12,900 years ago, the Northern Hemisphere entered an abrupt cold period known as the Younger Dryas. This glacial interval didn’t last long, just 1,200 years, but it coincided with the extinction of many large, iconic megafauna that lived in North America, such as mammoths. It’s also during this time that stone tools representing the Clovis culture vanish from the archaeological record.
For more than a century, climate change and overkill by humans were considered the two main drivers of megafaunal extinctions at the end of the last ice age. However, in 2007 a new hypothesis was proposed: one where a fragmented comet exploded over North America and ignited widespread wildfires that filled the atmosphere with smoke, soot and dust, resulting in an 'impact winter'.
This, combined with the rapid melting of ice sheets that shut down warm water currents in the North Atlantic, may be what ultimately plunged the Northern Hemisphere into the freezing Younger Dryas.
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In a study published in PLOS One in September 2025, UC Santa Barbara’s James Kennett and collaborators from The Comet Research Group (CRG) presented evidence they believe supports this impact hypothesis.
At three well-known archaeological sites in the US - Murray Springs in Arizona, Blackwater Draw in New Mexico, and Arlington Canyon in California’s Channel Islands - the CRG found shocked quartz grains in a layer of rock dating to the onset of the Younger Dryas. These kinds of grains are created by extreme pressures and temperatures and are thought to be indicative of crater-forming cosmic impact events.
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Typically, celestial objects that collide with Earth leave behind craters. Take the 200km-wide crater that lies beneath the Yucatán Peninsula for example; this giant pockmark was left by the asteroid that wiped out the dinosaurs 66 million years ago and serves as the ‘smoking gun’ for this particular impact hypothesis.
Kennett and his collaborators haven’t identified a crater associated with their proposed cosmic impact event. Instead, they point to the shocked quartz grains found at the aforementioned sites as evidence of a different kind of impact. They propose a fragmented comet hit Earth’s atmosphere and exploded above the surface, resulting in a 'touch-down' airburst that wreaked havoc but left little, if any, evidence on the landscape.
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The shocked quartz grains examined by this study show variations in shock patterns, something Kennett explains is expected of airburst-style impacts. “There are different levels of shocked quartz,” he said in a press release published by UC Santa Barbara. “There are going to be some very highly shocked grains and some that will be low-shocked. That’s what you would expect.”
Other signs of a cosmic impact event have been found in the same layer of rock, not only at the three sites investigated by this recent study, but dozens more sites across North America. These include an organic-rich 'black mat' that is said to be indicative of widespread biomass burning, as well as spikes in concentrations of platinum, meltglass, soot and nanodiamonds.
According to Kennett, these lines of evidence and the presence of shocked quartz grains at three key archaeological sites, “support a cosmic impact as a major contributing factor in the megafaunal extinctions and the collapse of the Clovis technocomplex at the YD (Younger Dryas) onset.”
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Despite a growing list of evidence, other scientists aren’t so convinced by the body of work present by the CRG. Ever since the Younger Dryas Impact Hypothesis (YDIH) was first proposed in 2007 it has faced a lot of backlash, with critics highlighting inconsistencies in results, the incorrect dating of certain archaeological sites, and the over-reliance on 'impact proxies' that can be otherwise explained by terrestrial or non-catastrophic factors.
There’s also evidence that populations of North American megafauna were already in a state of collapse prior to the proposed cosmic impact event, possibly as a result of anthropogenic activities, such as hunting.
In October of last year, a systematic review conducted by researchers working in Australia, Germany, Portugal and Malta, found that, out of the 360 papers published between 1959 and 2024 that examined megafaunal extinctions at the end of the last ice age, “only a few considered an extraterrestrial cause, such as a solar flare or comet impact” as a credible cause.
Overall, this review found 23% of papers cited humans as the primary driver of extinction, another 23% cited climate change as the primary driver, 20% proposed a combination of the two, and “a third” offered no explicit major driver of extinction.
While Kennett and others from the CRG may have discovered what they think is convincing evidence of a cosmic impact event roughly 13,000 years ago, it’s clear it currently cuts no ice with the wider scientific community.
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Top image: Herd of mammoths. Credit: Getty
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