A shadow biosphere | COSMOS magazine

A shadow biosphere | COSMOS magazine: "IF LIFE STARTED MORE than once on Earth, we could be virtually certain that the universe is teeming with it. Unless there is something very peculiar about our planet, it is inconceivable that life would have begun twice on one Earth-like planet but hardly ever on all the rest.

Until recently, biologists assumed that all life on Earth is of the same origin, with every organism that ever lived descended from a common genesis. But how do we know that is so? Could there be two or more different sorts of life on this planet? Has anybody actually looked?

When I began mulling these ideas a few years ago, I was amazed nobody had really thought much about evidence for multiple genesis events. Astrobiologists have been busy figuring out how to detect a different form of life on Mars, but it hadn't occurred to many people to hunt for alternative forms of life on our own doorstep."