Gary Rosenberg's Encyclopedia of Seashells (not right at hand, but easily
accessible at home if details are wanted) cites a study of a tribe in
northern Australia that eats a lot of shells. They recognized most of the
same species as do malacologists for the taxa utilized by them. Most (not
all) of these are smaller than rats.
If evolution within a "kind" is at the genus or family level, then there is
no problem with our evolving from Homo erectus or Ardepithecus ramidus,
respectively (if one accepts the nomenclatorially dubious way the latter
genus was introduced, else it should be called Australopithecus). For that
matter, Linnaeus in 1758 included chimps and ourangutangs in Homo, though
he did not have much material to go on (he seems to consider them the same
species, as best I can figure out the Latin).
David C.