For decades, scientists have worked under the assumption that life arose on Earth roughly 4 billion years ago, some 600 million years after the planet formed. But a new international study now challenges that timeline, proposing that life may have taken hold far sooner—just 400 million years after Earth’s birth, during one of its most hostile geological eras.
At the heart of this discovery is LUCA, the Last Universal Common Ancestor—a simple, prokaryote-like organism believed to be the shared ancestor of all life forms that exist today. According to the study, published in Nature Ecology & Evolution, LUCA likely lived around 4.2 billion years ago, placing it squarely in the Hadean Eon, a time once thought too volatile to support life.
A Single Origin for All Life
LUCA is not the first living thing to exist, scientists caution, but it is the most recent organism from which all current life descends. Its existence is inferred through shared biological traits found across all living species.
“The common ancestry of all extant cellular life is evidenced by the universal genetic code, shared protein synthesis machinery, the chirality of amino acids, and ATP as a universal energy currency,” the researchers wrote in the study. “Understanding LUCA helps us understand the earliest evolution of life on Earth.”
These shared traits act like molecular fingerprints, pointing back to a single biological source.
Rewriting the Timeline of Life
To determine when LUCA existed, scientists used a sophisticated genetic approach. They compared genes across modern species and calculated how many mutations had accumulated since those species diverged from a common ancestor. By applying evolutionary models to this data, researchers were able to work backward in time.
“The evolutionary history of genes is complicated by their exchange between lineages,” said Edmund Moody, lead author of the study from the University of Bristol. “We had to use complex evolutionary models to reconcile gene histories with the genealogy of species.”
Their conclusion: LUCA existed much earlier than previously believed, thriving in an era marked by intense volcanic activity, meteor impacts, and extreme heat.
A Surprisingly Sophisticated Organism
Perhaps the most surprising revelation from the study is not LUCA’s age, but its apparent biological complexity.
Although LUCA was likely a simple, single-celled organism without a nucleus, the researchers found evidence suggesting it may have already possessed an early immune system. This implies LUCA was capable of defending itself against viruses—meaning viral life had already emerged by that time.
“It’s clear that LUCA was exploiting and changing its environment,” said Tim Lenton, co-author and researcher at the University of Exeter. “But it is unlikely to have lived alone.”
This suggests that even at this early stage, life on Earth existed in interconnected microbial communities, recycling nutrients and shaping their environment.
Life Thriving in a Hostile World
LUCA likely lived in aquatic environments, possibly near hydrothermal vents where chemical energy could substitute for sunlight. Its metabolic waste may have served as food for other microbes, such as methanogens, helping establish some of the earliest ecosystems on Earth.
“These interactions would have created a primitive but functional recycling system,” Lenton noted, highlighting that life was already influencing Earth’s chemistry billions of years ago.
Big Questions Still Remain
While LUCA represents the oldest common ancestor scientists can currently identify, it does not explain how life first emerged from non-living matter. The steps that led from basic chemistry to organized, self-replicating life remain one of science’s greatest mysteries.
What this study does make clear, however, is that life began earlier, survived harsher conditions, and evolved faster than we once imagined.
As researchers continue to explore Earth’s deep past—and search for life beyond our planet—LUCA’s story offers a powerful insight: life is resilient, adaptable, and deeply interconnected.
Further research will aim to uncover what came before LUCA, how early life formed complex communities, and whether similar processes could be occurring elsewhere in the universe.
