Everything that exists today originates from a common ancestor known as LUCA (Last Universal Common Ancestor).

LUCA is a hypothetical common ancestor from which all modern organisms, from single-celled organisms like bacteria to giant redwoods (and humans), descend. LUCA represents the root of the tree of life before it diverged into the groups we know today: Bacteria, Archaea, and Eukaryotes. Modern life evolved from LUCA from various sources: the same amino acids used to build proteins in all cellular organisms, a common energy currency (ATP), the presence of cellular machinery like ribosomes and others associated with producing proteins from information stored in DNA, and even the fact that all cellular life uses DNA as a way of storing information.

The team compared all the genes in the genomes of living species, counting the mutations that occurred in their sequences over time since they shared an ancestor in LUCA.

The divergence time of some species is known from fossil records, so the team used a genetic equivalent of the well-known equation used to calculate speed in physics to determine when LUCA existed, arriving at an answer of 4.2 billion years ago, roughly four hundred million years after the formation of Earth and our solar system.

Co-author Dr. Sandra Álvarez-Carretero from the School of Earth Sciences in Bristol said: “We didn’t expect LUCA to be so old, within just a few hundred million years of Earth’s formation. However, our results fit with modern views on the possibility of life on early Earth.”

The team then worked out the biology of LUCA by modeling the physiological characteristics of living species through the genealogy of life back to LUCA. Lead author Dr. Edmund Moody explained: “The evolutionary history of genes is complex due to their exchange between lineages. We need to use complex evolutionary models to reconcile the evolutionary history of genes with the genealogy of species.”

Co-author Dr. Tom Williams from the School of Biological Sciences in Bristol said: “One of the real strengths here is applying the reconciliation approach of the gene tree and species tree to such a diverse dataset representing the primary domains of life Archaea and Bacteria. This allows us to estimate with some certainty and assess that certainty about how LUCA lived.”

Co-author Professor Davide Pisani said: “Our study showed that LUCA was a complex organism, not too different from modern prokaryotes, but what is really interesting is that it clearly possessed an early immune system, showing that even 4.2 billion years ago, our ancestor was battling viruses.”

Co-author Tim Lenton (University of Exeter, Geography School) said, “It’s clear that LUCA used and changed its environment, but it is unlikely that it lived alone. Its waste would have been food for other microbes, like methanogens, which would help create a recycling ecosystem.”

“The results and methods used in this work will also inform future studies that will explore the later evolution of prokaryotes in more detail in light of Earth’s history, including the less-studied Archaea with their methanogenic representatives,” added co-author Professor Anja Spang (Royal Netherlands Institute for Sea Research).

Co-author Professor Philip Donoghue said: “Our work combines data and methods from multiple disciplines, revealing insights into early Earth and life that could not be achieved by any single discipline alone. It also shows how quickly the ecosystem was established on early Earth. This suggests that life can thrive on Earth-like biospheres elsewhere in the universe.”

The study also involved scientists from University College London (UCL), Utrecht University, the Centre for Ecological Research in Budapest, and the Okinawa Institute of Science and Technology.

The research was funded by the John Templeton Foundation. The opinions expressed in this publication are the authors' and do not necessarily reflect the views of the John Templeton Foundation.

Source: University of Bristol