A new species of giant tortoise has been uncovered in the Galápagos Islands after DNA testing revealed that two different species had been accidentally mixed up for decades. 

The genetic analysis revealed that the family tree of the giant tortoises in the Galápagos is much more complicated than previously appreciated. It was thought that the 8,000 giant tortoises that currently lumber around San Cristóbal Island were a species known as Chelonoidis chathamensis, which was first described on the basis of bones and shells collected in a cave in 1906. 

However, it turns out, the DNA of present-day giant tortoises doesn’t match the old bones of C. chathamensis. Instead, the giant tortoises of San Cristóbal belong to another lineage that’s never been formally described or given a scientific name. This also indicates that C. chathamensis is almost certainly now extinct. 

“It seems likely that there were two species on San Cristóbal, not one, and if this is the case, the name of C. chathamensis should be assigned to the extinct species, and the extant taxa should be given a new name,” Galápagos Conservancy, a US-based NGO,  said in a statement.

The findings were recently published in the Nature journal Heredity. 

As the study notes, the Galápagos Islands are known as ”a natural laboratory for studying evolutionary processes.” Found some 1,000 kilometers (~600 miles) off the coast of Ecuador in the Pacific, the volcanic archipelago is perhaps best known as the location that helped to inspire and inform Charles Darwin's theory of evolution by means of natural selection. 

Darwin studied the endemic species of the islands during the second voyage of HMS Beagle in the 1830s. Through collecting samples and making observations, he came to deeply understand the diversification of species and the forces that guide this process. 

These are ideas that we’re still seeing play out today in the Galápagos. San Cristóbal Island was likely once two separate islands divided by high sea levels millions of years ago, each of which harbored its own tortoise species. Once sea levels dropped, the two islands merged, as perhaps did their tortoises.

The island still bears this legacy, with the southwestern section consisting of lushly vegetated highlands and the northeastern section being flatter and more arid. The southwestern region was once covered in giant tortoises, most likely C. chathamensis, but the population was decimated by hunting in the early 20th century. Today, giant tortoises can only be found in considerable numbers in the northeastern region. These two different environments, once divided by a geographic barrier, are likely to be how these two species — C. chathamensis and the newly discovered lineage — came into being.

The thousands of tortoises roaming around San Cristóbal recently received an injection of new life too. Back in March 2021, a group of 36 endangered giant tortoises reared in captivity were transported back to San Cristóbal. It was assumed this species was C. chathamensis, but the new genetic analysis shows they, in fact, belong to this never-before-documented lineage.