, we see ever-increasing patches of woodland and grassland interrupting the vast tracts of rainforest. The ongoing continental collisions had reached their zenith during the Miocene, with the major uplift of the Himalayas, the Tibetan Plateau, and the Ethiopian highlands, as continental plates crashed against each other, twisting and thrusting upward from the external land surface. Ecological instability resulted in rapidly fluctuating climatic conditions. This was not only a worldwide pattern but also occurred at a much finer scale, resulting in a patchwork of differing ecological niches within relatively small areas, which were always prone to rapid change or ecological extinction. Ecological change in earlier periods of the Cenozoic had settled down into long periods of stability, but this ceased at the onset of the Miocene (Isaac, 1976; Kennett, 1995; Partridge et al., 1995; Potts, 1996; Denton, 1999; Andrews & Humphrey, 1999).

The earliest Miocene also saw the genesis of the great African rift valleys, as a result of the formation of the Ethiopian highlands. There was massive faulting as the external land surface broke and slipped away, forming the fractured and broken valley floors and walls. This splitting of East Africa's land surface produced a vast increase in volcanic activity within this region (Isaac, 1976; Feibel, 1999). The uplift of the Ethiopian highlands was directly responsible for the formation of a rain-shadow zone because these highlands intercepted the eastward flow of precipitation across the continent. Thus, while western and central Africa continued to receive abundant rainfall, the rift valley systems and East Africa in general (which lay beyond the highlands) were marked by a significant rainfall reduction (Isaac, 1976; Potts, 1996; Andrews & Humphrey, 1999). Indeed, this was soon further exaggerated by the uplift of the Himalayas and the Tibetan plateau, which caused the air to rise and fall in the surrounding region and produced summer heating and winter cooling, and in turn increased the intensity of summer drying in East Africa (Quade et al., 1989; Cerling, 1992; Potts, 1996).

The East African early Miocene families Proconsulidae and Afropithe-cidae together represent a major biological radiation of apelike primates, arguably the earliest apes, though it is possible that the Proconsulidae at least may represent stem catarrhines (the population that gave birth to the Old World monkeys and apes) of modern aspect (Andrews, 1985, 1992; Harrison, 1987, 1988, 1993, 2002; Groves, 1989a; Begun et al., 1997; Harrison & Rook, 1997; Fleagle, 1999). Representatives of the Proconsulidae include the genera Proconsul, Rangwapithecus, and Turkanapithecus and have a temporal span from 23 to 15 million years ago, while the larger-bodied apes, allocated to the Afropithecidae, include the genera Afropithecus, Morotopithecus, and Heliopithecus, which date from between 18 and 15 million years ago (see Andrews, 1992; Harrison, 1992,2002; Cameron, in press a). (See Figures 2.2 and 2.3.) There is little to support a particularly close phylogenetic relationship between these early Miocene apes and the earlier Eocene and Oligocene primates from the Fayum depression of Egypt. This should not be particularly surprising, because they are separated in time by 10 million years and by almost 4,000 km, though some similarities between the Oligocene primate Aegyptopithecus and members of the Afropithecidae have been noted (R.E.F. Leakey et al., 1991).

Members of the Proconsulidae had been living in the forests of Africa long before the appearance of the Afropithecidae, and it is likely that the evolutionary divergence between these two groups began when some groups left the forest and moved out into more open areas. Species of Proconsul tended to occupy the more "closed forest" habitat and appear to

Figure 2.2 ► (a) Reconstruction of Proconsul heseloni female specimen KNM-RU 7290 (adapted from Walker et al., 1983). (b) Unreconstructed sideview of the same specimen.

Taken from Cameron (in press a).

Figure 2.2 ► (a) Reconstruction of Proconsul heseloni female specimen KNM-RU 7290 (adapted from Walker et al., 1983). (b) Unreconstructed sideview of the same specimen.

Figure 2.3 ► (a) Frontal view of Afropithecus turkanensis male specimen KNM-WK 16999. (b) Side view of same specimen.

Taken from Cameron (in press a).

Figure 2.3 ► (a) Frontal view of Afropithecus turkanensis male specimen KNM-WK 16999. (b) Side view of same specimen.

Taken from Cameron (in press a).

have spent most of their time in the trees. Their postcranial anatomy indicates an above-branch form of locomotion. They were also smaller than the Afropithecidae, and their facial and dental anatomy suggests that they focused on eating soft fruits, for which limited food preparation was required. The ongoing divergence of this group would be emphasized over time as habitat distinctions intensified (see Andrews et al., 1997; Walker, 1997; M.G. Leakey & Walker, 1997; Cameron, in press a).

As the Afropithecidae increasingly occupied more open woodlands and grasslands, they developed a dietary preference for hard and tough food items, which required extensive food preparation prior to digestion. This is evidenced by their strong and robust facial architecture and their large premolars and molars. Members of the Afropithecidae (including emerging daughter species) appear to have extended their behavioral and dietary preference for a more arid habitat and its associated dietary regime. The Afropithecidae adopted a primitive form of terrestrial or semiterrestrial locomotion, associated with increased body size. While a number of unique characters define each of the species within this family, they are united by a general adaptive trend, which is emphasized by selection pressures that are reinforcing these evolutionary-adaptive pathways of coarse and hard food object feeding in a more "marginalized" paleohabitat.

The divergence of these two distinct families can be seen as a direct result of ongoing climatic change, which resulted in a fragmentation of the previous relatively homogeneous ecological conditions expressed in Africa. This ongoing fragmentation starts to impact directly on the early Miocene landscape, but it has a history that extends back even earlier in time.

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