To comprehend the lives of the early humans who laid the foundations of civilisations, Steven Mithen sent a fictional observer to visit them

Late one afternoon last October, I climbed a hill in southern Turkey to visit one of the most significant new archaeological discoveries of recent years. Archaeologists are often tempted to overstate claims about their discoveries. But all who had seen G?bekli Tepe had acclaimed it. Indeed, I had heard a story - perhaps apocryphal - that when Ofer Bar Yosef, doyen of Near Eastern archaeology, visited the site, he had been heard muttering to himself: "We have got it all quite wrong" - the "all" being our understanding of how agriculture began.

G?bekli Tepe is awesome: circular stone structures built into the summit of the hill containing massive pillars of stone on which wild animals are exquisitely carved. Think of the art of Lascaux with the grandeur of Stonehenge in the heart of the Fertile Crescent. That is G?bekli Tepe.

Although Klaus Schmidt of the Institute for German Archaeology in Istanbul has been excavating the site since 1994, it is only in the past year that its true extent has become apparent. Schmidt seems as daunted by his discovery as anyone: a hilltop sanctuary dating to 9500 BC, just before the origin of farming. In fact, it is quite possible that the first farmers may have started tilling the land in the vicinity of G?bekli Tepe. As he showed me around his excavations, Schmidt remarked that the rolling hills a few kilometres away were the Karacadag - the spot geneticists had pinpointed as being where domesticated wheat had most likely evolved.

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The origin of agriculture is one of two key turning points in the 3 million-year human journey from ape-like hunter-gatherer-scavengers to modern city dwellers. The first occurred more than 150,000 years ago with the advent of Homo sapiens , who had more creative minds than other types of human. But even though early Homo sapiens had used pigments, made tools and believed in supernatural beings, they lived as hunter-gatherers until less than 10,000 years ago. The switch to an agricultural lifestyle was the second, some say more crucial, milestone in human history as it laid the foundations for the modern world. Within a few thousand years of the first harvests, people were living in towns, the first civilisations arose and nation-states and empires soon followed.

Archaeologists have been debating the origins of agriculture since the late 19th century. We have learnt that this happened not just once but at several different places across the world. Many species were domesticated: wheat, barley, sheep and goats in western Asia; maize, squash and beans in Mexico; rice and millet in China; and potatoes, alpacas and llamas in the Peruvian Andes.

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These quite independent developments occurred within a few thousand years of each other in the early Holocene, and they either immediately followed or immediately preceded other seminal events - the colonisation of the Americas and the high arctic, the extinction of mega-fauna such as mammoths and dramatic changes in geography caused by the melting of the great Pleistocene ice sheets. When viewed within the 2 million years of human existence, the 15,000 years between 20,000 BC (the height of the last ice age) and 5000 BC has strong claims to be the most momentous era of human history.

Climate change must be part of the explanation for the developments in human settlement, economy and society, especially the sudden surge of temperature and rainfall at around 9600 BC. But precisely how global warming engaged with factors such as population growth, technological developments, local ecology and the creative modern mind varied greatly around the world. Although the eventual outcomes may have been the same - sedentary farming communities - the specific trajectories by which these arose were as variable as one could imagine.

In Western Asia, Japan and the Ganges plain of India, hunter-gatherers settled in permanent villages before they began to farm. But in Mexico and New Guinea, plant cultivation leading to farming long preceded permanent settlements. In North Africa, cattle came before crops, just as domesticated vicuna (the ancestor of the alpaca) came before cultivated quinoa in the Peruvian Andes. The invention of pottery preceded the start of farming in Japan and the Sahara, whereas it occurred simultaneously with the origin of rice cultivation in China but was invented in western Asia only long after farming towns had begun to flourish.

