first, i'd like to request a link to the
online text whenever it's available. it's always handy to have when talking about the story but even more so during the hugos when the stories aren't always suited for audio (for instance, just knowing the the word 'mark' was capitalized during the first encounter would have helped clarified things at that point). this one was interesting but, as others noted, the constant introduction of new species & scenarios made it a challenge to follow in audio.
my take on the ending is that earth was actually the progenitor of these alien species. our situation is different than theirs since we developed in an isolated area during the early stages of the galaxy so we didn't have have the (radio) chatter of previous species to draw us out. the story lead us to believe that humanity had died out since that seemed to be the significance of the
moment, species came here in their last moments just as they became extinct.
instead it turns out that it was more of a homing instinct, returning to 'valley' of the original planet. sort of a reassurance that even though you (and your species) might slip away that you still leave your mark on the things to come.
edit: thanks for pecoging my request Norm =P