circa 65,500,000 BCE — The Cretaceous–Tertiary extinction event wipes out the dinosaurs

Everyone loves the dinosaurs. A lot of people – if the Jurassic Park films are to be believed – would like to see them come back. But without their extinction, we wouldn’t be here today.

Even now, it’s still not clear what exactly caused the extinction event – but the best known hypothesis is that of Luis and Walter Alvarez, which states that a meteoric or cometary impact caused a nuclear winter-like effect that altered the climate drastically, wiping out something like 75% of all species alive at the time. The effects were particularly felt by larger species – which included most dinosaurs.

In the wake of the event, now open evolutionary niches were occupied by mammals and birds, including our own ancestors.

circa 243,000,000 BCE — The earliest dinosaurs evolve

There remains some question which dinosaur was actually the earliest of its kind. The oldest claimant is of the order Saurischia (literally ‘lizard-hipped’ dinosaurs), the Nyasasaurus, dated at 243 million years ago. The next oldest saurischians are dated between 232 and 230 millions years ago.

The earliest true dinosaurs of the Ornithischia order (the other major order of the Dinosauria clade) are likewise dated at 230 to 220 million years ago, the oldest of which is the Pisanosaurus.