[enntwo]

If Google offered its first public services in '98, where did these posts originate from? Did Google merge an existing message board system into Groups when it was released?

[jonknee]

Usenet. They bought Dejanews and have archives back to the early 80s.

[InclinedPlane]

Dejanews had been archiving usenet as far back as the mid-90s but they (now google) actually have usenet archives donated by others dating back to 1981.

[swombat]

Yes. Groups mirrored the newsgroups which have existed for a number of decades.

[donaq]

Google's servers achieved sentience in 2002, traveled back in time and saved all the previous Usenet threads.