Oxygen levels on Earth reached a critical threshold to enable the evolution of complex life much earlier than thought, say scientists.

The evidence is found in 1.2-billion-year-old rocks from Scotland.

These rocks retain signatures of bacterial activity known to occur when there is copious atmospheric oxygen.

The microbes' behaviour is seen 400 million years further back in time than any previous discovery, the researchers tell the journal Nature.

The team is not saying complex life existed 1.2 billion years ago, merely that the conditions would have been right for it to start to take hold.

"We're recording a key stage in the evolution of life on Earth," said Professor John Parnell from the University of Aberdeen.

"The evidence relates to a particular group of microbes that have been very successful through Earth's history and are now found everywhere from glaciers to the deep ocean floor.

"These microbes made an important advance by becoming more efficient, which they did through using oxygen in their environment. So the occurrence of these microbes is a marker for increasing oxygen in the atmosphere," he told BBC News.

Yellow specks

The rocks that record their activity are sited today on the coast near Lochinver in the Northwest Highlands, but they comprise sediments that were once at the bottom of an ancient lake bed.

The researchers do not see the fossil evidence of the microbes themselves - only the chemical traces that they were present and using sulphur in the lake floor as a form of energy.

Image caption The sulphur atoms in the specks of fool's gold betray the activity of microbial life

These traces are evident in specks of the mineral iron pyrite, better known as "fool's gold" for its yellow appearance.

Analysis of the sulphur atoms in the pyrite shows them to be of different types, and in particular fractions, which could only have been produced through biological behaviour, say the scientists.

"There's a certain stage which is achieved by bacteria when they start to work in a more complex way, and they do this by forming a community where some bacteria are turning sulphate into sulphide and there's another lot of bacteria turning the sulphide back to sulphate," explained Professor Parnell.

"And so you start getting cycles of chemical reduction and chemical oxidation, and they drive up the level of fractionation."

Trigger events

Prior to the Lochinver discovery, the oldest rocks known to display this sulphur signature were 800 million years old. The new study now indicates oxygen levels in Earth's atmosphere were raised to such a level that the gas was permeating into lake waters and their sediment beds 1.2 billion years ago.

Image caption Coastal rocks near Lochinver were laid down at the bottom of a lake

The type of biochemistry pursued by complex life requires a lot of oxygen but the fact that levels were elevated at this time does not in itself prove such organisms were also present.

The precise timing of the emergence of complex life on Earth is a topic of intense debate, and marked by claim and counter claim.

Certainly, the earliest indisputable evidence for complex animal life - slug-like organisms called Kimberalla - are not seen until the Ediacaran Period, which came at the end of the last great global glaciation These fossil remains are found today in 555-million-year-old rocks in Australia and Russia.

"What we are now showing is that the conditions in the atmosphere were in place [1.2 billion years ago], so it probably needed some other factor to trigger the early evolution of complex life and the fact that the Ediacaran fauna occur after the 'snowball Earth' episode suggests those two are linked somehow," said Professor Parnell.

Jonathan.Amos-INTERNET@bbc.co.uk