Researchers measuring levels of carbon 13 - a byproduct of photosynthesis - in deep-sea core samples going back 205 million years have identified a sharp rise in oxygen levels about 50 million years ago. The increase in oxygen levels gave mammals the evolutionary boost they needed to dominate the planet, says researcher Paul Falkowski, from Rutgers University. Because photosynthesis produces oxygen and leaves carbon 13 behind, the presence of carbon 13 in the fossil samples allows precise measurements of how much oxygen was in the atmosphere at any given time. Falkowski said that when dinosaurs flourished the oxygen level was around 10 percent, but increased 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 explained. "The more oxygen, the bigger the mammals. We argue that the rise in oxygen content allowed mammals to become very, very large - mammals like… huge saber-toothed cats. They paved the way for all subsequent large mammals, including ourselves."
While there were placental mammals on Earth at the time of the great extinction of dinosaurs about 65 million years ago, they were small, limited creatures. The dinosaur extinction event itself 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's study in the journal Science.
Over the last 10 million years, the oxygen level has decreased to around 21 percent, which many scientists believe was caused by great fires that burned over the earth about 10 million years ago. These fires reduced the number of trees and, therefore, the amount of photosynthesis and oxygen.
Source: Rutgers University
