All living things that we see today have possibly descended from a microbe that existed on Earth about 4 billion years ago. This unicellular and bacterium-like organism is known as LUCA, or the Last Universal Common Ancestor.

In a new genetic study, William Martin, from Heinrich Heine University in Germany, and colleagues looked through 6.1 million protein encoding genes that are present in one-celled organisms.

Genetic analysis helped the researchers identify 355 genes that were most likely present in the ancestor microbe, from which bacteria and archaea (bacteria-like organisms with a different metabolism) have split from about 4 billion years ago.

Genes such as those in humans and mice are related by a common descent from an ancestral gene in the first mammals. By comparing the sequence of DNA letters, scientists can arrange genes in evolutionary family trees.

Using this technique, researchers were able to assign 6 million genes to a smaller number of families. Of these genes, only 355 met the criteria for having likely originated from the common ancestor of bacteria and archaea.

The study also unveiled the genetic structure of the primitive microbe and provided hints about its habitat. The researchers found evidence suggesting that LUCA likely thrived in a hot and non-oxygenated environment and fed on hydrogen gas.

This environment is comparable to that of a hot spring today, strengthening the idea proposed by some scientists that life on Earth began in extreme environments such as hydrothermal vents and undersea volcanoes.

Exotic life-forms are known to thrive in deep sea vents, and the genetic study supports the idea that life may have possibly originated in its extreme chemistry. Some scientists, however, argue that life may have originated elsewhere but was later driven and confined to a deep sea environment following a catastrophic event like the Late Heavy Bombardment, which happened between 4 billion and 3.8 billion years ago.

The 355 genes the researchers have identified include some that metabolize hydrogen to serve as a source of energy, and a gene for the reverse gyrase enzyme that is found only in microbes that thrive at extremely high temperatures.

"The 355 phylogenies identify clostridia and methanogens, whose modern lifestyles resemble that of LUCA, as basal among their respective domains. LUCA inhabited a geochemically active environment rich in H2, CO2 and iron," the researchers wrote in their study, which was published in the journal Nature Microbiology on July 25.

"The data support the theory of an autotrophic origin of life involving the Wood-Ljungdahl pathway in a hydrothermal setting."

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