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Structures found in 3.48-billion-year-old Australian rocks are the oldest evidence of life on Earth.
Detailed analyses of geological samples from the Dresser Formation conclude that, despite previous scientific controversy, they represent fossils formed by early life and could provide hints of what scientists should look for on Mars.
The Australian outback could help to establish whether life ever existed on the red planet.
Around 3.5 billion years ago, the area where Western Australia's Dresser Formation is now found would have featured shallow lagoons fed by water enriched in nutrients due to volcanism and hydrothermal activity.
These lagoons are believed to have been inhabited by photosynthetic organisms, with the fossilised remains of the structures they formed preserved within the sedimentary rocks of the Dresser Formation.
On Mars, very similar habitats could have existed more than three billion years ago when the planet is considered to have been habitable. If life ever existed on Mars, it is possible that similar fossilised remains could be found.
In a new study, published in the journal Geology, researchers examined samples from the Dresser Formation in greater detail than ever before.
Not only do they add weight to arguments that these structures represent some of the earliest traces of life on this planet, but also provide a dry run of the process that will be performed on Martian rocks when they are returned to Earth.
Dr Keyron Hickman-Lewis, a Research Fellow at the Museum who led the study, says, 'The Dresser Formation is one of Earth's oldest sedimentary layers and is well-preserved. It is also one of the oldest sites comparable to the kinds of environments we might expect on Mars.'
'Although our samples no longer contain microfossils or organic materials, they nonetheless have many characteristic structures that are consistent with having a biological origin. Imaging these structures with high-resolution techniques allows us to build a compelling case for how they might have formed due to the action of life.'
The biological structures found in the Dresser Formation are known as stromatolites, which are the preserved remains of 'microbial mats' stacked on top of one another. These mats form when communities of bacteria and other microbes secrete sticky substances that bind them together.
Over time, complex structures can develop as new mats grow on top of old ones. The mats nearest the top of these formations tend to contain photosynthetic organisms, while lower mats have different methods of sourcing energy such as the extraction of chemical energy.
The stacking of these mats can trap minerals between them, forming distinct structures that can be preserved. These structures, which can be observed in stromatolites formed today by living microbial mats, can then be used to identify similar forms in the fossil record.
'Dome-shaped structures with layers thickening toward the crest of the dome are characteristic of photosynthetic growth,' Keyron explains. 'Microbes do not grow at the same rate throughout a microbial mat, and these structures indicate growth towards a source of nutrients, which in this case is the Sun.'
'The other objects visible in the stromatolites are column-like structures separated by flat layers which appear very similar to a microbial growth texture called palisade structure. These are the result of upward microbial growth and are a major structural element of these layers.'
These distinctive characteristics, together with an ensemble of microscopic structures, can be used to identify potential life in the past even where no organic material remains.
A general lack of organic material in the Dresser Formation stromatolites has meant that their interpretation as biological structures is controversial. Other scientists have proposed that the structures could form non-biologically or have concluded that there is not enough evidence to decide either way.
The new paper uses a variety of techniques, including microscopy, chemical analysis and 3D scanning, to build a case that these structures have structural characteristics that could only represent evidence of life.
'If an archaeologist finds the foundations of a ruined city, they would know that it was something built by people as it would have all the hallmarks of having been built by people, such as bricks and doorways,' Keyron says.
'Similarly, stromatolites have structural elements that indicate their construction by microbes. This allows us to be archaeologists in deep time, even if the architects of the structures we study are much smaller.'
Certain characteristics of the environment of the Dresser Formation are similar to those which are believed to have existed at the edges of Mars's Jezero crater, which is currently being explored by NASA's Perseverance rover. Over three billion years ago, the crater would have formed a lake which could potentially have provided a home for Martian life.
'The Dresser Formation allows us a close comparison with early Mars,' Keyron says. 'We know that volcanism was once widespread on Mars, and we anticipate that hydrothermal activity was also present as they often co-occur.'
Finding evidence of Martian life is one of the goals of the Perseverance rover, which has a range of sophisticated equipment that could be used to find traces of past microbial life.
Professor Caroline Smith, the head of the Museum's Earth Science collections and co-author on the paper, says, 'Perseverance's sensors might be able to detect and analyse stromatolites in situ if they're present.'
'It depends on the size, preservation, and commonness of potential stromatolites on the planet. My view is that the rover's imaging systems should see them, but only if they are there and we're looking in the right place.'
At the time of writing, Perseverance has most recently taken samples from the Enchanted Lake region at the base of the Jezero delta. These samples are being stored, along with others taken by the rover, so that they can be brought to Earth in a future sample return mission.
This would give scientists a much better idea if traces of life are present, and they would be studied in a very similar way to how the Dresser Formation samples were analysed in this study. Caroline and Keyron work closely with NASA, and so could be among the researchers tasked with investigating returned samples in the future.
'This paper mimics how we hope to study the outcrops observed by the Perseverance rover,' Caroline adds. 'The rover's cameras would identify promising sites and then use close-up imaging systems like WATSON to analyse the samples. Other instruments, such as PIXL and SHERLOC, could be used to perform mineralogical and chemical tests as well.'
'However, the fine details would need to be investigated on Earth, and it is only when all of these techniques are added together that we would have the confidence to say that these are ancient stromatolites.'
It is hoped that the sample return mission will launch within the next six years, with the Martian samples arriving on Earth by 2033. Until then, scientists will continue to refine our methods of searching for extraterrestrial life so that they are prepared for the return of the first samples ever collected from another planet.