News in Science

Kiwis don't have Aussie roots

New Zealand origins The New Zealand kiwi bird's ancestors came from somewhere other than Australia, a new ancient DNA study has found.

The research undermines current wisdom about how flightless birds evolved, says Professor Alan Cooper of the University of Adelaide.

The emu and cassowary in Australia and New Guinea, the kiwi in New Zealand, the ostrich in Africa, and the rhea in South America are so-called 'ratite birds', which have lost the bone that wing muscles can attach to.

Traditionally it was believed that each ratite bird lineage evolved through the split up of the ancient supercontinent of Gondwana.

Nearly two decades ago, an ancient DNA analysis of ratite birds carried out by Cooper for his PhD, found the closest living relative to the kiwi was the emu and the cassowary in Australia.

Given that New Zealand split off from Australia as part of the break up of Gondwana, this suggested the ancestors of kiwis had actually come from Australia.

"This was a huge psychological blow in New Zealand and extremely unpopular," says Cooper, a New Zealander himself.

"Ever since then there's been this huge national guilt that the national bird was in fact Australian."

Now, however, in a paper published in today's issue of Science, Cooper and colleagues have set the record straight.

They have sequenced and analysed the mitochondrial genomes of two species of the now-extinct moa-like elephant bird from Madagascar and found that this giant herbivore is in fact the closest relative to the chicken-sized worm-eating kiwi.

The kiwi and elephant bird lineages separated about 50 to 60 million years ago, which is long after the continents came apart, says Cooper. Their relationship undermines the current idea that ratites evolved from a flightless ancestor and their evolution was driven by continental drift.

"The kiwi lineage had to get from Madagascar and clearly it must have flown because those two places were never joined," says Cooper.

"Clearly the ratites were flying when they dispersed."

Only after separating did each ratite lineage independently lose the ability to fly, says Cooper. And in most cases, they evolved into giant herbivores.

More importantly, says Cooper, the findings restore New Zealand's national pride.

"Twenty years later it's great to be able to show using ancient DNA that the kiwi is not an Australian bird. In fact its closest relative is the elephant bird from Madagascar," he says.

"The New Zealanders will be much more comfortable with that. It's their worst nightmare to be a derivative of Australia."

Exploiting a niche

Apart from the ratites, and the Australian Genyornis, flightless birds have only evolved on islands lacking mammalian predators.

Cooper and colleagues argue that ratites were able to evolve into large flightless herbivorous birds during a narrow evolutionary window just after the dinosaurs went extinct at 65 million years.

"When the dinosaurs went extinct it meant that the ecological niche for being a large vegetarian on the land was suddenly free because the only mammals around at the time were tree-dwelling rat-sized creatures," says Cooper.

"As far as we can tell, the ratites were flying around at that stage, landing on these different continents and finding that niche free, were rapidly expanding to become big flightless herbivores."

Once mammals increased in size about 50 million years ago, this evolutionary path was no longer an advantage as it made birds vulnerable to being eaten by mammals, he adds.

To survive mammalian predators large herbivorous ratites had to learn how to run very fast, says Cooper.