Whether giant sequoias, humans, or microscopic bacteria, every life form that inhabits Earth today shares a single, enigmatic ancestor. He is affectionately called Luca, an acronym for last universal common ancestor, and he lived about 4.2 billion years ago in a primordial ecosystem, on an Earth that is unrecognizable to us today. Every living being holds at least a piece of it in its genome. But Luca also carried in his DNA the imprint of an even more ancient world: a legacy of genes coming from those who preceded him on our planet. And as a new study published in Cell Genomicsthese genetic “fossils” can now be studied to discover something more about the origins of life and the forms it took before the appearance of the last universal common ancestor.
The roots of the tree of life
If we imagine evolution as a tree, Luca would be the trunk from which all the branches leading to plants, animals, fungi and bacteria originate. But as the authors of the new study explain, under that trunk there are deep and so far little studied roots: its ancestors. “Although the last universal common ancestor is the oldest organism we can study with evolutionary methods – explains Aaron Goldman, a researcher at Oberlin College and co-author of the study – some genes in its genome were much older”.
To reconstruct Luca’s genome, geneticists exploit his descendants: if a gene is present in every branch of current life, it has almost certainly been inherited from the last common ancestor. Studying the life forms that preceded it is not so easy, but Goldman and colleagues underline that scientific progress in recent years now makes it possible to do so – at least in part – by exploiting what they define as “universal paralogues”, genetic sequences resulting from gene duplications and present in every branch of the living world.
Ancestral genes
The reasoning is as follows: if these duplicated genes occur in homologous forms in creatures that are very different from each other today, it is probable that they come directly from Luca, and that they were also duplicated within his genome. And if this is the case, Luca must also have inherited them from even more ancient ancestors, in whose genome the original duplication process must have taken place. These elements therefore offer us precious clues to reconstruct what the world must have been like before the universal common ancestor.
The universal paralogues known today are not many, but they still offer interesting information. The study analyzes them all, demonstrating that they are linked either to the production of proteins, or to the movement of molecules across the cell membrane. What does it mean? That most likely, more than four billion years ago, living cell-like beings already existed on Earth, equipped with cellular machinery responsible for translating genes into proteins using amino acids as starting elements. Information that is anything but obvious going back so far in time, practically to the dawn of the history of life.
The first forms of life
“These data confirm that the transition towards a modern genetic code occurred before Luca and was a complex evolutionary process – write the three authors of the study – which involved multiple mechanisms and took place in parallel with the evolution of the amino acid production mechanisms”.
A promising start, in short, in what the three scientists hope will prove to be a much longer and more fruitful journey to discover the first life forms that appeared on Earth and the legacy they have left in today’s world. Thanks to artificial intelligence, in fact, it is becoming increasingly simple to identify these universal paralogues, study their functions and even reconstruct them to test their behavior live. “By following universal paralogues – concludes Betül Kaçar, microbiologist at the University of Wisconsin-Madison and co-author of the study – we can connect the very first steps of life on Earth with the tools of modern science. They offer us the opportunity to transform the deepest mysteries of evolution and biology into discoveries that we can actually test.”