Then there was a spike to about 23 percent during the Eocene period between 55 and 38 million years ago. Over the past 10 million years, the concentration of oxygen declined slightly to today's 21 percent.
The rapid decline in oxygen levels at the end of the Permian and the beginning of the Triassic was a major factor in contributing to the extinction of many land animals, mostly reptiles.
Mammals Debut
Then, the authors write, over the next 150 million years the concentration of atmospheric oxygen began to increase. By the middle Jurassic—roughly 150 million years ago—animals with high oxygen demands had evolved. These included small mammals and the theropod dinosaurs, the closest dinosaur relatives of today's birds.
Between 100 million and 65 million years ago, the fossil record reveals, there was a significant flourishing of placental mammals—species that develop in the womb.
Placental reproduction requires high ambient oxygen concentrations, and the spread of these species coincided with a period of high and stable levels of oxygen in the atmosphere.
The requirement persists today: Because they need significant concentrations of oxygen, few living mammals can live and reproduce at elevations greater than about 14,800 feet (4,500 meters).
Mammalian Opportunism
Sixty-five million years ago the catastrophic extinction of dinosaurs opened an ecological niche for placental mammals. But these small animals could not have taken advantage of the opportunity without an increase in oxygen concentrations.
Fortunately for them (and us), for 10 million years during the Eocene era, 50 to 40 million years ago, oxygen concentrations rose. Mammals began to increase in size, number of species, and geographic distribution.
Many of today's living placental mammals appeared during the early Eocene, and the large land-based plant-eaters spread widely during this time. Less dramatic increases in size occurred through the Miocene era, about 25 million to 5 million years ago.
The authors acknowledge that an increase in oxygen levels is not the only force guiding mammalian evolution, and other experts agree. Robert Asher, curator of mammals at the Berlin Museum of Natural History, said that the authors have found "a fascinating correlation of global oxygen levels with diversification and size of mammals."
But is global oxygen is the magic bullet that explains the evolution of mammals 50 to 40 million years ago? "My guess," Asher said, "would be no. … Like most other issues, there are a number of causative factors involved, including chance."
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