Falkowski and his colleagues have measured the abundance of carbon 13, a byproduct of photosynthesis, in deep-sea core samples that go back 205 million years. Because photosynthesis produces oxygen and leaves carbon 13 behind, the presence of carbon 13 in the fossil samples allows scientists to estimate precisely how much oxygen was in the atmosphere at any given time, Falkowski says.

From a steady 10 percent - the level at which dinosaurs flourished - the oxygen percentage rose to 17 percent 50 million years ago and then to 23 percent by 40 million years ago.

"In the fossil record, we see that see that this rise in oxygen content corresponds exactly to a really rapid rise of large, placental mammals," Falkowski says.

"The more oxygen, the bigger the mammals. We argue that the rise in oxygen content allowed mammals to become very, very large - mammals like 12-foot-tall sloths and huge saber-toothed cats. They paved the way for all subsequent large mammals, including ourselves."

The results described in Falkowski's article, "The Rise of Oxygen Over the Past 205 Million Years and the Evolution of Large Placental Mammals," stem from years of analysis of organic and inorganic core samples.

Scientists have been using deep-sea core samples for years, but Falkowski and his colleagues have achieved greater precision in their measurements, thanks to two high-precision, isotope ratio mass spectrometers housed in the geological sciences department at Rutgers.

There were placental mammals on Earth at the time of the great extinction of dinosaurs about 65 million years ago, Falkowski says. They were, however, tiny, limited creatures; the extinction event itself, while eliminating the dinosaurs, did little to further the mammalian domination of the planet.

It was the subsequent spreading of shallow seas, the increase in plant life - and photosynthesis - in addition to the consequent increase in oxygen content that gave the mammals the boost they needed, according to Falkowski.

In the last 10 million years, the percentage of oxygen in the earth's atmosphere has decreased to 21 percent. Falkowski says many scientists believed that great fires burned over the earth about 10 million years ago, reducing the number of trees and, therefore, the amount of photosynthesis and oxygen.

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