Another history question

I know that postgres was a project directed by Michael Stonebraker in
Berkeley (1986-1994) and that soon Jolly Chen and Andrew Yu did
postgres95.

I understand that the main change in postgres95 was to implement SQL
instead of POSTQUEL.  Then after the appearance of postgres95
postgreSQL 6.0 arises.  And what came later it is well-known history.

My question is if the architecture of postgreSQL were inherited of
postgres original project or postgreSQL were developed completely with
a new concept.  Thanks in advance.