Who knows what future scientists will find in the time capsules dropped by these caribou (Image: Design Pics Inc/Rex)

One day, 700 years ago, a caribou defecated on ice in what would become Canada. Today, scientists have opened this long-frozen time capsule and found an entire plant virus inside it.

Eric Delwart of the University of California, San Francisco, and his colleagues discovered the virus in samples of frozen caribou faeces collected from ice patches in the Selwyn mountains of the Yukon and Northern Territories. Over hundreds of years, caribou used the patches of ice to provide respite from irritating ticks and insects, so their copious faeces became preserved as successive layers of ice accumulated.

From faeces taken from a 700-year-old ice layer, about a metre down, they successfully extracted a complete virus. The virus resembles modern-day geminiviruses, which infect plants. Delwart’s team made an exact copy of the virus, and discovered that it could infect a type of tobacco plant. “We saw evidence of replication in the leaves,” he says.


The big sleep

“The find confirms that virus particles are very good ‘time capsules’ that preserve their core genomic material, making it likely that many prehistoric viruses are still infectious to plants, animals or humans,” says Jean-Michel Claverie of the Aix-Marseille University School of Medicine in France. “This again calls for some caution before starting to drill and mine Arctic regions at industrial scales.”

The discovery is the latest evidence suggesting that as ice, snow and permafrost melt through global warming, dormant but potentially infectious microbes could be revived. Such revivals could create unforeseen consequences both for people and for wildlife.

“There’s a theoretical risk of this, and we know that the nucleic acid of the virus was in great shape in our sample,” says Delwart. “But old viruses could only re-emerge if they have significant advantages over the countless perfect viruses we have at present.”

Blast from the past

The record for the oldest “resurrected” virus is currently held by the gigantic pithovirus, found last year in 30,000-year-old Siberian permafrost, which was found to infect amoebas.

The more recent plant virus was deposited on the icy ground back when the Black Death was raging through Europe. “Even if 700 years may not seem much compared with 30,000 years, it corresponds to a period when many diseases assumed to be ‘eradicated’ today were still in full bloom,” says Claverie, who led the team that discovered the pithovirus.

Delwart says that, among the old faeces, only fragments of caribou DNA were left. He speculates that the virus was hardier because it was protected by a tough coat called a capsid. “These are rock-hard, and make viruses extremely tough,” he says. “So the combination of the tough capsid and the cold conditions preserved the virus.”

Journal reference: PNAS, DOI: 10.1073/pnas.1410429111