PostgreSQL evolved from the Ingres project at the University of California, Berkeley. In 1982 under the leadership of Michael Stonebraker

Michael Stonebraker, then started a post-Ingres project to address the problems with contemporary database systems that had become increasingly clear during the early 1980s.

The actual implementation of Postgres began in 1986.

The team released version 1 to a small number of users in June 1989, then version 2 with a re-written rules system in June 1990. Version 3, released in 1991, again re-wrote the rules system, and added support for multiple storage managers and an improved query engine. By 1993, the great number of users began to overwhelm the project with requests for support and features.

In 1994, Berkeley graduate students Andrew Yu and Jolly Chen replaced the POSTQUEL query language interpreter with one for the SQL query language, creating Postgres95. The front-end program monitor was also replaced by psql.

Version 1.0 of Postgres95 was announced on September 5, 1995, with a more liberal license that enabled the software to be freely modifiable for any purpose.

In 1996, the project was renamed to PostgreSQL to reflect its support for SQL.

Since then, every subsequent release has a new set of features that came into existence.

The current version of PostgreSQL is PostgreSQL 12.1

Further reading can be found here