In search of lost life on Earth – 02/08/2024 – Fundamental Science

“Earth is the only planet where life forms have been found to date.” When we hear statements like this, we think of vibrant ecosystems, with complex food chains and the interaction of animals, plants, fungi and other organisms. Throughout much of the planet’s history, however, the life that thrived here was of a very different kind.

With the development of microscopes at the turn of the 16th century, the first microorganisms were soon discovered. In his excellent treatise “Micrographia”, from 1665, the famous English scientist Robert Hooke published the first description of the microfungus Mucor. Then, starting in 1674, Dutch microscope builder Anton van Leeuwenhoek described several protozoa and bacteria for the first time. Decades of study have allowed us to understand the importance of these microscopic beings, both as causes of disease and as key players in the recycling of crucial chemical elements throughout the biosphere.

The first records of life visible to the naked eye are the impressions that soft-bodied organisms, the so-called Ediacaran fauna, left on sedimentary rocks deposited in shallow seas about 550 million years ago. The Earth, however, is about 4.5 billion years old. What happened in the biosphere in the 4 billion years before these organisms emerged?

Evidence found in even older rocks suggests that, in fact, an abundant biosphere already existed on the planet, probably since shortly after its formation. As they were predominantly microscopic life forms, they were unable to leave visible impressions in the fossil record. Still, we found some evidence of its existence.

A rock known as a stromatolite – from the Greek stromalayer, and lithos, rock –, for example, it constitutes the evidence that is most frequently found in registry offices, or rather, geological records. Stromatolites are limestone rock formations with a column or dome shape that, in a cross section, reveal stacked millimetric layers. There are still places on the planet where similar columns of limestone sediment are forming today, such as Shark Bay in Australia and the Salt Lagoon in Rio de Janeiro. By examining these sites, it is possible to understand the formation process of this rock.

The millimetric layers of stromatolites are created by photosynthetic organisms that spread a kind of microbial mat on the surface of the sediment. To allow the entry of sunlight, essential for photosynthesis, organisms build a new mat when the old one is completely buried by sediment, and this new one settles on top of the previous one, and so on, hence the layered cake appearance. .

The oldest stromatolites are around 3.5 billion years old, and are found in Australia and South Africa. In 2016, a team led by Australian geologist Allen Nutman published, in the magazine “Nature”, the discovery of possible stromatolites of up to 3.7 billion years ago, in Greenland, but these occurrences are still being discussed.

Another type of evidence are microfossils of the organisms themselves, visible only through cutting slides just a few micrometers thick from rock samples, thin enough to be analyzed under a microscope. It is necessary to investigate hundreds of slides and have a trained eye to recognize structures that can be interpreted as ancient colonies or filaments of algae and other types of microorganisms. In another article in “Nature”, in 2017, a team led by Australian astrobiologist Matthew Dodd interpreted the presence of possible microfossils in Canadian rocks deposited between 3.8 and 4.3 billion years ago.

We also have indirect evidence for the presence of microscopic life on Earth, such as the isotopic signature of carbon remnants. Carbon has two abundant natural isotopes, carbon-12 and carbon-13, each with 6 protons (hence the name isotopes, which means the same number of protons), but with a different number of neutrons, resulting in different masses. In photosynthesis, which involves the reaction between carbon dioxide and water using sunlight, microorganisms prefer to use carbon dioxide molecules that have the lowest mass isotope, since breaking molecular bonds requires less energy. Therefore, a greater amount of carbon-12 in relation to carbon-13 may indicate that a sediment or mineral had interaction with microorganisms capable of selecting between the two isotopes.

In a study led by American geologist Elizabeth Bell published in 2015, micrometer-sized inclusions of carbon-rich minerals, such as graphite, in very small minerals such as zircons from Jack Hills in Western Australia indicate a carbon-12-enriched signature. , probable evidence of the presence of microscopic life on Earth since about 4.1 billion years ago. The biggest problem with isotopic evidence is that some mechanisms that occur in hydrothermal environments, such as oceanic fumaroles, and linked to volcanism, can also generate an enrichment in carbon-12, thus making the method not definitive as evidence.

Finally, the comparison of the genomes of bacteria and archaea (a kingdom that differs from that of bacteria due to the composition of its cell wall, genetics and biochemistry) made it possible to identify a set of genes common to both domains, which suggests that there was a last common ancestor – or LUCA, Last Universal Common Ancestor, in the original English acronym – for all living organisms. By knowing the rates at which biomolecules from these groups are currently mutating, scientists can estimate how long ago these biomolecules came from a common ancestor, a technique known as a molecular clock. Molecular clocks indicate that LUCA lived approximately 4.5 billion years ago – its age would be equivalent to the birth of the Earth. The method, however, also has its flaws, the main one being the premise that the mutation rates of a given group of biomolecules were constant in the past.

Although the evidence coming from different fields of knowledge is not definitive in itself, all presenting alternative interpretations, it is the set of evidence obtained by different methods of investigation that allows us to paint a picture of what primitive life was like on Earth, and where to look. for her. Apparently, the phenomenon of life is almost as old as the planet. This could open up new possibilities for the search for extraterrestrial life, which may, after all, not be as rare as we think, just different from what we expect to find, since conditions similar to those on Earth in its initial phase may be more common on other planets. .

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Fabrício Caxito is a professor of geology, main researcher in the GeoLife MOBILE project and philosopher at UFMG.

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