From LXF Wiki

Unix has been one of the most popular operating systems amongst professional computer users since the 1970s. The reasons for this really depends on your point of view; the fact that it was adopted early on in universities and large-scale businesses meant that it has had a profound influence on the way that computing, and business' use of computers, has developed since. Linux can be said to be a derivative of Unix in terms of design and features - but it shares no actual source code.

See Bell Labs' own Unix Story (http://www.bell-labs.com/history/unix/)