Researchers have discovered genetic traces of a giant tortoise species believed extinct for 150 years in living hybrids on the Galapagos island of Isabela.

A genetic analysis found DNA traces of the long lost tortoise species Chelonoidis elephantopus in their hybrid descendants. The hybrids have so much C. elephantopus DNA in them that scientists think that a few pure C. elephantopus tortoise still exist in the island.

"To our knowledge, this is the first rediscovery of a species by way of tracking the genetic footprints left in the genomes of its hybrid offspring," wrote researchers led by Yale University biologists Ryan Garrick and Edgar Benavides in a January 9 issue of Current Biology.

Garrick and colleagues went to Volcano Wolf on the northern part of Isabela Island and took blood samples from Chelonoidis becki, a tortoise species closely related to C. elephantopus. From the 1,600 individual C. becki tortoises they tested, the scientists found 84 tortoises that contained C. elephantopus DNA. In fact the 84 C. becki tortoises had so much DNA from the giant tortoises that at least one recent ancestor must have been a purebred C. elephantopus.

Although none of the purebreds were spotted, they could still be wandering around in the island just waiting to be found. The researchers estimate that in 30 cases breeding took place within the last 15 years. Given that tortoises from other giant Galapagos species could live for 170 years, there's a high probability the purebreds are still alive.

Galapagos turtles are famous for influencing Charles Darwin in his study on evolution and natural selection. There were an estimated 250,000 giant tortoises living in the Galapagos Islands in the beginning of the 16th century before humans arrived on the island. The tortoises had no natural predators until humans came along and saw a great food source in the giant tortoises. The Galapagos tortoises can weigh nearly 900 pounds and grow to almost 6 feet in length and their flesh was actually rich in oil. Sailors took the tortoises and kept them alive in their ships for future consumption.

Soon after Charles Darwin's historic voyage to the Galapagos the tortoises were declared extinct. Remarkably the sailors who took the tortoises aboard as a food source could also have saved the species.

"If a ship was under siege, sailors would unload it by throwing things overboard," said Garrick. "The first thing to go was stuff stored in the hull. Tortoises don't swim, but they float like wine corks, and it so happens that the prevailing current runs northeast through the islands. The last place a tortoise might catch land before being swept into the ocean was the northern part of Isabela island. This is where they would have washed up."

The researchers are hoping to establish a captive breeding program in the Galapagos Islands using the hybrids and any discovered pure C. elephantopus. The researchers think that through selective breeding the giant tortoises could be resurrected from near extinction.