Oxygen levels in the atmosphere of the ancient Earth reached the point where they could sustain complex life several hundred million years earlier than has been thought, a study of chemical signatures in Scottish rocks suggests.

The findings, detailed in today's (Nov. 10) issue of the journal Nature, could change our understanding of the timeline of evolution on Earth, potentially pushing the development of multi-celled bacteria and other organisms back in time.

Until oxygen became common enough in the atmosphere to support more complex life, only simple life forms existed on the planet.

Scientists have thought this important shift in oxygen levels took place about 800 million years ago, but the new findings suggest it had happened by 1.2 billion years ago.

"Our findings, which shift this key point in the evolution of life on Earth to a much earlier date than previously proven, will give impetus to further investigations into the timescale of the development of complex life, which followed this event," said study leader John Parnell of the University of Aberdeen in Scotland.

Parnell and his colleagues from the Scottish Universities Environmental Research Centre in Glasgow examined ancient rocks near Lochinver in the northwest Highlands of Scotland and found chemical signatures of bacteria that suggest oxygen from the atmosphere was being used in cellular processes.

"Investigations revealed that these bacteria – which, on a basic level, use sulfur to obtain energy – were also using oxygen in a much more complex and efficient chemical reaction in order to generate their energy and survive," Parnell said in a statement.

The signatures found in the rocks, which would have been located at the bottom of a lakebed at the time, suggest oxygen could have then supported complex life in near-surface lake environments.

Parnell said that more in-depth research is needed to see just how much the findings will affect the understanding of the timeline of evolution and just when life began to evolve into more complex forms.

The study was funded by the United Kingdom's National Environment Research Council.