Earth is the only home we know, but it was not always one we could live in.
The oxygen we depend on with every breath was essentially absent when Earth formed about 4.5 billion years ago. It wasn't until about 2.4 billion years ago, when our planet was nearly half its present age, that the gas became a lasting feature of its atmosphere in a transformation known as the Great Oxidation Event. For scientists, the harder question has been how oxygen stayed abundant for millions of years afterward — long enough, as ancient rocks suggest, to set the stage for the emergence of increasingly complex life. The answer may lie in the chemistry of ancient seas, according to a new study.
The research, led by Lewis Alcott, a biogeochemist at the University of Bristol in the U.K., describes a self-sustaining cycle in which phosphorus — a fundamental building block of all known life — was recycled in the oceans, helping maintain elevated oxygen levels after they first rose.
"Living things cannot grow or function properly without phosphorus," study co-author Andrey Bekker, a professor of geology at the University of California, Riverside, said in a statement.
A hidden period of Earth's history
The new study offers a clearer view of a period of Earth's history that has been difficult to reconstruct, according to the scientists. Until now, scientists could measure the total phosphorus preserved in ancient rocks, but determining the share that was actually available to marine life remained challenging.
Alcott, Bekker, and their colleagues got around that limitation with a new technique that separates phosphorus according to the type of mineral it is bound to. Analyzing ancient rocks from South Africa, the team dissolved each mineral type one at a time to determine whether the phosphorus had been available to living organisms or tied up in forms that made it unusable.
"We can now separate the phosphorus that was available to organisms from phosphorus that was essentially locked away," Bekker said in the statement. "That gives us a much clearer picture of nutrient levels in ancient oceans than we had before."
"A side effect of that process is that more oxygen continued to be released into the atmosphere," Bekker said.
Beyond shedding light on the history of our own planet, understanding how oxygen, nutrients and life evolved together on Earth could also help scientists recognize similar conditions on ocean-bearing worlds elsewhere, the researchers added.
"Earth's history shows that oxygen, nutrients, and life evolved together," Bekker said. "Understanding those connections gives us a more nuanced perspective on our own planet's future and what we might look for on other planets."
This research is described in a paper published on Aug. 27 in the journal Nature Communications.
