Scottish rocks record ancient oxygen clues

November 13 BBC

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. 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.