Sinkhole fossils might shed light on extinctions

Sawmill Sink is a flooded sinkhole on the island of Abaco in the Bahamas. Below 30 feet is a layer of opaque water saturated in poisonous, acidic hydrogen sulfide.

But in 2004, a retired military diver named Brian Kakuk figured out how to penetrate the murk and dive into the lightless depths of the hole.

Along ledges, he found a trove of bones, including those of dozens of species that have become extinct. A new report in the Proceedings of the National Academies of Science describes the discovery, and the authors think it has implications for one of the great mysteries in science — the wave of extinctions in the Americas at the end of the Pleistocene as the climate changed, and human beings arrived.

There are few issues in paleontology more controversial than the Pleistocene extinctions. Numerous species, including mammoths, saber-toothed cats, horses, camels and giant sloths, vanished about the time human beings arrived.

The Bahamas offers a special laboratory for separating the climate factors from the human factors. During the ice ages, when sea levels were about 250 feet lower, Abaco was a much larger, heavily wooded island pocked with sinkholes and inhabited by a great diversity of animals.

But there were no people. Abaco remained uninhabited by people until 1,000 years ago, long after the major climate changes, said David Steadman, curator of ornithology at the Florida Museum of Natural History and the lead author of the new paper.

The research shows two extinction spasms: One with climate change more than 10,000 years ago, and the second with the arrival of humans. The second one was slightly worse.

The divers retrieved more than 5,000 fossils from Sawmill Sink. The scientists were able to identify 95 vertebrate species represented among those bones. Thirty-nine of those had gone extinct on Abaco. It appears 17 of those disappeared during the climate change some 10,000 years ago, and then 22 species vanished roughly 1,000 years ago, when humans arrived.

The first wave of extinctions occurred when the pine forests of the ice age gave way to warmer, more tropical forests on the much smaller island. But some species proved adaptable to climate change and stuck around. Then the people arrived.