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Time! Life on Earth doesn't have a lot of time here. In a few hundred millions of years, the Sun's increasing luminosity will cleanse the surface of the Earth despite the fact that the sun's subgiant and then giant phases are billions of years away. Life here has spent the better part of 3 billion years in its unicellar stage. Maybe life's limit on how quickly it can luck out on advanced multicellularism is another time limit.
Or maybe our sample size is too small and all of this talk is pointless, and we are just trying to construct a palace from napkins with scribbles on them!
