Wednesday, July 12, 2006
Problems with “Out of Savannahstan”
A recent story in the popular science magazine “New Scientist” by Marek Kohn (Made in Savannahstan, NS July 1-7, 2006, pp 34-39) reviews a provocative article by Robin Dennell of the University of Sheffield, UK, and Wil Roebroeks of Leiden University, the Netherlands that appeared in the British journal “Nature” last December.
Dennell and Roebroeks hypothesize that hominins from the remakable Late Pliocene (~1.7 mya) Dmanisi site in the Republic of Georgia are the descendents of a yet unknown group of proto-humans that had previously dispersed into a belt of savannah extending from northern Africa across into southern Asia. It is this stock that adapted to the rigors of a grasslands environment and gave rise to Homo erectus, which then, much like the prodigal son, returned to Africa to smite its primitive cousins, the australopithecines (including early members of the genus Homo such as H. habilis and H. rudolfensis which they believe were advanced australopitecines.)
A linchpin of this scenario is the placement of australopithecines in Chad, far to the west of their previously known range of distribution, 3 to 3.5 mya, suggesting that australopithecines were capable of wide-ranging dispersals. This, in combination with early dates (> 1.5 mya) for Indonesian H. erectus from Sangiran and Mojokerto led the authors to speculate that there was an early dispersal of australopithecines out of Africa into Asia, where they were subjected to intensive selection resulting in the evolution of H. erectus. The sudden appearance of H. erectus in East Africa commencing ~ 1.8 mya, replacing species of so-called “early Homo,” was the result of their subsequent redeployment back into Africa.
There are a number of problems with the underlying premises of this “Out of Asia” hypothesis for the origins of Homo erectus. First, the presence of australopithecines in Chad is not unexpected given the recent discovery of 7 million year old Sahelanthropus in the same country. While I do not accept the hominin (aka hominid) status of Sahelanthropus, I do recognize it as a likely candidate for “Last Common Ancestor” of the African apes and humans (see previous post). As I see it, descendents of Sahelanthropus that stayed in northern Africa and dispersed east into Ethiopia and throughout the Rift Valley evolved into later australopithecines, while separate descendent lineages of Sahelanthropus that dispersed southward into equatorial Africa evolved into Gorillas and Chimps. In this scenario the presence of australopithecines in north central Africa at the time in question (3-3.5 mya) is to be expected, as it’s part of their ancestral homeland.
The small brain size, archaic facial morphology and Oldowan like tools of the Dmanisi hominins are also no surprise if one is familiar with fossil hominins from East Asia. I have previously favorably compared the Lantian skullcap from Gongwangling in China (dated at ~ 1.2 mya) with early H. erectus from East Africa. The Gongwangling calotte is estimated to have had a small cranial capacity (~ 780 cc) and a less derived facial morphology than later Chinese or Indonesian H. erectus. The hypothesis that there was a dispersal of an early grade of H. erectus out of Africa commencing ~ 1.8 mya is thus consistent with all available evidence and does not, at this time, require any premature modification. The quest for australopithecine fossils in Asia has been Quixotic at best and until there is some physical evidence for their presence outside of Africa hypotheses such as those of Dennel and Roebroeks will find only limited and I would suggest, ill-informed support (the presence of a "habiline" hominin at the Longgupo site in Sichuan, China dated to ~ 1.9 - 2.0 mya as proof of an early dispersal of pre-erectus hominins into Asia has been critiqued and is no longer generally accepted).
The article by Kohn makes reference to some preconceptions concerning the dispersal of H. erectus out of Africa perpetuated by commentators such as Ian Tattersall. One such idea is that humans expanded their range into Asia, or vice versa, due to a “typically insatiable human wanderlust.” This is just plain silly. Species, whether human or not, do not consciously decide to strike out on transcontinental treks, they expand into adjacent territory that they are equipped to exploit, both physically and behaviorally. Humans, incidentally, were not the first hominoids to disperse throughout the Old World. Did late Miocene apes, which ranged from Spain to China, have an “insatiable wanderlust?” No, they had the behavioral and physical adaptations that enabled them to exploit contiguous environments spread across the southern tier of Eurasia. This is, obviously, how all-major dispersals take place and humans are no different from other species in this regard.
If, as I have suggested in previous posts, the australopithecines are the lineal descendents of late Miocene European apes that were displaced southwards into northern Africa at the end of the Miocene because of climatic and environmental deterioration in their native clines, it would appear that the southern tier of Eurasia was inhospitable to large-bodied hominoids for a considerable span of time. Northern and northeastern Africa, which retained the open woodlands environment that suited these hominoids, was the cauldron in which the australopithecines evolved. As this environment slowly dried out and was replaced by expanding grasslands, robust australopithecines and the ancestors of the genus Homo diverged. The adaptations forged by early Homo for life on the savannah allowed for range expansion as soon as they emerged, hence the rapid dispersal of early Homo, commencing around 1.9 mya into the Middle East and the Caucus region and rapid subsequent eastward expansions into east and southeast Asia. This scenario is in keeping with the broad outline of human evolution generally accepted by most contemporary anthropologists and does not need any major revision based on presently available evidence.
As Templeton has documented using genetic data, there were apparently multiple dispersals out of Africa beginning ~ 1.9 mya. This was coupled with a considerable amount of back migration and multidirectional genetic admixing throughout the latter phases of human evolution. What the Dmanisi remains demonstrate is that between 1.7 and 1.9 mya there was what appears to have been a rapid transition from a “habiline to an “erectine” grade of hominin evolution. It is not surprising to find specimens during this period that show different mixes of primitive and derived features. What we’re witnessing may be the human lineage in the throes of evolutionary change as new genetic combinations are sorted out and eventually become channelized into the morphotype of classic Homo erectus, a pattern that once established remained virtually unchanged for nearly one million years.
Part of this transition was the development of new stone tool fabrication techniques, eventually leading to the emergence of a full-fledged Acheulean tradition by ~ 1.65 mya in East Afria. Earlier Oldowan-like traditions now appear to have spread into Eurasia prior to the spread of more sophisticated hand-axe traditions. But the idea that early Homo needed Acheulean tools and large brain size to expand their range out of Africa was never a viable hypothesis. For decades it was noted that Chinese and Indonesian H. erectus lacked major components of the Acheulean tradition and the so-called Movius line was established to separate the early Paleolithic into eastern and western traditions. The stone tool industries of Asian H. erectus were referred to as part of a chopper/chopping tool complex of a basically Oldowan nature. Early H. erectus in Asia is also noted for relatively small cranial capacities (i.e. the previously mentioned Lantian skullcap.). It’s been known for decades that hand-axes and large brains were not prerequisites for human expansion into the Far East.
In sum, australopithecines were not adapted to the savannah environments that were spreading across the southern tier of Eurasia at the end of the Miocene. It was the spread of these grasslands that spelled doom for the Eurasian hominoid antecedents of the African australopithecines. Africa seems to have been the incubator for the australopithecine adaptation to the woodland/grassland ecotone, a stable adaptation that remained little changed for nearly 2 million years (~ 4.5 – 2.5 mya). This environment does not seem to have prevailed in more northerly latitudes. As grasslands began sreading in Africa at ~ 2.5 mya generalized australopithecines seem to have split into robust lineages and those evolving towards the genus Homo. As of today genetic, archeological and paleontological evidence all points to an initial dispersal of Homo out of Africa at ~ 1.9 mya. Once this range expansion was achieved there seems to have been episodic genetic exchange between widely dispersed demes of an emerging H. erectus, the longest lasting hominin species and direct ancestor of all later humans.