Danish teacher sows doubts about earlier discoveries of traces of the world's oldest life

The oldest life we ​​know on earth is a simple unicellular bacteria.

Researchers believe they have found traces in the Isua Strait near Nuuk in southwestern Greenland.

But there are major disagreements about discovery.

"It's important to clarify what's going on before we know it's a matter of life, otherwise we risk finding false traces on Mars," says Minik Rosing, professor at the National Museum of Natural History. from Sweden and one of the researchers behind a study, which just appeared in the journal Nature.

It's no coincidence that he mentions Mars, as the first author of the new study, Abigail Allwood, is employed by NASA's Jet Propulsion Lab.

Traces of algae and bacteria

In 2016, an international research group described a sensational discovery. They had found stromatolites in Greenland stones 3.7 billion years old, the oldest traces of life.

Stromatolites are distinctive structures that algae and bacteria can form in minerals. When the algae are in the mud of the seabed and mineral dust covers them, they grow through the top layer several times to promote photosynthesis.

For a long time, they form a layered structure in sediments, which can be seen as cones that cross the layers. When stromatolite descends into the soil over time, the structure is encapsulated in stone due to pressure and heat.

And in 2016, the research group, led by Professor Allen P. Nutman of Wollongong University in Australia, could present the results of a fossil stromatolite preserved after 3.7 billion years in a stone found in Isua.

Stromatolites or not

This discovery caused the blocking of the eyes of many researchers. One of them was Minik Rosing, who, along with Abigail Allwood of NASA, returned to Isua to check the results.

The analyzes are being published in a new study that could return the 2016 results.

"The region is highly deformed by high temperatures, so fossils are unlikely to be preserved," said Rosing.

NASA's Rosing and Allwood had a new test, just half a meter from one of the places Nutman investigated.

– First, the limestone structure is part of the original foundation in the region and is not part of it, and the limestone is not so old. Second, it's not the layers in the rocks as it should be, and the conical shapes are actually oblong ridges, he says.

Reject criticism

In an email, Allen Nutman says that since Rosing and Allwood were in the area only on a day trip, they could not find the right area. He says that the stone was taken two meters away, although he offered them a guide.

This has led to a number of sources of error in the new study, Nutman believes. Among other things, he writes that:

• The stone he investigated is a rare discovery, even for the region, which survived by a number of coincidences and preserved the structure.
• The cone structures in the original stone can not be due to the fact that it was bent because it was flat in the bottom.
• The Nutman stone had a very rare composition of carbon isotopes and rare earths, which, unlike other stones, originate in the ocean – where algae and bacteria may have to be encapsulated several billion years ago.

Minik Rosing says that they made observations exactly the same place as Nutman and that the 2016 study measurements are too unpredictable.

"Chemical studies are done with a method in which each analysis covers a larger area than the minerals they wish to analyze – and therefore mix more things," writes Rosing in an email.

The oldest traces of life still have 3.7 billion years

Kills Hassenkam, associate professor at the Institute of Chemistry of the University of Copenhagen, also studied the oldest life in the mountains of Isua. He is also skeptical about the 2016 discovery.

"Looking at the images in the original article, you can see that the rocks are not particularly well preserved, nor do I think that it looks like stromatolites." It looks like folds in the stone, he says. he.

However, whether Nutman's results are correct or not, the oldest traces of life probably date back to about 3.7 billion years ago. Structures such as stromatolites are just one way to look for traces of life.

In 2017, Tue Hassenkam and Minik Rosing managed to find traces of carbon, phosphate and carbon-related nitrogen in small cavities in ancient granite stones that had been encapsulated in the sediment at 20 kilometers from where Nutman had found his stone.

By using carbon 12 and carbon 13 measurements, they could look for life. Carbon exists in two stable variants, also called isotopes, and living organisms prefer to use carbon 12 instead of carbon 13.

As a result, there is an overrepresentation of carbon 12 when life has been involved, says Hassenkam.

– Carbon traces can be a sign of life, but it can also act as meteorite residues. We found many of the building blocks of life in a few microns, and it would be an amazing coincidence if it's not due to life, says Hassenkam.

Hassenkam explains in particular that nitrogen in nature poses big problems of carbon bonding when there is no life.

In addition, phosphate does not normally bind to carbon, but to elements such as lime. At the same time as carbon, phosphate will crystallize normally if it is not associated with life.

"It's a bit like a criminal case." Each of the findings may not be enough, but when we compare them, we're sure that life is a matter of life, he says.

A controversial subject

However, there are also scientists who doubt that the tracks of Hassenkam and Rosing presented in 2017 truly testify to life.

In 2017, American geochemist Elizabeth Bell wrote in an email to science.dk that she was "worried" if the carbon could have found the way to the rocks without the help of life. For example, via heat from hydrothermal fissures.

Hassenkam and Rosing, however, rejected this idea and claimed that there were stones intact.

The details are important when we have to find life on other planets

One may wonder if it is important to specify the small details when researchers agree that the oldest life can be traced back to 3.7 billion years ago.

But this is not trivial, because we have to make sure that we are not advised to investigate remote and unavailable environments on other factories, where we can not just get new samples, "he said. Rosing.

"I do not pretend that one method is better than another.I suggest that all methods be used so that we can find the most accurate answer.We are therefore better prepared to look for life on other planets, "he says.

We can follow the life up to the attack by the meteors

Many geologists agree that there has been a large meteorological bomb over the period ranging from 4.1 billion to 3.8 billion years ago.

For Minik Rosing, it is therefore surprising that they find traces of life 3.7 billion years old.

"It's very fascinating because it means that we find life as far as possible, which contributes to the question of whether life is always in good condition or whether it is a random miracle, says -he.

If we can find it on other planets, we can learn more about it, he thinks.

"If we find a planet of living conditions, but we can not find traces of life, this may suggest lately," he concluded.

references:

A.C. Allwood et al .: "Reassessing Evidence of Life in Greenland Rocks 3,700 Million Years Old", Nature (2018), DOI: 10.1038 / s41586-018-0610-4.

AP Nutman et al .: "The rapid emergence of life revealed by the discovery of microbial structures aged 3,700 million years", Nature (2016), DOI: https://doi.org/ 10.1038 / nature19355.

T. Hassenkam et al .: "Elements of eoarchian life imprisoned in mineral inclusions", Nature (2017), DOI: https://doi.org/10.1038/nature23261. summary

© Videnskab.dk. Translated by Lars Nygaard for research.no.