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EVOLUTION: ON EARLY HUMAN DISPERSAL FROM AFRICA

The following points are made by R. Dennell and W. Roebroeks (Nature 2005 438:1099):

1) A key assumption in accounts of early hominin evolution is that the genus Homo originated in Africa, and an early form, classified either as Homo ergaster or H. erectus sensu lato, was the first to leave about 1.7-1.9 Myr ago (depending upon one's choice of dates and specimens), and then colonized southern Asia as far as 40 deg N. The identification of east Africa as the "core" area for the genus Homo (including H. ergaster) as well as tool-making seems secure to most palaeoanthropologists, and the most recent attempts at modelling early hominin dispersals start implicitly from the assumption that H. ergaster originated in east Africa and then dispersed across Asia[1,2].

2) In fact, the evidence that H. ergaster originated in east Africa is less convincing than it seems. H. ergaster marks such a radical departure from previous forms of Homo (such as H. habilis) in its height, reduced sexual dimorphism, long limbs, and modern body proportions[3] that it is hard at present to identify its immediate ancestry in east Africa[4]. Not for nothing has it been described as a hominin "without an ancestor, without a clear past"[5].

3) At present, we have very little information on where, when and which hominins first appeared in Asia, and the expansion of H. ergaster across Asia in the Early Pleistocene remains a massive assumption, even if it is routinely treated as a historical fact. It is assumed that H. ergaster hominins migrated out of Africa along the Nile Valley or across the southern end of the Red Sea, but there is no archaeological or fossil hominin evidence that hominins were in the Nile Valley in the Lower Pleistocene; and there are no Oldowan sites in the Sinai, southern Negev, or in southwest Arabia at the alleged point of entry to Asia. The only Asian Early Pleistocene fossil hominin evidence comprises three incisors from 'Ubeidiya, Israel (1.4-1.0 Myr ago), attributed to H. erectus sensu lato (s.l.) by default; the 1.7-Myr-old specimens from Dmanisi, Georgia, which have recently been classified as a very early type of H. ergaster and/or a new taxon, H. georgicus; and the specimens attributed to H. erectus sensu stricto (s.s.) in Java, 5300 miles away and regarded by many but not all as different from the east African H. ergaster.

4) In summary: The past decade has seen the Pliocene and Pleistocene fossil hominin record enriched by the addition of at least ten new taxa, including the Early Pleistocene, small-brained hominins from Dmanisi, Georgia, and the diminutive Late Pleistocene Homo floresiensis from Flores, Indonesia. At the same time, Asia's earliest hominin presence has been extended up to 1.8 Myr ago, hundreds of thousands of years earlier than previously envisaged. Nevertheless, the preferred explanation for the first appearance of hominins outside Africa has remained virtually unchanged. The authors demonstrate that it is time to develop alternatives to one of palaeoanthropology's most basic paradigms: "Out of Africa I".

References (abridged):

1. Antón, S. C. , Leonard, W. R. & Robertson, M. L. An ecomorphological model of the initial hominid dispersal from Africa. J. Hum. Evol. 43, 773-785 (2002)

2. Mithen, S. & Reed, M. Stepping out: a computer simulation of hominid dispersal from Africa. J. Hum. Evol. 43, 433-462 (2002)

3. Aiello, L. C. & Wells, J. C. K. Energetics and the evolution of the genus Homo. Annu. Rev. Anthropol. 31, 323-338 (2002)

4. Asfaw, B. et al. Remains of Homo erectus from Bouri, Middle Awash, Ethiopia. Nature 416, 317-320 (2002)

5. Walker, A. & Shipman, P. The Wisdom of Bones: In Search of Human Origins (Weidenfeld & Nicholson, London, 1996)

Nature http://www.nature.com/nature

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ANTHROPOLOGY: NEW EVIDENCE FOR OUT OF AFRICA MODEL

The following points are made by Chris Stringer (Nature 2003 423:692):

1) The idea that modern humans originated in Africa, with populations subsequently spreading outwards from there, has continued to gain support lately. But much of that support has come from analyses of genetic variation in people today, and from fossil and archaeological discoveries dated to within the past 120,000 years -- after our species evolved. Hard evidence for the inferred African origin of modern humans has remained somewhat elusive, with relevant material being fragmentary, morphologically ambiguous, or uncertainly dated. Thus the fossilized partial skulls from Ethiopia recently described by White et al (Nature 2003 423:742) are probably some of the most significant discoveries of early Homo sapiens so far, owing to their completeness and well-established antiquity of approximately 160,000 years.

