或许地球上就存在外星生命

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Paul Davies: I don't think anybody expects to go to Mars and find cats and dogs walking around, but I think there's a good chance that there's microbial life in the deep subsurface. And so, the same thing here on Earth.

You are listening to astrobiologist by Paul Davies of Arizona State University. Davies studies the possibilities of life beyond Earth. He said that scientists are now studying below the seafloor near deep-sea volcanic vents, and at Earth's icy poles, for microbes that might be analogs of extraterrestrial life.

Paul Davies: Most microbes haven't been characterized, let alone their genes sequenced. So it's entirely possible that in that microbial realm there's what we like to call a ‘shadow biosphere.' It doesn't mean that it's sort of shadowy in the mystical sense, it just means that we don't yet know what it is. But we can recognise what it is. We could be dealing just with ordinary-looking microbes, but with their innards completely different.

In other words, microbes with bizarre biochemistries live in a kind of ‘shadow' of the ordinary life we're used to. What's more, said Davies, these microbes suggest life might have emerged not just once, but many times throughout Earth's history.

Paul Davies: Life may have started 58 times on Earth, and we could have 27 ‘shadow biospheres' descended from those different geneses, but only one of these achieved multicellular life. And we're the product of that.

Asked by editor Paul Davies. Earthsky is a clear voice for science.

[背景知识]

至今，科学家仍未在地球上发现与已知生物不同的生命体，然而这并不代表地球上没有外星生命：虽然现代科学技术已经很先进，但还有很多生物我们无法观察到。

但这是一个激动人心但也很“异想天开”的设想：其他类型的生命体至今仍然存在，它们构成了一个“影子”生物圈(shadow biosphere)——这是美国科罗拉多大学的卡罗尔·克莱兰（Carol Cleland ）和谢利·科普利(Shelley Copley）发明的新词。

乍看起来，这个想法似乎很是荒谬：如果外星生物就在我们眼皮底下（甚至就在我们的鼻子内）大量繁殖，为什么科学家一直没能发现它们？但我们不能轻易否定这个设想。地球上，微生物的数量超乎想象，仅通过显微镜观察，很难区分它们。微生物学家必须分析某个微生物的基因序列，才能确定它在进化树上的位置。到目前为止，有明确分类的微生物，只占已知微生物很小一部分。