Well I've Never really believed in the "Out of Africa Theory" anyway.
This seems more plausible...but its a very long article. From this weeks 'New Scientist' July 1st.
START
THE archaeological excavations at Dmanisi, in the Republic of Georgia, are a glorious exception to the rule that if you are in a hole, you should stop digging. What began as the excavation of a medieval town has turned into a pivotal site for our understanding of human evolution. So far, palaeoarchaeologists working there have unearthed five ancestral human skulls and other remains: the individuals they represent are now the central characters in a story whose plot is poised to undergo a major twist.
The story is known as Out of Africa. It tells of Africa as the centre of evolutionary innovation in our ancestors, and the springboard from which some of these hominins struck out into other continents. There are two main parts to the tale. The most familiar one charts the evolution of our own species in Africa around 200,000 years ago, and the subsequent migration of these modern humans throughout the world. The less well known part of the story concerns the first migration of our ancestors out of Africa, more than 1 million years earlier. It is this part of the story that is now being challenged for the first time. Last December, Nature ran a provocative critique by Robin Dennell of the University of Sheffield, UK, and Wil Roebroeks of Leiden University, the Netherlands, that concluded: "Most probably, we are on the threshold of a profound transformation of our understanding of early hominin evolution."
The "Out of Africa 1" story begins more than 2 million years ago when small upright African apes, known as australopithecines, start evolving into large and recognisably human creatures - the first members of our own genus, Homo. Eventually one of these, Homo erectus, strikes out to conquer Eurasia. At the heart of the tale of this first transcontinental migration lies the assumption that what made us human also propelled us out across the rest of the planet. This idea has a powerful romantic appeal, suggesting that exploration and settlement are primordial and defining human instincts. H. erectus had "a typically insatiable human wanderlust", according to palaeoanthropologist Ian Tattersall of the American Museum of Natural History in New York. What enabled these beings to satisfy the urge to boldly go was their package of characteristically human traits that distinguished them from the australopithecines: longer limbs, increased body and brain size, an omnivorous diet and the use of stone tools.
Until quite recently, all the evidence seemed to support this view and version of events. The earliest remains of H. erectus in Africa are about 1.8 million years old. At first these beings seem to have produced only simple flaked stone tools, but around 1.5 million years ago these are joined in the archaeological record by teardrop-shaped hand axes, suggesting that their creators had reached a new level of sophistication. In addition, the various hominin fossils found in east Asia over the past century (see "The shifting spotlight") had been dated at a million years old at most. The timing of all this seemed to attest to the emergence of H. erectus in Africa, its growing ingenuity there and then gradual spread eastward.
In the past decade, however, this sequence has begun to unravel. Fossils of H. erectus found at the Indonesian sites of Sangiran and Mojokerto are now believed to be over 1.5 million years old - possibly as much as 1.8 million years old. Those at Dmanisi have been dated at 1.7 million years or more. With these startlingly early dates from both ends of Asia it looks as though H. erectus materialised almost simultaneously in Africa, east Asia and a point in between. What's more, hand axes have proved to be red herrings. The stone tools associated with the migrant populations are no technological advance on the first ones to appear in the archaeological record, half a million years previously (see "Tooled up and ready to go"). As for brain size: with an adult average of about 700 cubic centimetres these colonisers had the edge on australopithecines, whose brains were under half a litre, but they were at the bottom end of the H. erectus range, and had only about half the volume of a modern human brain. It looks as though increased intelligence was not a prerequisite for migration.
A still more radical challenge to the supposed role of superior cognitive abilities in the dispersal of hominins comes from a mid-1990s fossil discovery that Dennell considers one of the most important of the past two decades. Australopithecine fossils had hitherto been found in the Great Rift Valley of eastern Africa and in the south of the continent. Then one turned up in Chad, in the middle of the continent, 2500 kilometres away from the Rift Valley. If australopithecines were able to colonise that region between 3 and 3.5 million years ago, argues Dennell, there is no reason why they should have stopped at the Red Sea. Ancient hominins would not have distinguished between Africa and Asia, and neither should we, he and Roebroeks argue. Those australopithecines in Chad date from an era when grasslands stretched from northern Africa to eastern Asia. Other animals moved freely across this landscape, so why not hominins? "If you were a herbivore that took grass seriously," Dennell remarks, "you could munch your way all across south-west Asia to northern China." He and Roebroeks suggest that we should re-imagine this vast transcontinental band of grass as a zone throughout which our ancestors also roamed. Dennell has dubbed it "Savannahstan".
The savannahs were the product of global cooling, which dried out moist woodlands, shifting the balance to grass. Over millions of years, the global climate gradually cooled, but there were also times when conditions altered quite abruptly. These shifts rearranged the fauna - species vanished, new species emerged. One of these climatic pulses occurred around 2.5 million years ago. In the Arctic, ice sheets spread. In eastern Africa, forest-adapted antelopes were replaced by those suited to savannah. New, robust australopithecines appeared, as did somewhat larger-brained hominins, Homo habilis, the first members of the Homo genus, and we also find the earliest known stone tools.
In a bold challenge to the conventional story, Dennell argues that hominins migrated out of Africa before H. erectus even evolved, and long before the dates of the oldest known hominin fossils in Asia. These first migrants were either australopithecines or H. habilis - he, like some prominent palaeoanthropologists, regards these two as much the same kind of creatures. For evidence that small stature was no obstacle to dispersal he points to the Dmanisi hominins. Not only do their brain sizes fit within the H. habilis range, evidence from a femur and a tibia, as yet unpublished, indicates that one of them may have weighed only about 54 kilograms and stood just 1.4 metres tall. Although the stature of the individuals at Sangiran and Mojokerto is unknown, hominins clearly did not need long legs to stride out of Africa.
