(Patrick Orton/Getty Images)
At a point some 12,800 years ago, a tenth of Earth's surface suddenly became covered in roaring fires.
The firestorm rivalled the one that wiped out the dinosaurs, and it was likely caused by fragments of a comet that would have measured around 100 kilometers (62 miles) across.
As dust clouds smothered Earth, they kicked off a mini ice age that kept the planet cool for another thousand years, just as it was emerging from 100,000 years of being covered in glaciers. Once the fires burned out, life could start again.
"The hypothesis is that a large comet fragmented and the chunks impacted the Earth, causing this disaster," said Adrian Melott from the University of Kansas, who co-authored a 2018 study detailing this catastrophic event.
"A number of different chemical signatures – carbon dioxide, nitrate, ammonia and others – all seem to indicate that an astonishing 10 percent of the Earth's land surface, or about 10 million square kilometers [3.86 million square miles], was consumed by fires."
To peer back into the burning fires and shock waves of this major event, a large number of geochemical and isotopic markers were measured from more than 170 sites across the world, involving a team of 24 scientists.
One of the pieces of analysis carried out was on patterns in pollen levels, which suggested pine forests were suddenly burned off to be replaced by poplar trees – a species specializing in covering barren ground, as you might get when your planet has been hit by a series of massive fireballs.
In fact, parts of the comet that disintegrated in space are still likely to be floating around our Solar System 13,000 years later.
High concentrations of platinum – often found in asteroids and comets – and high levels of dust were also noted in the samples analyzed by the researchers, alongside increased concentrations of combustion aerosols you would expect to see if a lot of biomass was burning: ammonium, nitrate, and others.
Plants died off, food sources would have been scarce, and the previously retreating glaciers began to advance again, the team noted. Human culture would have had to adapt to the harsher conditions, with populations declining as a result.
The team hypothesized that such a widespread impact of comet fragments, and the ensuing firestorm, is responsible for that extra bit of cooling known as the Younger Dryas period. This relatively brief blip in the planet's temperature has sometimes been put down to changing ocean currents.
However, the comet hit isn't a completely new idea, even though this recent research goes into a great deal of depth to try and find evidence for it. Scientists have been debating whether a comet impact kicked off the Younger Dryas event for several years now.
Not everyone agrees that the data points to a comet strike, but this comprehensive work offers up more support for the hypothesis, as do the ancient carvings found in Turkey in 2017 – carvings which depict a devastating impact from an interstellar object.
"The impact hypothesis is still a hypothesis, but this study provides a massive amount of evidence, which we argue can only be all explained by a major cosmic impact," says Melott.
A version of this article was first published in 2018.