Tryptophan Was Supposed to Be Last… But It Ruled Earth BEFORE Life Even Started! Mind-Blown Discovery

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The provided text appears to be a popular science summary (likely from a social media post or article) describing a 2024 study by researchers at the University of Arizona, published in Proceedings of the National Academy of Sciences (PNAS). The original paper is titled “Order of amino acid recruitment into the genetic code resolved by last universal common ancestor’s protein domains” by Sawsan Wehbi and Joanna Masel (and collaborators), with the final publication dated December 2024 (though some references list it under 2025 or 2026 due to online-first availability or republication dates).The core findings challenge aspects of the long-standing “consensus” model for how the universal genetic code evolved—specifically, the stepwise addition of the 20 standard amino acids over time. Traditional views often ranked amino acids by factors like molecular complexity, biosynthetic pathways, or geochemical availability, with simpler/smaller ones (e.g., glycine, alanine) added early and more complex ones (e.g., tryptophan, tyrosine) added late.Key insights from the study include:

By analyzing ancient protein domains conserved back to the last universal common ancestor (LUCA, ~4 billion years ago), the researchers inferred amino acid usage patterns before and around LUCA’s time.
Smaller amino acids were indeed recruited earlier, but the previous consensus order added no extra predictive value beyond size.
Metal-binding amino acids (like cysteine and histidine) and sulfur-containing ones (cysteine and methionine) joined the code earlier than many models predicted.
Most strikingly for aromatic amino acids: Sequences predating or around LUCA showed higher frequencies of aromatics like tryptophan (W), tyrosine (Y), phenylalanine (F), and histidine (H) than expected—especially in even older, duplicated pre-LUCA protein families.
Tryptophan (long thought to be the last amino acid added to the code) was more abundant in pre-LUCA reconstructions (about 1.2% vs. 0.9% post-LUCA, a notable relative increase), hinting that earlier, now-extinct genetic codes or “protolife” systems may have used different amino acid sets or assignments.
This suggests a more chaotic early phase with competing or alternative genetic codes on ancient Earth, where some non-canonical or variant building blocks thrived temporarily before the modern code won out.

The idea of a “chaotic and vibrant era of protolife” aligns with the paper’s discussion of pre-LUCA sequences potentially reflecting older coding systems that were later replaced.Implications extend to astrobiology: If complex/aromatic amino acids formed readily in early Earth’s harsh conditions (possibly aided by abiotic processes or meteoritic delivery), similar chemistry could occur today in places like the subsurface oceans of Enceladus (Saturn’s moon), where hydrothermal activity might support prebiotic or even biotic processes.This isn’t a complete “rewrite” of life’s history but a significant refinement—showing the code’s assembly was likely more nuanced, gradual, and influenced by multiple competing systems than the simplified linear models suggested. The study uses phylogenetic reconstruction of protein domains (rather than full proteins) to peek deeper into pre-LUCA evolution, offering a window into life’s murky origins.