For the past 50 years, scientists have scoured¹ the skies for radio signals from beyond our planet, hoping for some sign of extraterrestrial life. But one physicist² says there's no reason alien life couldn't already be lurking³ among us - or maybe even in us.

在过去50年中，科学家们寻遍苍穹，希望发现来自外太空的无线信号，以期找到外星生命的一些迹象。但一位物理学家日前说，不能排除这样一种可能性，即外星生命已“潜伏”在我们中间，甚至可能存在于我们体内。

Examples of 'alien art' seen at an exhibition at the Science Museum in London in 2005. The law of probabilities backs theories that we are not alone in the Universe, scientists at a conference in the British capital have said.

Paul Davies, an award-winning Arizona State University physicist known for his popular science writing said Tuesday that life may have developed on Earth not once but several times.

Davies said the variant⁴不同的，多样的 life forms - most likely tiny microbes - could still be hanging around "right under our noses - or even in our noses."

"How do we know all life on earth descended⁵ from起源于 a single origin?" he told a conference at London's prestigious有声望的 Royal Society, which serves as Britain's academy of sciences. "We've just scratched the surface只做了肤浅的研究 of the microbial world微生物界."

The idea that alien micro-organisms could be hiding out躲藏 here on earth has been discussed for a while, according to Jill Tarter⁷, the director of the US SETI project, which 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概念，见解, which Davies earlier laid out in a 2007 article published in Scientific American in which he asked: "Are aliens among us?"

So far, there's no answer. And ever finding one would be fraught⁸ with充满，带有 difficulties, as Davies himself acknowledged.

Unusual organisms abound⁹ - including chemical-eating bacteria which hide out deep in the ocean and organisms that thrive in boiling-hot springs - but that doesn't mean they're different life forms entirely¹⁰.

"How weird¹¹ do they have to be suggest a second genesis as opposed to just an obscure模糊的，昏暗的 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外来的 elements which no other form of life had.

Such organisms have yet to be found. Davies also noted¹² that less than 1 percent of all the world's bacteria细菌 had been comprehensively包括地 studied - leaving plenty of time to find unusual organisms.