Long odds on space viruses seeding life

01 June 2009 by Anil Ananthaswamy

LIFE on Earth is unlikely to have come from space, says a new study on viruses. If life is ever found on another planet, however, the findings could help us judge whether it arrived from space or not.

Panspermia is the idea that life was seeded by extraterrestrial microbes in the form of hardy bacterial spores that hitched a ride on a space rock and landed on Earth. Jaana Bamford of the University of Jyväskylä in Finland and her colleagues say the key to testing this theory lies with viruses, which are thought to be tied to key steps in the evolution of complex life on Earth.

To find the likelihood of viruses stowing away in spores, the team induced a colony of bacteria hosting the Bam35 virus to form spores. Of the 83 spores that were revived to create new bacterial colonies, only 23 contained the virus (International Journal of Astrobiology, DOI: 10.1017/s1473550409004479).

This result suggests that, whatever the odds of a spore-laden meteoroid going from one planet to another, the odds of viruses hitchhiking within the bacterial spores are much longer, says team member Matti Jalasvuori.

Even if the odd virus did come along from space, it does not explain the huge diversity of viruses on Earth. Unlike cellular organisms, which are all thought to have had one ancestor, viruses are descended from more than one ancestral strain, so it is unlikely that enough virus-laden meteorites landed to seed the "virosphere" we know today. It also makes it unlikely that life on Earth itself came from space. "If life on Earth was because of panspermia, we would probably see a less diverse virosphere," says Jalasvuori.

Even if the odd virus did come from space, it does not explain the huge diversity on Earth
If we find life elsewhere, he adds, the absence of a diverse virosphere could suggest that life on Mars, say, might have originated in panspermia.

Tom Ray, an ecologist at the University of Oklahoma in Norman says the arguments are "plausible, not definitive". He believes that viral complexity on Earth could have evolved however viruses originated. "If viruses quickly emerge after the appearance - either by local origin or panspermia - of cellular life, it would probably be difficult to distinguish between the two cases billions of years later."

http://www.newscientist.com/article/mg20227104.100-long-odds-on-viruses-raining-from-space.html