Researchers have resurrected the genes of a tiny population of ancient woolly mammoths that mysteriously died out some 4,000 years ago on Wrangel Island, a remote 125-kilometer-wide (78-mile) Arctic refuge located off the coast of Siberia. It is likely that this group was among the last in the world.
During the Ice Age, Wrangel Island was connected to Beringia and modern-day Alaska and Canada via the Bering Land Bridge. Mammuthus primigenius were among the most abundant species adapted for cold weather, but as temperatures rose in the millennia following the Pleistocene, the bridge was eventually engulfed by rising seas, forever isolating the Wrangel Island mammoths.
Dramatic environmental changes associated with a warming planet saw the loss of dry steppe tundra, also known as Mammoth steppe, as the world entered the Holocene. A number of other cold-adapted species were also lost, including cave bears and hyenas as well as woolly rhinoceros. Meanwhile, mammoths went extinct in two waves, which saw mainland populations first die out, followed by those that inhabited St. Paul Island (5,600 years ago).
The Wrangel Island mammoths held on until a “fairly abrupt” extinction event wiped them out 4,600 years ago. To determine what that catastrophic event might have been, a collaborative team of researchers from several US-based universities compared the DNA of Wrangel Island mammoths to three Asian elephants and two other ancient mammoth populations. Specific genetic mutations unique to Wrangel Island mammoths were identified, each thought to play a role in important functions, like neurological development, male fertility, insulin signaling, and sense of smell. These altered genes were synthesized and placed into cells in Petri dishes for researchers to test whether the genes would function normally.
In short, they did not.
"Our results suggest that once populations get small they harbor deleterious mutations, once those mutations are there they will affect the ability of the population to thrive. Even if the population were to recover large numbers the mutations are still there, dragging the population down," study author Vincent Lynch, from the University of Buffalo, told IFLScience.
It is believed that the environmental conditions would have gradually killed off the Wrangel Island mammoths, dwindling their already isolated population. Individuals that survived would have bred with others over generations, eventually reducing the genetic pool and eliminating the ability for a species to “breed out” harmful genetic mutations, write the researchers in Genome Biology and Evolution.
"Evolution is only really effective in large populations," added Lynch, adding that as populations "get smaller, the ability of selection to remove bad mutations – and select for the good ones – is decreased because the strength of randomness is greater than selection."
The final nail on the Wrangel Island mammoth coffin remains mysterious, but the authors note that it is clear that it is likely no coincidence that the population declined shortly after isolation. The findings build on an understanding of the final days of the mammoths, highlighting that the last of their kind likely suffered from a variety of genetic effects that hindered their development, reproduction, and ability to smell. Lynch notes that there are several limitations associated with the study.
"Alas, we have only one Wrangel Island genome. It could be some of these mutations are unique to that individual rather than the population. More genomes would help us know this. Also, we don't have mammoth cells to test these genes in, so maybe the mammoth cells had ways to minimize the deleterious mutations we found," he said.
Even so, Lynch tells IFLScience that this is a “cautionary tale” for living species threatened with extinction who may face similar genetic challenges in the generations to come.