They discover the proteins that made life on Earth possible

• Biochemistry: the secret of the origin of life is hidden in salty lakes
• Space: a NASA probe will search for the origin of life on the asteroid Bennu

Shortly after the formation of Earth, about 4.5 billion years ago, the planet's surface was very different from what we know today. Large asteroids hit the Earth's crust. Temperatures were much higher and plate tectonics - which governs continental movements - began to kick in. Understanding how the processes that gave rise to the first organisms could appear in this hostile context, is a mystery that Science is still trying to solve.

Researchers from Rutgers University in New Jersey (USA) now provide one of the keys that could open the way to that first cell life. These are two protein structures that would be responsible for the first metabolisms. Their finding, published this Monday in the journal Proceedings of the National Academy of Sciences (PNAS ), reconstructs the anatomy and evolution of the first proteins , whose appearance they estimate between 3,500 and 2,500 million years ago.

Proteins are the simple molecules that fueled the first chemical processes from which primitive life arose. They are made up of chains of amino acids and play a fundamental role for biomolecules. When folded, these chains become a three-dimensional structure. In this case, the team has focused on two types of protein folding in particular: that of ferredoxin and Rossmann folding.

A puzzle of thousands of pieces

In their analysis, researchers have had to reconstruct the evolutionary path from the earliest proteins, over billions of years. A task comparable to "solving a puzzle with thousands of pieces", in the authors' words. With the aggravating circumstance that the solution to that puzzle required two pieces whose shape was unknown, and without which life on Earth could not have begun. But by examining and tracing the protein's network of connections, based on their role in metabolism, the team has been able to establish a robot portrait of the missing elements. And its description fits those two elements.

Ferredoxin is a protein that binds iron and sulfur compounds, abundant in that first stage of the Earth. "In addition to amino acids, compounds of carbon, nitrogen, oxygen, sulfur, and hydrogen, the earliest proteins probably used metals, primarily iron," explains co-author Vikas Nanda, professor of biochemistry and molecular biology at the Robert Wood Johnson Medical School at Rutgers. . "Iron can be used to move electrons, and it must have been key in electron transfer processes in early metabolisms."

In fact, ferredoxins transport electrons around cells to drive metabolic processes. Electrons that flow through solids, liquids, and gases and that feed living systems. The authors believe that the same electrical charge must be present in any other planetary system that has the possibility of supporting life.

The other key element was the Rossmann fold , a special type of nucleotide-binding protein structure (fundamental molecules for DNA and RNA). The combination of these two processes would have laid the foundation for the first blocks with which to create life. "We believe that life arose from very small blocks, such as a set of Legos, to later form more complex cells and organisms, such as humans," says lead author Paul Falkowski, director of the Laboratory for Environmental Biophysics and Molecular Ecology at Rutgers.

Furthermore, the evidence suggests that the two structures identified in the study may have shared a common ancestor . If confirmed, it would be the ancestor of the first metabolic enzyme of life. "Our results need to be verified in the laboratory to better understand the origins of life on Earth and to inform how life can originate elsewhere."

The authors are part of a group called ENIGMA (Evolution of Nanomachines in Geospheres and Microbial Ancestors) that tries to provide answers about the role of the simplest proteins that served as catalysts for the first stages of life. ENIGMA conducts its research under the umbrella of NASA's Astrobiology Program, a task the space agency hopes to use to search for life on other planets.

"Several recent studies have reported on proteins found in meteorites," says Nanda. According to the scientist, "if these results are confirmed, the question arises as to whether these molecules are of biological origin." The authors are currently working to identify which are the simplest proteins capable of carrying out a metabolic process. "Because although complex life is likely to be different on other planets, the first life-initiating molecules may, in fact, be common," says the researcher. "Our work will lead to the identification of organic biosignals that can be used to identify the life on other planets, "he says.

Looking for the origin of life

-Another study that appeared this year in PNAS pointed to the salt lakes as a possible scenario in which those first "blocks of life" flourished. The presence of phosphorus and cyanide would be ideal for the appearance of DNA and RNA.

-In 2018 the analysis of two meteorites found in the US and Morocco revealed for the first time remains of water and organic matter from outside the Earth. They included amino acids necessary for protein formation, in addition to nitrogen and oxygen.

-Scientists try to decipher how the first RNA sequences were translated into the first proteins. Laboratory experiments have shown that small sections of non-organic amino acids can spontaneously clump together.

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