Is oxygen really responsible for the rise of animals on Earth?

A team of scientists studying a prehistoric rock formation in China stumbled on an unexpected finding that, if confirmed, could upend our understanding of how animals first evolved.

By analyzing the chemistry of 1.4-billion-year-old rocks that once lay 300 feet below the sea, scientists concluded that deep, ancient oceans contained enough oxygen to support animal life 800 million years before the first animals actually appeared.

If their finding proves to be true, it will challenge the already murky role that oxygen is thought to have played in the origin of animals, and will raise questions about why, if there was enough oxygen, they didn't evolve sooner. 

For decades, many scientists thought that animals began to appear between about 800 million and 600 million years ago, billions of years after the Earth formed, because it took that long for the atmosphere to develop enough oxygen to support them.

"But sufficient oxygen in itself does not seem to be enough for animals to rise," said Don Canfield, professor of ecology at the University of Southern Denmark and a co-author of a January 4 paper on the topic in the Proceedings of the National Academy of Sciences.

The scientists from Denmark and China collected samples at a well-preserved rock formation 100 miles northwest of Beijing. In these ancient rocks, deposited in the deep ocean more than 1.4 billion years ago, the scientists measured the concentrations of trace metals to determine that the waters contained oxygen that was absorbed from the atmosphere.

“Our job was to figure out what was the minimal amount of oxygen in the atmosphere that was necessary to make its way to the bottom of the ocean,” says professor Canfield in an interview with The Christian Science Monitor.

They used a mathematical model that took into account things we know about the ocean today – such as the rate by which carbon is made on the surface of ocean, the rate by which photosynthetic organisms work, and how fast carbon settles and decomposes – to conclude that more than a billion years ago, the Earth’s atmosphere contained at least 4 percent of the oxygen found today. That amount is necessary for the oxygen to have persisted as the water circulated to the ocean depths, and it would have been more than enough to have sustained animals such as sponges, say the researchers.

"Sponges probably resemble some of the first animals on Earth,” said Canfield in a research announcement. “If they manage with less than 4 percent [of] today's oxygen levels, it is likely that the first animals could do with these concentrations or less."

These findings contradict research last year that concluded the opposite: Earth's atmosphere more than a billion years ago was not hospitable to animals.

In an October 2014 paper in the journal Science, a team of researchers from Yale University, the Georgia Institute of Technology, and other universities reported that by analyzing chromium isotopes in ancient sediments from China, Australia, Canada, and the United States, they calculated that oxygen levels during the same time period were 0.1 percent of what they are today, much lower than what most scientists assumed.

Noah Planavsky, an assistant professor of geology and geophysics at Yale, and a co-author of the Science paper, tells the Monitor that Monday's PNAS paper is "interesting," but questions its chemistry, which he says needs to be verified.

“I’m not saying it has to be wrong," he adds, "but they didn’t do all the steps that you need to convince the community that it’s right.”