

In 2000, the core operating system components of Mac OS X were released as open-source software under the Apple Public Source License (APSL) as Darwin the higher-level components, such as the Cocoa and Carbon frameworks, remained closed-source. At the time, interim CEO Steve Jobs alluded to British naturalist Charles Darwin by announcing "because it's about evolution".


In 1999, Apple announced it would release the source code for the Mach 2.5 microkernel, BSD Unix 4.4 OS, and the Apache Web server components of Mac OS X Server. This was developed into Rhapsody in 1997, Mac OS X Server 1.0 in 1999, Mac OS X Public Beta in 2000, and Mac OS X 10.0 in 2001. After Apple bought NeXT in 1997, it announced it would base its next operating system on OPENSTEP. The heritage of Darwin began with Unix derivatives supplemented by aspects of NeXT's NeXTSTEP operating system (later, since version 4.0, known as OPENSTEP), first released in 1989. 4.3 Darwin 16 onwards OS X rebranded into macOS.4.2 Darwin 12–15 Mac OS X rebranded into OS X.4.1 Darwin 0–11 and corresponding Mac OS X releases.
