The discovery of a bacteria that can exist in extreme environments adds weight to a new theory championed by the British cosmologist Professor Paul Davies.

He has argued that alien life might exist under our very noses, or indeed inside them, just in a form that we do not recognise.

He has talked about "weird" or "shadow" life that exists in tandem with our own but in hostile environments such as at the bottom of lakes, or in deep sea vents and deserts and caves.

At the heart of his theory, which he explained at the American Association for the Advancement of Science conference in 2007, is that life on Earth may have come and gone many times during the planet's existence.