Every living thing on Earth evolved from one common ancestor

The mystery of life’s beginnings has long captivated scientists. Central to that search is LUCA—the last universal common ancestor. LUCA sits at the root of the evolutionary tree, where two great domains of life—Bacteria and Archaea—split. This ancient microbe holds clues about how life first gained a foothold on Earth.

A global research team, led by evolutionary biologists at the University of Bristol, set out to uncover LUCA’s age. They used a mix of tools, including fossil records, isotopic data, and genetic timelines. By analyzing ancient genes that had duplicated even before LUCA existed, the team could track back further than ever before.

Their findings point to an organism that lived about 4.2 billion years ago. That’s earlier than many scientists expected—possibly before the violent asteroid storms of the Late Heavy Bombardment, which peaked between 3.7 and 3.9 billion years ago. The results suggest life may have endured even the most hostile conditions on early Earth.

The team relied on a refined dating method known as molecular clock analysis. Instead of using genes from more recent lineages, they focused on paralogues—gene pairs that split before LUCA emerged. This strategy reduced uncertainty and helped the team build a more accurate timeline.

To sharpen their estimates, they used a method called cross-bracing. It lets researchers apply fossil evidence multiple times throughout a genetic tree. The result was a surprisingly tight estimate for LUCA’s age: 4.2 billion years. This timeline supports the growing belief that Earth became habitable soon after it formed.

LUCA’s genetic makeup was far from primitive. Phylogenetic studies reveal a genome spanning at least 2.5 megabases, packed with around 2,600 proteins. That’s comparable in complexity to many present-day bacteria and archaea. In other words, LUCA was no simple cell.

Researchers believe it had an early form of an immune system. That suggests ancient viruses were already around, battling with the first forms of cellular life. LUCA also used anaerobic metabolism—likely acetogenesis—to generate energy. It fed on hydrogen and carbon dioxide in a geochemically rich environment.

LUCA didn’t live alone. Its metabolism created chemical byproducts that fed other microbes nearby. Meanwhile, photochemical reactions in the atmosphere helped recycle hydrogen, closing the loop. These early interactions hint at ecosystems forming quickly—complex, cooperative networks that set the stage for all life to come.

According to Professor Tim Lenton of the University of Exeter, LUCA’s waste products likely served as a food source for other microbes, highlighting its role in fostering a recycling ecosystem.

The universal genetic code, reliance on ATP for energy, and shared amino acid chirality among modern organisms all trace back to LUCA. These shared traits underscore its importance as the common ancestor of all cellular life. LUCA’s existence sheds light on how life’s fundamental processes emerged and evolved, providing a blueprint for the diversity seen in today’s biosphere.

Determining LUCA’s age was not without challenges. Fossil evidence from the early Archean period is sparse and often contested. To overcome these limitations, researchers used relaxed Bayesian node-calibrated molecular clock approaches.

This method integrates fossil and geochemical records with molecular data, offering a robust framework for estimating LUCA’s age. By calibrating their models with 13 fossil data points, including the Moon-forming impact and manganese oxidation indicative of photosynthesis, the team achieved a reliable timeline.

The researchers also avoided overreliance on the Late Heavy Bombardment hypothesis, which has been increasingly questioned. Instead, they based their maximum-age constraint on the Moon-forming impact, dated to 4.51 billion years ago. This approach ensured a more accurate estimate of LUCA’s timeline.

LUCA’s evolutionary significance extends beyond its age and genome. Its characteristics offer a glimpse into the biochemical pathways and ecological dynamics of early Earth. For example, LUCA’s ability to harness geochemical energy underscores the importance of Earth’s early environment in shaping life’s development.

Dr. Sandra Álvarez-Carretero from Bristol’s School of Earth Sciences noted that LUCA’s ancient origin aligns with the planet’s early habitability, emphasizing the rapid emergence of life after Earth’s formation.

The study’s interdisciplinary approach was key to its success. By combining molecular data, fossil records, and biogeochemical models, the researchers reconstructed LUCA’s characteristics and ecological role.

Dr. Edmund Moody explained that gene exchange between lineages complicated evolutionary history, requiring sophisticated models to align gene evolution with species genealogy. Dr. Tom Williams highlighted the importance of incorporating diverse data representing life’s primary domains, Archaea and Bacteria, to build a comprehensive picture of LUCA.

Professor Davide Pisani described LUCA as a complex organism, comparable to modern prokaryotes. Its early immune system points to interactions with viruses, suggesting that microbial warfare was a feature of early ecosystems. Such findings highlight the ecological sophistication present shortly after Earth’s formation.

The study’s implications extend to our understanding of life on other planets. Rapid ecosystem establishment on Earth suggests that life could arise and thrive on Earth-like planets elsewhere in the universe. As Professor Philip Donoghue observed, this research not only deepens our understanding of Earth’s history but also provides a framework for exploring life’s potential in the cosmos.

Future research will build on these findings, focusing on the evolution of prokaryotes, particularly Archaea and their methanogenic representatives. Professor Anja Spang emphasized the significance of these insights for studying Earth’s history and its broader implications for life science.

This study, led by the University of Bristol, included contributions from institutions such as University College London, Utrecht University, and the Okinawa Institute of Science and Technology. Their collaborative efforts offer a groundbreaking perspective on LUCA’s role in shaping life on Earth.

The study "‘The nature of the last universal common ancestor and its impact on the early Earth system" is available on Nature Ecology & Evolution.

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