Lookalikes: Emus' DNA tells the true
story of how they evolved (Image: Theo Allofs/Minden Pictures)
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and moas all belong to a group of flightless birds called ratites. Until now the
assumption was that early ratites spread around the world on foot while Africa,
New Zealand and Australia were still joined together. When this supercontinent
broke up, the birds were separated and evolved independently, producing
everything from Madagascar's huge extinct elephant birds to New Zealand's
hulking moas and medium-sized kiwis.
ancient DNA is telling a different story. The apparent similarity of these birds
is deceptive, says Alan
Cooper of the University of Adelaide in Australia. His team sequenced mitochondrial
DNA from the bones of Madagascan elephant birds, and compared it to the DNA
of other flightless birds, including moas.
DNA showed that elephant birds and moas are not evolutionary siblings at all,
but more like cousins. Each evolved separately from small flying birds.
Flying between continents
Madagascar's elephant birds, despite looking like moas, are in fact closely
related to the small flightless New Zealand kiwis. Meanwhile, the moas were most
closely related to aerial South American birds called tinamous.
"In both cases, the moa and the elephant bird, the nearest relative is on the
other side of the world," says Cooper.
shouldn't be surprising but it is," says David
Penny of Massey University in Palmerston North, New Zealand, who was not
involved in the study.
Madagascar and New Zealand separated about 50 million years ago, but the DNA
suggests the last common ancestor of elephant birds and kiwis lived more
recently than that. "That shows they can't have been separated by continental
drift," Cooper says. Instead, each must have each evolved from a bird that flew
across the oceans.
Somehow the ratites lost their flight on six occasions, and grew gigantic in
five of those instances. Cooper says the dinosaur extinction may explain this.
Suddenly there were no big plant-eating animals. "Nothing is eating trees and
browsing and grazing," says Cooper, so ratites could take over that role.
that's true it explains why ratites are the only birds to become massive and
flightless. By 50 million years ago, some mammals had become big, and the window
of opportunity had closed.
Similarly, kiwis never grew large because they only arrived in New Zealand
relatively recently, and there were already giant moas.
we think of birds as flying animals, Penny says their natural state is foraging
on the ground. If there are no predators and no competitors for food, it makes
sense for them to grow and lose the ability to fly.
Journal reference: Science,
Source: New Scientist URL: