Can't remember -- what makes them all start to freak out on the grups and go bonk bonk and all that? I've avoided that ep for years.
It was an unexpected effect of the engineereed virus that was supposed to prolong life. It ended up extending pre-adolescence for hundreds of years, but when the children hit puberty, they went bonkers. (I mean, even more bonkers than NORMAL teenagers.

) They they quickly burned out and died.
The unexplained DUPLICATE earth (as opposed to parallel) bugs me.
The “exact duplicate of Earth” is preposterous, of course, and has no relevance whatsoever to the story -- in fact, it doesn't get mentioned at all after the pre-credit teaser. It seems that it was just a cheap hook to pique viewers’ interest and to get them to stay tuned (this was still early in the show's first season).