This article on techradar pretty much tells the story as I remember and believe it happened.
“From the outset Apple had one goal in mind when OS X was designed, and that was to make the next great personal computer operating system.
The company wanted it to be the kind of operating system with which a beginner could feel intuitively at home, and a pro user could be taken to places they’d never thought they’d be. That’s a tall order, but eight major versions, millions of sales and a decade later, we think Apple did it. “
The article backs up from this statement and begins where OSX begun, as NeXTSTEP (aka OpenStep) from NeXT. Of course, it didn’t become part of Apple until Apple purchased NeXT and brought Steve Jobs back into the company as the iCEO.
He then lists off the iteration of OS X. Cheeta (10.0), Puma (10.1), Jaguar (10.2), Panther (10.3), Tiger (10.4), Leopard (10.5), Snow Leopard (10.6) and Lion (10.7).
He also admits that OS X was the basis for iOS as well.
What is really interesting, is the screen shots showing the evolution of OS X as it has progressed.
Mac OS X 10.0, Cheetah from March 2001.
And OS X 10.7 Lion from July of last year: