Magazine article Science News

Early Animals Couldn't Catch a Breath: Low Oxygen Levels May Have Hindered Evolution of Complex Life

Magazine article Science News

Early Animals Couldn't Catch a Breath: Low Oxygen Levels May Have Hindered Evolution of Complex Life

Article excerpt

The diversification of early animals may have been suffocated by a lack of oxygen. A new analysis of ancient rocks offers a glimpse of conditions in the millions of years leading up to the proliferation of animals. The data suggest that oxygen levels were less than 1 percent of today's levels, low enough that they may have stalled the emergence of animal life.

Scientists have been puzzled by a prominent lag in life's timeline. Around 2.3 billion years ago, cyanobacteria were producing such quantities of oxygen that scientists refer to that time as the Great Oxygenation Event. But then things got quiet, and Earth entered a period known as the boring billion. It wasn't until some 800 million years ago that multicellular animals appeared (SN: 12/31/11, p. 12).

Researchers have been unclear about the role of environmental factors in this delay, especially the availability of oxygen (SN: 9/7/13, p. 12), says paleobiologist Nicholas Butterfield of the University of Cambridge. "Large things don't appear on the scene until very late, not until 500 to 600 million years ago, which is sort of yesterday," he says.

Previous estimates of the boring billion's atmospheric oxygen levels--a proxy for oxygen in the shallow oceans, where animal life emerged--have varied from 1 to 40 percent of today's levels.

The new analysis examined the metal chromium in ancient marine sediments collected in China, Australia, the United States and Canada. When oxygen is present, chromium reacts with other metals. …

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