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January 10, 2017

Thousands of years ago, the Bahamas was home to a large tortoise that died out shortly after human occupation in the islands began.  Thanks to intrepid explorers probing the depths of a blue hole (water-filled sinkhole) on the island of Abaco, scientists have been able to study the astonishing remains of this extinct species.  The quality of preservation was high enough that the bones had potential to still have intact DNA.  A new study recently published in Proceedings of the Royal Society B titled "Tropical ancient DNA reveals relationships of the extinct Bahamian giant tortoise Chelonoidis alburyorum" highlights the team's findings.

Normally, fossils found in the tropics have little to no DNA left in them, as the thriving bacterial communities break down any strands of genetic information.  However, deep within this blue hole, fossils lay below a layer of hydrogen sulfide, which prevented bacteria from developing and allowed for the DNA to remain intact. The team, led by biologists from the Senckenberg Museum in Dresden and the University of Potsdam in Germany, was able to recover nearly the entire genome and compare this to their modern relatives in South America.

"The fact that there is any genetic information left at all is remarkable, but nearly the whole genome is unprecedented for this kind of fossil material," said Alex Hastings, paleontologist at the Virginia Museum of Natural History and co-author on the study.  “Preservation like this is exceptionally rare, but we hope this paves the way for more ancient DNA to be extracted from the other animals from the site”.

Today, South America is home to 3 species of living tortoises, as well as the giant tortoises of the Galápagos, which lie over 500 miles off the South American coast in the Pacific Ocean.  The fossils were recovered from just off the coast of Florida, with the expectation that they would have a closer relationship with the South American species.

"The genetic information showed a surprisingly close relationship with the Galápagos Tortoise and the Chaco Tortoise of southern South America," said Hastings.  "This means the two large island tortoises share an ancestral history much closer with the more distant South American tortoise than with the much closer tortoises of northern South America, despite two of them living in completely different ocean systems.  The study highlights the ability of these impressive reptiles to colonize island systems and shows that a previously unknown genetic stock gave way to both Caribbean and Pacific tortoises. Only recently, with human invasion into the Bahamas, did this unique tortoise of the Bahamas die out.  Studies of the evolution of rare island tortoises can aid in conservation efforts, as well as provide a greater understanding of the keys to success and survival in island environments."