It's a compelling story, offering a simple explanation to the mystery of why mammoths, mastodons, and Clovis humans vanished. But it's a controversial theory, and new research suggests the impact was far too small to have done any serious damage.
Doubts centre on the speed of extinctions, the fate of the Clovis culture, and the presence of supposed impact signatures. But advocates of the comet-blast theory say they will present their own new data at the American Geophysical Union (AGU) meeting in San Francisco, where they will share the stage with sceptics.
"Nothing special happened at 12,900 years ago," says John Williams of the University of Wisconsin-Madison. His data, reported in Science this week, suggest that large mammals were already rare well before the purported impact.
Scraping the bottom
Williams and his colleagues searched layered lake-bottom deposits in Indiana and New York State for the spores of the fungus Sporormiella found in the dung of large plant-eating mammals such as mammoths and horses.
From a decline in the spore counts, they conclude that the megafauna population dropped steadily between 14,800 and 13,700 years ago, making them rare 800 years before any comet strike. Williams says the data rule out a sudden, impact-driven extinction.
However, geologist and impact-advocate James Kennett at the University of California, Santa Barbara, calls that "a classic case of over-interpretation" because the data comes from only a small area and not from different sites across the whole of North America.
Some anthropologists are also unhappy and with the sudden extinction theory, taking issue with claims that an impact wiped out the Clovis people.
It's true that distinctive Clovis-style artefacts disappear and sites were abandoned at the start of a dramatic cooling event which began about 13,000 years ago called the Younger Dryas. But the people didn't die, says Vance Holliday of the University of Arizona in Tucson. "An artefact style was replaced by another style. You see that all over the world." Moreover, early North Americans were highly mobile hunter-gatherers who occupied sites only briefly before moving on.
What's more, the geological layer representing the Younger Dryas is missing the sort of extraterrestrial material that was a hallmark of the impact that wiped out the dinosaurs.
Isotope geochemist Mukul Sharma of Dartmouth College in Hanover, New Hampshire, says he has been unable to find any significant amount of platinum-group elements with the distinctive isotopic signature showing that they came from space. Sharma will detail his findings at the AGU meeting.
Yet something did crash into the Pacific Ocean around the period in question. In a separate AGU paper, Sharma will report finding traces of extraterrestrial osmium on the floor of the mid-Pacific dating from between 16,000 and 8000 years ago.
Some Younger Dryas deposits do contain residues similar to those from the