A species of giant tortoise thought to have been extinct for more than 150 years may still be living in the Galapagos Islands.

Scientists studying tortoises on the island chain have found genetic footprints of Chelonoidis elephantopus, the species that helped Charles Darwin formulate his opinions on evolution.

Genetic clues indicate that pure-bred members of the species have recently mated with giant tortoises of similar yet different species, suggesting that members of the Chelonoidis elephantopus may still be alive.

The genetic footprint was found in 84 tortoises from Isabela Island. Each of these hybrids had a parent from the long-thought extinct species. As tortoises live on average for more than 100 years, it is likely that members of the parent species are still alive, scientists predict.

Reported in the journal Current Biology, lead researcher Dr Ryan Garrick of the University of Mississippi, said: "To our knowledge this is the first report of the rediscovery of a species by way of tracking the genetic footprints left in the genomes (genetic codes) of its hybrid offspring."

Darwin visited the Galapagos archipelago in 1835, noting that the tortoises on different islands exhibited different shaped shells. From this and other observations Darwin extrapolated the theory of natural selection.

During the nineteenth century, Chelonoidis elephantopus was believed to have been hunted to extinction by workers at a heating oil factory on the nearby island of Floreana, the tortoises' original home.

However, traces of the lost DNA were first found in the species Chelonoidis becki on Isabela Island. Further traces were then found in a colony of tortoises living on the slopes of Wolf Volcano, also on Isabela Island.

Scientists believe that some of the giant tortoises could have been carried from Floreana to Isabela on whaling ships.

"If we can find these individuals, we can restore them to their island of origin," said co-author Dr Gisella Caccone, from Yale University in the US.

"This is important as these animals are keystone species playing a crucial role in maintaining the ecological integrity of the island communities."