Thursday, February 28, 2013

Did We Doom the Mammoths?

An artist's rendering of an Ice Age mammoth skeleton on display at the Page Museum at the La Brea Tar Pits in Los Angeles An artist's rendering of an Ice Age mammoth skeleton on display at the Page Museum at the La Brea Tar Pits in Los Angeles

Courtesy of the Page Museum/La Brea Tar Pits/Reuters

Climate change killed the last mammoths. Hunkered down on their final holdout, Siberia?s Wrangel Island, the shaggy herbivores died out as rising seas shrank their refuge and warm weather baked their cool grasslands. By 3,700 years ago, within the span of written human history, mammoths disappeared.

But the real reason there are no more mammoths?and cloning projects aside, the reason we?ll never get to hug an adorable baby mammoth at a petting zoo?is a hotly contested topic. Mammoths wound up hopelessly marooned on a Siberian island in the first place because they were wiped across the rest of their range?an impressively disheartening fact when you realize that the behemoths once trod the widespread ?mammoth steppe? grasslands from western Spain through northern Asia to the interior of North America. Explanations for their disappearance have historically involved climate change, ?hyperdisease,? comet impact, and hungry humans. Figuring out what happened to mammoths and other lost species is not just an exercise for academic keepers of the dead. If we can understand the mechanics of extinction and how life responds to fluxes like climate change, we might be able to use the dead to look to the future.

Just as non-avian dinosaurs were only part of a major mass extinction 66 million years ago, mammoths are but one icon of a deeper loss of biodiversity. The Pleistocene extinction saw the death of much of the world?s most spectacular megafauna (creatures that weighed more than 44 kilograms, or 97 pounds) from Australia, Asia, Europe, and the Americas. Mammoths, giant ground sloths, sabercats, and terror birds were among the losses. Some species that survived the die-off were swept from parts of their ranges; there are no more spotted hyenas in Europe, for example.

This global disaster did not occur all at once. There is a pattern in the geological record of extinction, one that seems to follow the path of wandering humans.

Mass extinctions are murder mysteries. In the case of the Pleistocene extinction, two culprits are likely intertwined: humans and climate change. Other suspects don?t hold up. The kind of indiscriminate hyperdisease that would kill smilodons (in the cat family), dire wolves (dog family), woolly mammoths (elephant family), and all the other lost forms wildlife is totally unknown. The proposal that North America?s extinction pulse was caused by a falling comet lacks solid support. ?

From Australia to North America, large mammals and assorted other creatures often disappeared shortly after the arrival of humans on their continent, whether it was 50,000 years ago or 10,000 years ago. We know that humans hunted at least some of the lost mammals. Butchered bones found at archaeological sites include the remains of mammoths and mastodons. At the same time, the world was going through its last oscillation from an Ice Age world to the one we know today. The cool, dry global climate gave way to a warmer, wetter one. Pleistocene creatures had to move with their favored environments, adapt to their new surroundings, or go extinct. What paleontologists continue to tussle over is how important humans and climate change were to the Pleistocene crisis.

From the perspective of Deep Time, the Pleistocene extinctions happened extremely rapidly. If we were able to travel 66 million years into the future and look back at the Pleistocene fossil record, the event might appear to be instantaneous. Since we?re so close to the massive bleed of species, though, we can pick through the evidence with more resolution.


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