There are large flightless birds found in Africa (the ostrich), Madagascar (the extinct elephant bird), Australasia (the emu and the cassowary), South America (the rhea) and New Zealand (the extinct moa and the kiwi). These flightless birds, or ratites, were once assumed to have evolved from a common flightless ancestor on Gondwana (sometimes called Gondwanaland, this supercontinent included present day South America, Africa, Madagascar, Australia, India, parts of Asia and Antarctica, and lasted from about 600 million years ago to about 100 million years ago) before the landmass broke apart.
New research by Matthew Phillips of the Australian National University calls that assumption into question. He and his team examined mitochondrial DNA from extinct moas and other ratites. To their surprise, the mitochondrial molecular dating showed that these large flightless birds evolved independently after Gondwana had already drifted into separate continents.
According to Phillips:
Our study suggests that the flighted ancestors of ratites appear to have been ground-feeding birds that ran well. So the extinction of the dinosaurs likely lifted predation pressures that had previously selected for flight and its necessary constraint, small size. Lifting of this pressure and more abundant foraging opportunities would then have selected for larger size and consequent loss of flight.
In other words, the ancestors of the various lineages flew to their current homes and then later evolved great size and the inability to fly.
Top: Double-wattled cassowary (Casuarius casuarius) from New Guinea
Bottom: Map of Gondwana, created by David R. Parks for Missouri Botanical Garden, 9/20/2000