Elephants today are confined to the African and Asian continents. But their extinct relatives once roamed far and wide across the planet. When they settled onto islands, some species’ evolutionary course changed direction in a dramatic fashion.
In a paper published earlier this month, scientists found clues to just how much island living can rapidly alter the evolution of these animals.
“Evolution on islands is a quite intriguing field of science, since it can be seen as an experiment of nature or evolution in action,” said Sina Baleka, the paper’s lead author and a paleogeneticist at McMaster University in Canada. She and her co-authors hope their findings can offer insights into 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 worldwide. Fossils of elephant species on islands off California and Siberia as well as in the Mediterranean and Indonesia show that these giants became much, much smaller. In some cases, these dwarves evolved down to the size of a large horse.
But much remains to be learned about how many millenniums of evolution it may take for mammals as massive as elephants to shrink to a horse-like size. To make sense of this mystery, the scientists focused on fossils of a species of dwarf elephant from Sicily, the large island off the toe of Italy’s boot. The fossils were excavated in the late 19th century from the Puntali Cave, not far from the city of Palermo, and are believed to be 50,000 to 175,000 years old.
This work wasn’t easy. It’s not as though Sicilian fossils neatly represented every single ancient elephant species at every phase of its size reduction, Dr. Baleka said. So she and her colleagues used a variety of techniques to study the rate at which the species’ ancestors became dwarves, including paleogenetics, paleontology, geochronology and different dating methods.
“We were able to define the dwarfing rate with much more accuracy than any source of evidence in isolation,” said Johanna L.A. Paijmans, a co-author and paleogenomics fellow at the University of Cambridge.
At the higher end, that rate was less than 352,000 years. But it might have occurred within 1,300 years, which equates to “about 40 generations,” of elephants, said Victoria Herridge, a co-author and evolutionary biologist at the Natural History Museum, London.
Ancient DNA from the Puntali elephant’s fossilized petrous bone indicated it descended from a mainland counterpart, Palaeoloxodon antiquus, around 400,000 years ago. Those beasts weighed an estimated 10 tons each and were about 12 feet tall.
Sicily has never been terribly distant from the rest of Italy, and there could have been a land bridge connecting the two in prehistory. Whether it was there or not, the ancient elephants could have swum to the island if they were anything like today’s elephants.
Descendants of the large elephants that colonized Sicily were almost 6.5 feet smaller, and almost 8 tons lighter. That change is comparable, the authors wrote, to a human becoming the size of a Rhesus monkey.
“The evolutionary rate they estimated the elephants to 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 rapid, but this is a striking example.”
While much work remains to pin down how long it took for the elephants to shrink, obtaining the DNA at all is a substantial accomplishment. Dr. Baleka made numerous attempts before she successfully pulled DNA from the petrous bone within the elephant’s skull. The difficulty resulted in part because ancient DNA begins to degrade from the moment of death, and survives best in frozen climates, but not in the heat of the Mediterranean.
That this team was not only able to extract viable DNA, but enough to sequence the genome offers the exciting possibility that this method might be replicated to study more fossils from similar climates. Dr. Bosse noted it was “very promising because we are now capable of making this travel through time much further back than previously anticipated.”