Haast's eagle was long assumed to be related to the large Aquila eagles on grounds of size. Ancient DNA from bones showed instead that its nearest relatives are small species such as the little eagle and booted eagle, weighing around one kilogram. Divergence is estimated at roughly 1.8 million years ago.
The implied rate of change is extreme. A small eagle arriving in New Zealand found an island with no mammalian competitors and an abundance of very large flightless prey, and its descendants scaled up to exploit it. The result was an eagle with relatively short broad wings suited to manoeuvring in forest rather than soaring, and a pelvis and talons robust enough to strike prey many times its own weight.
When Polynesian settlers hunted the moa out, the eagle lost its food supply and followed within about the same period, disappearing around 1400. Maori oral tradition preserves accounts of a great bird called pouakai, which is generally regarded as a memory of this species, though the correspondence is not provable.