Geologists have proven that fragments of graphite formed at the bottom of the primary ocean three and a half billion years ago represent traces of the existence of archaea - one of the two main types of microbes on Earth, according to the journal PNAS.
“Our measurements of isotope fractions have shown that these fossils are clearly of biological origin. We do not have direct evidence that life could have existed as early as 4.3 billion years ago, but there is no reason to believe that this was impossible in principle, and we plan to test this in the future, - said John Wally of the University of Wisconsin at Madison (USA).
Earth before the beginning of time
The first organisms appeared on Earth during the Archean era, but there is no generally accepted point of view about exactly when and how this happened. So far, only a few fossil evidence has been found that microbes existed in the primordial ocean about 3.4 billion years ago, but many scientists believe that life could have originated much earlier.
In 2015, Japanese geologists studying graphite samples from the Isua Formation, formed 3.7 billion years ago in Greenland, found hints of the existence of life already at that time. The first unequivocal evidence in favor of this was discovered last year, and a year earlier, scientists discovered in Australia traces that organisms existed on Earth even earlier - four billion years ago.
Many geologists, as Wally notes, fundamentally disagree with such estimates and believe that this happened much later - 2.5-3 billion years ago. They often criticize such finds, noting that deposits of graphite and other, presumably, biogenic rocks could have formed without the participation of microbes, and traces of bacteria and archaea could have been drawn by the imagination of researchers.
Wally and his colleagues have tried to prove the skeptics wrong. To do this, they studied the isotopic and chemical composition of graphite deposits found in Pilbara in western Australia three decades ago.
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These deposits formed about 3.5 billion years ago in the shallow waters of the primary ocean, as evidenced by the rocks surrounding the graphite. They are characterized by filamentous structures, similar to many microbes "glued" together.
"Talking" isotopes
Geologists drew attention to a well-known fact: a slightly different proportion of carbon isotopes is characteristic of living organisms and their remains than for deposits of inanimate organic matter. This makes it possible to unambiguously establish the origin of certain sedimentary rocks.
Remains of an ancient archaea found in western Australia / PNAS.
Guided by this idea, the scientists cut small layers from the pieces of graphite found in Pilbar and illuminated them with a particle accelerator. So they were able to accurately calculate the number of carbon-12 and carbon-13 atoms in the alleged "bacteria" and the surrounding matter of inorganic origin.
“The boundaries between microbes and inorganic sediments matched perfectly with how zones with different fractions of carbon isotopes were located. If these structures are not of biogenic origin, then such differences cannot be explained. The proportions of carbon-13 and carbon-12 in these remains perfectly match the way microbes metabolize and live in general,”Wally continues.
The same measurements, as the geologist notes, for the first time indicated that scientists are not dealing with the first bacteria, but archaea - distant relatives of modern staphylococci, E. coli and other representatives of the microcosm, which are a little closer to multicellular creatures than other microbes. Moreover, the relatively low proportion of carbon-13 in the remains suggests that these microbes were feeding on methane, which was then abundant in the atmosphere.
This discovery pushes the time of the appearance of archaea by almost 800 million years - earlier scientists believed that they appeared much later than bacteria, about 2.7 billion years ago. So, Wally says, life evolved much faster than anticipated, and could have appeared almost simultaneously with the birth of the planet.