"Civilisation did not arise as the result of a benign environment which allowed humanity to indulge a preference for living in complex, urban civilised societies," he told the annual meeting of the British Association for the Advancement of Science.

"On the contrary, what we tend to think of today as civilisation was an accidental by-product of unplanned adaptation to catastrophic climate change. Civilisation was a last resort," he added.

Brooks said he based his theory on close observation of archaeological remains of the Garamantian civilization in the Fezzan region of south-western Libya allied with evidence of changing rainfall patterns 3,000-5,000 years ago.

But he said the pattern could also be found in societies as diverse as South Asia, South America and China.

As the climate became steadily drier formerly nomadic people were forced to come together for mutual support and to eke out the dwindling natural resources.

But not all of the consequences of this merging movement were beneficial -- social inequality arose as did organised violence, there was no increase in life expectancy and autocratic governments emerged, Brooks said.

When climate conditions improved again there was no return to the former order.

"Once the cat is out of the bag, it doesn't go back. You can't uninvent technology," Brooks said.

And he warned against drawing comparisons with the global warming that is predicted to raise average temperatures by around three degrees this century, noting that the temperature rise was well above that which forced the societal change 5,000 years ago.