Stromatolite formed by ancient prokaryotic microorganisms. Image: totajla/

Complex Life Could Have Existed on Earth at Least Once Before

We might not be the first.

18 JAN 2017















"There is fossil evidence of complex cells that go back maybe 1.75 billion years," said astrobiologist Roger Buick. "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."

"This research shows that there was enough oxygen in the environment to have allowed complex cells to have evolved, and to have become ecologically important, before there was fossil evidence," he added. "That doesn’t mean that they did - but they could have."

That last point is pretty important - we need to make it clear that no one has found fossil or DNA evidence of this earlier complex life.

What the team is saying is that the ingredients were there for it to have evolved hundreds of millions of years before the first eukaryote, but we're still not sure if that actually that happened.

So why wouldn't this hypothetical life have survived until today if it did evolve? The research showed that although there was an early 'bubble' of oxygen in the atmosphere, it crashed shortly afterwards, which would most likely have killed the lifeforms off.


They were looking to see if the selenium had been changed by the presence of oxygen, or oxidised - a reaction that leaves a signature in the ratio of selenium isotopes stored in the rocks.

Traditionally, it was assumed that oxygen on Earth had a history of "none, then some, then a lot", said Buick.


It wouldn't have been enough time for complex life to diversify and take hold on the planet, but it's possible that some complex organisms could have arisen before dying off again when the oxygen crashed.

In the past, researchers had predicted that this early oxygen increase - known as the 'oxygen overshoot' - might have occurred, but this is the first time it has been studied in detail. 


So what caused Earth's oxygen levels to soar early on, only to crash again?

"That’s the million-dollar question," said one of the researchers, Eva Stüeken, from St Andrews University in Scotland. "It’s unknown why it happened, and why it ended."

"It is an unprecedented time in Earth’s history," added Buick. "If you look at the selenium isotope record through time, it’s a unique interval. If you look before and after, everything’s different."

We might never know for sure whether complex life originated and died out before the first eukaryotes, but it's interesting to note that the conditions that paved the way for complex life on Earth might not be quite as rare or unique as we previously thought.


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