Very low levels of oxygen on Earth during the its early days, only 0.1 percent of what it is today, delayed the rise of animals, new research shows.
The researchers wondered why animals began to prosper at the end of the Proterozoic period about 800 million years ago - and not about the billion-year stretch before that.
"We are providing the first evidence that oxygen levels were low enough during this period to potentially prevent the rise of animals," said co-author of the study, Noah Planavsky, professor at the Yale University in the US.
The scientists found their evidence by analysing chromium isotopes in ancient sediments from China, Australia, Canada and the United States.
Chromium is found in the Earth's continental crust and chromium oxidation is directly linked to the presence of free oxygen in the atmosphere.
Specifically, the team studied samples deposited in shallow, iron-rich ocean areas, near the shore.
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They compared the data with other samples, taken from younger locales, deposited in similar setting but known to have higher levels of oxygen.
It seems clear that there is a first-order difference in the nature of Earth surface chromium cycling before and after the rise of animals, the authors concluded.
The study appeared in the journal Science.