I like both of the “Earth is our final chance” ideas, but they don’t explain why we find skeletons.

Instead how about the message being “Go back through the portal before it closes!” That’s reminiscent of situations like the first planet found in Stargate: Universe where they learn that there’s a time limit, or the one Star Trek: TNG where they find a potential stable wormhole but Geordi sees it may not be and the Ferengi scientists don’t believe him.