Did Rising Oxygen Levels Fuel Mammal Evolution?

Nicholas Bakalar
for National Geographic News
September 30, 2005
Humans and other mammals have flourished on Earth, and one important change in their environment may help explain why: a large increase in the concentration of atmospheric oxygen.

Writing in the current issue of Science, researchers report that over the past 205 million years the concentration of oxygen in the air has more than doubled.

The successive increases in oxygen levels coincided first with the appearance of warm-blooded animals, then with the evolution of placental reproduction, and finally with the increasing size of mammalian species, the scientists say.

How can scientists know how much oxygen was in the air 205 million years ago? Atmospheric oxygen comes from the splitting of water molecules, a combination of hydrogen and oxygen, by sunlight.

How Plants Grow

Plants grow using the hydrogen part of the water molecule. The more plants grow, the more organic matter is buried in marine sediments when the plants die. Scientists can infer the amount of oxygen in the atmosphere from the amount of organics in the sediment.

Knowing the age of the sediments from the geologic record, the scientists were able to figure out how much oxygen was in the atmosphere at any given time.

The amount of atmospheric oxygen continues to gradually rise. "A major reason for the rise of oxygen," said Paul Falkowski, the lead author on the study and a professor of biochemistry and biophysics at Rutgers University, "is that continental margins along the Atlantic Ocean have been growing."

Long Cycle

Over the next 100 million years, the continents will move closer together, organic carbon at the edges of the continents will be absorbed into the Earth's crust, and oxygen levels will decrease.

"[A] cycle of rise and fall of oxygen occurs on time scales of about 350 million years," Falkowski said.

The amount of oxygen in the atmosphere was at a low point in the early Triassic period, about 245 million years ago, increasing to approximately 18 percent by the end of the Mesozoic 65 million years ago.

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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