All we needed was a little breathing room. Although the extinction of the dinosaurs may have made ecological space for mammals, increases in oxygen over the past 200 million years have allowed us to flourish, get bigger, and diversify, according to a new study.
Atmospheric oxygen levels have fluctuated throughout Earth's history, sometimes with catastrophic consequences. A sudden drop in oxygen, from roughly 30% of the atmosphere to about 10%, may have contributed to the mass extinctions at the end of the Permian period, 250 million years ago (ScienceNOW, 15 April). Since then, oxygen has been mostly on the rise. Today's level is 21%, helped along by several big boosts around 200 million and 50 million years ago. The exact causes for those jumps are unclear, but the most recent increase appears to coincide with a large, widespread bloom of tiny, ocean-dwelling plants called diatoms.
As it turns out, those increases correspond to significant changes in placental mammal evolution, says marine biologist Paul Falkowski of Rutgers University in New Brunswick, New Jersey. Falkowski and his team constructed a detailed 200-million-year record of atmospheric oxygen with carbon and sulfur isotopes from marine sediment cores. The rise of small mammals and birds around the 200 million year mark and the more recent rapid flourishing and diversification of large placental mammals are closely tied to large jumps in atmospheric oxygen, the team reports 30 September in Science.
It makes sense that more oxygen should be good for mammalian evolution, Falkowski says. Mammals are 4 to 6 times less efficient in turning food to energy than reptiles are, so they need higher oxygen concentrations for muscles to work effectively. Mammals are the Humvees of the animal kingdom, Falkowski says. Placental mammals also need more oxygen to nourish their embryos, he adds. In addition, larger animals need more oxygen than smaller ones because they have a harder time getting blood to their tissues, so higher levels in the atmosphere would have given them more opportunity to evolve.
The results are already sparking some fascinating new research directions, says astrobiologist and paleontologist Peter Ward of the University of Washington in Seattle. Scientists are now beginning to investigate how oxygen levels can affect the development of modern nonmammalian species, such as alligators and some invertebrates.