Ostrich why cannot 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 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, 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. The results were staggering, Baker says. The tinamous evolved within ratites, not as a separate lineage.

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, 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 tinamous' place on the evolutionary tree offers a glimpse into the origins of flightlessness. The researchers conjecture that in order to escape the threat posed by predatory dinosaurs, the ancestors of ratites kept flying.

As to why birds might evolve to lose flight, "wings are a big drain on resources if not being used and larger birds are basically better at converting food into growth and reproduction," Phillips told LiveScience. This growth in size and flightlessness enabled birds to fill some of the same niches their reptilian cousins once did.

For instance, once so-called " terror birds " roamed the land, now-extinct predators with curved beaks as much as 18 inches long that, while flightless, were only distant relatives of ratites. The mass extinction also is what scientists think allowed the rise of larger mammals. The scientists detailed their findings in the January issue of the journal Systematic Biology. Live Science. New research shows they may have evolved this way due to tweaks in DNA that bosses genes around.

Emus, ostriches, kiwis, rheas, cassowaries and tinamous all belong to a group of birds called ratites. So do the extinct moa and elephant birds. Of these, only tinamous can fly. The researchers found that mutations in regulatory DNA caused ratites to lose flight. The researchers reported their results April 5 in Science. Studying how this bossy DNA drives evolution could shed light on how closely related species can evolve such different traits.

Genes are pieces of DNA that hold instructions for making proteins. In turn, the proteins do tasks inside your body. Instead, it controls when and where genes turn on and off. Researchers have long debated how big evolutionary changes happen, such as gaining or losing flight.

Is it because of mutations — changes — to protein-making genes that are tied to the trait? Or is it mainly because of tweaks to the more mysterious regulatory DNA?

Scientists often had stressed the importance in evolution of changes in the genes that code for or make proteins. Examples are relatively easy to find. In general, mutations that change proteins are likely to do more damage than changes to regulatory DNA, says Camille Berthelot. That makes those changes easier to spot. One protein may have many jobs throughout the body. Each piece of bossy DNA might work in only one or a few types of tissue.

So changes can add up in those bits of DNA as animals evolve. And they may have changed a lot from species to species.