Let me suggest a rather weird idea (I'm not really serious, but who knows...): perhaps they have evolved to this not from more primitive species but from less primitive actually. Even humans don't really need strong hardcover bodies and strong limbs any more as now we enclose our bodies in all kinds of environment suits, structures, and vehicles, all kinds of physical work can be automated, all we really need is a brain and perhaps we can evolve into some sort of cephalopods occasionally as some hundreds millions years (I'm not really sure about the order of magnitude) pass if we don't go extinct and we can also start loosing some intelligence once too. BTW this direction (to the point of a rather-primitive amphibia) of human evolution as been suggested in the 15th episode of the second season of the StarTrek:VoyagerTV show (an episode popularly named among the worst although I find it being one of the most intriguing). The Guild navigators from the David Lynch movie are another clue, who knows what this weirdly evolved kind of human species could evolve into to adapt when there is no spice any more...