Some 24,000 years ago, a tiny creature was frozen in time in the Siberian tundra.
But being locked in permafrost in one of the coldest regions on the planet wasn't the end for the bdelloid rotifer — a microscopic animal that is found in freshwater environments.
The creature was still able to reproduce after being frozen for thousands of years, a team of Russian scientists has reported today in the journal Current Biology.
Offspring produced by their ancient parent were also able to survive being frozen and thawed in the lab, revealing the animal's amazing ability to tough it out in the most extreme conditions.
"[The ancient rotifer] did not show any signs of decline," said study co-author Stas Malavin, a biologist from the Soil Cryology Laboratory at the Institute of Physicochemical and Biological Problems in Soil Science in Pushchino, Russia.
"The longevity was a surprise."
A home of extremes
The ability to survive in a state of suspended animation is known as cryptobiosis.
While bdelloid rotifers have been known to bounce back after 6-10 years of freezing, the specimen reported in the new study breaks that record by a long shot.
These tiny, but mighty, animals can also make it through starvation, drying, and low oxygen conditions.
Other animals, plants, and microbes living in Arctic environments can also offer a window into the past, with some being frozen in time for tens of thousands of years.
Researchers have revived roundworms that have been frozen in Siberian permafrost that dates back 30,000 years.
Some plants and mosses can also regenerate after being covered in ice for thousands of years.
"Life on Earth has evolved in a very broad range of environments," said Rick Cavicchioli, who studies Antarctic microorganisms at the University of New South Wales.
"The natural environment one species calls home, another species may never survive in."
Waking up an ancient creature
Mr Malavin and his colleagues collected permafrost core samples from the Alazeya River in north-eastern Siberia.
Using radiocarbon dating, the researchers dated the permafrost to between 23,960 and 24,485 years old.
After thawing the permafrost, the researchers uncovered several rotifers and other microorganisms.
One of the ancient rotifers was able to reproduce itself in a petri dish through a process called parthenogenesis, a form of reproduction that doesn't require fertilisation through sex.
Examination of the ancient rotifer's physical features revealed it belonged to the Adineta genus.
Genetic analyses showed the ancient creature was a new species, most closely related to modern Adineta vagas rotifers collected in Belgium.
This species has been used to study how organisms survive conditions in space, such as doses of x-ray radiation and heavy ions.
Next, the researchers wanted to see whether the rotifers produced by their ancient parent could withstand the freezing and thawing process.
The team selected 144 of these rotifers and froze them at -15 degrees Celsius for one week.
The tiny creatures proved resilient once thawed, surviving the formation of cell-destroying ice crystals during the gradual freezing process.
Finding the secret to survival
Just how bdelloid rotifers are able to survive such extreme conditions is still a mystery, Mr Malavin says.
"There are different mechanisms, but we still don't understand the whole orchestration."
Other animals, such as Antarctic fish, have glycoproteins that produce an antifreeze effect.
These proteins lower the freezing point of fish and allow ice crystals to be filtered through the spleen, said Kerrie Swadling, a marine ecologist at the University of Tasmania's Institute for Marine and Antarctic Studies.
"To date, there has been no antifreeze identified in bdelloid rotifers," said Professor Swadling, who was not involved in the study.
"The jury seems to be out with respect to the mechanism behind their ability to survive freezing."
While more research is needed to figure out how bdelloid rotifers can recover from being frozen in time, looking at their DNA could be a good starting point, said Professor Cavicchioli, who was not involved in the study.
"The ability of the Siberian bdelloid rotifer to survive thousands of years in the permafrost may relate to the mechanisms that keep DNA repair enzymes functional," he said.
"The ability to survive desiccation may provide 'cross-protection' for these small animals when they find themselves exposed to nature's deep freeze."
As bdelloid rotifers are multicellular organisms like humans, animals, and plants, it's worth pinning down the mechanisms that help them survive freezing.
These insights could offer clues that could one day improve cryopreservation techniques — the process of freezing cells, tissues, and organs.
"We know for sure that this is possible," Mr Malavin said.
"This organism is quite a good model for studying this."