Look at that S car go!
https://thetfp.com/tfp/tilted-humor/87878-two-snail-jokes-forgive-me-if-theyre-repeats.html
I think snails are hermaphroditic and a single snail can reproduce.
So all it would take is ONE SNAIL stuck to a piece of wood or something, floating across the English channel, and you get the same result.
To turn this into some astounding revelation about human migration is lame... just lame.