National Geographic News
A photo of a female cassowary crossing a stream.

A female cassowary crosses a stream in Daintree National Park in Queensland, Australia.

PHOTOGRAPH BY CHRISTIAN ZIEGLER, NATIONAL GEOGRAPHIC

Alison Fromme

for National Geographic

Published May 13, 2014

Large flightless birds are scattered across all but one of the world's southern continents. Since Darwin's era, people have wondered: How are they related?

Ostriches, emus, cassowaries, rheas, and kiwis can't fly. Unlike most birds, their flat breastbones lack the keel that anchors the strong pectoral muscles required for flight. Their puny wings can't possibly lift their heavy bodies off the ground. These flightless birds, called ratites, are clearly different from other avian species. (Read "Big Bird" in National Geographic magazine.)

Darwin noticed, and he predicted that ratites were related to each other. His contemporary, Thomas Huxley, found another commonality among them: The arrangement of bones in the roofs of their mouths appeared more reptile-like than that of other birds.

At about the same time, another biologist, Richard Owen, assembled the remains of a giant ostrich-like fossil skeleton, the first extinct moa known to the western world. But a pesky detail puzzled Huxley: Small, ground-dwelling South American tinamous didn't seem to fit neatly with the ratites or other birds.

Tinamous fly, albeit reluctantly. And they possess keeled sternums, suggesting that they evolved with flying birds. But their palate bones match the ratites. Where do they belong?

Scientists have debated this question for 150 years. Now, a new study in the journal Molecular Biology and Evolution, analyzing the largest molecular dataset to date, clarifies the tinamous' place on the evolutionary tree and offers clues about the origins of flightlessness.

To sort out the details, scientists probed almost 1,500 DNA segments from tinamous, emus, ostriches, the extinct little bush moas, and others. After sandblasting and pulverizing an ancient moa toe bone to chemically extract and sequence the DNA, scientists compared its DNA with that of the other species and ran multiple computer models simulating molecular evolutionary changes.

Some earlier studies, which have generally showed tinamous on the outskirts of the ratite group, relied solely on morphological traits like skeletal details. Other investigations of limited genetic information suggested tinamous were evolutionarily tangled with the flightless birds. "Fundamentally, the recent debate is about molecular data versus morphology," says Allan Baker, lead author of the study. "We can't both be right."

DNA Settles Relationship

The results were staggering, Baker says. The tinamous evolved within ratites, not as a separate lineage. "And the DNA says absolutely that moas and tinamous are closely related," says Baker, who first encountered moa fossils as a kid tromping through New Zealand caves.

Moa breastbones, toe bones, leg bones, and even the occasional skull rested in the mud, the final resting place for birds chased and slaughtered by humans about 12,000 years ago. Today, a cast of a Dinornis robustus skeleton towers over visitors to the Royal Ontario Museum where Baker is the senior curator of ornithology.

The Origins of Flightlessness

The tinamous' place on the evolutionary tree offers a glimpse into the origins of flightlessness. All ratites, including tinamous, probably trace their ancestry back to a flying relative, according to Baker. Tinamous retained their ability to fly, while the other lineages each lost flight independently. "It's very unlikely that tinamous would re-evolve flight from a flightless ancestor," says Baker.

The study upends an alternate, oft-cited story. Scientists speculated that the breakup of the supercontinent Pangaea's southern section split up a population of flightless ratite ancestors. Each landlocked group evolved in place, creating the awe-inspiring and quirky birds known today: ostriches in Africa, rheas in South America, emus and cassowaries in Australia, the extinct elephant bird in Madagascar, and kiwis and extinct moas in New Zealand. The story conveniently explained how flightless birds dispersed across the oceans. "Growing up, we were told that moas and kiwis were sisters," Baker says.

But now it looks like each group invaded New Zealand separately. The new evidence doesn't align with the timing of Pangaea's split more than 100 million years ago. The ratites evolved into separate lineages between 90 and 70 million years ago, and the tinamous and moas diverged about 45 million years ago, according to the study. "We can't rule out that the birds flew to each continent," says Baker, and then independently evolved their flightless features.

The debate about these birds has been contentious, Baker says. "But I think this study will put it to rest."

Follow Alison Fromme on Twitter.

22 comments
Rodolfo Alonzo
Rodolfo Alonzo

The cassowary hunts people in packs like a velociraptor. Way cool! Evolution and instinction seem to work hand in hand. Birds and and shrews made it past the asteroid 66 million years ago. After a cataclysmic event happens on Earth. What will survive? And what will evolve from that survivor. Let's get back to the basics of life. Let's go to Luckenbach Texas.

Robyne Jones
Robyne Jones

Could you people stop writing about evolution as if it is deliberate, PLEASEEEEE ( Science teacher)

James Oxton
James Oxton

Interesting, New Zealand has only had a human population for approximately 500 to 600 years.So I'm at a loss as to 12,000 year old moa being chased by people.

Brian Garland
Brian Garland

The science is settled - no more talk!


(please censor this again)

Владимир Предоев
Владимир Предоев

I wonder why the birds have leaned to the flightlessness despite that their avian features are in their disadvantage for such a change. For example - hollow bones means easily broken bones, air pockets out of the lungs are against them because they take room, otherwise needed for a vital organs, bipedal walk with needless wings, they are not like the old-time predatory dinosaurs, they don`t use either their beak, or their wings to grasp prey. Simply for me settling on the ground is a downside, instead of an advantage. Is it easier for them to behave like ground-dwelling mammals than sustaining their avian way of life? If yes, what is the price? What pushed them to leave the skies for all the lurking land predatory animals? 

Bryan M.
Bryan M.

