In a quest to recover ancient viruses, Eric Delwart of the Blood Systems Research Institute in San Francisco and colleagues analyzed layers of Caribou feces in a 700-year old ice layer in Canada's Selwyn Mountains.

The scientists successfully identified two viral sequences that could shed light on how viruses have evolved over time.

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As yet, scientists have limited knowledge about viral evolution. Understanding the structure of ancient viruses could help fill the gap.

However, reconstructing ancient viruses for experimental purposes is a major challenge among scientists because this kind of viruses change quickly, making it difficult to relate new sequences to one another. The nucleic acid content of ancient viruses can also degrade rapidly.

To solve these problems, scientists used modern techniques in finding traces of well-preserved ancient viruses in a 4000-year-old patch of ice. They found the two well-preserved viral sequences while probing nucleic acids in frozen caribou fecal pellets.

The scientists then characterized the two viruses in replicated experiments performed in two separated laboratories.

One of the viruses was part of an RNA virus genome, which was identified as belonging to the insect-infecting genus Cripavirus.

Caribou may have consumed insects infected with the virus or the insects themselves may have deposited the virus on the feces and the surrounding snow. Having the possibilities, scientists gave the name "Ancient Northwest Territories cipavirus" (aNCV) to the partial RNA viral genome.

The other viral sequence was a complete genome of DNA virus, which was named "Ancient Caribou Feces Associated Virus" (aCFV). Experimental analysis identified aCFV as a distant relative of a group of plant-infecting viruses called geminiviruses and a fungi-infecting gemycircularviruses.

Delwart and his team also introduced the complete DNA virus to the species of tobacco plant vulnerable to viral infections for them to learn more about it. The virus infected the plant's leaves, confirming that viruses can still become active even after being frozen for centuries.

"As climate change accelerates the melting of arctic ice, it is possible that ancient viral particles and the associated nucleic acids could be released into the environment," Delwart warned.

The findings, which could give additional knowledge to prepare for possible future reemergence of ancient viruses, were detailed in the journal Proceedings of the National Academy of Sciences. 

Tags700-year-old viruses, Caribou, Caribou feces, Selwyn Moutains, Viral Evolution