Monday, July 05, 2010
A Chinese Ape in Our Ancestry?
As paleoanthropologists home in on the Last Common Ancestor (LCA) of apes and humans, it seems that many have been barking up the wrong tree. Rather than being rather chimp-like, it's turning out that the LCA was perhaps more hominid-like (in the old sense of the word) than previously thought. This has been highlighted by the recently published account of the Ardipithecus ramidus partial skeleton, which at 4.4 mya is considered by many paleoanthropologists to be one of our earliest direct ancestors.
A. ramidus retains a suite of primitive upper body traits and a mix of lower body traits indicative of an incomplete adaptation to bipedalism. According to Tim White et al. Ardi does not show any signs of a knuckle-walking morphology, casting doubt on the long held supposition that its recent ancestor, the LCA of chimps and humans, was a knuckle-walker. White’s team suggests that knuckle-walking evolved in parallel between gorillas and chimpanzees. Based on the results of the Ardipithecus research it can now be hypothesized that the chimp/human LCA was an incipient biped or at least had pre-adaptations that were later exapted for bipedalism. As Oreopithecus bambolii (a Late Miocene ape from Italy with a pelvis and lower limbs that suggest adaptations for bipedality) demonstrates, traits generally associated with bipedalism are found in at least one non-hominid Late Miocene ape lineage. Taking these considerations into account it may well be the case that the hominid-like traits of Ardipithecus, Sahelanthropus and Orrorin (all thought by many to be early pre-Australopithecine human ancestors) are actually what we should expect in the LCA.
Another Late Miocene ape that may shed light on the nature of the LCA is found not in Africa but in Asia. I'm speaking of the "enigmatic" ape Lufengpithecus from Yunnan in southern China. Specimens of Lufengpithecus have been known since the 1970s and were first attributed to Ramapithecus lufengensis and Sivapithecus yunnanensis. At that time ramapithecines were still thought to be basal hominids while sivapithecines were seen as ancestral pongids. This hypothesis was challenged in 1979 with the discovery of GSP 15000, the partial skull of a sivapithecine from the Siwaliks Hills of Pakistan, remarkably orang-like in morphological features. A thorough revaluation of ramapithecine and sivapithecine specimens in the early 1980s led to the realization that Ramapithecus fossils were female sivapithecines and that the sivapithecines included a number of genera broadly ancestral to the modern great apes, including humans. Sivapithecus itself is generally thought to be an early relative of extant orangutans.
Genetic research, combined with an enhanced record of hominid evolution, reinforced the idea that the African apes and humans shared a unique common ancestor to the exclusion of the Asian orangutan and that Chimpanzees and humans were sister lineages. The temporal and geographical placement, ecological setting and physical adaptations of the LCA of chimpanzees and humans have thus become the “holy grail” of paleoanthropology. The time and place of this LCA is equivocal. The time is thought to be between 5-7 mya although some would suggest a more recent age (~ 4 mya) and others an earlier one (~8 mya). The place is generally thought to be the horn of Africa although some would extend the region to include adjacent areas of North and East Africa and perhaps southeastern Europe. The ecological setting and physical nature of the Chimpanzee/human LCA is more controversial. Until the analysis of Ardipithecus was recently published it was generally assumed that the LCA was more chimpanzee-like than not. The partial skeleton of Ardipithecus however shows that the LCA retained a suite of primitive locomotor characters not seen in either of its descendants, holding out the possibility that it was actually more bipedal that previously thought. Hence some 5-7 million year old fossils with seeming adaptations to bipedalism such as Sahelanthropus, Orrorin and Ardipithecus itself may in fact be morphologically representative of the LCA. In my estimation this is indeed the case and the LCA is already known but unrecognized. The Last Common Ancestor is thus in reality the Lost Common Ancestor.
