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The first edition of Unix was developed for the PDP-11/20 at Bell Labs by Ken Thompson, Dennis Ritchie and others. It provided a process-oriented multitasking environment with a hierachical filesystem for about two or three users.
The source code for the kernel came from a document entitled Preliminary Release of Unix Implementation. Al Kossow from the Computer History Museum unearthed a paper copy of the document, scanned it in and donated it. The manual pages were OCR'd by Dennis Ritchie.
For more information about First Edition Unix, see The Evolution of the Unix Time-sharing System by Dennis Ritchie.