As is well known, a number of Xerox PARC researchers joined Apple after the famous 1979 visit to PARC. They brought with them deep knowledge of mice, windows, and user interface research, and often joined Apple with the hope of creating simpler systems than had been developed at Xerox.
But there's another source of expertise in these technologies that hasn't been looked at so closely: Apple had a number of employees who had worked with Douglas Engelbart's Augmentation Research Laboratory at the Stanford Research Institute, or its incarnation at Tymshare. Engelbart's group had developed early mouse and windows technology in the 1960s; a number of ARC alumni later worked at Apple, Sun, other Silicon Valley companies, or in academia.
Within Apple, ARC alumni worked on the Apple, Lisa, and Macintosh. David Casseres appears to have been the first ARC alum who joined Apple. He worked in Douglas Engelbart's group at SRI from 1966 to 1969. He joined Apple in 1979 as a technical writer, and later became a programmer. As a technical writer, he became friends with Jef Raskin, and contributed an essay to The Book of Macintosh entitled "Beyond Word Processing," which made the case for a structured work processor like that developed by ARC for its oNLine System.
Ken Victor was the first who moved from Engelbart's group at Tymshare to Apple. (Tymshare and Apple were only a few blocks away from each other at the time, so it was a short move.) He became a manager in the Apple II and Apple III divisions. Harvey Lehtman, a colleague of Victor's from the SRI days, also worked on the Apple II, before becoming a researcher in Apple's Advanced Technology Group-- where David Casseres was a colleague.
Other ARC alumni worked on the Lisa project. Dirk van Nouhuys and Sandy Miranda, both SRI alumni ("I was the first tech support" on the ARPANET, Miranda later joked), worked as technical writers on Lisa. Van Nouhuys came to Apple through the ARC network-- "Ken Victor was instrumental in my going to work at Apple," he said-- while Miranda applied for a job there after reading Jef Raskin's BASIC manual. Miranda was later recruited by Macintosh publications manager Chris Espinosa to write the script for the "Test Drive a Macintosh" promotion.
Finally, two ARC alumni was even more closely involved in the Macintosh project. Caroline Rose, who as editor of Inside Macintosh worked closely with Andy Hertzfeld and other Macintosh software developers, worked with Engelbart's group at Tymshare, and was contacted by van Nouhuys about joining the Macintosh group. Finally, Bob Belleville, worked at both SRI and Xerox PARC, was Macintosh's software manager, and one of the main figures behind Apple's effort to develop a laser printer.
The dynamics of this group illustrate two things. First, it's a good example of the way informal recruitment networks function in Silicon Valley. Many of the people who moved from Engelbart's lab to Apple seem to have had a personal connection to the company in the form of another ARC alum. Once there, they were not shy about recruiting former colleagues: Sandy Miranda recalled that Apple "was so exciting," and the pay so much better there (especially for women writers), "that I immediately called two of my friends back at the [U.S. Geological] Survey, and said, you've got to come right away." Second, it suggests how unsustainable the "Promethean myth" of Apple's theft of Xerox PARC technology is. No one in this network worked on the Apple mouse, but their presence in the company shows how widespread knowledge of-- even personal contact with-- the mouse was by 1980. The fact that it hadn't been widely adopted isn't proof that it had been forgotten; rather, the history of the Apple mouse suggests that economics and ease of use, not obscurity, were what hindered its acceptance.