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How life evolved is one of the central questions in science and has fascinated humans for centuries. Early Earth was undoubtedly a harsh environment, but we know surprisingly little about the conditions at the time. “The problem is that due to geological recycling and so on, everything has been destroyed, so it’s really hard to model how life would have behaved in those early days,” explains Christof Mast, a biophysicist at Ludwig Maximilian University.
To navigate this lack of concrete data, researchers have instead focused on developing codon chronologies: timelines mapping out the order in which different components of the genetic code most likely developed. By hypothesizing how aspects of the environment called selection pressures would have impacted the emergence of DNA sequences, scientists have proposed more than 50 different chronologies.
Researchers can consider various factors as selection pressures, including which amino acids were accessible and the effect of meteorite hits. But the influence of high-energy ultraviolet light, which would have bombarded the surface of early Earth because the planed lacked an ozone layer, has never been considered as a selection pressure before now (ACS Cent. Sci. 2025, DOI: 10.1021/acscentsci.4c01623). Mast and colleagues fed experimental data about the UV susceptibility of short DNA sequences (eight base pairs long) into a computer model and extrapolated these core results to simulate UV damage on much longer and more realistic estimations of early genomes, known as protogenomes.
In previous work, the team had prepared samples of single-stranded DNA, each containing a central section of eight randomized nucleobases, and irradiated them to determine the probability of UV damage. The experiment probed 65,536 possible combinations of these random fragments and revealed the influence of neighboring nucleobases on the UV stability of different sequences in the chain.
With these data in hand, the researchers computationally generated DNA sequences 150 base pairs long and simulated UV exposure on random sections of the DNA strand. Repeating this randomized process millions of times within the simulation, the team was able to build a comprehensive picture of which possible early DNA sequences are most susceptible to UV damage.
As a result, Mast’s team proposed a new codon chronology. It centers on the hypothesis that UV-susceptible codons most likely decomposed rapidly under the conditions of early Earth—meaning that the first genomes would be biased toward UV-resistant sequences. Seeking to corroborate this theory, the researchers compared their UV chronology with others reported in the literature.
“These chronologies have very different approaches, but most are actually more on the UV-stable side,” Mast says. In particular, the team’s results closely correlated with the consensus chronology—an average of around 50 different reported chronologies—suggesting that UV radiation was probably a significant factor.
It’s important to remember that even the consensus chronology does not provide a definitive answer, however. “Certain chronologies or certain selection biases might be more important than others,” Mast says. “I’m superclear that we made a lot of assumptions here. This is our hypothesis.”
The hypothetical nature of this field does leave space for a variety of opinions among researchers. Joanna Masel, an evolutionary biologist at the University of Arizona, remains unpersuaded by the assumption that codons arose sequentially. “Some might have been used more than others, but I expect they all would have been present at some frequency,” she says via email.
Despite this reservation, Masel is intrigued by the notion of UV as a selection pressure and is keen to see the work expanded to the earlier genetic molecule, RNA. “The work was performed on DNA, whereas the biologically more meaningful molecule for this set of questions is RNA,” she says. “I would be interested in how the ancestral makeup of ancient RNA molecules that still exist today [for example, ribosomal RNAs and transfer RNAs] might show signs of being shaped by selection on UV sensitivity.”
Mast’s team sees this as the next step. “We are working on method development to use the same approach to calculate the UV dependency of RNA protogenomes,” he says. “The big impact of that would be that we could actually predict the UV damage on certain ribosomes in prebiotic chemistry.”
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