IPhone OS 1

iOS 1 was the first iOS made(duh.) It is the main operating system for First generation iPod touches and First generation iPhones. Not much is known about it.

Similarities with iOS 5
iOS 1 is very similar with iOS 5 and iOS 4. Mostly, it lacked a lot of the newer features like notification center, multitasking and reminders. The user interface was slightly different too, namely the lack of a 3D-like dock. It also had alot wallpapers built in. The OS was definitely faster then today's, but at the time it was revolutionary.

Triva

 * This was origanally called iPhone OS 1.0
 * Before the name iPhone OS Apple said the iPhone runs a version of OS X.