Most scientists now believe that, due to the fossil evidence, birds are an evolutionary offspring of dinosaurs. Now they've discovered that 175 to 275 million years ago, when this evolution began to take place, oxygen fell to dangerously low levels on Earth. This may have led to the major extinction event which eliminated 90% of all the species. It would have made breathing the air at sea level the same as breathing at high altitudes today and could have led to the development of the unusual breathing system found in some dinosaurs, including the brontosaurus, which still exists in today's birds.
Instead of a diaphragm forcing air in and out of their lungs, birds have lungs attached to a series of thin-walled air sacs that move air throughout their bodies. Biologist Peter Ward says, "The literature always said that the reason birds had sacs was so they could breathe when they fly. But I don't know of any brontosaurus that could fly."
Ward thinks the low oxygen level and greenhouse gas conditions were caused by high levels of methane from intense volcanic activity. The methane could also have been released from the ocean floor, where large amounts of it are stored.
Scientists know about five mass extinctions in the Earth's history. One of these occurred about 250 million years ago and another 200 million years ago. The first one killed 90% of all species, while the second killed about 50%. Dinosaurs evolved between these two extinctions and weren't affected by them. "?Dinosaurs skated across the second of these mass extinctions, actually increasing in number as they went along, while everything else was dropping around them," says Ward.
We only know the reason for one of these extinctions, that occurred 65 million years ago when an asteroid impact finally killed off the big dinosaurs. It left only the smaller, birdlike creatures that could breathe in an atmosphere that temporarily contained less oxygen. Ward says, "The reason the birds developed these systems is that they arose from dinosaurs halfway through the Jurassic Period. They are how the dinosaurs survived."
Birds survived by getting in the gap. The gap between Thanksgiving and Christmas is coming up soon, so shop early on our Christmas store.
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