"It requires that life elsewhere should be modeled along similar lines to life here, which is that it should be cellular, it should be carbon-based. It should be in water. If those things are not true, then there's no reason why that numbers game would apply anywhere else. But if those things are true, then yes, I think the fact that photosynthesis only arose once, that Eucaryotes only arose once, that what Nick [Nicholas J.] Butterfield calls organ grade multicellularity, which is to say quite serious differentiation with scores of different cell types and specializations. We don't see that in fungi. We don't see that in algae. ...[Y]ou see two or three different types of cell. So that's rare. It's in plants and it's in animals. It begins to look less likely. I think it's reasonable to say it's less likely, but I wouldn't like to rely too much or put too much weight on it."