When you start to look in detail at the evolution of life on Earth, it appears to be one mystery after another.  Why did life appear so quickly after the Earth became hospitable to it? Why did life spend billions of years exclusively in the form of single-celled organisms without a nucleus (bacteria and archaea)? Why are all complex cells (eukaryotes) apparently descended from a single ancestral cell? Why did it take so long for complex multicellular organisms to evolve? (I've taken a crack [perhaps crackpot] shot at that one myself.) Why did evolution favour sexual reproduction, where two parents are required to produce offspring, while clonal reproduction is twice as efficient?  Why just two sexes (among the vast majority of species) and not more? What drove the apparent trend toward greater size and complexity in multicellular organisms?  Why are the life spans of organisms so accurately predicted by a power law based upon their metabolic rate?  Why and how does metabolic rate fall with the size of an organism?  Why did evolution favour warm-bloodedness (endothermy) when it increases an organism's requirement for food by more than an order of magnitude?  Why do organisms age, and why is the rate of ageing and the appearance of degenerative diseases so closely correlated with metabolic rate?  Conversely, why do birds and bats live so long: a pigeon has about the same mass and metabolic rate as a rat, yet lives ten times as long?

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