Does Earth harbour a 'shadow biosphere' of alien life?

A 'shadow biosphere' of 'weird life' – unrelated to life as we know it – might exist on Earth, giving new insight into how common life is elsewhere in the universe, astrobiologists say.

Finding life that doesn't fit with the types we already know would be a strong indication that life developed more than one time here on Earth, increasing the chances of finding it elsewhere, said Paul Davies, an astrophysicist at Arizona State University in Tempe.

But nobody has ever seriously searched for microorganisms - or any form of life - different from the carbon-based, DNA-centred type of life about which we have long known.

Unique biochemistries

If we do look, Davies said, "It's entirely feasible that we'll find a shadow biosphere," he told reporters at the annual meeting of the American Association for the Advancement of Science (AAAS) in Chicago.

"Our search for life [has been] based on our assumptions of life as we know it. Weird life and normal life could be intermingled, and filtering out the things we understand about life as we know it from the things we don't understand is tricky."

The tools and experiments researchers use to look for new forms of life - such as those on missions to Mars - would not detect biochemistries different from our own, making it easy for scientists to miss alien life, even if was under their noses.

"When you don't know what you're looking for or what it'll look like, you have to come up with a whole scientific method for how to go about [looking for] it," added Steven Benner, a Fellow at the Foundation for Applied Molecular Evolution and The Westheimer Institute for Science and Technology in Gainesville, Florida.

Scientists are looking in places where life isn't expected - for example, in areas of extreme heat, cold, salt, radiation, dryness, or contaminated streams and rivers. Davies is particularly interested in places that are heavily contaminated with arsenic, which, he suggests, might support forms of life that use arsenic the way life as we know it uses phosphorus.

Piggybacking on a meteorite

If we do discover exotic life unrelated to ours, it might not have developed here, Davies said. Instead, it might have originated elsewhere, then hitchhiked to Earth by piggybacking on a meteorite.

But it doesn't matter where it originated, Davies argued, because it's still an indication that life has cropped up from scratch in more than one place.

"If it's happened more than once in the Solar System, then the Universe will be teeming with life," said Davies.

Scientists assign a probability to the possibility of life elsewhere in the universe on a scale of 0 to 1, with 0 being no chance to 1 being a 100 per cent likelihood. "We'd like to think it's 1, but who knows," said Davies.