And emus are Australian, not New Zealand "emus, kiwis and extinct moas in New Zealand"

Adam Wu
Adam Wu

@Владимир Предоев  

Hollow bones sure didn't mean more easily broken bones for the theropod dinosaurs, and maybe even the sauropods, who also had them. Air pockets out of the lungs were no problem for the theropod dinosaurs either, who also had them. And many, many, many birds do use their beaks to grasp prey.

Oedhel Setren
Oedhel Setren

@Владимир Предоев  

I think that you're a bit confused on the evolutionary steps of birds. Birds like the Emu are closest to velociraptors. They didn't evolve from flying dinosaurs; its not that they evolved to become ground based, but rather they never evolved to fly.

Most flightless bird don't have a full skeleton of hollow bones. Many of the more robust bones (like the femur) aren't hollow and actually have marrow. 

Why fly if you're too big or too fast to be eaten. Flying can be incredibly taxing if your aren't built for it. In fact, we see many birds prefer to be ground based even though they can fly (geese and ducks.) As for being ground-based as a disadvantage, you are over-looking that these birds are generally among the larger fauna in their biome. These birds also tend to dwell in areas where flight based behavior isn't an option for larger birds. Either due to lack of trees for roosting or too many trees for flying.

You should also be aware that birds do use their beaks and many of these large, land birds do have simple teeth.

Charlie Ewing
Charlie Ewing

@Владимир Предоев My first guess would be that food was so plentiful that 10000 generations or more were able to lounge on the beach and just eat fish all day. Small fish, and they might have had to work a little bit. But relative ease would have done this. The lack of need to migrate, or the lack of ability to detect a migratory direction (such as a period of not being able to determine north and south) might have also done this.

Oedhel Setren
Oedhel Setren

@Bryan M.  I'm not seeing where you got that quote. 

"emus and cassowaries in Australia, the extinct elephant bird in Madagascar, and kiwis and extinct moas in New Zealand."

It looks like you skipped the better part of a sentence.

Oedhel Setren
Oedhel Setren

@Ben Creisler  Actually, humans had reached much further than we originally assumed. Carbon dating on bones as well as genetic testing has shown that there are skeletons predating the Maori. 

In South America there is a foot print in sediment that predates the Bering Land Bridge. Meaning Humans came to America long before we initially thought. Similar genetic testing was done on bones in the Americas. Test results showed that the original peoples of America came from Europe along kelp beds over 15k years ago. 

If they can cross a very turbulent ocean then there's no reason they couldn't have island hopped to New Zealand. There's also the issue of the devil being in the details. If a tribal people is just passing through an area, they tend not to leave much evidence in terms of cultural references. It's not until people settle into an area that we are left with evidence of their existence.

Gr Hurlburt
Gr Hurlburt

@Oedhel Setren @Владимир ПредоевRatites evolved from fllying ancestors. THAT IS EXACTLY WHAT THIS PAPER STATES! ALL these flightless birds had a common ancestor with tinamous. |None of these birds evolved from Velociratpor. NO BIRDS HAVE TEETH. Please cite any evidence. Birds have the genetic potential for teeth and they have been experimentally induced in chickens. but that is it. The "egg tooth" you  may be thinking of is not an actual tooth.

Drew mac
Drew mac

@Oedhel Setren @Владимир Предоев  

Beaks and feathers both are highly complex evolutionary steps to take from reptilian jaw bones and scales, with very specific advantages that ONLY benefit flight. The beak is much lighter weight but is hard and lacks versatility. Feathers are just a hugs step to take from scales with a complex system of interlocking strands that serve primarily to be aerodynamic and light weight. 


If they never evolved to fly it seems almost impossible that they would have developed these 2 traits because they're so complex and and aimed so specifically at flight. For an organism that never left the ground the benefits would be negligible, if not disadvantageous.


The substitution of functional arms and claws for useless little wings is flat out disadvantageous.


If they'd never left the ground why develop all the traits for flight? 


Also, FTFA: "We can't rule out that the birds flew to each continent," says Baker, and then independently evolved their flightless features.


I think your direct from velociraptor to Emu theory has some holes.

Charlie Ewing
Charlie Ewing

@Владимир Предоев At or around 70 Ma, it also appears that there was a land bridge connecting Australia, Antarctica and South America. The quantity of extra shallow shoreline would definitely prompt evolution of a series of birds able to fish without flight. It would also increase the biomass of smaller trees and shrubs in the area into which these creatures could have hidden. Also note the color of the some of those flightless species and the shape of their feathers. It seems somewhat clear that they were in a position where ground-based camouflage was more important than flight, if in fact their feather type has been around as long as they have.

At or around 20 Ma, it also appears that the varieties no longer had a land connection and so species left in one place diversified more from others, such as possibly the penguin. The shaggier birds probably became even more shaggy to avoid frost damage over the many ice ages that have occurred since then, if their feathers were not that way when they first evolved.

Oedhel Setren
Oedhel Setren

@Drew mac Not only are scales and feathers the same gene that is switched by a hormone, but velociraptors had feathers. You should stay a little more on top of paleontology. This information has been out for over a decade. We even see this transitional stage in chickens in the egg. 

You are also overlooking the fact that feathers aren't build for flight, they are build for reducing drag; which applies to runners as well as swimmers and flyers.

You can claim that my theory has holes, but its not my theory. This is the common consensus among experts. Go watch a few TED talks on the topic. Specifically anything Jack Horner talks about since he's the leading expert in the field of dinosaur-ornithological evolution.

You seem to be backwards-engineering you ideology of birds based on common traits of all bird. That's not how evolution works. Every species needs to be considered independently with more weight put on its habitat. You may consider many of these features of large bird a disadvantage, but I ask you this: if its such a hindrance to have flightless wings, why are these species still around?

Share

Popular Stories

The Future of Food

See more food news, photos, and videos »