Earth's oldest rocks reveal life may have thrived on planet 3.2bln years ago

Washington: A new research looking at some of the planet's oldest rocks has revealed that life may have thrived on planet 3.2 billion years ago.

The ability to use atmospheric nitrogen to support more widespread life was thought to have appeared roughly 2 billion years ago. Now research from the University of Washington looking at some of the ancient rocks finds evidence that, 3.2 billion years ago, life was already pulling nitrogen out of the air and converting it into a form that could support larger communities.

The authors analyzed 52 samples ranging in age from 2.75 to 3.2 billion years old, collected in South Africa and northwestern Australia. These are some of the oldest and best-preserved rocks on the planet.

The rocks were formed from sediment deposited on continental margins, so are free of chemical irregularities that would occur near a subsea volcano. They also formed before the atmosphere gained oxygen, roughly 2.3 to 2.4 billion years ago, and so preserve chemical clues that have disappeared in modern rocks.

Even the oldest samples, 3.2 billion years old - three-quarters of the way back to the birth of the planet - showed chemical evidence that life was pulling nitrogen out of the air. The ratio of heavier to lighter nitrogen atoms fits the pattern of nitrogen-fixing enzymes contained in single-celled organisms, and does not match any chemical reactions that occur in the absence of life.

Genetic analysis of nitrogen -fixing enzymes has placed their origin at between 1.5 and 2.2 billion years ago. The authors hypothesize that this may be further evidence that some early life may have existed in single-celled layers on land, exhaling small amounts of oxygen that reacted with the rock to release molybdenum to the water.

Future work will look at what else could have limited the growth of life on the early Earth. Stueken has begun a UW postdoctoral position funded by NASA to look at trace metals such as zinc, copper and cobalt to see if one of them controlled the growth of ancient life.

The study is published in Nature.