Out of Africa and into Asia
Bernard Wood and Alan Turner
Few doubt that Africa was the birthplace of the hominid lineage, but there is no equivalent consensus about when hominids first moved out of that continent. Despite the announcement of early dates for a juvenile Homo erectus from Indonesia1, the circumstances surrounding the recovery of many of the fossil hominids from the island will always hinder attempts to date them. Thus the excavation of hominid remains, in combination with crudely fashioned artefacts in what are claimed to be early Pleistocene deposits at Longgupo Cave in central China (Huang and co-workers, page 275 of this issue2), is of major importance. Most notably, the remains lend support to the idea3 that representatives of the hominid lineage were established in mainland Asia as early as about 1.9 million years (Myr) ago.
Africa has been the focus for research into human evolutionary history for the past three decades, but it was not always thus. A century ago, space in the correspondence column of Nature was regularly claimed to debate the significance of the finds Eugene Dubois had made, eginning in 1891, at Trinil in Indonesia. Although initially allocated to Pithecanthropus erectus, the species distinction of the Trinil hominid has survived but the genus has long since been sunk into Homo.
Two decades later, excavations were instigated by the Canadian anatomist Davidson Black in the cave deposits at Choukoutien, now called Zhoukoudian, and the first of the series of remains of what became known as Peking Man was discovered. Despite being allocated to a new genus and species, their affinities with the hominids from Trinil, and with similar material that was subsequently recovered at Sangiran, also in Indonesia, was evident, and the Chinese remains have also been subsumed within H. erectus. There have been sporadic attempts to demonstrate both that the hominid remains from the Indonesian sites are from more than one species4,5,and that they include specimens that should be allocated to Australopithecus6 or Paranthropus7, and thus to an earlier, more primitive phase of hominid evolution. But none of these claims has survived close scrutiny8. Likewise, until recently there has been little compelling evidence to suggest that any of the Asian hominid sites were yielding hominids more than one million years old3.
The importance of the material from Longgupo Cave is twofold. Not only does it support an early date for the hominid occupation of Asia, but the morphological details of the admittedly fragmentary fossil evidence also means that it may represent not H. erectus but a more primitive species akin to H. ergaster, thus far known only from Africa.
Of course, dating the material is crucial to the argument. Longgupo Cave has several lines of evidence, none of them contradictory. Paleomagnetic stratigraphy shows a reversed polarity for most of the sediments, with the hominid fossils and lithic items associated with the lower of two normal events and therefore referred to the Olduvai magnetic event. The magnetic evidence is broadly supported by analysis of tooth enamel from the sediments, using electron spin resonance, which gives a minimum age of 0.75 ± 0.09 Myr based on an early uranium uptake model. It could be argued that the normal magnetic event associated with the material is therefore likely to be Jaramillo, but the associated mammalian fauna is really too archaic and point instead to the earlier Olduvai event. Of particular interest here is the presence of Nestoritherium, a genus of the family Chalicotheriidae, an extinct, bizarre, claw-hoofed member of the Perissodactyla, today represented by tapirs, rhinos and horses.
The lithic items identified as primitive stone tools do seem to be exotic, and they are notably larger than the rest of the sediments. They look as much like stone tools as anything of this age ever does, and they fall into the category of items in finer sediment deposits that, as Gamble9 has pointed out, tend to categorize genuine archaeological assemblages as opposed to naturally bashed stones. Moreover, the uneroded state of the bone in clay facies channels is consistent with primary deposition rather than intrusive burial. But we are unlikely to be dealing with a site of hominid occupation. The giant hyaena, Pachycrocuta, is a perfectly plausible agent of accumulation10 (it is less likely that the sabre-toothed Homotherium did much bone destroying).
The authors draw attention to the presence of Gigantopithecus, a large, gorilla-like and presumably herbivorous primate, in the same level as the hominid fossils, and stress that this is the third such co-occurrence at Asian localities over a time span of some 1 Myr. Such co-occurrences are always intriguing, but the evidence of hyaena activity reduces the likelihood that Gigantopithecus was prey to the more advanced hominid. The remaining elements of the mammalian fauna at Longgupo shed little light on the local environment of the site, although both woodland and more open-country taxa seem to be represented.
The hominid remains -- part of the left side of an adult mandible and an isolated upper incisor -- are meagre pickings froma taxonomic point of view. However, the mandibular fragment includes both the crown and the root of a premolar tooth (P4), and they provide the best evidence about the affinities of the material. The crowns of the P4 teeth of H. erectus are generally relatively simple and the teeth are usually single-rooted, like those of modern humans. In contrast, the Longgupo P4 root is bifid for most of its length. This, and other features of the mandible and the dentition, suggest that the Longgupo hominid may be much more primitive than H. erectus11. This opens up the possibility that the first hominid to leave Africa was at least as primitive as H. ergaster12, and implies that H. erectus may have evolved within Asia and then spread back into Europe and Africa.
In terms of overall patters of mamalian movement, there is nothing inherently implausible about the age of the material and the implications that it holds for human dispersions from Africa. Hominid remains and lithic items from Dmanisi13 in Georgia point to at least an initial presence at the gates of Europe by around the same time as the age of the Longgupo evidence. And it is clear that a Late Pliocene dispersion across Arabia, probably via the Levant, and perhaps through the Bab-el-Mandab straits, was possible for several mammalian taxa14,15, while the presence of hominids in the Levant itself by 1.4 Myr ago is evident at the Israeli site of Ubeidiya14. The new report from Huang et al. adds weight to other, less well-substantiated claims that hominids travelled even further, and occupied China in the latest Pleistocene some 1.9 Myr ago.
The evidence from Longgupo Cave in China, described by Huang et al.2 and discussed here, suggests that hominids were established in Asia just after two million years ago. Given the primitive nature of the premolar teeth, it seems that the first hominid to occupy Asia may not have been Homo erectus, but perhaps a variant of H. ergaster or even H. habilis.
Bernard Wood and Alan Turner are in the Department of Human Anatomy and Biology, The University of Liverpool, PO Box 147, Liverpool L69 3BX, UK.