In mythology, opening Pandora's Box released evil into the world instead, the Pandoravirus opens up a host of questions about the origins of life on Earth. Its discoverer, Jean-Michel Claverie at Aix-Marseille University in France , says: "We believe that those new Pandoraviruses have emerged from an ancestral cellular type that no longer exists."

Each virus is around 1 micrometre long and 0.5 micrometres across, and their respective genomes top out at 1.9 million and 2.5 million bases — making the viruses larger than many bacteria and even some eukaryotic cells. These viruses are more than mere record-breakers — they also hint at unknown parts of the tree of life. Just 7% of their genes match those in existing databases.

How did this odd cellular form turn into a virus? Chantal Abergel says it may have evolved as a survival strategy as modern cells took over. "On Earth it was winners and it was losers, and the losers could have escaped death by going through parasitism and then infect the winner," she says.

The method used to isolate theses viruses reminds me of an undergraduate virology experiment I used to carry out with students to isolate viruses from the environment.  Claverie and his team  scooped out sediment samples from the coast of Chile and a freshwater pond in Australia. They brought back the samples and placed them in a solution filled with antibiotics, to kill any bacteria that might have been along for the ride. Then they exposed the samples to their laboratory amoebas. "If they die, we suspect that there's something in there that killed them." he said. It worked - the infected amoebas spawned lots of Pandoraviruses.

The article appeared in the journal Science.