Will Dunham
WASHINGTON (Reuters) – Scientists have discovered some of the oldest signs of life on Earth using a new method that recognizes the chemical fingerprints of living organisms in ancient rocks. This approach also holds promise in the search for life beyond our planet.
Researchers have found evidence of microbial life in about 3.3 billion-year-old rocks from South Africa, when the Earth was about a quarter of its current age. They also found molecular traces left by microbes that participated in oxygen-producing photosynthesis (converting sunlight into energy) in about 2.5 billion-year-old rocks from South Africa.
Scientists have developed an approach using machine learning to distinguish between organic molecules of “biological origin” (such as microbes, plants and animals) and organic molecules of non-living origin in ancient rocks with more than 90% accuracy. The method was developed to identify chemical patterns unique to biology.
“The remarkable discovery is that we can identify signatures of ancient life from highly decayed molecules,” said Robert Hazen, a mineralogist and astrobiologist at the Carnegie Institution for Science in Washington and co-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 look for ancient life.”
“We collect and concentrate carbon-rich molecules, analyze them in a way that identifies thousands of tiny molecular fragments, and then look at their distribution using machine learning. The human eye simply sees hundreds or thousands of little “spikes” of different molecules, but the machine learning method reveals subtle patterns that distinguish molecules that were once alive from those that were not,” Hazen said.
Scientists searching for evidence of ancient life on Earth have relied primarily on finding fossil organisms. The earth was formed approximately 4.5 billion years ago. Its first living organisms may have been microbes, which arose perhaps hundreds of millions of years later in marine hydrothermal vents or terrestrial hot springs.
The oldest definitive fossils of living organisms are mounded microbial deposits called stromatolites, about 3.5 billion years old in Australia, and microbial mat structures of similar age in South Africa. But such fossils are extremely rare.
Another way to find evidence of early life is to look for traces of biomolecules—chemicals associated with living organisms—in ancient rocks. The new approach follows this path.
For example, the researchers found organic molecular evidence that the oxygen-producing photosynthesis that eventually oxygenated the planet's atmosphere and fueled the evolution of complex aerobic life was carried out by marine bacteria more than 800 million years earlier than previously documented by this type of data.
“It was well known from other evidence that the Earth began to become saturated with oxygen 2.5 billion years ago, and maybe even a little earlier. “We have thus provided the first compelling fossil organic molecular evidence with the prospect of pushing the record even further,” said Hazen.
All the ancient biomolecules, such as sugars or lipids such as fats, disappeared and fragmented into small pieces containing only a few carbon atoms. However, the distribution of these fragments differs markedly for sets of organic molecules in living and nonliving nature.
“First, we roughly doubled the age at which we can identify signs of life using organic molecules from 1.6 billion to 3.3 billion years,” said study co-author Anirudh Prabhu, a mineralogist, astrobiologist and data scientist at the Carnegie Institution.
“Secondly, this biosignature method 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 fingerprints of life in ancient rocks even when all the original biomolecules have decayed,” Prabhu said.
NASA rovers have collected rock samples on Mars to find out whether life ever existed on Earth's neighboring planet. Other destinations in our solar system also have the potential to search for life, including Saturn's moons Enceladus and Titan, and Jupiter's moon Europa.
The researchers received a NASA grant to develop their approach to identifying evidence of life.
“One of the key applications of our project is astrobiology,” Prabhu said.
Hazen said: “We are very excited about the prospects of using this technique on samples from Mars, ideally those returned to Earth, but perhaps on a future rover mission. We're 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)
