New research challenges long-held assumptions about amino acid evolution — and could reshape the search for life beyond Earth.

The Building Blocks Were There Before Life Was

For decades, scientists believed they had a reasonably firm grasp on the sequence of events that gave rise to life on Earth — a tidy origin story involving amino acids assembling themselves into proteins, proteins begetting genes, and genes eventually producing everything from single-celled bacteria to human beings. But a new peer-reviewed study from researchers at the University of Arizona is quietly upending that narrative, suggesting that some of the most fundamental assumptions about early genetic history may have been shaped less by evidence than by bias.

The study, published in the Proceedings of the National Academy of Sciences, focuses on protein domains — chains of dozens or more amino acids that serve as the structural workhorses of biology. Think of them the way one researcher described them: like a wheel on a car. Wheels existed long before automobiles did, and protein domains existed long before the life forms we recognize today.

Rethinking LUCA and the Amino Acid Timeline

At the center of this research is the last universal common ancestor, or LUCA — the single ancient life form from which all life on Earth eventually branched. Scientists have long used LUCA as a kind of temporal landmark, a dividing line between what came before biology and what came after it. Using specialized software and data from the National Center for Biotechnology Information, the research team built an evolutionary tree of protein domains stretching back roughly four billion years to LUCA’s era.

What they found challenged a prevailing assumption: that the frequency of an amino acid in early life forms indicates when it first appeared. Under the current model, the amino acid found most abundantly in ancient life was presumed to have arrived earliest on the scene. The University of Arizona team argues this logic is flawed — that it conflates what was common in early living systems with what was available in the broader prebiotic chemical environment.

Tryptophan’s Surprising Cameo

The amino acid tryptophan — best known, somewhat unfairly, as the compound blamed for post-Thanksgiving drowsiness — emerged as a striking case study. Scientists have long held a consensus that tryptophan was the last of the 20 canonical amino acids to be incorporated into the genetic code. Yet the researchers found it appeared at a rate of 1.2% in pre-LUCA data, compared to just 0.9% after LUCA. That gap, modest in absolute terms, represents a 25% difference — a figure that demands explanation.

Why would the so-called latecomer amino acid be more prevalent before the emergence of life than after it? The researchers theorize that the chemistry involved may point to an even older, more primitive version of genetic logic — one that predates LUCA itself and may have operated under entirely different rules.

Multiple Codes, Competing Simultaneously

One of the more provocative conclusions in the paper is the suggestion that genetic codes did not develop in a single linear sequence. Instead, the researchers propose that stepwise construction of the current code and competition among multiple ancient codes may have unfolded at the same time. Even more striking, they raise the possibility that these ancient codes made use of noncanonical amino acids — ones that didn’t survive into the modern genetic alphabet — potentially emerging near alkaline hydrothermal vents on the ocean floor, environments long theorized as cradles of early life.

This reframing matters not just for understanding Earth’s past, but for thinking about what life might look like — or have looked like — elsewhere.

Enceladus and the Search for Life Beyond Earth

The paper closes with a prospect that stretches the imagination without straining it. The researchers note that the abiotic synthesis of aromatic amino acids — the kind that may have existed before life formally began on Earth — could theoretically occur at the water-rock interface of the subsurface ocean on Enceladus, one of Saturn’s moons. That moon is already considered one of the solar system’s more promising candidates for extraterrestrial life, given its liquid water and active geology.

In this light, studying the chemistry of four-billion-year-old Earth is not simply an exercise in ancient history. It is, potentially, a rehearsal for knowing what to look for 800 million miles away.