Mammoths may have disappeared much earlier than DNA suggests
Some ancient DNA may lead paleontologists astray in their attempts to date when woolly mammoths and woolly rhinos became extinct.
In 2021, an analysis of plant and animal DNA from Arctic sediment samples, spanning roughly the past 50,000 years, suggested that mammoths survived in north-central Siberia about 3,900 years ago (SN: 1/11/22). This is much later than when the youngest mammoth fossil found in mainland Eurasia suggests the animals became extinct; it dates from around 10,700 years ago. Only on Wrangel Island off Siberia and the Pribilof Islands in the Bering Sea are mammoths known to have survived later.
The discovery was one of many in recent years to use ancient DNA found in sediments and other environmental materials to suggest new information about animal extinctions. Genetic evidence from woolly rhinos in Eurasia and horses in Alaska has also indicated that these animals stayed thousands of years longer in some areas than previously thought.
But thousands of years is also how long large animal bones can linger on the ground in the frigid north, slowly weathering and shedding tiny bits of DNA, write two researchers on 30 november in Nature.
This means that the youngest ancient DNA in sediment samples may have come from such bones, not from living mammoths, woolly rhinos and other megafauna. Studies that rely on this genetic evidence could skew estimates of when these animals died out by thousands of years to the present, say paleontologists Joshua Miller of the University of Cincinnati and Carl Simpson of the University of Colorado at Boulder.
When and why mammoths and some other Ice Age creatures died is a lingering mystery. Dating when these animals went missing could help reveal what led them to their death – humans, a warming climatea combination of the two or something else entirely (SN: 11/13/18; SN: 08/13/20).
But getting a good idea of when a species disappeared from its range or the planet is not straightforward. For long-extinct animals, fossils can help, but it would be a huge coincidence if the youngest fossil ever found of an extinct species was also the last living individual.
Where fossils give way, DNA has begun to take over. Over the past two decades, environmental DNA, or eDNA, has become a go-to technique for discovering what organisms livewhere once lived, in a certain place (SN: 01/18/22).
Paleontologists usually focus on a variant of eDNA that attaches to minerals and other materials and buries itself over time. This “ancient sedimentary DNA”, or sedaDNA, is what evolutionary geneticist Yucheng Wang of the University of Cambridge and his colleagues analyzed in the 2021 mammoth study.
“DNA can come from a living animal, but it can also come from poo, from bones,” says Miller. “In our case, we focus on the bones.”
In warmer climates, a bone lasts long enough to propagate DNA for at most a few decades, which usually isn’t important for getting a general extinction date, he says. “But in these cold environments, you’d expect a much, much larger spread, even on a millennial scale.”
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Miller and Simpson base their estimates of how long dead mammoth bones can release DNA into the environment on radiocarbon dating of bones of large animals found on the Earth’s surface in cold places today. . 2,000-year-old caribou antlers have been found on the islands of Svalbard in Norway and Ellesmere Island in Canada, and 5,000-year-old remains of elephant seals near the coast of Antarctic.
Wang and his colleagues disagree that the mammoth eDNA in their sample may have come in part from cold, old bones that are weathering. In a response in the same issue of Nature, they point out, for example, that the youngest mammoth eDNA they found shows low genetic diversityprecisely what you would expect if the DNA actually came from a declining population at the end of the mammoth’s time on Earth, instead of a thriving population earlier.
“I think Miller and Simpson make a valid point for further testing and analysis,” says evolutionary geneticist Hendrik Poinar, a pioneer in eDNA research who was not involved in the mammoth study of 2021. “But I don’t think their analysis is enough to combat the multiple lines of evidence that suggest a late persisting megafauna,” says Poinar, of McMaster University in Hamilton, Canada. He points out, for example, that in Wang’s study, DNA evidence traces plants back to the time. This suggests that the woolly mammoths of north-central Siberia could persist thanks to the steppe-tundra, which was their natural habitat, clinging to it.
For Miller, the time lapse between the youngest known mammoth skeletal remains from north-central Siberia and the youngest mammoth eDNA reported by Wang and his colleagues is just too suspicious.
“This paper gives us scientific permission to really expect bones to be much younger than what we have seen. There should be dozens or hundreds of [such relatively recent] dead mammoths somewhere,” he says. “People were looking for them…. And you can’t find anything younger.