According to two geologists at the University of Oregon, Dr Douglas Kennett and Dr Jon Erlandson, there is reason to believe a large chunk of a comet exploded above or crashed directly into the Laurentide ice sheet, rupturing the ice dams on the easterly margin of Lake Agassiz and causing frigid water to flood into the North Atlantic. The primary evidence consists of a carbon-rich layer of soil, dating to around 13,000 years ago, found at 50 Clovis-age sites across North America. The Clovis people, the first wave of human colonists to reach North America from Siberia, had only recently colonised the Americas.

The carbon-rich layer contains microspherules of metal, including iridium, mixed with charcoal particles. The composition and extent of the layer suggests a massive, continental-scale wildfire. The Cretaceous-Tertiary boundary layer, deposited by the massive comet or asteroid strike in the Caribbean that wiped out the dinosaurs 65 million years ago, is similar in composition - the iridium in the layer has an isotope composition that differs from iridium occurring naturally in the Earth's crust. The iridium from the sooty Clovis layer has yet to be "typed" to determine whether it is of extraterrestrial origin, but the spectrum of materials in the layer (and the associated ecological upheaval) have all the hallmarks of an extraterrestrial impact.

"The highest concentrations of extraterrestrial impact materials occur in the Great Lakes area and spread out from there," Kennett says. "It would have had major effects on humans. Immediate effects would have been in the north and east, producing shockwaves, heat, flooding, wildfires, and a reduction and fragmentation of the human population." No crater has been found yet - it may be hidden at the bottom of one of the Great Lakes. Alternatively, an explosion at high altitude might have shattered the ice sheet without leaving a crater.

According to Kennett, 35 animal genera (groups of species) became extinct in North America at the end of the Pleistocene epoch, and 15 of these were clearly wiped out close to 12,900 years ago, at the beginning of a 1500-year mini-glacial period known as the Younger Dryas. This mini-ice age was triggered by the Agassiz megaflood, which appears to have shut down the Gulf Stream, the tropical ocean current that circles the North Atlantic, warming the climates of eastern North America and north-western Europe. The megaflood resulted from the collapse of natural ice dams confining two vast glacial lakes, Lake Agassiz and Lake Ojibway, on the southern margin of the retreating Laurentide ice sheet that covered the northern US and Canada during the last glacial period, which ended around 15,000 years ago.

As the ice melted, the lakes filled - at the time, Lake Agassiz contained more fresh water than all of the world's modern lakes combined. The collapse of the ice dams possibly sent frigid fresh water riding over the denser, salty waters of the northerly-flowing Gulf Stream, dramatically reducing evaporation and rainfall, and sending temperatures plunging by five to 10 degrees Celsius across North America and Northern Europe within just a few years.

The impact of the sudden cooling was felt worldwide, and caused the northern hemisphere ice sheets to advance southwards again. In his book, First Farmers, Australian National University archaeologist Professor Peter Bellwood argues that this mini-glacial period, the Younger Dryas, was the catalyst for the development of the first permanent farming communities in the Middle East in an arc stretching from modern Israel and Jordan through Syria and Iraq, to southern Anatolia in Turkey. Bellwood's book draws together archaeological and environmental evidence indicating that the sudden return of cold, dry conditions to the Middle East during the Younger Dryas confronted the pre-agricultural Natufian culture with a momentous decision.

The Natufians, descendants of ice age hunter-gatherers, had begun to harvest the wild ancestors of rye, barley and, later, wheat, which flourished in the region as the climate became warmer and wetter after the last glacial period, between 15,000 and 13,000 years ago. The warming oceans were releasing large amounts of dissolved carbon dioxide into the atmosphere, accelerating plant photosynthesis and causing the ancestors of modern cereals to produce much larger grain.

Bellwood estimates that human numbers in the region expanded about six-fold during this time, and the Natufians established small settlements of rounded, mud-brick huts in areas where cereals were most abundant. They fashioned stone sickles to harvest grain and cut reeds to thatch their huts. Bellwood says modern geneticists have estimated that intense selection pressure for large-grained varieties of crops that retained their seeds in the head late into the season could have shaped semi-domesticated versions in as few as 30 seasons. Bellwood believes that when the Younger Dryas plunged the Middle East back into the cold, dry conditions of the most recent glacial period, the Natufians had to choose between abandoning their settlements and reverting to the harsh, nomadic life of hunter-gatherers, or staying put and becoming full-time farmers.

The sharply reduced productivity of wild food plants, dwindling numbers of game animals and the Natufians' own burgeoning numbers left them with only one realistic choice: they went farming. First Farmers records that the Natufians abandoned many of their pre-agricultural settlements in drier areas and retreated to sites with permanent water where they began cultivating primitive varieties of familiar crops such as rye, barley, wheat and lentils.

Bellwood says the Natufians also began to domesticate the wild ancestors of cattle, sheep, goats and pigs, which were native to the region and congregated in herds or flocks, making them easy to domesticate and manage, providing a permanent source of meat and milk. According to Bellwood's book, by around 8400 years ago, large, complex settlements such as the ancient, walled city of Jericho, with several thousand inhabitants and diverse, specialised labour forces, had developed at permanently watered sites. Meanwhile, as the Earth warmed, the Mediterranean Sea, reduced to a huge hypersaline lake when low global sea levels cut it off from the Atlantic during the Younger Dryas, slowly refilled and around 8200 years ago swamped over a silt dam that had plugged the Bosphorus Strait between Europe and Asia.

The overflow became a raging torrent that, within a few days, cut a deep channel through silt plugging the strait. With the weight of the newly filled Mediterranean behind it, the water cascaded 150 metres into the Black Sea. The massive waterfall, estimated at 300 times the volume of Niagara Falls, would have created a deafening roar that could have been heard 500 kilometres away.

Submarine imaging surveys have revealed the geometric outlines of ancient settlements on the margins of the former lake shore that drowned as the lake's rapidly rising waters advanced several kilometres a day across its gently sloping shores. Historians suspect the Black Sea flood 8200 years ago was the source of the biblical tale of Noah's Ark. First Farmers, by Peter Bellwood, is published by Blackwell.