The Original Miocene Out of Africa

The paleobiogeography of the earliest representatives of our own family, the Hominidae, during the Middle and Late Miocene has recently been discussed by a number of researchers (Andrews, 1992; Ciochon & Etler, 1994; de Bonis & Koufos, 1994; Andrews et al, 1996; Pilbeam, 1996, 2002; Begun et al., 1997; Begun & Gulec, 1998; Stewart & Disotell, 1998; Andrews & Bernor, 1999; Agusti et al., 1996, 2001; Begun, 2001, 2002; Heizmann & Begun, 2001; Kohler et al, 2001; Kelley, 2002; Cameron, in press a). Most paleoanthropologists who have discussed an

African or Eurasian origin for the hominids tend to agree with the second scenario; that is, the immediate ancestor to the hominids probably emerged from Eurasia around 14-15 million years ago (Ciochon & Etler, 1994; Begun et al., 1997; Begun & Gulec, 1998; Stewart & Disotell, 1998; Agusti et al., 1996, 2001; Begun, 2001, 2002; Heizmann & Begun, 2001; partly Andrews & Bernor, 1999), the clear implication being that the three hominid genera Pongo, Gorilla, and Pan are not endemic to their respective extant geographical distributions.

Much of the original evidence used to support a Eurasian origin for the hominids is based not so much on the hominid fossil record but on the well-documented dispersal into Africa of a number of faunal groups, including the Eurasian hipparionine horses and murid rodents that first appear in the African continent around 10-12 Ma (Bernor et al., 1987; de Bruijn, 1986; Flynn & Sabatier, 1984; Ciochon & Etler, 1994; Bernor et al., 1996; Gentry & Heizmann, 1996; Woodburne et al., 1996). Indeed, many "African" mammal faunal groups (e.g., proboscideans, giraffoids, bovids through to rodents) are now recognized as not having any African early or middle Miocene ancestors; rather, their origins appear to be from outside of Africa (Barry et al., 1985; Thomas, 1985; de Bonis et al., 1992; de Bonis & Koufos, 1994; Bernor et al., 1996; Gentry & Heizmann, 1996; Woodburne et al., 1996; Begun, 2001). Solounias et al. (1999) have recently argued that much of the extant African savanna fauna migrated into Africa from more northerly latitudes, including Greece and Iran. These migrant fauna are suggested to have replaced much of the African endemic fauna because these new immigrant species were already adapted to the new conditions prevailing in Africa, as a result of global cooling (see later). So much of the previously argued "endemic African savanna fauna" now appears to have its origins within Eurasia (Solounias et al., 1999; see also de Bonis et al., 1988; de Bonis & Koufos, 1994).

Stewart and Disotell (1998) have recently reinterpreted hominoid biographical origins based on extant and fossil catarrhine molecular and fossil evidence (Figure 2.10). When first examining the extant ape molecular data and their geographical distributions alone (i.e., not taking into account the fossil evidence), they recognized two major evolutionary scenarios. The first was that the lineage leading to the extant gibbons dispersed out of Africa to Eurasia around 18-20 Ma, leaving the last common ancestor of the orangutan, gorilla, and chimpanzee in Africa. Later the lineage leading to the orangutan dispersed into Eurasia, while the last common ancestor to Gorilla and Pan remained in Africa. The second scenario

Scenario A

Living species rC

Chimpanzees

Humans

Gorillas

Orangutans Gibbons

Old World monkeys

Living and fossil species r¿

Chimpanzees

Humans

Gorillas

Eurasian fossil A Eurasian fossil B Orangutans Eurasian fossil C Eurasian fossil D Gibbons African fossils

Old World monkeys

Scenario B

Chimpanzees Humans Gorillas Orangutans Gibbons Old World monkeys

Chimpanzees ►_p— Humans Gorillas

Eurasian fossil A Eurasian fossil B Orangutans Eurasian fossil C Eurasian fossil D Gibbons African fossils Old World monkeys

Figure 2.10 ► Two dispersal patterns for the hominoids, both assuming an African origin for the Old World primates (catarrhines). Arrows indicate the intercontinental dispersal events required to explain the distribution of the living and fossil species. Scenario A: Separate dispersal events from Africa to Eurasia for each of the Eurasian hominoid lineages. Scenario B: The common ancestor of all living hominoids dispersed out of Africa, and later the common ancestor of the extant African hominids (gorillas, chimpanzees, and humans) dispersed back into Africa. For just the extant species (top cladograms), these two scenarios are equally parsimonious. When the fossil clades are included (bottom cladograms), scenario B is favored, because scenario A now requires a minimum of six independent dispersal events, while scenario B requires just two (see text for more details).

From Stewart and Disotell (1998), p. 585.

is defined by the dispersing of the last common ancestor of the extant hominoids out of Africa, around 18-20 Ma, which then speciated into the lesser and great ape lineages while in Eurasia. Later, one of these populations moved back into Africa, giving rise to Gorilla and Pan. Both of these scenarios are equally parsimonious, in the sense that each requires two migratory events. When they included the available fossil hominid material, however, they determined that the first scenario (e.g., African origins)

is far less parsimonious, for it required at least six dispersals out of Africa, while the Eurasian origin still only required two such events.

