| | Big Mammals Evolved Thanks to More Oxygen: Scientific American |
 | | 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. |
 | | 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. |
 | | 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. |
| www.sciam.com /article.cfm?chanID=sa003&articleID=00065C28-A06A-133D-A06A83414B7F0000 (604 words) |