Well, let’s get back to Lufengpithecus. When first described the female Lufengpithecus remains were attributed to Ramapithecus lufengensis and were thought to be ancestral hominids (aka hominins of current usage). This was warranted given the very australopithecine-like appearance of its dentition. Of course we now know that the hominid-like dental traits of R. lufengpithecus, i.e. the relatively short canines, incipiently bicuspid lower premolars, and thick enameled, bunodont molars, are character states commonly seen in late Miocene female apes. The male Lufengpithecus remains were much more pongine-like with tusk like canines, semi-sectorial lower P3s and large, thick-enameled, crenulated molars. Back in 1989 Jay Kelly and I demonstrated that the two sets of teeth were statistically uni-modal and hence represented females and males of a single highly dimorphic species. Nonetheless, the Lufengpithecus teeth, like the dentitions of many late Miocene apes, are characterized by thick molar enamel and have striking similarities to australopithecine homologues.
Two recent monographs, one edited by Xu Qinghua and Lu Qingwu (2008), describing the major discoveries of Lufengpithecus lufengensis in the late 1970s and early 1980s from the Lufeng site in Yunnan, and the other edited by Qi Guoqin and Dong Wei (2004), reviewing hominoid material collected in the Yuanmou basin, go a long way towards clarifying the temporal relationships of the two sites and the morphological characteristics of their fossil hominoids. Lufeng is shown to be 6.2-6.9 million years old and the sites in the Yuanmou basin between 7.2 and 8.2 mya. The Lufeng material is assigned to L. lufengensis and the Yuanmou material to L. hudienensis. I will focus on the 2008 monograph dealing with L. lufengensis as it is much more comprehensive in describing the hominoid fossils collected.
PA 644, the holotype of the L. lufengensis, consists of a nearly complete splanchnocranium (minus the lower jaw) the anterior portion of the neurocranium (frontal bone delimited by the frontal trigone), and a substantial portion of the basicranium.
Besides the craniofacial features preserved in PA 644 a largely intact but crushed basicranium is also present. It seems to be nearly as well preserved as the recently described basicrania of Ardipithecus ramidus and Sahelanthropus (see illustrations below: left Ardipithecus ramidus, center Sahelanthropus tchadensis, right Lufengpithecus lufengensis).
Until the publication of the Lufengpithecus lufengensis monograph in 2008 there had been no description of the cranial base of the PA 644 cranium. This is unfortunate because much has been made of the basicranial morphology of both Ardipithecus and Sahelanthropus. According to Brunet et al. in Sahelanthropus, “The basicranium has small occipital condyles associated with an apparently large foramen magnum. Despite damage, the foramen magnum seems to be longer than wide, and not like the rounded shape typical of Pan. As in A. ramidus, the basion is intersected by the bicarotid chord; the basion is posterior in large apes and anterior in some of the later hominids.” These basicranial traits are taken to be strong indicators of bipedalism in both Sahelanthropus and Ardipithecus. According to White et al. in Ardipithecus ramidus “The occipidal condyle is small, measuring 16 x 17.5 mm. The anterior border of the foramen magnum (basion) is intersected by a bicarotid chord connecting the centres of right and left carotid foramina, and the endocranial opening of the hypoglossal canal is placed more anteriorly relative to the internal auditory meatus than in great apes. This condition, as in other fossil hominid taxa, reflects a shortened basioccipital component of the cranial base relative to modern African ape crania.” These basicranial traits are taken to represent derived characters shared among all hominids indicative of bipedality (see this review of James Ahern's paper: "Foramen Magnum Position Variation in Pan troglodytes, Plio-Pleistocene Hominids, and Recent Homo sapiens: Implications for Recognizing the Earliest Hominids," by Afarensis).The appearance of the PA 644 male Lufengpithecus cranial base is consistent with these descriptions. The conclusion that must be drawn is that either Lufengpithecus was as well-adapted to bipedalism as both Sahelanthropus and Ardipithecus or the features seen in all three are not indicative of obligatory bipedalism but of facultative bipedalism and that these traits are to be expected in the LCA of the great ape and human lineages.
More to come.