In Search of Lost Life on Earth – 08/02/2024 – Basic Science

“Earth is the only planet on which life forms have been found so far.” When we hear such phrases, we think of vibrant ecosystems, with complex food chains and interactions between animals, plants, fungi, and other organisms. However, throughout most of the planet's history, the life that flourished here was of a very different kind.

With the development of microscopes at the beginning of the sixteenth 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 microscopic fungi. Mukur. Then, beginning in 1674, Dutch microscope maker Anton van Leeuwenhoek described many protozoa and bacteria for the first time. Decades of study have allowed us to understand the importance of these microorganisms, both as causes of disease and as key players in the recycling of important chemical elements throughout the biosphere.

The first records of life visible to the naked eye are the impressions left by soft-bodied creatures, so-called Ediacaran animals, in sedimentary rocks deposited in shallow seas about 550 million years ago. However, the Earth is about 4.5 billion years old. What happened in the biosphere during the four billion years before these organisms appeared?

Evidence in older rocks suggests that an abundant biosphere already existed on the planet, perhaps shortly after its formation. Since they were mostly microscopic life forms, they were unable to leave visible impressions in the fossil record. However, we found some evidence of its existence.

A rock known as stromatolite – from the Greek In vainlayer, and lithos, Stone – For example, it constitutes the evidence frequently found in registration offices, or rather, geological records. Stromatolites are column- or dome-shaped limestone rock formations that reveal, in cross section, stacked millimeter layers. There are still places on the planet where similar columns of limestone deposits form today, such as Shark Bay in Australia and the Salt Lagoon in Rio de Janeiro. By examining these sites, the formation process of this rock can be understood.

The millimeter layers of stromatolite are created by photosynthetic organisms that spread a type of microbial mat on the surface of the sediment. To let in sunlight, which is necessary for photosynthesis, organisms build a new mat when the old one is completely buried by sediments, and this new one settles on top of the previous one, and so on, hence the layer cake.

The oldest stromatolites are about 3.5 billion years old and are found in Australia and South Africa. In 2016, a team led by Australian geologist Allen Notman published in the journal Nature the discovery of the possibility of the existence of stromatolites up to 3.7 billion years old. Before that, in Greenland, but these events are still under discussion.

Another type of evidence is microfossils of the organisms themselves, which can only be seen through cut slices only a few micrometers thick from rock samples, which are thin enough to be analyzed under a microscope. It is necessary to examine 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 explained the presence of possible microfossils in Canadian rocks deposited between 3.8 and 4.3 billion years ago.

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

In a study led by American geologist Elizabeth Bell and published in 2015, the presence of micrometer-sized inclusions of carbon-rich minerals, such as graphite, in very small minerals such as zircon from Jack Hills in Western Australia, indicates a carbon-12-rich signature. Possible evidence of microscopic life on Earth. Earth 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, associated with volcanic activity, can also generate carbon-12 enrichment, making this method inconclusive as evidence.

Finally, comparison of the genomes of bacteria and archaea (a kingdom that differs from the bacterial kingdom due to its cell wall composition, genetics, and biochemistry) allowed the identification of a set of genes shared in both domains, suggesting that there was a last common ancestor – or LUCA, the last universal common ancestor, in Original English abbreviation – for all living things. By knowing the current mutation rates of biomolecules from these groups, scientists can estimate how long it has been since these biomolecules came from a common ancestor, a technique known as a molecular clock. Molecular clocks indicate that LUCA lived about 4.5 billion years ago, and its age would be equivalent to the birth of Earth. However, this method also has its drawbacks, the most important of which is the assumption that mutation rates for a given set of biomolecules have been constant in the past.

Although the evidence coming from different fields of knowledge is not definitive in itself, they all offer alternative explanations, it is the body of evidence obtained through different methods of investigation that allows us to paint a picture of what primitive life on Earth was like, and where it was. Primitive life on Earth? To search for it. It is clear that the phenomenon of life is almost as old as the planet. This may open up new possibilities for the search for extraterrestrial life, which may not be as rare as we think, but rather very different from what we expect to find, since conditions similar to those on Earth in its initial stage may be more difficult. Common on other planets.

*

Fabricio Caxito is a professor of geology, and the principal investigator on the project Geolife Mobile And a philosopher from UFMG.

The Basic Science Blog is edited by Serrapilheira, a private, non-profit institute that promotes science in Brazil. subscription In the Serrapilheira newsletter to follow the latest news from the institute and the blog.