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 have found evidence of microbial life in South African rocks that are about 3.3 billion years old, when the Earth was about a quarter of its current age. They also detected molecular traces left by microbes involved in photosynthesis – converting sunlight into energy – in South African rocks that are around 2.5 billion years old.
Scientists have developed an approach, leveraging machine learning, to distinguish in ancient rocks between organic molecules of biological origin — such as from microbes, plants and animals — and organic molecules of non-living origin with more than 90% accuracy. The method was designed to discern chemical patterns unique to biology.
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“The remarkable discovery is that we can detect whispers of ancient life in highly degraded molecules,” said Robert Hazen, a mineralogist and astrobiologist at the Carnegie Institution for Science in Washington and co-lead author of the study published this week in the journal Proceedings of the National Academy of Sciences.
‘This is a paradigm shift in the way we search for ancient life.’
‘We collect and concentrate carbon-rich molecules, analyze them to identify thousands of tiny molecular fragments, and then observe their distributions with machine learning. The human eye only 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 weren’t,’ said Hazen.
Scientists searching for evidence of ancient life on Earth have relied primarily on fossil organisms. The planet was formed approximately 4.5 billion years ago. Their first living organisms may have been microbes that emerged perhaps hundreds of millions of years later in marine hydrothermal vents or terrestrial vents.
The oldest definitive fossils of living organisms are mound-like microbial deposits in Australia called stromatolites, about 3.5 billion years old, and microbial mat structures of similar 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 follows this path.
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Researchers have discovered, for example, organic molecular evidence that photosynthesis, which over time oxygenated the planet’s atmosphere and enabled the evolution of complex aerobic life, was carried out by marine bacteria more than 800 million years earlier than had previously been documented with this type of data.
‘It was well known from other evidence that the Earth became oxygenated 2.5 billion years ago and perhaps even a little earlier. We therefore provide the first convincing fossil organic molecular evidence, with the prospect of taking the record even further back,’ said Hazen.
All the old biomolecules, like sugars or lipids, disappeared and fragmented into small pieces with just a handful of carbon atoms. However, the distribution of these fragments is remarkably different in sets of organic molecules when there is life.
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“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,” said study co-author Anirudh Prabhu, a mineralogist, astrobiologist and data scientist at the Carnegie Institution for Science.
‘Secondly, 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,’ said Prabhu.
NASA rovers have collected rock samples on Mars to find out whether Earth’s planetary neighbor once harbored life. Other destinations in our solar system also have potential in the search for life, including Saturn’s moons Enceladus and Titan and Jupiter’s moon Europa.
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The researchers received a grant from NASA to develop their approach to identifying evidence of life.
‘One of the main areas of application of our project is astrobiology,’ said Prabhu.
Hazen said he was ‘excited’ about the prospect of applying the method to samples from Mars, ‘preferably those returned to Earth, but possibly on a future rover mission’.
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“We are also thinking about ways to sample the organic-rich plumes of Enceladus or the surface of Titan or Europa.”
