I don’t think anybody expects to go to Mars and find cats and dogs walking around, but there’s a good chance that there’s microbial life in the deep subsurface. 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. 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 get know what it is, recognize for 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 that life might have emerged not just once, but many times throughout Earth’s history. 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.