A tortoise species thought to have gone extinct more than a century ago is happily living on the Galápagos Islands today.
Researchers have confirmed that a female tortoise - named Fernanda after the island Fernandina she was discovered on - is from a species last identified in 1906.
She was spotted in 2019 but it's only now that experts have been able to prove it is a Fernandina Island Galápagos giant tortoise - Chelonoidis phantasticus, or "fantastic giant tortoise".
The species had only been known from a single specimen and was assumed extinct.
By sequencing the genomes of both the living individual and the museum specimen, and comparing them to the other 13 species of Galápagos giant tortoises, Doctor Stephen Gaughran showed that the two known Fernandina tortoises are members of the same species, genetically distinct from all others.
Dr Gaughran, of Princeton University in the United States, is co-first author of the study, published in the journal Communications Biology, confirming the species' continued existence.
Princeton's Professor Peter Grant, who has spent more than 40 years studying evolution in the Galápagos islands, said: "For many years it was thought that the original specimen collected in 1906 had been transplanted to the island, as it was the only one of its kind.
"It now seems to be one of a very few that were alive a century ago."
He explained that when Fernanda was discovered, many ecologists doubted that she was actually a native phantasticus tortoise as she lacks the striking "saddleback" flaring of the male historical specimen, though scientists speculated that her obviously stunted growth may have distorted her features.
Tortoises can't swim from one island to another, but they do float, and they can be carried from one Galápagos island to another during hurricanes or other major storms. There are also historical records of seafarers moving the tortoises between islands.
Dr Gaughran, a postdoctoral research fellow in ecology and evolutionary biology, said: "Like many people, my initial suspicion was that this was not a native tortoise of Fernandina Island."
To determine Fernanda's species definitively, he sequenced her complete genome and compared it to the genome he was able to recover from the specimen collected in 1906.
He also compared those two genomes to samples from the other 13 species of Galápagos tortoises - three individuals from each of the 12 living species, and one individual of the extinct C. abingdonii.
Dr Gaughran said: "We saw - honestly, to my surprise - that Fernanda was very similar to the one that they found on that island more than 100 years ago, and both of those were very different from all of the other islands' tortoises."
Study senior author Professor Adalgisa Caccone, of Yale University in the US, said: "The finding of one alive specimen gives hope and also opens up new questions, as many mysteries still remain.
"Are there more tortoises on Fernandina that can be brought back into captivity to start a breeding programme?
"How did tortoises colonise Fernandina, and what is their evolutionary relationship to the other giant Galápagos tortoises? This also shows the importance of using museum collections to understand the past."
Dr Gaughran said: "Part of my postdoc is developing a tool that analyses DNA from ancient museum specimens so we can compare them to modern samples.
"The software doesn't care if it's a seal or a tortoise or human or Neanderthal. Genetics is genetics, for the most part. It's in the interpretation where it matters what kind of creature the DNA comes from."
He said that, since 1906, scant but compelling evidence has hinted that giant tortoises might still live on Fernandina Island, an active volcano on the western edge of the Galápagos Archipelago that is reputed to be the largest "pristine" island on Earth.
A single specimen of C. phantasticus - "the fantastic giant tortoise" - was collected by explorer Rollo Beck during a 1906 expedition.
The "fantastic" nature refers to the extraordinary shape of the males' shells, which have extreme flaring along the outer edge and conspicuous saddlebacking at the front.
Saddlebacking is unique to Galápagos tortoises, and the phantasticus tortoise shows it more prominently than the other species.
Since its 1906 discovery, the survival of the Fernandina tortoise has remained an open question for biologists.
The island has remained largely unexplored, due to extensive lava fields blocking access to the island's interior.
Scientists estimate that Fernanda is well over 50 years old, but she is small, possibly because the limited vegetation stunted her growth.
Dr Gaughran says the tortoise populations were decimated by European seafarers who hunted them for food, having discovered that they could keep tortoises alive on their ships with minimal effort, as the reptiles could survive with little food or water.
He said: "They were a great source of fresh meat for the sailors, but it meant that many of the species were severely overhunted.
"And because they have such a long generation time, the populations have a hard time recovering quickly."
Prof Grant said: "The genetic work provides intriguing hints of a mixing of genes with members of another population.
"It would be fascinating if confirmed by future detective work on the genome.
"Another thought-provoking finding is the nearest relatives are not on the nearest very large island, Isabela, but on another, Espanola, far away on the other side of Isabela.
"The question of how the ancestors reached Fernandina is left hanging."
Fernanda is now at the Galápagos National Park Tortoise Centre, a rescue and breeding facility, where experts are seeing what they can do to keep her species alive.
Prof Grant added: "The discovery informs us about rare species that may persist in isolated places for a long time.
"This information is important for conservation. It spurs biologists to search harder for the last few individuals of a population to bring them back from the brink of extinction."
