Modern elephant species genetically distinct, though ancient ancestors interbred
An illustration of elephant is shown in a handout. (THE CANADIAN PRESS/HO-Carl Buell)
Sheryl Ubelacker, The Canadian Press
Published Monday, February 26, 2018 6:11PM EST
Last Updated Monday, February 26, 2018 6:13PM EST
TORONTO -- For many people, one elephant may seem much like another -- majestic trunked and tusked creatures that roam the African savannah and forests or often serve as domesticated beasts of burden in India and parts of southeast Asia.
But an international study shows the three modern species of elephants have distinct genetic profiles, despite a complex evolutionary history spanning millions of years that includes interbreeding between some groups of their ancient forebears, including mammoths and mastodons.
The team of scientists meticulously sequenced 14 genomes from both living and extinct elephant species from Asia and Africa, two American mastodons, a 120,000-year-old straight-tusked elephant and a Columbian mammoth.
"We provided extracts in genomes of several woolly mammoths from across Siberia and North America and then the Columbian mammoth, which were the larger (animals) living south of the ice sheets, in southern Canada through the United States down into Mexico," said evolutionary geneticist Hendrik Poinar, director of McMaster's Ancient DNA Centre and a senior author on the research.
"And then we also provided mastodon genomes, which are relatives of elephants but are about 20 million years outside of that elephant family."
A comparison of the different genomes -- the genome is an organism's complete set of DNA, including its genes -- revealed a complexity the scientists had been previously unaware of, said Poinar.
Analysis of the ancient straight-tusked elephant, for example, showed it was a hybrid with portions of its genetic makeup stemming from an ancient African elephant, the woolly mammoth and present-day forest elephants.
"This is one of the oldest high-quality genomes that currently exists for any species," said Michael Hofreiter at the University of Potsdam in Germany, a co-senior author who led the work on the straight-tusked elephant, also known as .
Researchers also found more evidence of interbreeding among the Columbian and woolly mammoths, first reported by Poinar and his team in 2011. Despite their vastly different habitats and sizes, researchers believe the woolly mammoths encountered their Columbian counterparts at the boundary of glacial and more temperate regions of North America.
"As glaciers grew and contracted across the northern expanse of our lands here in Canada, the woollies who were living happy as pigs in feces up on the open tundra were probably pushed south into what we think of as the Midwest United States and the Great Lakes regions," Poinar speculated. "And there they meet up with Columbians.
"It looks like the hotbed for mammoth dating was right around the Great Lakes, right around here," he said. "So Toronto was date night 12,000 years ago."
Mammoths lived during the Pleistocene epoch several million years ago in Africa, Europe, Asia and North America, going extinct about 11,000 years ago. Mastodons appeared on Earth about 20 million years ago and roamed North America. They predate mammoths by millions of years, but died out about the same time as their larger cousins, both likely due to a combination of climate change and human hunting.
Surprisingly, the scientists found no genetic evidence of interbreeding between two of the world's three existing species, the savannah and smaller forest elephants in Africa, suggesting they have lived in relative isolation for the last 500,000 years, despite being in neighbouring habitats.
"There's been a simmering debate in the conservation communities about whether African savannah and forest elephants are two different species," co-senior author David Reich of the Broad Institute and Harvard Medical School in Boston, said in a release.
"Our data show that these two species have been isolated for long periods of time -- making each worthy of independent conservation status."
In the early 20th century, there may have been as many as three million to five million elephants in Africa, both savannah and forest species. Today, the World Wildlife Fund estimates their numbers have dropped to about 415,000 overall, largely due to the ivory trade and habitat loss, and both species are considered vulnerable.
Asian elephant numbers have dropped dramatically over the last three generations and continue to decline. With only 40,000 to 50,000 left in the wild, the WWF classifies these elephants as endangered.
"There is a really important conservation point right now, which is to say that these are distinct entities, so they should be conserved as distinct entities," said Poinar, arguing against deliberate interbreeding as a way to boost overall elephant numbers.
"So I think it's a cry to arms," he said of the genomic findings, published Monday in the journal Proceedings of the National Academy of Science.
"It just shows that biology was really messy in the past and that's probably what led to the adaptability of these giant and lovable creatures. But they're now at a critical point in their history."