2) There are two broad theories about the origins of H. sapiens. A few researchers still support a version of the "multiregional" hypothesis, arguing that the anatomical features of modern humans arose in geographically widespread hominid populations throughout the Pleistocene epoch (which lasted from around 1.8 million to some 12,000 years ago). But most researchers now espouse a version of the "out of Africa" model, although there are differences of opinion over the complexity of the processes of origin and dispersal, and over the amount of mixing that might subsequently have occurred with archaic (non-modern) humans outside of Africa. Within Africa, uncertainties still surround the mode of modern human evolution -- whether it proceeded in a gradual and steady manner or in fits and starts (punctuational evolution). Other questions concern the relationship between genetic, morphological and behavioral changes, and the precise region, or regions, of origin.

3) For instance, possible early H. sapiens fossils, dating from about 260,000 to 130,000 years ago, are scattered across Africa at sites such as Florisbad (South Africa), Ngaloba (Tanzania), Eliye Springs and Guomde (Kenya), Omo Kibish (Ethiopia), Singa (Sudan) and Jebel Irhoud (Morocco). But the best dated of these finds, from Florisbad and Singa, are problematic because of incompleteness and, in the latter case, evidence of disease. Meanwhile, the more complete or diagnostically modern specimens suffer from chronological uncertainties. So the most securely dated and complete early fossils that unequivocally share an anatomical pattern with today's H. sapiens are actually from Israel, rather than Africa. These are the partial skeletons from Skhul and Qafzeh, dating from around 115,000 years ago. Their presence in the Levant is usually explained by a range expansion from ancestral African populations, such as those sampled at Omo Kibish or Jebel Irhoud around 125,000 years ago.

4) The new cranial material from Herto, Ethiopia -- described by White et al -- adds significantly to our understanding of early H. sapiens evolution in Africa. The fossils are complete enough to show a suite of modern human characters, and are well constrained by argon-isotope dating to about 160,000 years ago. Three individuals are represented by separate fossils: a nearly complete adult cranium (skull parts excluding the lower jaw), a less complete juvenile cranium, and some robust cranial fragments from another adult. All display evidence of human modification, such as cut marks, considered to represent mortuary practices rather than cannibalism. Associated layers of sediment produced evidence of the butchery of large mammals such as hippopotamuses and bovines, as well as assemblages of artefacts showing an interesting combination of Middle Stone Age and late Acheulean technology.

Nature http://www.nature.com/nature

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PALEOLITHIC HUMAN POPULATION EXPANSION IN AFRICA

Notes by ScienceWeek:

Human populations have undergone dramatic expansions in size, but other than the growth associated with agriculture, the dates and magnitudes of those expansions have never been resolved. Genetic approaches to the study of human population expansions have focused on variation at a single genetic locus, the "control region" of *mitochondrial DNA. But in the study of demographic history, single-locus investigations suffer from pronounced statistical and biological limitations. The statistical problem is that the conclusions rely on only one particular realization of a gene genealogy, the "tree" determining the ancestral relationships among a set of *alleles. The biological problem is that there are a large number of functional genes in the mitochondrion, and due to a complete linkage, a selective sweep for any one of the genes may lead to a spurious signal of expansion.

The following points are made by Reich and Goldstein (Proc. Natl. Acad. Sci. 1998 95:8119):

1) The authors present two new statistical tests for population expansion, using variation at a number of unlinked genetic markers to study the demographic histories of natural populations.

2) The authors report that analysis of genetic variation in various aboriginal populations throughout the world reveals highly significant evidence for a major human population expansion in Africa, but no evidence of expansion outside of Africa. The inferred African expansion is estimated to have occurred between 49,000 and 640,000 years ago, certainly before the Neolithic expansions, and probably before the splitting of African and non-African populations.

3) The authors suggest that in showing a significant difference between African and non-African populations, their analysis supports the unique role of Africa in human evolutionary history. The authors also suggest that the missing signal in non-African populations may be the result of a population bottleneck associated with the emergence of these populations from Africa, as postulated in the "Out of Africa" model of modern human origins.

Proc. Nat. Acad. Sci. http://www.pnas.org

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