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.

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.