For 24,000 years, tiny creatures lay dormant, entombed in the frozen depths of Siberia’s permafrost. These microscopic creatures, known as bdelloid rotifer, were in a suspended state between life and death — until Russian scientists thawed them.

As the microscopic animals “zombie” activated from their icy slumber, they not only survived but also began cloning themselves. Previously, scientists thought that such creatures couldn’t survive being frozen for more than 10 years. This prediction was blown out of the water by this evidence that was first reported in 2021. It suggests that the rotifers can last thousands of years frozen, if not indefinitely.

The Ultimate Ice Age Survivors

Rotifers are multicellular animals that belong to their own distinct ancient phylum (meaning they are morphologically and genetically distinct from other animals at a very fundamental level). They’re split into three major groupings, which are further split into more than 2,200 species. They are found across the world, mostly in freshwater environments and in moist soil.

Rotifers are sometimes called “wheeled animals” due to the beating cilia that surround their mouths, which resembles the rotation of a wheel (hence their name). The edge of this “wheel”, called a corona, is used for feeding on organic waste, algae, and protozoans.

The creatures are microscopic and have soft bodies, which is why they are not favored for fossilization. But that doesn’t mean they’re fragile. Actually, far from it.

Like the more famous tardigrades (water bears), rotifers are extremophiles, capable of surviving low oxygen environments, severe dehydration, and freezing temperatures. Modern rotifers have been known to endure freezing temperatures as low as minus 4 degrees Fahrenheit (minus 20 degrees Celsius) and revive after a decade.

But the rotifers recently recovered from Siberian permafrost are in a league of their own. Frozen during the Pleistocene epoch, when woolly mammoths roamed the Earth, these ancient rotifers were thawed in a lab—and promptly showed signs of life, as reported in the journal Current Biology.

“The takeaway is that a multicellular organism can be frozen and stored as such for thousands of years and then return back to life – a dream of many fiction writers,” Stas Malavin, of Russia’s Institute of Physicochemical and Biological Problems in Soil Science, told the Press Association.

Clones of the Past

Once revived, the rotifers began cloning themselves through parthenogenesis, a form of asexual reproduction. This created a population of genetically identical offspring, making it impossible to distinguish the ancient rotifers from their modern descendants. Since rotifers typically live only about two weeks, the researchers studied the clones to understand the survival mechanisms of their Ice Age ancestors.

To ensure the rotifers were truly ancient and not modern contaminants, the researchers carefully analyzed permafrost samples drilled from 11.5 feet (3.5 meters) below the surface near Siberia’s Alazeya River. Radiocarbon dating confirmed the soil was around 24,000 years old. 

The findings suggest that rotifers have evolved remarkable tools for enduring extreme conditions. They can repair DNA damage and protect their cells from harmful molecules called reactive oxygen species. Complex animals like mammoths or humans are killed when frozen due to the formation of ice crystals that penetrate cells, destroying them. But rotifers are somehow able to control the formation of ice crystals on the surface of their cells. These adaptations make them ideal subjects for studying cryobiology—the science of freezing and reviving living organisms.

“Organisms isolated alive from permafrost potentially represent the best models for cryobiology research,” Malavin said. Understanding how rotifers survive could inform efforts to preserve human cells, tissues, and organs for medical purposes. However, Malavin cautions that humans are unlikely to replicate the rotifers’ deep-freeze feats anytime soon.

“The more complex the organism, the trickier it is to preserve it alive frozen,” he said. “For mammals, it’s not currently possible.”

A Frozen Time Capsule

These findings are among the most definite bits of evidence today that multicellular organisms could withstand thousands of years of cryptobiosis, which the Russian scientists describe as a “state of almost complete arrested metabolism.” The frozen Siberian rotifers aren’t the record holders for the oldest multicellular creatures to be brought back from the dead, though. Gizmodo reports that nematode worms dated to at least 40,000 years ago were resurrected, also by Russian scientists.

This marks “the longest reported case of rotifer survival in a frozen state,” according to the Russian biologists at Soil Cryobiology Lab who regularly drill the Siberian permafrost in search for ancient frozen life forms and fossils.

A better understanding of this biochemical mechanism may help scientists develop new cryopreservation methods for storing cells, tissues, and perhaps even entire organs.