Scientists have discovered the last universal common ancestor (LUCA) of all life on Earth, dating back four billion years. This ancient microbe, which lived in a violent, hot, and unstable environment, is the foundation of the tree linking all modern bacteria and archaea, and indirectly, all life forms on Earth. LUCA was not the first life form but the last shared ancestor of every cellular organism today. It represents a pivotal moment in the evolution of life, when it had already diversified but had not yet split into the major domains we see today.
For decades, scientists have debated the characteristics of LUCA. Some studies suggest a simple organism with around 80 core proteins, while others propose a genome rivaling modern microbes, with over 1,500 gene families. A recent study published in Nature Ecology & Evolution takes a novel approach by combining molecular clocks, genome reconstructions, and models of early Earth to estimate LUCA's age, biology, and environment.
The research, conducted by an international team, highlights the importance of understanding LUCA's characteristics and timeline in deciphering the evolution of life on Earth. Edmund R. R. Moody, a senior research associate in computational evolutionary biology at the University of Bristol, emphasizes the significance of this discovery. Sandra Álvarez-Carretero, a research fellow at UCL, also contributed to the study.
Dating LUCA is challenging due to the scarcity and dispute over fossils from Earth's earliest era. Molecular clocks can drift over billions of years, and earlier studies often relied on single-copy genes. The new study focuses on duplicated genes, called pre-LUCA paralogues, which provide more accurate age estimates by comparing both copies across modern species.
The researchers analyzed 700 modern genomes, split evenly between bacteria and archaea, to reconstruct LUCA's genome. They estimated that LUCA likely had a genome of about 2.75 megabases, encoding around 2,657 proteins. This genome size falls within the range of many modern microbes. The study also identified a core of 399 gene families that strongly support their presence in LUCA, forming the backbone of its metabolic network and cellular machinery.
LUCA's metabolism suggests it lived in a hydrogen-rich environment, utilizing the Wood-Ljungdahl pathway for carbon fixation. It lacked oxygen-based respiration and likely relied on simple organic molecules. The study also found evidence of early CRISPR-Cas systems, indicating that LUCA faced viral threats and used immune-like tools to survive.
The findings portray LUCA as a complex, fully cellular organism with a genome and metabolism similar to modern microbes. It lived in hot, oxygen-free environments, utilized hydrogen-based chemistry, and already battled viruses. This rapid rise to complexity from a molten planet to complex cellular life is a striking revelation, suggesting that many steps between chemistry and biology may have unfolded in a narrow window of time.
The authors note that this reconstruction is not final, and many early lineages may have vanished without leaving genetic traces. As more genomes are sequenced and models improve, our understanding of LUCA will become clearer, offering one of the most comprehensive views of life's shared starting point.
