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

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 variant 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 descended from a single origin?" he told a conference at London's prestigious 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 out here on earth has been discussed for a while, according to Jill Tarter, 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, 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 fraught with difficulties, as Davies himself acknowledged.

Unusual organisms abound - 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 entirely.

"How weird do they have to be 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 which no other form of life had.

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

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。

知名科学作家、亚利桑那州立大学获奖物理学家保罗•戴维斯于本周二称，地球上的生命可能进化了不止一次，而是好几次。

戴维斯说，生命变体形式——特别是极小的微生物——可能就出没于“我们的鼻子底下甚至鼻子里”。

他在伦敦皇家学会的一次研讨会上说：“我们怎么知道地球上的所有生命只有一个起源？我们对于微生物世界的了解只是一点皮毛而已。”皇家学会是英国的科学学会，享有很高声望。

据美国“寻找外太空星球智慧生命项目”的主管吉尔•塔特说，有关外星微生物体可能隐居在地球上的观点已经讨论了一段时间。“寻找外太空星球智慧生命”项目主要监听来自遥远星球的声音讯息，以判断是否有外星文明存在。

她说，参与该项目的几位科学家对探讨这一观点很感兴趣。戴维斯在2007年发表于《科学美国人》的一篇文章中提出：“外星生命会不会就在我们身边？”

到目前为止，这一问题还没有答案。戴维斯自己也承认，找到这样一个生命十分困难。

地球上存在很多奇异的生物体，其中包括藏在深海中以化学品为食的细菌和生存在滚烫的温泉中的生物体。但这并不意味着它们是完全不同的生命形式。

戴维斯说：“它们到底要多怪异，才能说明它们是二次进化，而非生物系统中一个鲜为人知的分支呢？”戴维斯指出，证明一种生物体不是“我们所知道的生命”的唯一办法就是看它是否是由其他生命形式所没有的外来元素组成的。

这样的生物尚未发现。戴维斯还指出，目前世界上所有的细菌中，仅有不到1%得到了深入研究，因此寻找这些奇异生物体还需要大量时间。