Not to mention Aepyornis ('Elephant bird') and Dinornis (Moa), two colossal birds in their own right, and not far removed from Emus either. The Moa was the largest of the two and only very recently extinct as well, probably around 1500.
But 100 million years ago in Australia there was a carnivorous Dinosaur described in 2009 called Australovenator, if you compare that Dinosaur's bones to an Emu or a Cassowary the only real difference is the size. Till you get to the head that is.
Fossilized Australian Dinosaurs are a rarity, so this find was big news in paleontology and still is because they keep finding more of the animal.
The thing that really strikes me is Australovenator is a Carnosaur (This group contains animals like Allosaurus), Emus are Coelurosaurs (This group contains animals like Tyrannosaurus and Velociraptor as well as all modern birds), both are Theropods but those two groups split sometime during the Jurassic which means Ratites carry features which are at least 170 million years old, perhaps older than that by an additional 30 or 40 million years.
What's more is Ratites were around at the same time Australovenator was, but unable to compete with their large cousins few Ratites got larger than a Turkey until the Dinosaurs disappeared.
This video goes into the relationship between Dinosaurs like Australovenator and modern birds like the Cassowary and Emu:
Just a warning though, the documentary does focus on an autopsy of a Cassowary killed after being hit by a car but that link skips over that part until it hits the 33 minute mark when they go back to it.