The pattern for African hominid migrations as proposed by Stewart and Disotell, however, is rather narrow in its interpretation, in the sense that they believe it requires every hominid to have its origins in Africa. That is, each is marked by its own migration out of the African continent. For example, they propose that Griphopithecus must have moved out of Africa into Eurasia, which was then followed by another separate migration out of Africa by Dryopithecus, which was then followed by yet another separate migration out of Africa by Graecopithecus. It is doubtful that anyone supporting an African origin for the hominids would agree with such a narrow reading of the "Out of Africa" scenario. Indeed, an African origin can be interpreted in any number of ways. Perhaps the least complicated scenario is that the Asian apes dispersed out of Africa around 18 Ma, followed by the Eurasian hominid ancestor around 16-17 Ma. This Eurasian ancestor then gave rise to the Eurasian hominids, including Dryopithecus, Griphopithecus, and Graecopithecus, while the ancestor to the African apes, Gorilla and Pan, remained in Africa. There are three possible middle to late Miocene African hominids, Kenyapithecus, Otavipithecus, and Samburupithecus, which have been considered by some experts to represent an African hominid ancestor (see Ishida et al., 1984; Conroy et al., 1992, 1993; Hill, 1994; Ishida & Pickford, 1997; Nakatsukasa et al., 1998; Ishida et al., 1999; Cameron, in press a). As such, the extant hominids have been considered endemic to their respective regions, while the Eurasian fossil hominids are thought to have ultimately originated from an African ancestor but are largely endemic to Eurasia. This scheme, like the Eurasian origin for extant and fossil hominids, requires only two dispersal events.

Agusti et al. (1996, 2001), however, believe that the pattern of Miocene hominid radiation can be explained as a result of the evolution of the Alpine belt during the Neogene (Figure 2.11). They suggest that around 16 Ma, with the collision of the African and Eurasian plates, an ancestral form of Dryopithecus moved into Europe. This was followed by the divergences of Dryopithecus from the Pongo clade. The rising of the Alpine belt and the expansion of the central European inland sea (the Paratethys) led to the diversification of the later and more specialized southern European hominid Graecopithecus (and eventually Sivapithecus) from the western European dryopithecine populations. Further, the uplift of the Turkish Plate, producing the Zagros mountain chain, and the continued uplift of the Himalayas and Tibetan Plateau resulted in the independent

Mountain Uplift Speciation

Figure 2.11 ► Pattern of vicarious (range-splitting) speciation explaining the independent evolution of the fossil hominids. After the hominids had dispersed into Europe, morphological stasis and persistence of primitive populations took place in western Europe. Isolation by the rising of the Alpine belt, however, may have led to the diversification of more specialized taxa (e.g., Graecopithecus, Sivapithecus). The Vallesian extinction event affected all these apes, although some descendants of the Sivapithecus-Pongo lineage could persist in refugial areas. Zoogeographic barriers: (1) Tethys-Mediterranean realm, (2) Paratethys realm, (3) Himalayan system (see text for more details). Agusti et al. (1996), p. 153.

Figure 2.11 ► Pattern of vicarious (range-splitting) speciation explaining the independent evolution of the fossil hominids. After the hominids had dispersed into Europe, morphological stasis and persistence of primitive populations took place in western Europe. Isolation by the rising of the Alpine belt, however, may have led to the diversification of more specialized taxa (e.g., Graecopithecus, Sivapithecus). The Vallesian extinction event affected all these apes, although some descendants of the Sivapithecus-Pongo lineage could persist in refugial areas. Zoogeographic barriers: (1) Tethys-Mediterranean realm, (2) Paratethys realm, (3) Himalayan system (see text for more details). Agusti et al. (1996), p. 153.

evolution of Sivapithecus and Pongo as a consequence of their long-term isolation. This scenario requires the hominids to be established in both Europe and Asia within a 2-million-year-period, that is, appearing in Europe around 16 Ma and in Pakistan around 14 Ma (this date is based on the requirement to be present in Asia before the major geological barriers). Finally, they suggest that Dryopithecus and Graecopithecus became extinct in Europe around 9 Ma, while Sivapithecus survived to give rise to the Asian extant hominid, Pongo. The African extant apes they consider to have evolved from an unknown African ancestor, thus being endemic to this continent (see Table 2.1).

Begun (1992a, 1994a, 2001, 2002) and Begun and Gulec (1998) have recently published a number of papers supporting a Eurasian origin for the African and Asian extant hominids. As discussed earlier, Begun has argued that Dryopithecus, defined mostly by the Hungarian Rudabanya specimens, is the basal hominid that gave rise to the African hominids (though Begun [1994a, 2002] does incorporate a discussion of the Spanish specimen CLl-18000 in his hypodigm of Dryopithecus). Begun (1992a, 1992b, 1994a, 2001, 2002) and Begun and Gulec (1998) argue that the Asian hominids diverged before the emergence of Dryopithecus and, as such, had nothing to do with the origins of the Asian clade. The basal ancestor of the Asian hominid clade (Sivapithecus, Lufengpithecus, and Pongo), they suggest, may be represented by the late Miocene Turkish hominid Ankarapithecus or a hominid very much like it (see Kappelman et al., 2003). The paleobiogeographical interpretation provided by Begun (2001, 2002; see also Begun & Gulec, 1998) is for an African hominoid migration into Europe by 16.5 Ma, as shown by the Engelswies hominoid molar from Germany (Heizmann & Begun, 2001). It is still unknown how closely related this thick molar enamelled hominoid is to

TABLE 2.1 ► Spatial and Temporal Information for the Miocene Hominids Considered Here

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