A Cosmic Impact and the Beginning of Farming at Abu Hureyra in Syria

As our study of the plant remains, artifacts, and chronology proceeded we could also see that the transition from hunting and gathering to initial cultivation of rye then wheat and legumes took place precisely at the moment when a dramatic, near-instantaneous change in climate and vegetation occurred as the warm, moist conditions of the Late Glacial gave way to a cold, dry Ice Age-like climate that persisted for over 1,200 years. This episode of Northern Hemisphere cooling is called the Younger Dryas. It was the onset of the Younger Dryas that triggered the beginnings of farming at Abu Hureyra.

There matters rested until 2007 when a group of scientists posited that an extra-terrestrial impact by a comet had caused part of the Laurentide ice sheet to melt, redirecting melt water flow that shut down the North Atlantic circulation system and so caused the Younger Dryas. Soon after Douglas Kennett of the University of California, Santa Barbara, and a member of the scientific team, contacted me to ask if we had retained soil samples from Abu Hureyra that dated to the impact event 12,800 years ago. It turned out that we had. So began several years of detailed microscopic and geochemical analyses of these samples to see if they contained evidence that would document the cometary impact event at Abu Hureyra.

The crucial samples came from a level of heavy burning that I had previously interpreted as the remains of cooking fires. It radiocarbon dated to precisely 12,800 years ago, the same date as the comet collision. This level contained a variety of materials that were produced by the heat of the impact: nanodiamonds, spherules, meltglass, and traces of iridium, platinum, nickel and cobalt that came from the comet itself. All of these have been found and described at many other sites, thus recording the effect of this impact throughout the world. The platinum traces, in particular, are important because they also occur in a contemporary, well-dated layer in an ice core from the Greenland ice sheet that independently documents this event. The meltglass was formed by melting of the silica in the subsoil at temperatures over 2,000°C that could only have been reached in an airburst explosion. At Abu Hureyra the meltglass was splattered throughout the soil samples from the impact level as minute discrete lumps and on pieces of animal bone and building clay.

What of the wider regional implications of this discovery? Only open-air sites would have been exposed to similar airbursts, and few of these are known across the Near East that date precisely to the Younger Dryas onset. Those that might be considered for further investigation are in locations where, given present circumstances on the ground, archaeological excavations cannot be conducted. So it will be many years before we can take the inquiry further within the region. Worldwide, however, the Younger Dryas comet impact not only caused massive surface burning of vegetation, it is also implicated in large-scale human population declines and, in North and South America and elsewhere, the demise of many species of megafauna that had flourished during the Pleistocene. The cometary impact was an event with significant global effects on human societies and the natural world.