A new study of some of the world's oldest rocks suggests life on Earth could be 1.2 billion years older than we thought.
Life requires gene-building nitrogen to exist, and these new findings show the element appeared on Earth as far back as 3.2 billion years ago, the University of Washington reported.
"People always had the idea that the really ancient biosphere was just tenuously clinging on to this inhospitable planet, and it wasn't until the emergence of nitrogen fixation that suddenly the biosphere become large and robust and diverse," said co-author Roger Buick, a UW professor of Earth and space sciences. "Our work shows that there was no nitrogen crisis on the early Earth, and therefore it could have supported a fairly large and diverse biosphere."
To make their findings a team of researchers analyzed 52 samples ranging from 2.75 to 3.2 billion years old gathered in South Africa and northwestern Australia. Even the oldest samples showed chemical evidence that life was pulling nitrogen out of the air at the time of formation. In these samples the ratio of heavier to lighter atoms indicated the presence of nitrogen-fixing enzymes found in single-celled organisms.
"Imagining that this really complicated process is so old, and has operated in the same way for 3.2 billion years, I think is fascinating," said lead author Eva Stüeken, who did the work as part of her UW doctoral research. "It suggests that these really complicated enzymes apparently formed really early, so maybe it's not so difficult for these enzymes to evolve."
The findings come as a surprise because genetic analysis of nitrogen-fixing enzymes suggested this type of life did not appear until at least 2.2 billion years ago. The researchers believe some early life may have existed on land in single-celled layers in the Earth's early days; these organisms would have exhaled small amounts of oxygen that reacted with the rock releasing molybdenum, which was detected in the samples.
"We'll never find any direct evidence of land scum one cell thick, but this might be giving us indirect evidence that the land was inhabited," Buick said. "Microbes could have crawled out of the ocean and lived in a slime layer on the rocks on land, even before 3.2 billion years ago."
The findings were published in a recent edition of the journal Nature.