The universal genetic code evolved in unexpected ways

Every living being we know of, from the smallest bacteria to the largest blue whale, hinges on the same remarkable genetic code – a universal blueprint that underpins the diversity of life on Earth.

Our understanding of how this intricate code evolved, however, has sparked intense debates and fueled ongoing scientific inquiry.

Is the universal genetic code an ancient feature that emerged fully formed at the very onset of life, or is it a product of gradual evolution shaped by billions of years of adaptation and change?

Sawsan Wehbi, a student at the University of Arizona, has provided compelling evidence that the widely accepted theory of how this genetic code evolved requires a rethink.

And it seems the answer lies not in how we’ve translated the code, but in the order in which its building blocks – the amino acids – were added.

Established views of the genetic code

“The genetic code is this amazing thing in which a string of DNA or RNA containing sequences of four nucleotides is translated into protein sequences using 20 different amino acids,” said Joanna Masel, the study’s senior author.

“It’s a mind-bogglingly complicated process, and our code is surprisingly good. It’s nearly optimal for a whole bunch of things, and it must have evolved in stages.”

The team’s findings clash with the conventional view, which suggests that early life preferred smaller, simpler amino acid molecules.

The study shows that complex molecules were added as well. Furthermore, the researchers found that amino acids that bind with metals joined the genetic code earlier than previously thought.

This offers a whole new perspective on how the genetic code came to be.

Rethinking laboratory approaches

The accepted view of the evolution of the genetic code largely leans on lab experiments which attempted to recreate early Earth’s conditions – most notably the famous Urey-Miller experiment of 1952.

However, as valuable as this experiment was, it had its limitations. No sulfur-containing amino acids emerged from the experiment, despite the abundance of this element on early Earth.

This lack of sulfur led many to believe that sulfuric amino acids joined the genetic code at a later stage. But as critics point out, this isn’t surprising given that sulfur was not included in the Urey-Miller experiment.

Searching for life beyond Earth

According to study co-author Dante Lauretta, a professor of planetary science and cosmochemistry at the U of A Lunar and Planetary Laboratory, the sulfur-rich nature of early life offers fascinating possibilities for astrobiology.

“On worlds like Mars, Enceladus and Europa, where sulfur compounds are prevalent, this could inform our search for life by highlighting analogous biogeochemical cycles or microbial metabolisms,” said Professor Lauretta.

New perspectives on the genetic code

The researchers from the University of Arizona adopted an innovative approach to analyze sequences of amino acids dating back to our common ancestor, LUCA, hypothesized to have lived around 4 billion years ago.

Unlike previous studies, they focused not on full-length protein sequences, but on protein domains – shorter stretches of amino acids.

“If you think about the protein being a car, a domain is like a wheel. It’s a part that can be used in many different cars, and wheels have been around much longer than cars,” said Wehbi.

By using statistical data analysis tools, they were able to trace the incorporation of individual amino acids into the genetic code.

Significantly, the team found that ancient sequences included amino acids with aromatic ring structures, like tryptophan and tyrosine, even though these were late additions to our code.

“This gives hints about other genetic codes that came before ours, and which have since disappeared in the abyss of geologic time,” Masel said. “Early life seems to have liked rings.”

This breakthrough not only reframes our understanding of early molecular evolution but also provides a foundation for exploring how genetic systems might arise on other planets.

The evolution of the genetic code, which previously seemed to be well understood, now once again opens up exciting new possibilities for future research and discoveries.

The full study was published in the journal Proceedings of the National Academy of Sciences.

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