Elephants today are restricted to the African and Asian continents. But their extinct relatives once roamed far and wide across the planet. When they settled on islands, the evolutionary course of some species changed dramatically.
In a paper released earlier this month, scientists found evidence of how much island life can rapidly change the evolution of these animals.
“Evolution on islands is quite a fascinating field of science as it can be viewed as an experiment of nature or evolution in action,” said Sina Baleka, lead author and paleogeneticist at McMaster University in Canada. She and her co-authors hope that their results can shed light on how species living today are affected by geographic isolation on islands and in other habitats.
Evidence of smaller versions of extinct elephants has been found around the world. Fossils of elephant species on islands off California and Siberia as well as in the Mediterranean and Indonesia show that these giants have become much, much smaller. In some cases, these dwarfs evolved to the size of a large horse.
However, much remains to be learned about how many millennia of evolution it can take for mammals as massive as elephants to shrink to horse-like size. To solve this puzzle, the scientists concentrated on fossils of a pygmy elephant species from Sicily, the large island off the tip of the boot of Italy. The fossils were excavated in the Puntali Cave near the city of Palermo at the end of the 19th century and are believed to be 50,000 to 175,000 years old.
This work wasn’t easy. It’s not that Sicilian fossils cleanly represented every single ancient species of elephant at every stage of its size reduction, said Dr. Baleka. Hence, she and her colleagues have used various techniques to study the rate at which the species’ ancestors became dwarfed, including paleogenetics, paleontology, geochronology, and various dating methods.
“We were able to define the dwarfism rate with much more precise accuracy than any isolated source of evidence,” said Johanna LA Paijmans, co-author and paleogenomics fellow at the University of Cambridge.
At the high end, this rate was less than 352,000 years. But it could have occurred within 1,300 years, which equates to “about 40 generations” of elephants, said Victoria Herridge, co-author and evolutionary biologist at the Natural History Museum in London.
Old DNA from the fossilized petrous bone of the Puntali elephant suggested that it descended from a mainland counterpart, Palaeoloxodon antiquus, some 400,000 years ago. These animals were estimated to weigh 10 tons each and were approximately 12 feet tall.
Sicily has never been this far from the rest of Italy, and there could have been a land bridge in prehistory that linked the two. Whether there or not, the ancient elephants could have swam to the island if they were like today’s elephants.
Descendants of the large elephants who colonized Sicily were nearly 6.5 feet smaller and nearly 8 tons lighter. This change is comparable, the authors wrote, with the fact that a person reaches the size of a rhesus monkey.
“The rate of evolution at which elephants shrink (adapt) is remarkable,” wrote Mirte Bosse, a conservation genomics researcher at Wageningen University in the Netherlands who was not involved in the research. “We know that evolution can be fast, but this is a striking example.”
While there is still a lot of work to be done to figure out how long it took for the elephants to shrink, preserving DNA in the first place is quite an achievement. Dr. Baleka made numerous attempts before successfully extracting DNA from the temporal bone in the elephant’s skull. The difficulty resulted in part from the fact that ancient DNA begins to crumble from the moment of death and survives best in frozen climates, but not in the heat of the Mediterranean.
That this team was able to extract not only viable DNA but enough to sequence the genome offers the exciting possibility that this method could be replicated to study more fossils from similar climates. Bosse said it was “very promising because we are now able to take this journey through time much further than previously thought”.