I believe there's also a hypothesis that our ancestry back to the Devonian or Cambrian was based on neoteny/paedomorphism where a particular creature living on the sea floor, when in the infant stage, has a limb for propulsion which it loses when it finds a nice spot to settle. Our very distant ancestors didn't lose those limbs and evolved into the distant precursors to placaderms, bony fish, and, well, terrestrial fauna.