Ever been groggy after an extra-long nap? Then imagine waking up after spending 41,000 years frozen in Siberian permafrost.

Nematodes may have experienced this extreme version of a Rip Van Winkle tale, according to paper published last summer in the journal Doklady Biological Sciences.

Led by Anastasia Shatilovich of the Russian Academy of Sciences, the study describes samples collected in 2015 from about a site 11 feet underground, near the Alazeya River in Yakutia. Radiocarbon dating revealed that the sediment was about 41,700 years old, give or take 1,400 years.

That’s why Shatilovich’s team was excited to find that nematodes in the sample were revived once they were fed and thawed at a laboratory. The story of the alleged 41,000-year-old worms went viral last week after the Washington Post mentioned the study in an article.

"It is obvious that this ability suggests that the Pleistocene nematodes have some adaptive mechanisms that may be of scientific and practical importance for the related fields of science, such as cryomedicine, cryobiology, and astrobiology," the team concluded in the study.

If the nematodes really do hail back from an era when Neanderthals and dire wolves roamed our planet, they would be by far the oldest living animals ever discovered.

But many scientists have suggested that the worms might have been a more modern interloper that contaminated the soil. Jacquelyn Gill, a paleoecologist at the University of Maine, outlined this perspective in a comprehensive Twitter thread.

While Gill noted that 32,000-year-old plant seeds have successfully bloomed into flowers, she said it was trickier to confirm that the nematodes originated so far in the past. She pointed out that it was the permafrost sample—not the nematodes—that Shatilovich’s team dated, so it’s possible that the worms might have a different origin.

“Basically, we need to both 1) prove that there are nematodes in the sediment, and 2) rule out the possibility of modern contamination,” Gill said in an email.

“I'd want to sieve the sediment to look for worms or eggs, and then get enough to date the nematodes themselves,” she added. “Then I'd cultivate the sediments under highly controlled conditions where I knew there was no possibility of them having been affected by modern water,” such as rivers, tap water, or mud.

Tatiana Vishnivetskaya, a biochemist at the University of Tennessee who co-authored the 2018 study, said that contamination was unlikely because the samples came from “syncryogenic” permafrost sediments. While some permafrost samples may have thawed out and refrozen over the past 41,000 years, syncryogenic samples are thought to have remained frozen, along with any microorganisms living inside them.

“By definition, it was assumed that nematodes were frozen along with sediment deposition, so we used the sediments to obtain radiocarbon age,” Vishnivetskaya said in an email.

“Our team is very cautious about sterility and aseptic techniques especially when we collect samples for microbiology and molecular biology studies,” she added. “We are pretty sure no contamination with upper soil happens during sampling.”

Nematodes are the most numerically abundant animals on Earth, so it’s not a stretch to think they could find their way into all kinds of samples. At the same time, they are also among the most resilient creatures known to scientists, so it’s possible the nematodes really were frozen into the soil long ago. As Gill said in her Twitter thread, “if there's a betting pool for animals that could survive 40,000 years being frozen, nematodes would definitely be a candidate.”