London ? For decades, scientists have scanned the heavens in search of extraterrestrial life. Perhaps they should have looked closer to home.

Variant life forms — most likely tiny microbes — could still be hanging around “right under or noses — or even in our noses,” Paul Davies, an award-winning Arizona State University physicist, told a group of scientists Tuesday.

“How do we know all life on earth descended from a single origin?” he said, speaking at London’s Royal Society, which serves as Britain’s academy of sciences. “We’ve just scratched the surface of the microbial world.”

The idea that alien micro-organisms could be hiding on Earth has been discussed for a while, according to Jill Tarter, the director of Search for Extraterrestrial Intelligence, a U.S. project that listens for signals from civilizations based around distant stars.

She said several of the scientists involved in the project were interested in pursuing the notion, which Davies laid out in a 2007 Scientific American article, “Are Aliens Among Us?”

So far, there’s no answer. And finding one would be fraught with difficulties, as Davies himself acknowledged.

Unusual organisms abound — including chemical-eating bacteria which dwell deep in the ocean and organisms that thrive in boiling-hot springs — but that doesn’t mean they’re different life forms entirely.

“How weird do they have to be to suggest a second genesis as opposed to just an obscure branch of the family tree?” he said. Davies suggested that the only way to prove an organism wasn’t “life as we know it” was if it were built using exotic elements that no other form of life had.

Such organisms have yet to be found. Bruce Jakosky, an astrobiologist at the University of Colorado at Boulder, said the notion of alien life on Earth was “an interesting theoretical idea” but one that would be impossible to put odds on because “we have no idea what we’re looking for.”

Jakosky added that, if such life forms existed, humans would probably have little to fear, as their different biochemistry would tend to mitigate against infection or disease.