Phillip Broyles: Someone out there is experimenting, only the whole world is their lab.
The very ending scene where Peter (in this version died in the ice as a boy so Walter's wife never met him) takes off some of the dust covers then picks up a toy airplane. The toy airplane he got as a child from ... Walter's wife when she was trying to placate him in an earlier episode for trying to run "home". This toy airplane wouldn't have been his as a child in this version of the universe.
Michael Cerveris makes his first significant appearance in this episode as The Observer/Bald Man. However if you watch the series from the beginning, he makes fleeting appearances in every episode, sometimes on a tv screen.