Friday, July 16, 2010
Could There Have Been Non-Hominan “Hominids”?
To begin with let me clarify that by “hominid” I mean a hominoid primate that has the appearance of being a possible human ancestor and by hominan I mean a member of the subtribe Hominina that includes direct human ancestors and collateral relatives descended from the Last Common Ancestor (LCA) we share with the chimpanzee.
The last few essays that I’ve posted have dealt with Lufengpithecus and other indeterminate hominoid remains from China. These include a number of dentognathic specimens that have been compared to fossil orangutans, a non-orang large bodied ape (possibly allied to Lufengpithecus), or early species of Homo cum Meganthropus.
The recent descriptions and analyses of L. lufengensis and L. hudienensis by Chinese colleagues show that this late Miocene hominoid was distinct from living and fossil orangs and orang relatives such as Sivapithecus. Lufengpithecus had a suite of craniofacial and dentognathic features long associated with African apes and humans. It also possessed basicranial and femoral traits that have been used to infer bipedalism in a number of late Miocene apes such as Sahelanthropus and Orrorin purported by their respective discoverers to be early hominans.
The questions then arises, was Lufengpithecus a bona fide hominan or something else, and if something else what? Given its age (6.1-6.9 mya) and morphological features, L. lufengensis, if found in Africa would most likely be accepted into the ranks of human ancestry. Its geographical location and affinities to L. hudienensis (~ 8 mya) from the Yuanmou basin and L. keiyuanensis (~ 11 mya) from Kaiyuan complicate the issue as this lineage of Asian apes can be traced back in time towards the middle Miocene, long before the genetic evidence suggests that the LCA of chimpanzees and humans lived.
I have a solution to the riddle of these Asian “mystery apes” and their affinities to the extant hominoids including humans, but first let’s look at some previous solutions that have been proffered. Jeff Schwartz and his colleagues have a unique solution to the above quandary. They propose that the orangutan (Pongo) not the chimpanzee (Pan) is our closest living relative. This throws everything for a loop and the “hominid”-like features of Lufengpithecus then fall nicely into place. An early ancestor of humans should have a surfeit of derived orang-like traits mixed in with pre-orang-like retentions and ipso facto - presto problem solved.
Another solution is that Lufengpithecus is a relic of the pre-orang divergence great apes. It would thus retain a suite of primitive features some of which are retained in the African hominines and other in the Asian pongines. If that is the case then the features that Lufengpithecus shares with the African apes and humans are symplesiomorphies (shared primitive features) and the features it shares with the Asian orangutan are likewise a different set of symplesiomorphies. Lufengpithecus should then have none of the shared derived features of either the African ape/human clade or the Asian ape clade. This would make the basicranial and femoral traits for bipedalism that it shares with Sahelanthropus, Orrorin and Ardipithecus symplesiomorphies of the Great Ape/human clade. That has been my preferred solution to a problem that no one else probably even recognizes.
But I’ve been thinking of late (this has been my downfall) that maybe Lufengpithecus is an early pongine, and its hominine-like traits are primitive retentions from the hominid (sensu lato) LCA. The bipedal adaptations its shares with African hominines could well be parallelisms. Thus “hominid”-like features may have evolved independently in various lineages of both African hominines and Asian pongines. The present day apes are highly derived and adapted to a closed canopy tropical forest environment and their adaptations for suspensory locomotion in the trees and knuckle walking/fist walking on the ground are likewise parallelisms.
So here is where the title of this essay comes into play. The early “hominid” grade of evolution may have been entered by two or more separate hominoid lineages. Lufengpithecus may have been a “hominid grade” pongine. The implications are rather staggering. There may well have been bipedal pongines. They may have begun to use very simple, barely diagnosable stone tools. There may have been an adaptive radiation of pongine “hominids” thus explaining some of the indeterminate hominid-like remains (Longupo, Jianshi, Meganthropus, etc.) found in China and Java. Of course Gigantopithecus fits nicely into this scenario as an Asian analogue of robust australopiths.
The above scenario, which elaborates on my previous post resurrected from i5 years ago, is in keeping with recent trends in interpreting the hominoid fossil record. Rampant parallelisms, a bushy phylogenetic tree with a proliferation of hominoid, hominid, hominine, hominin and hominan lineages. Why not a proliferation of Asian pongine lineages evolving in ways parallel to the African hominines? Then when Homo erectus entered East Asia it would have encountered other “hominid grade” hominoids. Sort of an early version of the “Out of Africa” replacement hypothesis but a couple of million years earlier.