?In a bold challenge to the conventional story, some argue that hominins migrated out of Africa before H. erectus evolved?What's more, Dennell has the makings of a story set in Savannahstan that could explain a key mystery of human evolution - what spurred the evolution of H. erectus itself. While H. habilis seems to have evolved in response to the cold snap around 2.5 million years ago, there is no such climate change in Africa coinciding with the emergence of the earliest known examples of H. erectus, around 1.8 million years ago. Nor does H. erectus have any clearly identifiable immediate predecessors. "Not for nothing has it been described as a hominin 'without an ancestor, without a clear past'," observe Dennell and Roebroeks.
Dennell's solution to the problem is beguilingly simple: perhaps we have been looking in the wrong place. "Maybe the Rift Valley was a cul-de-sac," Dennell suggests. Tongue in cheek perhaps, but the remark conveys his strong conviction that the importance of Asia has been unfairly neglected. At around the time H. erectus emerged some 1.8 million years ago, selective pressures to evolve would have been greater in Asia than in Africa, he argues. Traces of the global cooling pulse starting around 2.5 million years ago have been detected in the soils of China's Loess Plateau.
Beneath the silty loess are layers of red clay, which appear to have been blown there by westerly winds before the cooling began. Above these, the particles of loess decrease in size from north to south, indicating that they were deposited by northerly winds, the heavier particles falling to the ground first. So it appears that the winds changed when the climate cooled. This would have brought monsoons and polarised the years into seasons, with summers becoming increasingly arid over subsequent millennia, causing the grasslands to expand. Asia was the core of this process and Africa was peripheral, according to Dennell.
In this perspective the Dmanisi hominins may represent a missing link in the evolution of H. erectus, responding to climatic pressures but still retaining much in common with H. habilis. Australopithecines were adapted to open spaces in woodlands, ranging around relatively small areas, living off plants, seeds, small mammals and perhaps carcasses. As these open spaces expanded into savannah, the Dmanisi hominins would have faced pressures to evolve more human-like traits, increasing the distances over which they ranged, and turning more to animals as a source of food.
Dennell even goes so far as to suggest that the Dmanisi hominins might be ancestors of the later H. erectus in Africa. The most celebrated representative there is the 1.6-million-year-old "Turkana Boy". His tall stature, long limbs and body proportions epitomise adaptation to a hot, dry climate. In other words, African H. erectus might have Asian roots. If this is the case, Out of Africa 1 is a crucial part of the story of our own evolution, since H. erectus is generally thought to be a direct ancestor of modern humans.
?African H. erectus might have had Asian roots, adding a crucial twist to the story of our own evolution?Since Dennell and Roebroeks wrote their Nature review, American and Georgian researchers studying the Dmanisi finds have published a paper that points in a similar direction (Journal of Human Evolution, vol 50(2), p 115). Suggesting the finds be classed as Homo erectus georgicus, Philip Rightmire of Binghamton University, New York, and his colleagues conclude that Dmanisi may be "close to the stem from which H. erectus evolved". They also point to the possibility that the Dmanisi population's ancestors were H. habilis emigrants from Africa, and that the dates do not rule out the possibility that H. erectus evolved in Asia. "For me, the evidence from Dmanisi is critical," says Rightmire. "It seems to me that such a population could well be ancestral to H. erectus in Africa and also to H. erectus in the Far East." But he anticipates that rewriting the origin and dispersal of H. erectus will be a slow process. "We're not likely to see a major breakthrough immediately."
Further research that broadly chimes with Dennell and Roebroeks's arguments comes from Alan Templeton of Washington University, St Louis (Yearbook of Physical Anthropology, vol 48, p 33). By comparing clusters of DNA that vary between individuals and tend to be inherited together, geneticists can identify when particular mutations arose, and use these to map relationships within or between species. Until a few years ago, they had to rely on DNA from mitochondria or sex chromosomes, but it is now becoming possible to increase the resolution of such maps by using data from the rest of the genome. Comparing 25 DNA regions in the genomes of people from across the world, Templeton found evidence for an expansion out of Africa around 1.9 million years ago, and that gene flow between African and Eurasian populations - in both directions - was established by 1.5 million years ago. Not only do these findings suggest that migration began earlier than previously thought, it also looks as though hominins were moving back and forth between Eurasia and Africa.
"The hypotheses Dennell and Roebroeks present are testable with molecular genetic data," Templeton says, "so I think that the prospects for testing some of their alternatives to 'Out of Africa 1' will be excellent in the near future." Only four years ago, when he first conducted an analysis of this kind, there were insufficient results for him to detect any expansion out of Africa between 1 and 3 million years ago. Increasingly, however, researchers looking for genetic variation among individuals are also recording their geographical origins - just the information Templeton needs to do his analysis. "I anticipate greater and greater statistical resolution of these older events in human evolutionary history," he says. "Genetics will play an increasing and important role in testing their ideas in conjunction with new fossil and archaeological discoveries."
For Dennell, however, the objects in the ground are what matters. He is keen to look for hominin remains in Asia to balance the generous legacies of the Rift Valley and southern Africa. Unfortunately, the countries he most wants to search - Saudi Arabia, Iran, Afghanistan - read like a list of places not to visit these days. A site in Pakistan where he found stone tools in the 1980s dating from 1.9 million years ago is also now off limits because of the political turbulence that has spread across the region. It seems that Asia will not give up its secrets easily, but Dennell is convinced that in this case, absence of evidence is not evidence of absence. The search may prove difficult but the rewards are potentially enormous, amounting to nothing less than the rewriting of human prehistory.
From issue 2558 of New Scientist magazine, 01 July 2006, page 34
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THERE ARE MANY FURTHER Pictures and comments by others. But I have not included those to save space. Since the full article will be put on the Net, by New Scientist next week.
