Tiny balls of fungus and feces may disprove the theory that a huge space rock exploded over North America about 12,900 years ago, triggering a thousand-year cold snap, according to a new study.
The ancient temperature drop, called the Younger Dryas, has been well documented in the geologic record, including soil and ice core samples.
The cool-down also coincides with the extinction of mammoths and other Ice Age mammals in North America, and it's thought to have spurred our hunter-gatherer ancestors in the Middle East to adopt an agricultural lifestyle.
But the theory that a comet or asteroid explosion is behind the cooling event is wrong, according to study leader Andrew C. Scott, a paleobotanist at Royal Holloway University in London.
For years proponents of the impact theory have cited tiny spherules of carbon found in a layer of charred sediment throughout North America that dates to the Younger Dryas period.
According to the theory, these spherules are organic matter subjected to intense heat after debris from an exploded meteor rained down on Earth, sparking massive wildfires.
The new research, however, detected carbon spherules in soil layers from before, during, and after the Younger Dryas, making it hard to argue that the particles are products of a sudden impact.
What's more, Scott's team found that most of the spherules are similar to tightly packed balls of fungus found in modern soils that have been exposed to low to moderate heat during wildfires. Plant and soil fungi are known to create these balls of material to help them survive extreme conditions.
Other elongated forms of the spherules match modern fecal pellets from insects.
"All these particles are of natural biological origin and are not related to either intense wildfires or cosmic impacts," Scott said in an email.
"The press and public are very interested in catastrophic explanations," he added. "But it is important that when evidence stacks up to show the theory does not work, then it should be abandoned."
What About the Nanodiamonds?
In fact, most experts acknowledge that carbon spherules are found throughout the geological record, including biological forms associated with wildfires, said James Kennett, an emeritus geologist at the University of California, Santa Barbara, who supports the cosmic-impact hypothesis.
However, the spherules are not often found in large quantities, he said, and there is "a peak in carbon spherules at the Younger Dryas boundary."
What's more, those spherules are found alongside microscopic diamonds, or nanodiamonds, which often form under the conditions caused by extraterrestrial impacts.
The new study does not report evidence of nanodiamonds, Kennett noted, which is expected, since the team wasn't directly looking for them.
"So their [reported] data is consistent," Kennett said.
Study leader Scott said that his team has studied the nanodiamond issue, but he's not yet able to discuss the results.
He did, however, hint that the particles might not be nanodiamonds at all: Fungal spores the team examined have similar microscopic features.
And, Scott said, "obviously [spores] are not nanodiamonds."
The carbon spherule study has been accepted for publication in the journal Geophysical Research Letters.