New evidence has come to light in one of prehistory’s greatest “whodunnit” stories—a spate of animal extinctions more than 10,000 years ago.

Overhunting by humans in North America has been proposed as a possible reason for the die-off, which wiped out species such as the mammoth and the wild horse Equus ferus.

But new dating of more than 600 animal bones suggests that other factors, such as natural climate shifts, were to blame, according to R. Dale Guthrie of the University of Alaska in Fairbanks, Alaska.

Data provided by Guthrie in this week’s issue of the research journal Nature gave what he argued is a more precise account of the sequence of events surrounding the mass extinction.

Guthrie employed radiocarbon dating, a chemical analysis used to determine the age of organic materials based on their content of the radioactive element Carbon-14. He wrote that he was able to obtain more samples than past researchers had used, and in this way to identify “previously unrecognized patterns” in the data.

The extinctions occurred during a period known as the Pleistocene-Holocene transition, from 13,000 to 10,000 years ago. The dated samples included bones from bison, wapiti, moose and humans, all of which survived the period, together with horse and mammoth, which did not.

The evidence suggests climate shifts or more subtle mechanisms were behind the die-offs, Guthrie argued—not a “blitzkrieg” of hunting by humans, who were newly arrived in the New World.

Guthrie also said his data contradicts the “keystone removal” hypothesis, which suggested that killing off of grazing mammoths by humans led to changes in vegetation that caused a domino-effect extinction of other beasts.

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