New Study Changes Thinking on Evolution of Flightless Birds

evolutionAt the bottom of the world there lives a small, shy flightless species of bird. They only come out at night, and are rarely seen by anyone.  However, this bird, the kiwi, holds the key to new thoughts on flightless birds. A new study has shown a link between the kiwi of New Zealand and the giant, aptly-named and extinct elephant bird of Madagascar. This link has changed scientists’ thinking on the theory of continental drift as well as the evolution of flightless birds.

The Madagascan elephant bird died out just a few centuries ago. It was said to be nine feet tall and weighed as much as 600 pounds. One of its eggs would be the equivalent of around 160 normal chicken eggs. But, like the emu, its wings were tiny and useless. Instead of flying, it spent its days with its head down, scratching in the dirt for food.

News of the giant birds was told to Marco Polo when he visited the island in the Middle Ages. It is possible that there were still sightings of the bird as late as the 1800s, but the last available record was said to be from a French colonial governor living in Madagascar in 1658.

Scientists in the 1990s began looking at what the little DNA left behind by the birds could tell them, mainly via shell fragments and pieces of bone. However, due to recent developments in DNA extraction, scientists have been able to gather larger samples of DNA, which have allowed them to study elephant birds in more depth. They were surprised to find that the closest living relative to the massive Madagascan was the small nocturnal kiwi, living half a world away.

The discovery made by the new study is changing the way that scientists are viewing the evolution of flightless birds. The new thinking is not solely concerned with kiwis, but the whole history of the origins of all ratites in general.

Species of ratites are not only set apart by their lack of wingspan.  There are significant differences in the bones of their skulls that show them to be different from other bird types.  They are missing a keel to their sternum, which means that even if they developed significant wings, they still could not fly. Current ratite species include the Australian emu and cassowaries, which can be found in Australia and New Guinea. Other extinct types include the moa.

Theories of the evolution of flightless birds have long baffled scientists. It had been thought that continental drift was responsible for the fact that the birds are found throughout the Southern Hemisphere in seemingly isolated pockets. It was presumed that when Gondwana began to separate, the different sections took their own animals along for the ride. This has been backed up by the differences between the African ostrich, the first to break away, and other ratite forms.

In the 1990s, scientists began testing this theory of evolution by comparing DNA of the elephant bird and the moa, an extinct ratite of New Zealand, to other living species. They were able to come up with the first piece of evidence that seemed to cast doubt on the continental drift theory. The moa’s closest living relative was the tinamou, a small but flying bird located in Central and South America. The fact that the birds were closely related, although one was winged while the other was flightless, was a source of confusion to the scientific community.

The connection found between the kiwi and the elephant bird further upsets the theory. The extracted DNA from both the kiwi and the elephant bird puts their common ancestor as living around 50 million years ago. The evidence shows that Gondwana broke up before this time, so New Zealand and Madagascar were already separated by an ocean. Thus the only way the kiwi could have found its way to New Zealand is if it flew there.

This would suggest a counter-type of evolution where both the kiwi and the elephant bird evolved to be flightless as they adapted to their new environments. This idea remains as a simple postulation as this time, but the new study is allowing scientists to gather more data to map the evolutionary tree of flightless birds, which could change current thinking. They say it will not be long before the mystery is fully unraveled.

by Sara Watson

Sources:

NY Times
Waikato Times
San Jose Mercury News

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