Many theories. Mine: “Whatever floats.” I believe the wretched refuse of distant teeming shores occasionally had the phenomenal luck to wash up here on the way to elsewhere. There was a first time for this; and there was Columbus; and a lucky few in between.
Moreover, there’s a lot of easy food on a beach. If I were an ancient man that’s where I’d hang out.
In addition, the continental shelf of all the continents has been exposed to various degrees most of the past 2 million years or so (during glaciations), and it was ice-free and warmer down there. Most of our ancestry came from down there, which is probably one reason there’s been much more interest in research of the submerged areas, during recent decades.