A newly compiled comprehensive record of Earth¿s atmospheric oxygen shows a large peak 50 million years ago. The abundance of oxygen, which came on the heels of the dinosaur decline, could have fueled not only the evolution of giant, placental mammals such as the 10-foot sloth but also large-brained creatures, including humans.

The data come from deep-sea sediment cores dating to 205 million years ago that contain inorganic carbon-rich minerals as well as the organic remains of single celled marine phytoplankton. These organisms generated oxygen through photosynthesis and in the process, left behind a chemical signature by changing the ratio of the two stable isotopes of carbon--carbon 13 and carbon 12--in the sediments.

By comparing the amount of carbon 13 in the inorganic particles with the absence of carbon 13 in the organic matter, scientists can estimate how much oxygen was present in the atmosphere at that time. Paul Falkowski and his team at Rutgers, the State University of New Jersey analyzed such evidence as well as related data published earlier by a German team, and concluded that oxygen levels have essentially doubled over the last 205 million years.

At the beginning of this time period, the atmosphere contained only 10 percent oxygen by volume. And mammals, which appeared around 190 million years ago, were tiny, weighing in at about two grams. Around 50 million years ago, in contrast, oxygen levels in the atmosphere measured 23 percent--2 percent higher than today. "It's exactly at this time, when we look at the fossil record, that we see the extreme radiation of large orders of new placental mammals," Falkowski says.

In general, mammals require large amounts of oxygen just to move and to metabolize their food. Mammalian brains also burn about one third of the total calories consumed. And mammal mothers use up about 60 percent of the oxygen in their blood before it gets to the placenta. More oxygen in the air would have contributed to better conditions in the womb.

Since the mammalian heyday, oxygen levels have dropped to about 21 percent, a reduction that may have been caused by great fires that burned over the earth about 10 million years ago, destroying large swaths of oxygen-producing vegetation. A paper detailing these findings appears in the current issue of Science.