Humans and other mammals have flourished on Earth, and one important change in their environment may help explain why: a large increase in the concentration of atmospheric oxygen.
Writing in the current issue of Science, researchers report that over the past 205 million years the concentration of oxygen in the air has more than doubled.
The successive increases in oxygen levels coincided first with the appearance of warm-blooded animals, then with the evolution of placental reproduction, and finally with the increasing size of mammalian species, the scientists say.
How can scientists know how much oxygen was in the air 205 million years ago? Atmospheric oxygen comes from the splitting of water molecules, a combination of hydrogen and oxygen, by sunlight.
How Plants Grow
Plants grow using the hydrogen part of the water molecule. The more plants grow, the more organic matter is buried in marine sediments when the plants die. Scientists can infer the amount of oxygen in the atmosphere from the amount of organics in the sediment.
Knowing the age of the sediments from the geologic record, the scientists were able to figure out how much oxygen was in the atmosphere at any given time.
The amount of atmospheric oxygen continues to gradually rise. "A major reason for the rise of oxygen," said Paul Falkowski, the lead author on the study and a professor of biochemistry and biophysics at Rutgers University, "is that continental margins along the Atlantic Ocean have been growing."
Long Cycle
Over the next 100 million years, the continents will move closer together, organic carbon at the edges of the continents will be absorbed into the Earth's crust, and oxygen levels will decrease.
"[A] cycle of rise and fall of oxygen occurs on time scales of about 350 million years," Falkowski said.
The amount of oxygen in the atmosphere was at a low point in the early Triassic period, about 245 million years ago, increasing to approximately 18 percent by the end of the Mesozoic 65 million years ago.
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