Every organism alive on Earth, from oak trees to octopuses to the bacteria in our gut, belongs to a single extended family. Genetic evidence points back to one ancestral cell, a last common forebear scientists call LUCA, that links all known life in a continuous chain. Yet the story does not stop there. Some of the genes that power life today may be older than that ancestor itself, carrying traces of the chemistry that existed before biology took off.
That idea reshapes how we think about evolution. Rather than a clean starting gun, the origin of life looks more like a long dawn, with molecules trying many paths until a stable lineage emerged. LUCA marks the point where that lineage became recognizable as life as we know it, but the genetic tools it used likely evolved earlier, in a world that was still mostly chemistry rather than cells. In that world, simple replicators may have copied themselves, competed, and left behind a few successful molecular tricks that later became part of modern genomes.
The single ancestor behind all life
Biologists now treat common descent as a working fact: all life descended from a common ancestor, rather than appearing in separate acts of creation. The universality of features like the genetic code and the way cells read DNA suggests that everything alive today traces back to one evolutionary line. That line converges on LUCA, the last universal common ancestor, a single cell that sat at the root of the tree from which bacteria, archaea, and the ancestors of animals and plants later branched.
Discussions of a shared origin often point to the consistency of molecular machinery across species. A summary of this notes that the same core script appears in every genome scientists have checked. If life had started many times in isolation, we would expect very different biochemical systems. Instead, the match between organisms is so strong that researchers now see a single ancestral cell as the simplest explanation for the patterns we observe in DNA, proteins, and cell structures.
Why LUCA is not the first life
It is tempting to treat LUCA as the moment life began, but the evidence points to a deeper past. LUCA does not represent the origin of life, and researchers are careful to separate the last universal common ancestor from the first living chemistry. The origin of life was the moment when some mix of molecules on early Earth crossed a threshold and became self-maintaining, self-copying systems that could evolve. That first step may have involved many false starts and short-lived lineages that left no direct descendants.
In that sense, LUCA sits partway along a longer path. Work described in a recent overview of argues that this ancestor marks the point where life as we know it took off, meaning cells had already refined their genetic and metabolic tools. Before that, there must have been a series of transitional stages: primitive replicators that copied themselves poorly, networks of reactions that almost counted as metabolism, and early genetic molecules that stored information without the full complexity of modern DNA-based cells. LUCA is the last shared node of today’s life, not the first spark.
Genes older than our last common ancestor
If LUCA was already a fairly advanced cell, then some of the genes it carried must predate it. When scientists compare genomes from across the tree of life, they find families of genes that are present in bacteria, archaea, and eukaryotes, which implies that LUCA already had them. Those shared genes likely arose earlier, in a pool of ancestral molecules that existed before the final lineage leading to LUCA had fully formed. In other words, certain genetic tricks are older than the last ancestor that used them.
This idea matches how evolution works at the molecular level. Natural selection acts on genes and genetic elements that help them persist, an idea often called the “selfish gene” view of evolution. A reflection on this argues that it permeates all biology right back to the beginnings of life on Earth, because any replicator that can copy itself and improve its own chances will tend to spread. If selfish genes were already at work in the chemical world that preceded LUCA, then some of the sequences that proved useful could have been passed into the lineage that became our universal ancestor and survived ever since.
From chemistry to biology
To see how genes could be older than LUCA, it helps to picture the transition from non-living chemistry to living systems. The origin of life is sometimes described as a kind of chemical alchemy. At some point, molecules that simply reacted with each other crossed a threshold and started to behave as a unit that could grow, divide, and evolve. That shift likely involved molecules that stored information, catalysts that sped up reactions, and compartments that kept helpful reactions close together. Each part made the whole system more stable and more likely to persist.
Researchers studying the beginnings of life on Earth say we have made great strides in understanding the steps by which chemistry on Earth became biology. Experiments show that simple membranes can form on their own, that RNA molecules can copy parts of themselves, and that mineral-rich environments can drive complex reactions. In this picture, early Earth may have hosted many small “protocells” that grew and split. Some of them may have carried genetic fragments that later became part of LUCA’s toolkit, while others faded away, leaving only chemical traces in rocks and oceans.
Numbers that frame LUCA’s world
Researchers often use numbers to give a sense of the scale of this deep history. One estimate suggests that LUCA lived at least 3.8 billion years ago, not long after Earth cooled enough to support stable oceans. That figure, 3,800,000,000 years, helps show how much time evolution has had to work with since our last universal ancestor. It also reminds us that LUCA itself was already the product of earlier evolution, not a sudden arrival. The long gap between Earth’s formation and LUCA’s time leaves room for many generations of pre-cellular chemistry.
Other numbers highlight the complexity that had already emerged by LUCA’s era. Some reconstructions suggest that this ancestor may have had on the order of 698 genes that coded for core functions like copying DNA, building proteins, and managing energy. Modern bacteria can carry thousands of genes, but even a set of hundreds shows that LUCA was no simple blob. It had enough parts to run a full living cell, yet it still sat close enough to life’s beginning that some of its genes likely came from older, more scattered molecular systems.
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*This article was researched with the help of AI, with human editors creating the final content.
