Science: March of the timewalkers: New evidence suggests that man's precursors set out to travel the globe much earlier than previously believed. Clive Gamble reports

IMAGINE a world where humans inhabit just a small corner of a single continent - a fact of geography for most other species. For Homo sapiens, however, the situation is almost unthinkable. One of the distinguishing characteristics of human beings is their global ubiquity, which came about many thousands of years ago. Anthropologists have long argued about how this early migration could have occurred; recent research is beginning to provide the answers. Just as importantly, scientists have begun to form a picture of what drove humans to travel to the four corners of the earth.

The story appears to begin about 1.8 million years ago, in the region of Africa south of the Sahara that is said to be the cradle of humanity, judging from the wealth of ancestral bones that have been found there. It was here that a species of early human lived a simple foraging existence, although its way of life was no doubt considerably more sophisticated than that of any ape-like animal which had lived before.

Homo erectus had a relatively large brain - two-thirds the size of ours - a robustly built body, broad feet, nimble hands and, as the name implies, an upright, two-legged stature. They patrolled large territories in search of food and mates and fashioned simple stone tools; they communicated using gestures, facial expressions, sounds and probably groomed each other to reinforce social bonds. They resembled modern humans enough for the anthropologists who first discovered their fossilised remains to classify them alongside humans rather than apes.

Until very recently one of the great puzzles about H. erectus was why, with all these obvious advantages, the species appeared to have remained in sub-Saharan Africa for as long as 800,000 years? Why, with their brains and clever hands, which could manipulate stone tools, and presumably with a sophisticated social network, did they live only in one place for so long before moving on to populate other parts of the world?

Fossil bones and tools of H. erectus have been found in the Middle East, South-east Asia, China and Europe, so archaeologists have presumed that the species eventually did make its way past the natural barrier of the Sahara desert and over the land bridge between Africa and the Middle East to Asia and Europe. In the past month, however, new dates of a site in Java where H. erectus fossils have been found have suggested that the early hominids may well have migrated much earlier than previously believed - the Java fossils seem to be about 1.8 million years old, making them at least as old as the oldest H. erectus fossils in Africa.

Scientists are now trying to validate these new dates, which appear to make H. erectus a far older traveller than anthropologists previously supposed. This could explain the apparent conundrum of why they stayed so long in Africa - they didn't. Instead of sitting around in their sub-Saharan 'home' for nearly a million years, they expanded rapidly. On these timescales, the overland distance of 13,000 kilometres from Nairobi to Jakarta could have been covered very slowly indeed, perhaps less than a kilometre a year, and yet this still seems instantaneous compared to the couple of million years of human evolution.

This new find suggests that H. erectus had all the accoutrements necessary for migration early in their evolution. Since they strode, rather than waddled (like the earlier hominids belonging to the group known as the Australopithecines), they were not limited by locomotion. They were omnivorous and well suited biologically to a range of environments away from the tropics where they were first found. Their large social groups provided defence against carnivores such as lions and hyenas and their survival was helped by primitive technology. The durable stone implements that have been found could well be tools used to make other artefacts from wood, leaves, bamboo and animal tissues.

Whether the early migration was slow or fast (the new dates have to be double-checked), there remains the mystery of how and why these primitive hominids travelled such vast distances. I believe an important factor influencing movement at this time was climatic change. Rainfall and drought would have had drastic effects on the availability of food for these early foraging omnivores. If the Sahara had become temporarily greened it would have enabled these early 'timewalkers' to cross this great natural barrier and emerge from the other side to continue their journey.

A struggle for food, however, was not the only factor limiting long-

distance migration. Those migrating to new habitats had to overcome two social difficulties for permanent colonisation to occur. These were the problems of living apart and of living together for extended periods of time. It is for these reasons that baboons (who can't cope with even temporary strangers) and chimps (who do best in small social groups) did not become timewalkers. They are restricted as much by their societies as by the environment.