Although archaeologists have made remarkable progress in understanding such developments, new discoveries can still have a devastating effect on their theories. This is the case with G?bekli Tepe. Jericho and other settlements in the Jordan Valley are widely believed to be where cereal farming began, and this had been readily explained by the impacts of the climatic fluctuations of the late Pleistocene followed by the spurt of global warming at 9600 BC. But with the discovery of Gobekli Tepe, people appear to have been driven as much by ideology as by the need to cope with environmental stress.

The intensive cultivation of wild wheat may have first occurred to supply sufficient food to the hunter-gatherers who quarried 7-ton blocks of limestone with flint flakes, decorated their surfaces and then used them to create the earliest known megalithic site on that hilltop in southern Turkey.

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A visit to G?bekli Tepe, just like a visit to Jericho or Guil¨¢ Naquitz, the Mexican cave where the New World's earliest domesticated plants have been found, is for a prehistorian not only deeply exciting but also immensely frustrating. New scientific techniques allow us to reconstruct the activities at these sites in ever-increasing detail. But we will never know the identities of their occupants. With no written documents left to work from, the writing of prehistory is particularly difficult. By this I mean prehistory in terms of what happened in the past and why, a narrative that interprets rather than just documents archaeological evidence. Good popularising historians convey complex ideas, facts and figures by hanging them around the lives of individuals because we like to read about the nature of human experience.

This option is not available to prehistorians. When I peer at the deserted dwellings at my own excavations at WF16 in southern Jordan, a site contemporary with G?bekli Tepe and Jericho, I often feel utterly frustrated at not knowing the identities of the people who had built the reed-covered huts whose traces I have found. I become haunted by the words of the great travel writer Wilfred Thesiger, who had lived with the Marsh Arabs of southern Iraq. He later recalled his first visit - canoes on the waterways, the crying of geese and boys singing in the dark - and wrote: "I experienced the longing to share this life, and to be more than a mere spectator." As an archaeologist, I am not even a spectator on prehistoric life; all I have to look at is the rubbish that was left behind and has survived the ravages of time.

What would it have been like to have travelled through the prehistoric world in the same way that Thesiger travelled through Africa and Asia? To have been at G?bekli Tepe when the huge stone pillars were carved or at WF16 when the huts were built?

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A few years ago, I decided to vent my archaeological frustration in the only manner that an academic can - by writing a book. In After the Ice I have used the narrative of a journey as a means to write a global history between 20,000 and 5000 BC. Rather than merely describing one archaeological site after another, I imagined visits by a traveller through prehistoric times to the living settlements that we know only through their archaeological remains. All such visits were based on a meticulous study and interpretation of the archaeological evidence.

By placing a spectator in prehistory, one who remains unseen and can ask no questions, I could write about the sights, sounds and smells, not only at the settlements themselves but in the landscapes of prehistory that my spectator travelled through as they underwent environmental change. This literary device turns what might have otherwise been a dry catalogue of archaeological sites into an account of human history during those 15,000 years of great change for planet Earth. But it carries an inevitable risk - a spectator must see more than I am able to know, creating the danger of crossing the boundaries between legitimate interpretation, poetic licence and complete fiction. No one knows precisely where those boundaries exist and whether or not they should be crossed.

In academic terms, not only is it risky to write in this fashion, but I had to stretch my imagination further than ever. What would it have been like at Monte Verde in Chile at 12,500 BC, the earliest known settlement in the Americas? Or at Zhokhov in the Siberian arctic at 6500 BC where polar bears were hunted? Or at Jericho at 9500 BC and Gulia Naquitz at 8500 BC when the first steps to agriculture were being taken. I placed my unseen traveller at all of these sites and many more throughout the prehistoric world to watch history being made through the daily activities of people whom we do not know. There was only one site that I could not send him to visit because it is too extraordinary for my mind to understand: G?bekli Tepe.

Steven Mithen is professor of early prehistory and head of the School of Human and Environmental Sciences at the University of Reading. His book After the Ice is published by Weidenfeld & Nicholson next week (?25.00).

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