Complex life on Earth is generally thought to have appeared at least 1.75 billion years ago. But a new study suggests there may have been an earlier period where complex life could have evolved, before disappearing and then reappearing again.
The theory was put forward by a study led by the University of Washington, published today in the Proceedings of the National Academy of Sciences. They describe how isotopic ratios in the element selenium in sedimentary rocks suggest a high presence of oxygen in Earth’s atmosphere between 2 and 2.4 billion years ago.
The suggestion is that for this relatively brief period in Earth’s 4.5-billion-year history, conditions may have been favorable for complex life. Previously, it had been thought that oxygen on Earth went through a period of none, then some, then a lot, when eukaryotes – animals, plants, fungi, and protists – came into existence. But this research suggests there was a spike before “none” and that it dropped down again.
“There is fossil evidence of complex cells that go back maybe 1.75 billion years,” said study co-author Roger Buick from the University of Washington in a statement. “But the oldest fossil is not necessarily the oldest one that ever lived – because the chances of getting preserved as a fossil are pretty low.”
However, that’s not to say life did exist in this earlier period. Buick added that the research showed there was enough oxygen to allow complex cells to evolve and become ecologically important, but that does not necessarily mean that they did.
This isn’t the first time this theory of increased oxygen earlier in Earth’s history has been proposed, but it does provide some additional possibilities, such as Earth’s atmosphere and surface ocean experiencing an increase in oxygen, but not the deep ocean.
What’s not clear, though, is why this happened. Eva Stüeken from the University of St Andrews, another study co-author, said that was the “million-dollar question”.
Finding out more about this possible event could have implications for studying planets outside the Solar System, too. The researchers noted that if we find oxygen in the atmosphere of a distant exoplanet, it may not necessarily hint at a complex biosphere.