LUCA - Last Universal Common Ancestor

Everything alive today derives from a single common ancestor known affectionately as LUCA

LUCA is the hypothesized common ancestor from which all modern cellular life, from single-celled organisms like bacteria to gigantic redwood trees (as well as humans), descend. LUCA represents the root of the tree of life before it splits into the groups recognized today: Bacteria, Archaea, and Eukarya. Modern life evolved from LUCA from various sources: the same amino acids used to build proteins in all cellular organisms, the shared energy currency (ATP), the presence of cellular machinery like the ribosome and others associated with making proteins from the information stored in DNA, and even the fact that all cellular life uses DNA itself as a way of storing information.

The team compared all the genes in the genomes of living species, counting the mutations that have occurred within their sequences over time since they shared an ancestor in LUCA. The time of separation of some species is known from the fossil record, so the team used a genetic equivalent of the familiar equation used to calculate speed in physics to determine when LUCA existed, arriving at the answer of 4.2 billion years ago, about four hundred million years after the formation of Earth and our solar system.

The researchers did not expect LUCA to be so old, within just hundreds of millions of years of Earth’s formation. However, their results fit with modern views on the habitability of early Earth. Next, the team worked out the biology of LUCA by modeling the physiological characteristics of living species back through the genealogy of life to LUCA. The evolutionary history of genes is complicated by their exchange between lineages, so the team had to use complex evolutionary models to reconcile the evolutionary history of genes with the genealogy of species.

One significant advantage of this study is the application of the gene-tree species-tree reconciliation approach to a diverse dataset representing the primary domains of life Archaea and Bacteria. This approach allowed the researchers to confidently assess how LUCA lived. The study revealed that LUCA was a complex organism, not too different from modern prokaryotes, and possessed an early immune system, indicating that even by 4.2 billion years ago, our ancestor was engaging in an arms race with viruses.

It is clear that LUCA was exploiting and changing its environment but likely did not live alone. Its waste would have been food for other microbes, like methanogens, which would have helped to create a recycling ecosystem. The findings and methods employed in this work will inform future studies looking into the subsequent evolution of prokaryotes in light of Earth’s history, including the lesser-studied Archaea with their methanogenic representatives.

This research draws together data and methods from multiple disciplines, revealing insights into early Earth and life that could not be achieved by any one discipline alone. It also demonstrates just how quickly an ecosystem was established on early Earth.

"This suggests that life may be flourishing on Earth-like biospheres elsewhere in the universe." - Philip Donoghue