In Why Evolution is True Jerry Coyne asked five good questions about some facts of nature in the section called Palimpsests in Embryos (pages 73 thru 80).
Why do different vertebrates, which wind up looking very different from one another, all begin development looking like a fish embryo?
Why do mammals form their heads and faces from the very same embryonic structures that become the gills of fish?
Why do vertebrate embryos go through such a contorted sequence of changes in the circulatory system?
Why don't human embryos, or lizard embryos, begin development with their adult circulatory systems already in place, rather than making a lot of changes in what developed earlier?
Why does our sequence of development mimic the order of our ancestors (fish to amphibian to reptile to mammal)?
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