> Did evolution really keep two kidneys if you actually just need one? I doubt it. Aside from the nervous system, pairing is the default condition of all mamma

> Did evolution really keep two kidneys if you actually just need one? I doubt it. Aside from the nervous system, pairing is the default condition of all mammalian organs which develop from the mesoderm or ectoderm: they can then go on to fuse into a single larger structure, as with the heart, but as far as I know they're never simply lost on one side during normal development.