In a discovery that deepens our understanding of life's origins, researchers have pinpointed the existence of our last universal common ancestor (LUCA) to about 4.2 billion years ago, just a few hundred million years after Earth first formed. This discovery shows that LUCA lived further back in time than we thought. It reveals that life on Earth started in a surprisingly complex and active environment, full of other life forms and viruses.
LUCA, as biologists have affectionately named it, was not so different from the more complex bacteria that exist today. What’s particularly fascinating is the early development of an immune system in LUCA, indicating that even at this early stage, our ancient ancestor was locked in a struggle with viruses, according to Davide Pisani, a genomics researcher at the University of Bristol and a co-author of the study.
A Shared Origin: Unifying Traits in All Life
All cellular life on Earth shares several key characteristics, such as the use of protein building blocks, reliance on ATP for energy, and DNA for storing genetic information. These similarities suggest that all life as we know it descended from this single origin. Previously, scientists estimated that LUCA lived around 3.9 billion years ago, but dating such ancient events accurately has always been a challenge.
Published in the journal Nature Ecology & Evolution on 12 July, the new study aimed to provide a more precise timeline for LUCA’s existence. The research team compared genes from 700 existing species of bacteria and archaea—microbes thought to be among the oldest forms of life on Earth. By counting the mutations in these genomes and analysing 57 shared genes, they calculated when LUCA likely lived. Fossils containing ancient life traces, such as 3.48-billion-year-old microbial mats from Australia, helped anchor their estimates.
LUCA's Environment: Thriving in Extreme Conditions
This comprehensive analysis led to the surprising conclusion that LUCA lived around 4.2 billion years ago. "We did not expect LUCA to be so old, within just hundreds of millions of years of Earth formation," remarked Sandra Álvarez-Carretero, a research fellow at UCL in the UK and co-author of the study. At that time, Earth was still in its Hadean phase—a period marked by extreme conditions, with hot oceans and a near lack of atmospheric oxygen.
The study also provided insights into LUCA’s environment and lifestyle. Although the exact habitat remains unclear, LUCA likely thrived in an oceanic setting, possibly around shallow hydrothermal vents or hot springs. LUCA was resilient, capable of surviving extreme temperatures and “breathing” without oxygen, relying instead on the waste products of other organisms in its ecosystem.
LUCA’s metabolic pathways suggest it utilised organic material pre-digested by other microbes for energy. Intriguingly, the research also uncovered evidence that LUCA had already developed genes for defending against viral infections, further indicating the complexity of its ecosystem.
The fact that LUCA existed in such a dynamic and bustling environment opens up tantalising possibilities about life beyond Earth. As Professor Philip Donoghue, senior author of the study and a paleobiology professor at the University of Bristol, noted, “This suggests that life may be flourishing on Earth-like biospheres elsewhere in the universe.”
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