In the vast history of Earth, billions of years ago, an arid landscape dominated by volcanoes and shallow pools may have witnessed the beginning of something extraordinary: life. An illustrative scene generated by artificial intelligence captures this vision, with a primitive coastline serving as the backdrop for the early evolutionary steps that would define the planet’s destiny. However, the how and when of the emergence of the genetic code that unites all living organisms remains a mystery shrouded in scientific controversy.
A new study led by Sawsan Wehbi, a doctoral student at the University of Arizona, is shaking the foundations of classical theories about the evolution of the genetic code. Published in the journal Proceedings of the National Academy of Sciences (PNAS), the research challenges the traditional model of the development of this “universal language” and offers a new perspective on the evolutionary steps of the building blocks of life as we know it.
The genetic code is, in the words of Joanna Masel, co-author of the study and professor of ecology and evolutionary biology, an astonishingly complex and surprisingly efficient process. This system translates DNA or RNA sequences into proteins using 20 specific amino acids, and its optimization for diverse functions makes it a marvel of molecular biology. But this “language” did not emerge overnight. The evolution of the genetic code occurred in stages, a process that Wehbi and her team argue is far more intricate than previously assumed.
The research concludes that in its early stages, primitive life preferred smaller, simpler amino acids. As the genetic code evolved, larger and more complex molecules were incorporated. Surprisingly, amino acids that interact with metals, such as cysteine, appeared much earlier than previously theorized.
Moreover, the researchers suggest that the current genetic code is the result of the survival of one among several primitive genetic codes, now extinct. This raises fascinating questions about the biological systems that preceded our “modern version” of life.

One of the most striking aspects of this research is its critique of historical experiments that have shaped the conventional view of the origin of life. The famous 1952 Urey-Miller experiment, which demonstrated how organic molecules could form from inorganic compounds under simulated conditions of early Earth, plays a central role in the debate.
While the experiment was groundbreaking, it failed to produce sulfur-containing amino acids, an element abundant in primordial oceans. This omission led scientists to assume that sulfur-containing amino acids, such as methionine, were integrated into the code in later stages. However, Wehbi’s research argues that this conclusion underestimates the early role of sulfur in life’s chemistry, given that the element was deliberately excluded from the experiment.
According to Dante Lauretta, co-author of the study and planetary science expert, the sulfur richness of early organisms could be crucial to understanding possible extraterrestrial life forms. On worlds like Mars, Enceladus, or Europa, where sulfur compounds abound, this could guide our search for life by highlighting similar biogeochemical cycles or analogous microbial metabolisms, Lauretta explained.
To trace the evolution of the genetic code, the team adopted an innovative approach, focusing on protein domains rather than complete sequences. Domains, akin to the wheels of a car, are reusable parts that existed long before the more complex structures of proteins.
Using advanced statistical tools, the researchers analyzed amino acid sequences from the last universal common ancestor (LUCA), a hypothetical organism that lived about 4 billion years ago and from which all current life forms descend. This analysis revealed that some amino acids, especially those with aromatic ring structures like tryptophan and tyrosine, were present in genetic codes older than LUCA.
One of the most intriguing findings of the study is that primitive life seemed to favor amino acids with aromatic rings. These molecules, although late additions to the repertoire of the current genetic code, were prominent in earlier, now-extinct codes. According to Masel, this discovery points to the existence of other genetic codes that have disappeared over geological time.
These findings provide clues about biological systems that preceded ours and no longer exist, she explained. It’s fascinating to think that evolution experimented with different codes before arriving at the one all living organisms share today.
This rethinking of the origin of life challenges the linear perception of evolution, revealing a much more dynamic path, full of experimentation and adaptations lost to time. By looking back, beyond LUCA, scientists are beginning to uncover a rich tapestry of evolutionary possibilities that could redefine our understanding of universal biology.
SOURCES
Sawsan Wehbi, Andrew Wheeler, et al., Order of amino acid recruitment into the genetic code resolved by last universal common ancestor’s protein domains. Proceedings of the National Academy of Sciences, 2024; 121 (52) DOI: 10.1073/pnas.2410311121
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