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 evolutionary 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 pretty quiet and Earth entered a period known as the boring billion. It wasn’t until some 800 million years ago that proper multicellular animals started to appear on the scene (SN: 12/31/11, p. 12).
Researchers have been unclear about the role of environmental factors in this delay, especially the availability of oxygen, a requirement for animal metabolism (SN:9/7/13, p12), 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,” says Butterfield, who was not involved with the new research. “So the idea is, what’s been delaying the rise of animals?”
Previous estimates of the boring billion’s atmospheric oxygen levels — a proxy for oxygen levels in the shallow oceans, where animal life really got going — have varied widely, from 1 to 40 percent of today’s levels. The new analysis examined versions of the metal chromium in ancient sediments collected in China, Australia, the United States and Canada. In an oxygen-free environment, chromium exists in one form. When there’s oxygen around, it reacts with other metals. Heavier versions of naturally occurring chromium are more likely to undergo these oxygen-related reactions and get washed into the oceans. By looking for evidence of this chromium cycling in ancient rocks, scientists can deduce how much oxygen was present.
There was a marked increase in chromium-53 in samples that date to roughly 800 million to 750 million years ago, researchers report in the Oct. 31 Science. This shift suggests a rise in environmental oxygen that would have enabled the rise of complex life, the team proposes. The team also looked for the heavy chromium in four samples from shale deposits that ranged from 1.7 billion to 900 million years old and found no evidence of the chromium cycling during the boring billion.
“This breathes life, so to speak, into the idea that the diversification of life on the planet was controlled by the environment,” says geochemist Noah Planavsky of Yale University, who led the research.
Butterfield disagrees. Invoking some external environmental trigger to allow diversification isn’t necessary, he says. He maintains that the difficult task of evolving complex animals’ gene and regulatory networks would have taken a long time even in ideal conditions.
Framing the debate as one or the other — environmental versus internal developmental and genetic constraints — is probably overly simplistic, says Doug Erwin, a paleobiologist at the Smithsonian Institution in Washington, D.C. The new research adds to data suggesting that the environment during this period was highly unstable, and rather than a tidy, gradual rise in oxygen levels, concentrations may have been highly dynamic.
“This is really valuable data,” says Erwin. “These geochemical studies are critical to improving our understanding the environmental context of animal divergence.”
Editor’s note: This story was updated on November 7, 2014, to correct the description of chromium cycling. A previous version suggested that oxygen causes reactions that convert chromium-52 to chromium-53, which is incorrect.