3.42 billion years ago, the oldest evidence of life

3.42 billion years ago, the oldest evidence of life

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In the 1970s and 1980s, scientists discovered a vibrant “black chimney” ecosystem on the bottom of the ocean. In these places, mineral-rich fluids interact with the hot volcanic crust to form hydrothermal vents, just like big smokestacks with black smoke.

Since then, people have come to realize that even in such corners of the earth, where the sun is not visible and far away from the surface, life can still flourish under the drive of chemical energy.

The approximate distribution map of the global submarine hydrothermal activity area. | Image source: Wikicommons

We now know that this hydrothermal system is widely distributed across the seabed, and the ecosystems near many vents are even more colorful.

In these places, the interaction of colder seawater and warmer underground hydrothermal fluids will produce a rich “chemical soup”. Changes in these environmental conditions will also lead to the emergence of a variety of potential micro-habitats, and many microorganisms even rely on weak Chemical energy can survive.

Recently, new discoveries by a group of international researchers have once again expanded the boundaries of life. They discovered fossils of methane-cycling microorganisms that lived in the hydrothermal system on the seafloor 3.42 billion years ago . These microfossils are the oldest evidence of this type of life, and they extend the boundaries of potentially habitable environments on the early Earth and other planets such as Mars. The research has been published in “Science Progress” recently.

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Biologists infer that the metabolic pattern that consumes or produces methane should have evolved very early, but they are not yet sure when this type of organism appeared.

Previous studies have found some indirect evidence of microorganisms that carry out the methane cycle, suggesting that microorganisms may exist in ancient rocks filled with fluids about 3.5 billion years ago. But previous research did not find real microorganisms. Although in theory these archaea can leave fossils, direct evidence is very limited.

In this new study, the team analyzed rock specimens collected from the Barberton Greenstone Belt in South Africa . This area is close to the border between Swaziland and Mozambique and contains some of the oldest and best-preserved sedimentary rocks found on our planet.

The rocks collected in the study came from the Barberton Greenstone Belt in South Africa. |Image source: A. Hofmann

The team cut the rock samples into 30-50µm thick sections and observed them under a microscope. At the same time, mass spectrometry and a special imaging technique called Raman microspectroscopy were used for detection. Researchers found some tiny filamentous structures in the rock, with an average length of about 42 µm and a diameter of only 0.77 µm. These filamentous fossils appear in hydrothermal veins, which are cracks in the bedrock that contain heated water.

Optical microscope image of filamentous microfossils. |Image source: B. Cavalazzi

After detailed chemical analysis, scientists believe that these filamentous structures can be interpreted as remnants of cell structures, which are relics of life. They seem to have thrived on the walls of the cavity formed by the hydrothermal system a few meters below the sea floor.

Confocal Raman imaging shows that the filamentous structure contains carbonaceous material inside. |Image source: B. Cavalazzi

Filament fossils contain a carbon-based shell. Structurally and chemically, this outer shell is not the same as the preserved inner part, as if the inside of the cell is wrapped by an outer layer of cell wall or cell membrane.

Chemical analysis shows that filaments contain most of the main elements needed for life. The concentration of nickel in organic compounds is relatively high, and nickel may come from enzymes containing nickel in microorganisms, which provides further evidence of primitive metabolism. The team also noticed that this concentration is very similar to the nickel levels found in modern archaeal prokaryotes, which often live in hypoxic environments and use methane for metabolism.

Barbara Cavalazzi, the first author of the study, believes that the evidence is a good indication that they have found unusually well-preserved microbial fossils. She also said, “The underground habitat heated by volcanic activity may be the place where the earliest microbial ecosystems appeared on the earth, and this is the oldest example we have found so far. This discovery can convert this type of archaeal fossil The record extends to the era when life first appeared on Earth”.

Outcrops of rock samples from the Barberton Greenstone Belt, South Africa. |Image source: Cavalazzi et al.

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Some scientists believe that these filaments are indeed strong evidence of Archean methanogenic fossils . Such ancient fossils may contain clues about the early inhabitants of the earth. Some people suspect that life on our planet may have arisen in a similar environment.

However, there have also been some false signals in the exploration of early life forms. Some researchers still doubt the authenticity of these fossils. Julie Cosmidis, a geobiologist at the University of Oxford, said in an interview with “Science News” that in a hydrothermal environment rich in silica, some components may be mixed together to form a seemingly lifelike appearance through chemistry, and they are more The actual cells are more likely to leave “fossils.” She also pointed out that nickel was very common on the early earth and easily attached to organic matter. In fact, scientists do not know enough about the process of creating false biological traces.

In any case, follow-up research and experiments may give us a better understanding of the oldest methanogens. The researchers also said that the new research may bring some inspiration. For example, scientists currently looking for signs of early life on Earth have explored surface water deposits far beyond these hydrothermal systems. Maybe we should broaden our horizons. Life and its origin may be far more miraculous than we thought.

Reference source:

https://phys.org/news/2021-07-oldest-fossils-methane-cycling-microbes-frontiers.html

3.42-billion-year-old fossil threads may be the oldest known archaea microbes

https://www.the-scientist.com/news-opinion/microbial-fossils-found-in-3-4-billion-year-old-subseafloor-rock-68975

 

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