Professor Gordon Hillman, at University College London, has spent over 20 years investigating the remains of ancient food plants at a unique site at Abu Hureyra, in the middle Euphrates.

"Nowhere else has an unbroken sequence of archaeological evidence stretching from hunter-gatherer times to full-blown farming," he told BBC News Online.

Hunter-gatherers

The evidence for cultivated crops comes from seeds carefully sifted from the material excavated at Abu Hureyra. These had survived because they had been accidentally charred in domestic fires before eventually becoming buried.

Farming crisis: drought drove the hunter-gatherers into cultivation

Many years of ecological field work assessing present day vegetation was also necessary to provide a basis for interpreting the material found.

"What we expected to find from the hunter-gatherer levels at the site was lots of wild cereals. These are characteristically very skinny and we found plenty of them," explains Professor Hillman.

"But then, at higher and later levels, we found things that did not belong there. There were these whacking, great fat seeds, characteristic of cultivation."

The cultivated seeds found at Abu Hureyra are the oldest yet found.

A dry death

Professor Hillman and his team found that, as they looked through the archaeological record, the wild seed varieties gathered as food gradually vanished, before the cultivated varieties appeared. Those wild seeds most dependent on water were the first to die out, followed one by one by the more hardy ones.

This was a clue to why the hunter-gatherer people turned to cultivating some of the foods they had previously collected from the wild, and prompted Professor Hillman to look at independent climate records for the period.

What he found was evidence for a terrible drought: "It was very sharp and would certainly have been felt within a human lifetime, perhaps even in the space of 10 or so years."

Geologist call this period the Younger Dryas, a 1000-year spell of cold and dry weather with interrupted the planet's gradual warming from the last ice age.

The land had to be cleared before planting

Professor Hillman's team suggest that as the wild grasses and seeds that the people relied on for food died out, they were forced to start cultivating the most easily-grown of them in order to survive.

Professor David Harris, also at UCL, said: "There came a point when this community had no option - they were stuck with agriculture."

The archaeologists found no evidence that the irrigation was used to grow the first crops as the drought set it. Professor Hillman explains: "What they did was to take seed of the wild cereals from higher areas to the West, and sowed it close to Abu Hureyra in areas such as breaks in slope, where soil moisture was greatly enhanced naturally."

"Wild stands of these cereals could not have continued to grow unaided in such locations because they would have been out-competed by dryland scrub. Therefore, these first cultivators had to clear the competing vegetation."