#1 First Berkeley Software Distribution (1BSD) Unix

Came into existence: March 9, 1978

It was not Linux which that was the first open-source operating system; the honor goes to Bill Joy’s first version of BDS Unix. When Unix first showed up in 1969, it was open source, it was closed later, but it kept the free software flag flying. Though Linux enjoyed the lime light, Unix was important too.

BSD is a Unix operating system derivative developed and distributed by the Computer Systems Research Group (CSRG) of the University of California, Berkeley. Today the term "BSD" is often used non-specifically to refer to any of the BSD descendants which together form a branch of the family of Unix-like operating systems. Operating systems derived from the original BSD code remain actively developed and widely used.

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Courtesy: Wikipedia