By Richard Black
Environment correspondent, BBC News website
An extinct Galapagos tortoise species could walk again, scientists believe.
Writing in Proceedings of the National Academy of Sciences, researchers report finding relatives of Geochelone elephantopus alive and well.
Cross-breeding these living tortoises might re-create the extinct species - though it could take a century.
The distribution of related tortoises between the islands was one of the pieces of evidence Charles Darwin used in formulating his theory of evolution.
But of 15 known Galapagos species, four have since gone extinct - elephantopus less than two decades after Darwin visited the island.
Now, according to Gisella Caccone from Yale University in New Haven, US, there is a chance that its former island home of Floreana could one day feel its footsteps again.
"We might need three or four generations to do this," she told BBC News.
"But in theory it could be done, and I think it's pretty exciting to bring back from the dead a genome that we thought was gone."
When HMS Beagle visited the Galapagos in 1835, Darwin noted that many of the islands were home to giant tortoises that shared many features, yet were distinct from island to island.
The vice-governor of the archipelago told the naturalist that he could identify which island a tortoise came from, just by its appearance.
Yale's Nikos Poulakakis examines a museum specimen of elephantopus
Darwin later surmised that the animals had been carried to the Galapagos from mainland South America, where similar species could be found. As with other groups of animals, he believed that once a group of tortoises reached an island, they would then maintain a relatively isolated existence, and populations on the various islands would evolve in subtly different ways.
"Thus the several islands of the Galapagos Archipelago are tenanted, as I have elsewhere shown, in a quite marvellous manner, by very closely related species; so that the inhabitants of each separate island, though mostly distinct, are related in an incomparably closer degree to each other than to the inhabitants of any other part of the world," he noted in his masterwork On the Origin of Species.
But he also saw at first hand that some of the islands were being depleted of their marvellous inhabitants by whaling ships, which would cart the tortoises away to be killed and eaten at a later date.
Dr Caccone believes that about 250,000 tortoises may have been removed in this way.
Floreana, as a relatively low-lying island where the wildlife was therefore relatively easy to hunt, was depleted more than most, leading to the disappearance of elephantopus.
The biggest island of all, Isabela, was less ravaged. And around one of its volcanoes, researchers found a group of animals that did not look like the others.
Now, genetic analysis shows they are close to the Floreana lineage.
The likely explanation is that sometimes, whaling vessels would find they had picked up more tortoises than they needed, and so would jettison the excess animals in shallow water as they returned through the archipelago.
So some of the Floreana tortoises made it to Isabela, where their genes have - albeit slowly, for these animals take about 25 years to produce a new generation - mixed with those of other species.
Finding the relatives is one thing; but using their genetic heritage to bring back the extinct Floreana species is quite another.
The Yale team plans next to mount a more exhaustive survey of the Volcano Wolf region of Isabela to identify more individuals carrying elephantopus genes.
"Then we would have to look at individuals of interest, and genotype them and maybe use marker-assisted selection to help the process along," said Dr Caccone.
Marker-assisted selection involves choosing which individuals to cross according to which versions they carry of which genes, so taking away some of the randomness involved in conventional cross-breeding.
But the long intervals between generations mean that even if the project does start, it will not be concluding any time soon. A century ahead would be a fair bet.