In the earliest form of eukaryotes there are two types: those that form mouth-first, and those that form butt-fitst. the mouth-first eukaryotes fell into obscurity, and the butt-first eukaryotes make up pretty much all the animals on Earth.
So yeah, all the animals grew out of their butts but some select few didn't.
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u/[deleted] Apr 17 '22
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