Here's how much the iPhone's software has changed over time

iPhone OS 2 launched alongside the iPhone 3G in 2008 and had a big addition: The App Store.

According to Apple, the App Store is now worth around $31 billion (£21 billion) per year and has 1.5 million apps and it all started on iPhone OS 2.

Apple rolled out the SDK, the software developers use to make apps, alongside the iPhone 3G.

iPhone OS 3 launched with the iPhone 3GS in 2009 and had some new apps, including Compass and Voice Memos, both of which have persisted until today.

The operating system was also the first to support copy-and-paste, a feature that many thought Apple had missed.

The design remained much the same as versions 1 and 2, however.

iOS 4 came with the iPhone 4 in 2010 and changed the look of the operating system as well as introducing the iOS name, which Apple still uses.

The update introduced the option of having a background image, changing the look of the operating system dramatically, and allowed for multi-tasking in apps.

The iPad, which also launched in 2010, ran a modified version of iOS 4, making it the first version of Apple's operating system to run across both devices, a tradition that exists until today.

iOS 5 launched alongside the iPhone 4s in 2011 and was a big update for the operating system that included iCloud, iMessage, and deep social media integration.

The update also introduced Notification Centre, which grouped all notifications into one place, and the ability to access the camera app by swiping up on the lock screen.

Newsstand, the magazine and newspaper reading app, made its debut in iOS 5, too. However, the app was killed in iOS 8.

iOS 6 launched with the iPhone 5 in 2012 and added a new row of icons on the home screen because of the larger 4-inch screen.

The operating system was the last version to be designed by Scott Forstall and increased the capacity of Siri, introduced a new Apple-made Maps app, and the Passbook app, which is used to store tickets.

Facebook was also integrated deeper into iOS, with an option to log in through the Settings app.

iOS 7 launched alongside the iPhone 5s in 2013 and was the second big redesign of the operating system after iOS 4.

New, flatter icons replaced the older ones and Spotlight search was tweaked to be accessible via a swipe down, not as a separate screen.

The multi-tasking view was also changed to focus on panels rather than app icons and all Apple-made apps were given a new design.

iOS 8, which launched alongside the iPhone 6 in 2014, continued to refine iOS 7 and toned down some of the more radical changes, like the Safari icon.

Widgets, such as Weather, also became available in Notification Centre which was simplified to two tabs: Notifications and Today.

Apple introduced a new feature called Continuity that lets a user open apps on an iPhone or Mac in the same state they were on the other.

The latest version of iOS, which launched alongside the iPhone 6s in 2015, added a new app switcher, updated Spotlight search, and changed the system font to San Francisco.

Find My Friends also became part of the operating system.

Apple introduced a range of new battery features, including a power save mode.

The next version of iOS will likely come this year. Apple usually unveils new versions of the operating system at its World Wide Developer Conference (WWDC) in June and then the first device to get them is the new iPhone.

Apple worked Siri, the virtual assistant, more heavily into iOS 9 and that trend could continue with iOS 10. Both Microsoft, with Cortana, and Google, with Now, are developing comprehensive virtual assistants that do more and more and Apple won't want to get left behind.