New method spots signs of Earth’s primordial life in ancient rocks

By Will Dunham

WASHINGTON (Reuters) – Scientists have detected some of the oldest signs of life on Earth using a new method that recognizes chemical fingerprints of living organisms in ancient rocks, an approach that also holds promise in the search for life beyond our planet.

Researchers found evidence of microbial life in rocks about 3.3 billion years old in South Africa, when Earth was about a quarter of its current age. They also discovered molecular traces left by microbes involved in oxygen-producing photosynthesis (converting sunlight into energy) in rocks around 2.5 billion years old in South Africa.

Scientists have developed an approach, leveraging machine learning, to distinguish organic molecules of biological origin – such as those from microbes, plants and animals – and organic molecules of non-living origin in ancient rocks with greater than 90% accuracy. The method was designed to discern chemical patterns unique to biology.

“The remarkable finding is that we can tease out rumors of ancient life from highly degraded molecules,” said Robert Hazen, a mineralogist and astrobiologist at the Carnegie Institution for Science in Washington and co-senior author of the study published this week in the journal Proceedings of the National Academy of Sciences. “This is a paradigm shift in how we search for ancient life.”

“We collect and concentrate carbon-rich molecules, analyze them to identify thousands of tiny molecular fragments, and then examine their distribution using machine learning. The human eye only sees hundreds or thousands of small ‘spikes’ of different molecules, but the machine learning method brings out subtle patterns that distinguish molecules that were once alive from those that weren’t,” Hazen said.

Scientists searching for evidence of the earliest life on Earth have relied primarily on the discovery of fossil organisms. The Earth was formed approximately 4.5 billion years ago. Its first living organisms may have been microbes that appeared perhaps hundreds of millions of years later in marine hydrothermal vents or terrestrial hot springs.

The oldest definitive fossils of living organisms are mound-shaped microbial deposits called stromatolites, approximately 3.5 billion years old in Australia and microbial mat structures of the same age in South Africa. But these fossils are exceptionally rare.

Another way to find evidence of early life is to look for traces of biomolecules – chemicals related to living organisms – in ancient rocks. The new approach takes this route.

For example, researchers discovered organic molecular evidence that oxygen-producing photosynthesis, which over time oxygenated the planet’s atmosphere and enabled the evolution of complex aerobic life, was underway by marine bacteria more than 800 million years earlier than this type of data had previously documented.

“It was well known, through other evidence, that the Earth had been oxygenated 2.5 billion years ago and perhaps even a little earlier. So we provided the first compelling fossil organic molecular evidence, with the prospect of pushing the record even further,” Hazen said.

All the old biomolecules, such as sugars or lipids like fats, have disappeared and are fragmented into small pieces containing only a handful of carbon atoms. However, the distribution of these fragments is remarkably different between the suites of organic molecules of life and those of non-life.

“First, we have roughly doubled the age at which we can identify signs of life using organic molecules, from 1.6 billion to 3.3 billion years ago,” said study co-lead author Anirudh Prabhu, a mineralogist, astrobiologist and data scientist at the Carnegie Institution for Science.

“Second, this biosignature technique can distinguish not only life from non-life, but also different types of life, such as photosynthetic organisms. Third, our paper shows how machine learning ‍can identify the fingerprints of life in ancient rocks⁠, even when all the original biomolecules are degraded,” Prabhu said.

NASA rovers have collected rock samples on Mars in an effort to find out whether Earth’s neighbor once supported life. Other destinations in our solar system also hold potential in the search for life, including Saturn’s moons Enceladus and Titan and Jupiter’s moon Europa.

The researchers received a grant from NASA to develop their approach to identifying evidence of life.

“Astrobiology is one of the key application areas of our project,” said Prabhu.

Hazen said: “We are very excited about the prospect of using this method on samples ‌from Mars, ideally those returned to Earth, but possibly on a future rover mission. We are also thinking about ways to sample the organic-rich plumes of Enceladus ‌or the surface of Titan or Europa.”

(Reporting by Will Dunham; editing by Daniel Wallis)