Given what we now know of how birds evolved from the therapod dinosaurs, it would be tempting to look at the big flighteless birds like the emu, ostrich, moa, the extinct giant elephant bird or Aepyornis maximus of Madagascar, and not so big New Zealand kiwi, which we collectively call the ratities, and assume they may have missed out on flight altogether and simply distributed themselves on foot from their ancestral homelands somewhere around Africa when the major landmasses were still joined up, marking then out as not very far removed from the early proto-avians and feathered dinosaurs.
However, DNA analysis, which is proving such a powerful tool for answering these little questions and resolving disputes about the precise details of evolution, shows they may not be closely related at all and may have evolved from flying birds on at least six different occasions. Their similar appearance may simply be an example of convergent evolution where broadly similar environments produce broadly similar solutions.
A team lead by Alan Cooper of the University of Adelaide in Australia has sequenced the mitochondrial DNA of the Madagascan Aepyornis maximus and other flightless birds and has shown the the closest relative of A. maximus is the New Zealand kiwi and not the moa as had been assumed from their appearance. Kiwis and A. maximus shared a common ancestor about 50 million years ago, which is some time after New Zealand and Madagascar were last in contact, so the only way they could have their current distribution was by flying. Similarly, the moas, which were thought to be more closely related to A. maximus turns out to be closer to the South American aerial tinamou. Again, this separation is more easily explained if both shared a flying common ancestor.
The evolution of the ratite birds has been widely attributed to vicariant speciation, driven by the Cretaceous breakup of the supercontinent Gondwana. The early isolation of Africa and Madagascar implies that the ostrich and extinct Madagascan elephant birds (Aepyornithidae) should be the oldest ratite lineages. We sequenced the mitochondrial genomes of two elephant birds and performed phylogenetic analyses, which revealed that these birds are the closest relatives of the New Zealand kiwi and are distant from the basal ratite lineage of ostriches. This unexpected result strongly contradicts continental vicariance and instead supports flighted dispersal in all major ratite lineages. We suggest that convergence toward gigantism and flightlessness was facilitated by early Tertiary expansion into the diurnal herbivory niche after the extinction of the dinosaurs.
Kieren J. Mitchell1, Bastien Llamas1, Julien Soubrier, Nicolas J. Rawlence1, Trevor H. Worthy, Jamie Wood, Michael S. Y. Lee1, Alan Cooper.
Ancient DNA reveals elephant birds and kiwi are sister taxa and clarifies ratite bird evolution
Science 23 May 2014: Vol. 344 no. 6186 pp. 898-900 DOI: 10.1126/science.1251981
The suggestion is that these birds evolved during a brief spell between the extinction of dinosaurs and the evolution of large mammals when they moved into a vacant niche for large terrestrial animals. In all but one instance this involved becoming large in the process. The reason kiwis remained small was because the niche had already been occupied by moas.
So here we see how well the DNA evidence is meshing neatly with the geological evidence for continental drift and the paleontological evidence for the extinction of dinosaurs and the rise of large mammals to replace them. Just like every other test of Darwinian evolution thrown up by new scientific discoveries (Darwin knew nothing of DNA or continental drift of course) the theory is not only passing with flying colours but is strengthened and confirmed by it. There are probably no other scientific theories that can claim that, not even fundamental 'laws' like Newton's Laws of Motion, the theory of gravity and the Law of Conservation of Matter which were all overthrown by Relativity.
Creationists still like to pretend this theory is no more than a guess with no supporting evidence, teach this denialism to their unfortunate children and want to be able to teach it to our more fortunate children at public expense.
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