By Brian Switek of Nature magazine

About 13,800 years ago, a mastodon in North America met a somewhat ironic end. It died at the hands of humans wielding a bone projectile made from the skeleton of another mastodon. A study of the remains of this unfortunate trunk-bearing beast add weight to the theory that the downfall of the megafauna may have been more gradual than previously proposed.

The skeleton of the mastodon (Mammut americanum) was excavated from the Manis site in Washington state in the late 1970s. Initial examinations found cut marks on the bones, and a projectile point made of bone was embedded in one of the mastodon's ribs. This mastodon had been hunted.

Exactly when this struggle occurred was disputed. Organic material associated with the mastodon skeleton was radiocarbon dated to about 14,000 years ago, a date thought to be 2,000 years too early for humans to have been hunting mastodons. According to an extinction scenario known as the 'blitzkrieg' model, first proposed by paleontologist Paul Martin in the 1960s, humans quickly spread through North America and eradicated mastodons and other large Pleistocene mammals after about 12,000 years ago.

Now geoarcheologist Michael Waters of Texas A&M University and his colleagues have taken another look at the Manis material. Carbon dating of collagen from the mastodon's rib and tusks yielded dates of approximately 13,800 years, confirming the animal's antiquity. Additionally, DNA and protein sequencing of the rib and the bone point matched mastodon genetic material. The researchers report on the implications of this finding today in Science.

Most previously discovered mammoth and mastodon kill sites have been attributed to humans of the Clovis culture, who used finely honed stone tools. These people were thought to have been the first major hunters of Pleistocene wildlife in North America. But the bone tool found in the mastodon rib was created by a culture that was hunting mastodons at least 800 years earlier.

The humans who killed the mastodon were "carrying a toolkit that was durable, lethal and portable", Waters says. The Manis site may even be a two-for-one discovery. "These people either scavenged bone from a fresh carcass," Waters suggests, "or they had killed another mastodon to make the bone point."

The Manis site is not unique. Waters points out that two mammoth kill sites in Wisconsin, dated to roughly 14,200 and 14,800 years ago, also indicate the presence of pre-Clovis hunters.

The indirect signature of hunters can also be seen in the ecological record. In 2009, Jacquelyn Gill of the University of Wisconsin, Madison, and her colleagues found that fluctuations in the dung-dwelling fungus Sporormiella sampled from Pleistocene bogs in Indiana and New York tracked changes in the abundance of large herbivores. Pleistocene plant-eaters began to decline about 14,800 years ago before being locally extirpated around 13,000 years ago. Waters suggests that this ecological pattern matches the archaeological record.

Just what the pre-Clovis kill sites indicate about the triggers of the Pleistocene extinction are not entirely clear. "One hunted mastodon obviously does not demonstrate that hunting caused the extinction of this species or any other," says paleontologist Adrian Lister of the Natural History Museum in London. But he notes that the new findings are consistent with the dung-spore record.

Perhaps, Waters suggests, the arrival of humans pushed already weakened populations of megafauna over the brink. "These animals were on their way," Waters says, "being stressed because of climate and vegetation, and there were hunting pressures put on these animals earlier than we thought."

This article is reproduced with permission from the magazine Nature. The article was first published on October 20, 2011.