The oldest life forms on Earth appeared thanks to viruses

Life as we know it on Earth might never have arisen if not for viruses. Such

a hypothesis was put forward by scientists studying "living stones" - stromatolites - billions of years old.

In an article published in the journal Trends in Microbiology, a group of scientists from the University of South Wales atSydney and the United States examined the evidence for the oldest life forms in the fossils known as stromatolites. These are layered limestone rocks that are often found in shallow waters around the world. The researchers sought to understand the mechanism that led to the creation of colonies of unicellular organisms (bacterial mats) and, subsequently, stromatolites.

Scientists believe that it is viruses that can bemissing piece of the puzzle. Perhaps it was thanks to them that the soft microbial mat was transformed - or lithified - into solid stromatolite elements. They are predominant in places like Shark Bay and Pilbara in Western Australia.

Stromatolites are one of the oldestknown microbial ecosystems, which are about 3.7 billion years old. They are widespread in the fossil record and are some of the earliest examples of life on Earth.

Scientists wanted to understand the mechanism of transformationmicrobial mats into stromatolites, not only because very little is known about this process, but also because it can add to the knowledge of scientists about life on Earth and, possibly, on other planets.

A fragment of stromatolite rock found in Shark Bay shows layered deposits formed by microbial mats billions of years ago. Credit: UNSW / Brendan Burns.

The authors postulate that the transition of bacterialmat from soft cells to stone was enhanced by interaction with viruses. Scientists have suggested that viruses may have a direct or indirect effect on the metabolism of bacteria, which regulates the transition from mat to stone.

In a direct impact scenario, viruses enterthe nucleus of cyanobacteria and affect the metabolism of the host by inserting and removing genes that simultaneously increase the fitness of the virus and the host. This, in turn, increases the survival rate of the microbial mat and selects genes that potentially affect carbonate deposition.

In an indirect scenario, scientists talk about a processknown as viral lysis, when viruses invade living cells and cause their membranes to break down and release their contents. This actually leads to cell death.

Bacterial mats - biocenoses, consisting ofprokaryotes and located at the bottom of water bodies or in their coastal zone. In shape, they are most similar to mold films. Ancient bacterial mats were the only biocenoses on Earth.

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