From the tiniest bacteria to the largest blue whales, life on Earth exhibits breathtaking diversity, yet most organisms share a common thread—the universal genetic code. The origins and development of this genetic code have long been a topic of debate among scientists.
A new study led by Sawsan Wehbi, a doctoral student at the University of Arizona, challenges the prevailing theories about how the genetic code evolved. Wehbi’s research, published in the journal PNAS, suggests a revised timeline for the incorporation of amino acids, the essential components of the genetic code.
Joanna Masel, the study’s senior author and a professor of ecology and evolutionary biology at the University of Arizona, described the genetic code as “a mind-bogglingly complicated process” that is “nearly optimal for a whole bunch of things.” According to Masel, this complex system likely evolved in stages.
The research uncovered that smaller amino acids were initially favored by early life forms, while larger, more complex amino acids were integrated later. Intriguingly, amino acids that interact with metals were incorporated sooner than previously believed. The study also posits that the modern genetic code was preceded by other codes now extinct.
The researchers argue that traditional views on genetic code evolution are flawed due to a reliance on laboratory experiments that may not accurately reflect evolutionary history. A key piece of evidence they dispute is the Urey-Miller experiment of 1952, which attempted to recreate the conditions of early Earth.
While this experiment showed that life’s building blocks could emerge from nonliving matter, it failed to produce sulfur-containing amino acids, a missing element in the experiment’s setup. This oversight has led to assumptions that sulfuric amino acids were latecomers to the genetic code.
Dante Lauretta, a co-author of the study and Regents Professor of Planetary Science, highlighted the implications for astrobiology. He noted that sulfur’s role in early life on Earth could inform the search for extraterrestrial life, especially on planets and moons rich in sulfur compounds, such as Mars, Enceladus, and Europa.
The research team employed a novel method to trace amino acid sequences back to the last universal common ancestor (LUCA), which lived approximately 4 billion years ago. Unlike previous studies that analyzed full protein sequences, Wehbi’s team focused on protein domains, which are shorter and more versatile segments of amino acids.
Wehbi likened protein domains to wheels on a car, noting their long-standing presence and adaptability. By using statistical tools to analyze the enrichment of amino acids in sequences from LUCA and earlier, the team identified when specific amino acids likely joined the genetic code.
They discovered over 400 families of sequences dating back to LUCA, with more than 100 predating LUCA and featuring amino acids with aromatic ring structures. Despite being late additions to the genetic code, these findings suggest alternative genetic codes may have existed in the distant past. “Early life seems to have liked rings,” Masel remarked.
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