From trees to humans, from elephants to the tiniest bacterium, all living beings today have a common ancestor, a small cell that the scientific community has named LUCA, for last universal common ancestor.

But LUCA is not the origin of life. It's the oldest being that we can trace back to, by analyzing the genes of today's living species. This is a huge challenge for phylogeneticists, the genealogists of living organisms. An international team has proposed a reconstruction of the LUCA genome, and a new dating, in work published in July in the scientific journal Nature Ecology and Evolution.

"According to our estimate, this LUCA is a fairly complex organism," said Edmund Moody, a researcher in evolutionary biology at the Paleobiology Group, University of Bristol, UK, and lead author of the study. Its genome contains the blueprints for 2,600 proteins. This is a higher number than previously estimated.

To arrive at this number, the authors first built up a database from the genomes of prokaryotes, the single-celled beings of today's world, such as bacteria. Then they used a new algorithm. "For each gene family, it establishes a probability of being present in the LUCA genome," said co-author Tom Williams, professor at the University of Bristol.

"If we have an idea of the phylogenesis [genealogy] of species, this method enables us to reconstruct an evolutionary scenario for each gene," said Vincent Daubin, CNRS research director at the Laboratoire de Biométrie et Biologie Evolutive, in Villeurbanne (east-central France), who worked on the method but was not involved in the study.

Imagine a tree. LUCA is at the base of its trunk. And all current species form its leaves. Researchers reconstruct a tree for each gene, to understand its history. And they deduce which genes of today's prokaryotes were already present in LUCA.

Genes are passed on to descendants, from roots to leaves. But they can also be transmitted between species, horizontally, between the branches of the tree. "This new method makes it possible to detect horizontal transfer," said the phylogeneticist.

"These horizontal transfers are still difficult to see, especially when they are long-standing. The risk is to overestimate the quantity of genes present in LUCA," said Purificacion Lopez-Garcia, director of research at the Laboratoire d’Ecologie, Systématique et Evolution at Université Paris-Saclay. The biologist highlighted the study's "major uncertainties."

"The further back we go, the more uncertain we are," said Hervé Philippe, bioinformatician and research director at the Station d'Ecologie Théorique et Expérimentale (SETE) in Moulis, Ariège. "The results very much depend on the starting hypotheses."

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