Tuesday, July 27, 2010
Late Miocene Hominoids: Back to the Future
The discovery of a 7 million year old hominoid tooth in Bulgaria may not sound like much but it could be a “big deal.” For one, it demonstrates that late Miocene (Turolian) apes persisted in Europe much later than previously thought. The Turkish ape, Ouranopithecus turkae, with a biochronological age of 8.7–7.4 mya and fossil hominoids from Yunnan attributed to Lufengpithecus, dated between 6-11 mya, also last well into the Turolian. It is also during the Turolian that purported basal hominans (i.e. Sahelanthropus) begin to appear in the African fossil record.
Ouranopithecus, a well-known late Miocene European hominoid, has been identified as a hominine (a member of the African ape/human clade) by many paleoanthropologists. Based on evidence recently reviewed at this blog many Chinese paleoanthropologists now recognize Lufengpithecus to, likewise, be a stem hominine. What the new Bulgarian tooth demonstrates is that the radiation of these purported stem hominines extended both in time and space to a much greater extent than once thought.
The question has arisen, however, as to whether these potential hominines could be direct human (hominan) ancestors. If Sahelanthropus, at 7 mya, is accepted as a hominan, there is no biochronological reason not to accept the others as well. Biogeography should not enter into the picture either, as primates are known to disperse as readily as any other order of mammals. As I have attempted to show in these posts many, if not all, the cranial and post-cranial features used to diagnose early hominans (aka hominins) in Africa have been identified in Lufengpithecus. Both Ouranopithecus and Lufengpithecus represent fossil lineages that extent back to 10-11 mya. The same may be demonstrated in Africa for the lineage represented by Sahelanthropus and Orrorin. Thus purported hominans, i.e. direct human ancestors, may be documented in the fossil record at time scales and across continental areas equivalent to those proposed in the 1960s and 70s for Ramapithecus.
Well, then, it may be asked, what of the molecular clock? Geneticists have long argued for a late divergence of chimpanzees and humans between 4-6 mya. Human paleontologists, however, have recently wanted to push this divergence further back in time to accommodate the likes of Sahelanthropus. The increasingly dense and diagnostic late Miocene hominoid fossil record may thus force us to either acknowledge an early divergence hypothesis for hominan origins or accept the fact that stem hominines (i.e. the ancestors of the extant African apes and humans) were indistinguishable from early hominans. The poster child for this latter hypothesis may be Ardipithecus.
The implications of either of these hypotheses are quite significant. If an early divergence hypothesis for hominan origins on the order of 12 mya is accepted, the molecular clock would need to be recalibrated. This is in keeping with those who suggest that primate origins can be traced back to the Cretaceous on the order of 80+ mya. For instance, If we double the divergence time of apes and humans from 4-6 mya to 8-12 mya we should also double the dates for the origins of anatomically modern Homo sapiens and their “Out of Africa” dispersal. Thus, the common ancestor of Neanderthals and H. sapiens would be on the order of 0.8-1.2 mya rather than 0.4-0.6 mya, AMHS would have originated 400 kya rather than 200 kya, and the “Out of Africa” dispersal would have begun 120-140 kya rather than 60-70 kya. These revised dates are actually more in keeping with the emerging human fossil record than the dates now commonly accepted. The second hypothesis, that the common ancestor of extant hominines was more hominan than not, would overturn the conventional wisdom that human ancestors are uniquely derived relative to our shared ancestor with the African apes. Of course it can always be argued that none of the evidence presented here has any direct relevance to the question of human evolution, that late Miocene Eurasian apes with hominan-like traits have nothing to do with the origins of the African apes or humans. Any similarities they share with hominines or hominans, for that matter, are just examples of homoplasy, i.e. parallel evolution. I've argued that hypothesis and it's implications in a previous post. For the sake of clarity let me reiterate the three alternative hypotheses regarding late Miocene hominoids.
1. Late Miocene Eurasian apes, with features that have been favorably compared with hominans (i.e. direct human ancestors and their collateral kin), are in fact early members of the human clade. This would entail an early divergence of the African apes and humans perhaps as long as 12 mya.
2. Late Miocene Eurasian apes, with features that have been favorably compared with hominans, are actually stem hominines (African ape/human ancestors either direct or collateral). This would mean that nearly all the features currently used to identify members of the human clade, such as cranial and post-cranial adaptations for some form of post-facultative, pre-obligate, habitual bipedalism are to be found in the last common ancestor (LCA) of the African apes and humans. Hence, given the constraints of the late divergence hypothesis of human origins (i.e. the LCA of chimpanzees and humans lived 4-6 mya), the likes of Sahelanthropus, Orrorin and Ardipithecus would be very close or identical to the the LCA of the African ape/human clade.
3. Late Miocene Eurasian apes, with features that have been favorably compared with hominans, are either early pongines or stem hominoids that evolved adaptations for bipedalism and other hominan-like traits in parallel with our African ancestors, suggesting that features that have long been thought to distinguish the human lineage from other hominoids are not so unique after all.
All three of these hypotheses are heterodox as they challenge the prevailing orthodoxy that the human lineage emerged from a bunch of jungle dwelling African apes who had never left their continental homeland and that human-like adaptations for bipedalism were unique to ouirselves amongst the hominoids. In other words all those late Miocene Eurasian apes were nothing but an interesting but ultimately futile evolutionary sideshow. Any so-called hominan-like traits they possessed have been over-interpreted, misinterpreted or simply don't exist.
This position is epitomized by Cartmill and Smith (2009) who have this to say about the late Miocene Eurasian apes:
All these supposed orangutan relatives are of approximately the same age (late Miocene, around 10 Mya). Because they all resemble Pongo, albeit in different ways and to different degrees, they all have been placed in the orangutan family Pongidae by some authorities. But if Sivapithecus and its relatives are pongids, then the arm-swinging adaptations shared by the living apes must have evolved at least three separate times: once for orangutans, once for gibbons, and once for hominids. It seems equally likely that orangutan-like facial morphology evolved twice – once in the orang lineage, and convergently in the ancestry of Sivapithecus. We provisionally interpret Sivapityhecus and most of the other Miocene pseudo-orangutans as comprising an extinct family of specialized proconsuloids, the Sugrivapithecidae. However, Khorapithecusmay may prove to be a true member of the orangutan family.This scenario seems far-fetched to me, as I would expect adaptations for arm-swinging locomotion to be a common parallelism in suspensory hominoids, while the autapomorphic facial features of orangutans seem to be much less adaptive, and as a consequence, may have much greater phylogenetic weight. Nevertheless it highlights the prevailing notion that early human evolution occurred in Africa and Africa alone and that any attempt to look father afield is misguided.
Cartmill, M. and F. Smith. 2009. The Human Lineage, Wiley-Blackwell.