Insight into one of life’s earliest ancestors revealed in new study

LUCA was one of the earliest forms of life, but was probably part of an ecosystem. (Credit: Paulista)

An international team of researchers, including Dr James Clark from the Milner Centre for Evolution at the University of Bath, has shed light on Earth's earliest ecosystem, showing that within a few hundred million years of planetary formation, life on Earth was already flourishing.

Everything alive today derives from a single common ancestor known affectionately as LUCA (Last Universal Common Ancestor).

LUCA is the hypothesised common ancestor from which all modern cellular life, from single celled organisms like bacteria to the gigantic redwood trees (as well as us humans) descend.

LUCA represents the root of the tree of life before it splits into the groups, recognised today: Bacteria, Archaea and Eukarya. Modern life evolved from LUCA from various different sources: the same amino acids used to build proteins in all cellular organisms, the shared energy currency, 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 and so the team used a genetic equivalent of the familiar equation used to calculate speed in physics to work out 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 team was led by the University of Bristol, with researchers from the University of Bath, University College London (UCL), Utrecht University, Centre for Ecological Research in Budapest, and Okinawa Institute of Science and Technology Graduate University.

Their study is published in Nature Ecology & Evolution.

Co-author Dr Sandra Álvarez-Carretero of Bristol's School of Earth Sciences said: "We did not expect LUCA to be so old, within just hundreds of millions of years of Earth formation. However, our results fit with modern views on the habitability of early Earth."

Next, the team worked out the biology of LUCA by modelling the physiological characteristics of living species back through the genealogy of life to LUCA.

They used complex evolutionary models to reconcile the evolutionary history of genes with the genealogy of species.

Dr James Clark, Prize Fellow from the Milner Centre for Evolution at the University of Bath, said: "The more distantly related two species are, the more differences you see in their DNA, so we can effectively combine molecular data with fossil data to count backwards in time and piece together a timeline."

The study showed that LUCA was a complex organism, not too different from modern prokaryotes, but interestingly their results showed clearly that it possessed an early immune system. This suggests that 4.2 billion years ago, our ancestor was part of an ecosystem, engaging in an arms race with viruses.

Co-author Tim Lenton (University of Exeter, School of Geography) said "It's clear that LUCA was exploiting and changing its environment, but it is unlikely to have lived alone. Its waste would have been food for other microbes, like methanogens, that would have helped to create a recycling ecosystem."

The findings and methods employed in this work will also inform future studies that look in more detail into the subsequent evolution of prokaryotes in light of Earth history, including the lesser studied Archaea with their methanogenic representatives.

Co-author Professor Philip Donoghue, from the School of Earth Sciences at the University of Bristol, said: "Our work 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."

The research was funded by the John Templeton Foundation. The opinions expressed in this publication are those of the author(s) and do not necessarily reflect the views of the John Templeton Foundation.

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