Of life on Earth there are 3,95 billion years ?

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AFP

Wednesday, 27 September, 2017 15:02

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Wednesday, 27 September, 2017 15:02

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A rudimentary form of life could have been already present on the Earth there are 3,95 billion years ago, when the Earth was undergoing intense bombardment of comets and asteroids, say researchers in a study published Wednesday in Nature.

“We have found the oldest evidence of life on Earth,” the Canada “in sedimentary rocks of the Labrador dating back to 3.95 billion years ago”, said to AFP Tsuyoshi Komiya of Tokyo University, one of the authors of the study.

At this time, the Earth, that has formed there is 4,567 billion years ago, was bombarded by comets, he noted.

For the past year, the ads on the date of the appearance of life on Earth follow one another in Nature and they are the subject of heated debates among scientists.

In September 2016, a team of researchers announced in the journal british have discovered in Greenland, stromatolites (structures limestone formed by microbial colonies) old 3.7 billion years ago.

And then in march 2017, scientists have shown, always in Nature, have discovered micro-organisms in the fossils that have between 3,77 and 4.29 billion years. They have been spotted in the Belt, Nuvvuagittuq in Canada.

This time, japanese researchers have worked in the area of Saglek Block, northern Labrador, of which the rocks are of about 3.95 billion years.

They studied the isotopic composition of grains of graphite (carbon) whether it was of organic origin or not.

Isotopes are atoms that have the same number of protons but different number of neutrons.

Carbon has several natural isotopes (including the famous carbon-14, radioactive, used for dating, but that was not to be found in ancient rocks).

For their work, the researchers are interested in the ratio of carbon 13 (6 protons, 7 neutrons) carbon-12 (6 protons, 6 neutrons), two stable isotopes.

“The agencies, to develop themselves, prefer to isotopes of light, in this case, carbon-12, rather than carbon-13 is heavier”, explains Tsuyoshi Komiya.

His team has found that graphite grains were substantially enriched in carbon-12.

Tsuyoshi Komiya concludes that “the signature” of this graphite is “biogenic,” that is to say, it comes from living organisms.

But Sylvain Bernard, géochimiste at the Muséum national d’histoire naturelle (France), shows itself to be very circumspect about these conclusions.

“It is not only the living that has the isotopic signature”. It can come from reactions of minerals between them, or of fluids between them, ” he stressed.

“The arguments advanced by these researchers are far from being sufficient to determine unambiguously the +biogénécité+ of these graphites. They use arguments that may be necessary but are not sufficient,” continues Sylvain Bernard.

“For the moment, we still do not know when or how life appeared on Earth,” he says. “But progresses” through advanced techniques, he notes.