Tracing the genealogical tree of life to its deepest roots: this is the feat achieved by a multidisciplinary team led by Edmund Moody of the University of Bristol. Their work, published in July in the review Nature Ecology & Evolutionsketch the contours of an ancestral organism much more complex than imagined.
Named LUCA (Last Universal Common Ancestor), it was a single-celled organism that lived 4.2 billion years ago. It was believed to be simple, limited from a functional point of view, but this is no longer the case ; our understanding of the origins of life on Earth is thus somewhat shaken up. Just like Quaestio simpsonorum has just shaken up our beliefs about the theory of evolution, a little over a month ago now.
An organism already equipped to survive and evolve
LUCA does not represent the very origin of life, that moment when molecular alchemy generated the first self-replication mechanisms. Rather, it embodies the moment when life as we know it has truly taken flight. Greg Fournier, an evolutionary biologist at MIT, rightly points out: “ It is neither the first cell nor the first microbe. It is, in a way, the culmination of the story of the origins of life “. In reality, it represents a point of convergence where all forms of life have a common ancestor and not the first organism to emerge from the primordial soup.
Genomic analysis shows that LUCA had approximately 2,600 proteins, a complexity comparable to that of certain current bacteria. Another rather remarkable fact, LUCA already had a rudimentary immune system composed of 19 CRISPR genes (Clustered Regularly Interspaced Short Palindromic Repeats, a bacterial DNA sequence which serves as “immune memory”) intended to fight viral attacks.
Its metabolism was based on the exploitation of hydrogen and carbon dioxide, and that was quite good. The atmosphere of the early Earth was very different from what we know today. since it was mainly composed of gases like methane, ammonia and carbon dioxide, and therefore contained very little oxygen. LUCA was like this extremely well adapted to the harsh conditions of our planet.
A dazzling emergence in a hostile world
The dating of LUCA constitutes one of the most surprising discoveries of this research. Molecular analyzes place its existence, as previously written, around 4.2 billion years ago, barely 300 million years after the formation of the Moon. In reality, this new timeline completely turns things upside down our preconceived ideas about the emergence of primitive life.
At that time, our planet was going through a particularly chaotic period : the earth’s surface, still unstable, was undergoing intense meteorite bombardments. Extreme temperatures and incessant volcanic activity created an environment that scientists considered, until recently, incompatible with the appearance of living organisms. Estimates suggest that it would have taken between 100 and 200 million years, at a minimum, for the Earth to stabilize enough to support the first forms of life.
This chronology therefore reveals an evolution of astonishing speed: in the space of only a few hundred million years, life would have crossed the gulf separating the first self-replicating molecules from sophisticated cellular organisms. To make a temporal comparison, this is a shorter interval than that separating the appearance of the first dinosaurs from their extinction.
Phil Donoghue, co-author of the study, draws a conclusion from these observations that echoes our questions about extraterrestrial life: “ Our work suggests that these early stages of evolution were not difficult. If you’re interested in the origin of microbial life, it seems relatively simple and should be fairly common in the universe “. This hypothesis therefore implies that the fundamental mechanisms of life could emerge relatively easily as long as environmental conditions permit, even in circumstances that we previously considered prohibitive.
This accelerated timeline however, is not unanimous in the scientific community. Patrick Forterre, former director of microbiology at the Pasteur Institute, expresses reservations about the possibility of such an early emergence of life: “ It is very difficult to imagine that LUCA lived before 4 billion years ago “. In his view, the Earth had not yet cooled enough to allow life to appear at the time of LUCA, and he doubts the accuracy of our estimates of the rate of evolution at that time.
LUCA goes under the microscope
To reconstruct the portrait of LUCA, the researchers developed a unique probabilistic approach. By analyzing the evolutionary trees of nearly 10,000 gene families shared between 350 bacterial species and 350 archaeal species, they were able to estimate the probability of the presence of each gene in the ancestral genome. It’s a bit like comparing thousands of puzzles to try to piece together a complete picture. By comparing the pieces of these puzzles (the genes), researchers were able to assess which genes were present in LUCA.
This methodology made it possible to overcoming the biases of previous approacheswhich tended to either overestimate or underestimate the complexity of LUCA. The team notably took into account several complex genetic phenomena which can confuse evolutionary tracks.
Last point: LUCA was probably not an isolated organism, but was part of a complex microbial ecosystem of which he is the only survivor. This hypothesis is reinforced by its metabolism, compatible with symbiotic or competitive interactions with other microorganisms.
The discoveries on LUCA therefore invite us to fundamentally reconsider our understanding primitive mechanisms of life. Beyond questions about our terrestrial origins, this work traces new avenues of reflection on the possibility of other forms of life in the Universe. The image of a complex ancestral organism, capable of thriving in extreme conditions, therefore considerably enriches our perception of the potential of life. Understanding LUCA perhaps brings us closer to answering the fundamental question: how did life appear ? Question for which we still have no real answer, hence the importance of continue our research.
- LUCA, a complex single-celled organism, lived 4.2 billion years ago and possessed an advanced metabolism and a rudimentary immune system.
- LUCA was not isolated, but evolved in a complex microbial ecosystem.
- Its rapid emergence in a harsh Earth environment suggests that life may be easier to develop than previously thought, even elsewhere in the Universe.
