Answer:
The answer to this question, according to research recently published by investigators, as recently as 2017, is that, according to DNA findings of bones from the extinct ancestors of all these birds, and from ancestors who did not go extinct, but migrated to different regions due to landmass shifting, these different birds found themselves facing environments, including climate changes on the regions where they moved to after the Crutaceous period, that initiated genetical changes in them to adapt to their new living conditions.
Explanation:
This research was published in the journal Current Biology, volume 27, issue 3, on February 6, 2017, by Florian Maderspacher, but it cites the recent findings by investigator Masami Hasegawa on what happened to ratite birds, how they ended up spread out like they did on the Southern hemisphere, but in such different locations, and with so many phenotypical and genetic differences. In the end, what they discover from different types of DNA, is that originally the ancestors of these ratities, the paleognaths, moved from the Northern Hemisphere towards the South through the joint lands of South America, Antartica and towards New Zeland, Australia, Africa and Madagascar, which formed then the super continent of Gondwana. Taking also the theory of land shifting, and the changes in conditions that these birds had to face, it seems that they started developing phenotypical and genetic differences that helped them to adapt to their new climates and environments. When these land masses separated and migrated, each of these birds had to face severe changes in their conditions, including the appearance of predators, that forced them to adapt. This is the reason why the rhea of South America, and the Emu of Australia, are now so different.
