Evidence found in ancient rocks that once lay on a Scottish lake floor show the Earth's atmosphere was able to sustain complex life 1.2 billion years ago - 400 million years earlier than previously accepted.
According to the researchers, the findings published today in Nature could lead to new understandings of when complex life, from which humans and animals developed, evolved on Earth.
Lead author Professor John Parnell, of the University of Aberdeen, says previously it was believed a dramatic rise in oxygen levels in the Earth's atmosphere took place about 800 million years ago.
This increase in oxygen marked the beginning of a move from simple organisms to the development of complex multi-cellular organisms, which eventually led to higher life forms.
However, Parnell says chemical signatures of bacteria found in ancient rocks near Lochinver in the north-west Highlands of Scotland, push this key event in evolution back in time by about 400 million years.
Analysis of the 1.2 billion-year-old rocks show the bacteria that use sulphur to generate energy in order to survive were also using oxygen in a more complex chemical reaction that formed part of a so-called sulphur cycle of oxidation and reduction.
He says evidence of this chemical reaction shows levels of oxygen in the atmosphere were, at this much earlier stage in Earth's history, already at this key point for evolution.
Time to rethink the timescale
"Our findings will give impetus to further investigations into the timescale of the development of complex life, which followed this event," Parnell says.
"The whole story indicates that when animal life was kicked off - probably after the Snowball Earth episode at about 600 million years ago - the atmospheric composition was already suitable and not a barrier to development."
Previously ancient sediments have shown increases in atmospheric oxygen concentration about 2.3 billion years ago (the Palaeoproterozoic era) and 0.8 billion years ago (the Neoproterozoic era).
It is this latest rise in oxygen levels that is thought to be connected to the subsequent evolutionary expansion of animal life.
Professor Malcolm Walter, director of the Australian Centre for Astrobiology at the University of New South Wales -whose team is internationally known for its work on West Australian stromatolites - says the paper "adds a significant new data point in the oxygenation of the atmosphere".
He says the finding could change long-held views on animal evolution as it indicates the oxygen that was needed for animal life was now known to be present earlier.
However the earliest known animal fossils date back to about 600 million years.
Walter says it is important other researchers follow up on these findings and add to this study.
"So far this [finding] relates to only one deposit," he says.
He says one of his students was now working at ancient lake sites in the Pilbara in north-west Australia, that were dated at more than 2.7 billion years old.
Walter believes this paper suggests it would be worth looking for similar oxidising bacteria there.