However, the earliest timewalkers developed intense social interactions which produced partial solutions to living away from the sub-Saharan African 'home'. Such intensification came from social competition, now recognised as universal among primates, and widely regarded as the basis of our common, socially manipulative, Machiavellian-style intelligence. Social competition upped the stakes when negotiating with others for social position, mates, food and support in conflicts. The success of such alliances relied on enhanced memory, which unintentionally allowed the prolonged social separation essential for migrating in deserts. The timewalkers could now move north through the Sahara.

Whatever success H. erectus had in populating the world outside Africa, he still failed to reach the places modern humans have lived in for tens of thousands of years. H. erectus and his regional descendants, such as the Neanderthal people in Europe and Western Asia, never reached Australia, the Pacific islands or the New World. They never colonised Madagascar or the islands of the Indian Ocean. The Arctic was off-limits, as, apparently, were the tropical rainforests of Africa and the high mountain chains of the Old World.

Global colonisation was achieved by ourselves, Homo sapiens, in a series of population waves that began only some 60,000 years ago. It was accomplished by people with our

anatomical features and a foraging lifestyle based on fishing, gathering and hunting. Populations were small in size but always capable of phenomenal rates of increase, especially when new continents such as Australia and the Americas with their giant marsupials and 'megamammals' - now sadly extinct - lay before them.

The earliest fossils of modern- looking humans appeared, again in eastern Africa, and date to about 150,000 years ago. However, it seems that apart from head shape and build, very little distinguished them from other Old World populations of H. erectus. These first moderns did not immediately begin another major wave of colonisation to new continents, but instead fitted in among and around existing populations of other hominids in Africa. However, these early modern humans that lived in Africa were without doubt the direct ancestors of all humans today and at a crucial point began to emulate the migratory history of H. erectus with greater success.

Another and more fundamental social revolution was needed to give us both the biological unity of humankind as well as our global diversity. Perhaps spoken language played a part, which also enhanced memory and so helped us to store information about being and belonging to groups. This can be seen in the shape and design of tools, weapons and the appearance of art, jewellery and figurines, which only become commonplace after 40,000 years ago.

So when did the timewalkers become modern? I would identify the transition neither by skull shape nor technology but instead by the great push towards world colonisation that started 60,000 years ago. Australia, which was always separated by water, was peopled at least 55,000 years ago, while by 30,000 years ago humans were moving into Siberia and, then, 15,000 years later, they crossed the Bering land bridge and moved into the Americas. The unexpected consequence of this second social revolution was our universal humanity.

Once the expansion was under way these modern timewalkers took less than 60,000 years to complete what H. erectus had not managed in hundreds of thousands of years, making our almost universal distribution one of the great hallmarks of our species. Probably one of the last niches that man occupied was the tropical rainforests of South America. They were colonised very late in human prehistory - probably as little as 10,000 years ago - and it was only 10 years ago that one group of foragers, the Nukak people living in the rainforests of Colombia, were first contacted.

I believe this global colonisation was one of the great achievements of prehistory, certainly as significant as the invention of agriculture or the foundations of urban civilisation. And yet these early timewalkers have received little attention. The western explorers from Columbus to Cook expected to find people everywhere and the tradition grew that the timewalkers reached even such remote spots (to Europeans) as Tasmania, Easter Island or Tierra del Fuego under natural pressures. These included hunger, population numbers and the push of climate.

The novelist Jack London put it in its most dramatic form in his short story called The Human Drift. He wrote in 1919 that: 'Dominated by fear, and by their very fear accelerating their development, these early ancestors of ours, suffering hunger pangs very like the ones we experience today, drifted on, hunting and being hunted, eating and being eaten, wandering through 1,000-year- long odysseys of screaming primordial savagery.'

London's invocation of environmental pressures contains only part of the answer to global colonisation because he denied these prehistoric peoples their social revolution. It was that spark of humanity which made us global timewalkers.

Clive Gamble is Reader in the Department of Archaeology at the University of Southampton. His book, 'Timewalkers, the Prehistory of Global Colonization', is published by Alan Sutton at pounds 19.99.

(Photograph omitted)