All Life on Earth Today Descended From a Single Cell. Meet LUCA.

 The clearest picture yet of our “last universal common ancestor” suggests it was a relatively complex organism living 4.2 billion years ago, a time long considered too harsh for life to flourish.

If you follow any path of ancestry back far enough, you’ll reach the same single point. Whether you begin with gorillas or ginkgo trees or bacteria that live deep in the bowels of the Earth — or yourself, for that matter — all roads lead to LUCA, the “last universal common ancestor.” This ancient, single-celled organism (or, possibly, population of single-celled organisms) was the progenitor of every varied form that makes a life for itself on our planet today, quantamagazine.org.

LUCA does not represent the origin of life, the instance whereby some chemical alchemy snapped molecules into a form that allowed self-replication and all the mechanisms of evolution. Rather, it’s the moment when life as we know it took off. LUCA is the furthest point in evolutionary history that we can glimpse by working backward from what’s alive today. It’s the most recent ancestor shared by all modern life‚ our collective lineage traced back to a single ancient cellular population or organism.

“It’s not the first cell, it’s not the first microbe, it’s not the first anything, really,” said Greg Fournier(opens a new tab), an evolutionary biologist at the Massachusetts Institute of Technology. “In a way, it is the end of the story of the origin of life.”

Still, understanding LUCA — whether it was simple or complex, and how quickly it emerged after life’s origin — could help answer some of our deepest questions about where we come from and whether we’re alone in the universe.

“[LUCA] tells our own story,” said Edmund Moody(opens a new tab), an evolutionary biologist at the University of Bristol. “It gives us a point from which we can look even further back.”

For half a century, biologists have focused on different kinds of physiological, genomic and fossil evidence to paint portraits of LUCA that sometimes clash dramatically. In 2024, Moody and a team of interdisciplinary researchers, including geologists, paleontologists, system modelers and phylogeneticists, combined their knowledge to build a probabilistic model that reconstructs modern life’s shared ancestor and estimates when it lived.

The analysis, published in Nature Ecology and Evolution in July, sketched a surprisingly complex picture(opens a new tab) of the cell. LUCA lived off hydrogen gas and carbon dioxide, boasted a genome as large as that of some modern bacteria, and already had a rudimentary immune system, according to the study. Its genomic complexity, the authors argue, suggests that LUCA was one of many lineages — the rest now extinct — living about 4.2 billion years ago, a turbulent time relatively early in Earth’s history and long thought too harsh for life to flourish.

The analysis reaches two conclusions that seem in conflict with each other, according to Aaron Goldman(opens a new tab), who studies the molecular evolution of early life at Oberlin College and wasn’t involved in the new research. “The first is that LUCA was a complex cellular organism that likely lived in a complex ecological setting,” he said. “The second is that LUCA dates to a time that is pretty early in the history of Earth.” The results could mean that life evolved from a simple replicator into something resembling modern microbes remarkably quickly, he said. “That’s really exciting.”

“You get a picture of this fairly complex organism.”

Edmund Moody, University of Bristol

“Our work suggests that those early steps of evolution weren’t hard; they’re pretty easy,” said co-author Phil Donoghue(opens a new tab), an evolutionary biologist at the University of Bristol. “If you’re concerned with the origin of microbial-grade life, then that’s apparently very easy, and it should be quite common in the universe.”

Not all experts in the field agree, however. Some argue that a few hundred million years is not enough time for complex life to have evolved. The authors stress that their analysis is a first attempt to paint a fuller, admittedly fuzzy, picture of LUCA. “I fully expect and hope people prove us wrong in certain aspects,” said Moody, the paper’s lead author, especially if those new results offer a clearer view of the ancient ancestor of all life we know.

A Probabilistic Portrait

It’s no small task to puzzle out the nature of an entity that lived so long ago on fragmentary, inferential evidence. Microscopic fossils can get researchers part of the way, but the oldest traces of life date to only around 3.5 billion years ago, likely long after LUCA lived. That leaves LUCA’s descendants for us to study. “The way to study LUCA is to compare the diversity of genes and physiologies and metabolisms today, and then work backward,” said Fournier, who was not involved in the new work.

In the 1970s — well before the genomic era — researchers thought the last common ancestor of all life (not yet named LUCA) was quite primitive(opens a new tab), a replicating entity that hadn’t yet mastered the ability to translate genes into proteins. But as genetic sequencing technology advanced and more genomes became available, researchers became able to look for shared genes among many different organisms to infer which are the most ancient and conserved through time.

That’s how Moody’s team started. They built an evolutionary tree representing the relationships among 350 living bacterial species and 350 archaean species. Then, they analyzed the evolutionary histories of nearly 10,000 gene families shared by those species to create an evolutionary tree for each gene.

When we compare two closely related gene lineages, it’s natural to conclude that any shared genetic material would have been present in their common ancestor. However, that inference gets more complicated as you delve deeper in time. If one of LUCA’s genes disappeared in some later lineages, it could appear to have evolved more recently. Alternatively, a gene that evolved long after LUCA could have spread among unrelated lineages through a process known as horizontal gene transfer; in that case, a genetic analysis would make it appear as if something that arose much later had originated in LUCA. “There’s a lot of evolutionary processes that can get in the way of a clear signal,” Goldman said.

One way to isolate the signal from evolutionary noise is to select only genes and proteins that show little evidence of horizontal gene transfer. The most prominent analysis of this sort, from 2016, suggested that LUCA was a relatively simple entity(opens a new tab) that was only “half alive,” dependent on the geochemistry of hydrothermal vents for energy. However, such a conservative focus on only the most obviously shared genes and proteins could bias researchers, leading to a conception of LUCA that is too simple, Donoghue said.

LUCA was likely part of a broader ecosystem, from which its lineage was the sole survivor.

So, instead of a binary, in-or-out approach, Moody and his colleagues computed the probability that a given gene was present. By comparing the gene trees to the species trees, the researchers estimated rates of horizontal gene transfer, gene loss and other evolutionary processes that could muck up the picture. At the end of the process, they assigned each gene a probability of having been part of LUCA’s genome.

“They’ve taken the best-practice approach for a single family and scaled it up,” Goldman said.

The team identified 399 gene families with a high chance of having been in LUCA, a number roughly in line with the conservative 2016 analysis. By also integrating the probabilities of thousands of other gene families, they estimated that LUCA’s genome likely encoded about 2,600 proteins — making it similar in size to the genomes of some modern-day bacteria.

Given what’s known about these proteins, “you get a picture of this fairly complex organism,” Moody said. LUCA likely existed without oxygen by converting carbon dioxide and hydrogen gas into energy. Those inputs might have come from nonliving sources, such as hydrothermal vents or atmospheric gases at the ocean’s surface.