The oldest known film ever made is a short piece titled ‘Roundhay Garden Scene’. In it four individuals stand around in a garden, two of them even walk a little. If that sounds boring, just remind yourself that Buried came out in 2010, and that film is just a guy trapped in a box.
Innovations tend to occur in very concentrated bursts, once something is developed it is inevitably improved upon. Film tends to be one of the best ways of recording innovation as, by its nature, it records.
Of course early celluloid was a fragile material: the first 3D film is said to be a movie called The Power Of Love in 1922, but this cannot be definitively confirmed as no surviving copies of the film exist. So although movies lend themselves to explaining their own history there is always the chance there is some lost film out there from the 30s with CGI. Doubtful, but you never know.
The entries included are game changing not just for their technical attributes, but for their progression in the development of storytelling; and although there are hundreds of other films that could justifiably find themselves in a discussion on the works that defined the medium, the ones on this list helped lay the groundwork for film as we know it today: a universally-accepted and startlingly lucrative industry.