The Last Universal Common Ancestor was already sophisticated. Following pre-LUCA “universal paralogues” allows us to link the first steps of life on Earth to experiences testable today.
For decades, the LUCA — English acronym for Last Universal Common Ancestor — was treated as the most remote point that biology can reconstruct with classical evolutionary tools.
When we go back in time, beyond the dinosaurs, the Cambrian explosion and the first multicellular forms, the scientific narrative converged on a microscopic organism that would have lived around four billion years ago.
This “horizon” seemed to mark the limit of what is possible to infer from family trees of genes and organisms.
But there is a problem: when researchers try to reconstruct the LUCA genome with bioinformatics and molecular phylogeny, a “rudimentary” being does not emerge. Instead, LUCA appears as an already sophisticated cell: complex metabolism, genetic code based on DNA and ribosomes — the cellular “factories” responsible for producing proteins.
For some scientists, this portrait raised a fundamental question: if LUCA was already a complete cellular machine, then the decisive phase in the origin of life would have occurred much earlier.
“Universal Paralogues”
A new approach, advocated by Aaron Goldman, Gregory Fournier e Betül Kaçarproposes a way of peeking into this even deeper past.
In a perspective text in Cell Genomics, the authors highlight the potential of a rare class of genes they call “universal paralogues” (universal paralogs), which would function as a kind of molecular fossil.
The idea is that some genes will have undergone duplication before LUCA existed. The two copies — the paralogs — were inherited by LUCA and, by extension, preserved throughout evolution in virtually all branches of current life, from bacteria to humans.
Typically, when reconstructing the evolutionary history of a gene, the “trunk” of the tree ends up at LUCA. There is no reference point to decide what happened before. But when there are two ancient versions of the same gene, present universally, it becomes possible to compare them to each other and infer characteristics of the ancestral state, pre-LUCA.
Fournier, a geobiologist at the renowned MIT, cited here by , describes these genes as the only information that may survive about the oldest cell lineages. Goldman, on the other hand, summarizes the consequence: although LUCA is the oldest organism accessible by evolutionary methods, some of the genes in its genome could be substantially older.
What “fossil genes” reveal about life before LUCA
A recurring pattern is the transition from generalist to specialist proteins. In the modern world, enzymes and proteins tend to perform very specific tasks; in the remote past, the authors suggest, there would predominate more versatile moleculescapable of performing multiple functions, albeit less efficiently.
One example comes from the system of translating the genetic code into proteins. Today, two distinct proteins help manage different stages of the process: initiation and elongation of the protein chain. But by reconstructing the pre-LUCA ancestor of these proteins, researchers found evidence of a more “polyvalent” form, capable of performing both roles before duplication and evolutionary divergence created separate functions.
In the reconstruction, the pre-LUCA ancestor related to leucine, isoleucine and valine would be poorly selective: a “promiscuous” enzyme, which did not distinguish well between these amino acids. Survival, at this early stage, may have depended less on precision and more on flexibility.
So: did life begin inside cells, or as a set of reactions in a chemical “broth” without borders? For the three researchers, the type of genes that appear repeatedly in this category points to the early existence of cellularity. Known universal paralogs tend to cluster around three vital abilities: protein production, amino acid processing and, crucially, transport and membrane functioning.
Among the examples cited is Signal Recognition Particle (SRP), a system that directs proteins to the cell membrane. The pre-LUCA ancestor of this complex, according to the authors, would already be capable of recognizing membrane targets and acting as a receptor. For the team, the presence of these ancient “transporters” reinforces the idea that pre-LUCA organisms were membrane-bound entities, not just diffuse chemistry in a primordial ocean.
Goldman and collaborators have already reconstructed and synthesized ancestral proteins related to membrane insertion in the laboratory and observed that they can interact with modern protein production machinery. Following these universal paralogues allows us to connect the first steps of life on Earth to experiences testable today.
