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.
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