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Other applications and/or programs may indeed have been 'acquired' as you stated. (These few details were noted in Google, by way of: Did Bill Gates buy Excel).

However, Microsoft developed the program entirely in-house. Excel was the brainchild of Bill Gates, along with Microsoft developers Doug Klunder, Charles Simonyi, and Jabe Blumenthal.

Microsoft originally created a spreadsheet program called Multiplan. Because Multiplan was losing in the market to competitors like Lotus 1-2-3, Microsoft decided to build a brand-new, vastly improved spreadsheet application.

They named it Excel to imply it was "excellent" and superior to Lotus.

Excel was first announced jointly by Bill Gates and Apple's Steve Jobs on May 2, 1985, and officially released for the Apple Macintosh later that year on September 30, 1985.