History of MacOS

The heritage of what would become macOS had originated at NeXT, a company founded by Steve Jobs following his departure from Apple in 1985.

The Unix-like NeXTSTEP operating system was developed and then launched in 1989. The kernel of NeXTSTEP is based upon the Mach kernel, which was originally developed at Carnegie Mellon University, with additional kernel layers and low-level user space code derived from parts of BSD. Its graphical user interface was built on top of an object-oriented GUI toolkit using the Objective-C programming language.

Throughout the early 1990s, Apple had tried to create a "next-generation" OS to succeed its classic Mac OS through the Taligent, Copland and Gershwin projects, but all of them were eventually abandoned. This led Apple to purchase NeXT in 1996, allowing NeXTSTEP, then called OPENSTEP, to serve as the basis for Apple's next-generation operating system. This purchase also led to Steve Jobs returning to Apple as an interim, and then the permanent CEO, shepherding the transformation of the programmer-friendly OPENSTEP into a system that would be adopted by Apple's primary market of home users and creative professionals. The project was first code-named "Rhapsody" and then officially named Mac OS X.

Apple rapidly developed several new releases of Mac OS X. Siracusa's review of version 10.3, Panther, noted: "It's strange to have gone from years of uncertainty and vaporware to a steady annual supply of major new operating system releases." Version 10.4, Tiger, reportedly shocked executives at Microsoft by offering a number of features, such as fast file searching and improved graphics processing, that Microsoft had spent several years struggling to add to Windows with acceptable performance.

Mac OS X was originally presented as the tenth major version of Apple's operating system for Macintosh computers; current versions of macOS retain the major version number "10". Previous Macintosh operating systems (versions of the classic Mac OS) were named using Arabic numerals, as with Mac OS 8 and Mac OS 9. The letter "X" in Mac OS X's name refers to the number 10, a Roman numeral, and Apple has stated that it should be pronounced "ten" in this context. However, it is also commonly pronounced like the letter "X".

The consumer version of Mac OS X was launched in 2001 with Mac OS X 10.0. Reviews were variable, with extensive praise for its sophisticated, glossy Aqua interface, but criticizing it for sluggish performance. With Apple's popularity at a low, the makers of several classic Mac applications such as FrameMaker and PageMaker declined to develop new versions of their software for Mac OS X. Ars Technica columnist John Siracusa, who reviewed every major OS X release up to 10.10, described the early releases in retrospect as 'dog-slow, feature-poor' and Aqua as 'unbearably slow and a huge resource hog'.

In 2005, the first Intel Macs released used a specialized version of Mac OS X 10.4 Tiger.

A key development for the system was the announcement and release of the iPhone from 2007 onwards. While Apple's previous iPod media players used a minimal operating system, the iPhone used an operating system based on Mac OS X, which would later be called "iPhone OS" and then iOS. The simultaneous release of two operating systems based on the same frameworks placed tension on Apple, which cited the iPhone as forcing it to delay Mac OS X 10.5 Leopard. However, after Apple opened the iPhone to third-party developers its commercial success drew attention to Mac OS X, with many iPhone software developers showing interest in Mac development.

In 2007, Mac OS X 10.5 Leopard was the sole release with universal binary components, allowing installation on both Intel Macs and select PowerPC Macs. It is also the final release with PowerPC Mac support. Mac OS X 10.6 Snow Leopard was the first version of OS X to be built exclusively for Intel Macs, and the final release with 32-bit Intel Mac support. The name was intended to signal its status as an iteration of Leopard, focusing on technical and performance improvements rather than user-facing features; indeed it was explicitly branded to developers as being a 'no new features' release. Since its release, several OS X or macOS releases (namely OS X Mountain Lion, OS X El Capitan, and macOS High Sierra) follow this pattern, with a name derived from its predecessor, similar to the 'tick-tock model' used by Intel.

Apple removed the head of OS X development, Scott Forstall, and design was changed towards a more minimal direction.

In 2012, with the release of OS X 10.8 Mountain Lion, the name of the system was shortened from Mac OS X to OS X.

From 2012 onwards, the system has shifted to an annual release schedule similar to that of iOS.

Apple also steadily cut the cost of updates from Snow Leopard onwards, before removing upgrade fees altogether from 2013 onwards. Some journalists and third-party software developers have suggested that this decision, while allowing more rapid feature release, meant less opportunity to focus on stability, with no version of OS X recommendable for users requiring stability and performance above new features.

Apple's new user interface design, using deep color saturation, text-only buttons, and a minimal, 'flat' interface, was debuted with iOS 7 in 2013.

With OS X engineers reportedly working on iOS 7, the version released in 2013, OS X 10.9 Mavericks, was something of a transitional release, with some of the skeuomorphic design removed, while most of the general interface of Mavericks remained unchanged.

The next version, OS X 10.10 Yosemite, adopted a design similar to iOS 7 but with greater complexity suitable for an interface controlled with a mouse.

Apple's 2015 update, OS X 10.11 El Capitan, was announced to focus specifically on stability and performance improvements.

In 2016, with the release of macOS 10.12 Sierra, the name was changed from OS X to macOS to streamline it with the branding of Apple's other primary operating systems: iOS, watchOS, and tvOS. macOS 10.12 Sierra's main features are the introduction of Siri to macOS, Optimized Storage, improvements to included applications, and greater integration with Apple's iPhone and Apple Watch. The Apple File System (APFS) was announced at the Apple Worldwide Developers Conference in 2016 as a replacement for HFS+, a highly criticized file system.

Apple previewed macOS 10.13 High Sierra at the 2017 Worldwide Developers Conference, before releasing it later that year. When running on solid-state drives, it uses APFS, rather than HFS+.

MacOS 10.13 High Sierra successor, macOS 10.14 Mojave, was released in 2018, adding a dark user interface option and a dynamic wallpaper setting.

macOS 10.14 Mojave was succeeded by macOS 10.15 Catalina in 2019, which replaces iTunes with separate apps for different types of media, and introduces the Catalyst system for porting iOS apps.