Looking for LUCA, everyone’s shared ancestor

Scientists want to identify your earliest ancestor. And to do that, they have to look back in time. Way, way back.

Let’s climb up your family tree. The first people we’ll meet are your parents, then your grandparents and their parents. Keep climbing and we’ll meet more parents of parents, and parents of parents of parents.

Eventually we’ll zip through hundreds of thousands of years and ancestors. This tour will cross continents and oceans. Modern humans first walked the planet more than 300,000 years ago, so you have a lot of family to get through.

Scientists Say: Evolution

If we keep going back — say 6 million to 8 million years — we’ll find the first upright-walking creature that’s not only your ancestor, but also the ancestor of other primates living today, such as monkeys, chimps and lemurs. So if you want to plan a family reunion, you’ll need to invite not only every person alive, but also every primate you can track down. (That will be a lot of invitations!)

Further up the family tree, about 180 million years back, you’ll find a creature that probably looked like a mouse. It’s the ancestor of you and every other mammal, from whales to marmots. Ascend that family tree a little higher, and you’ll find a common ancestor to every animal. (You won’t recognize it, though. It won’t look remotely like anyone you know.)

And we’re still not done.

Eventually, if we travel back billions of years, we’ll reach LUCA. It’s just one cell, probably shaped like a rod. Scientists suspect it once lived near a hydrothermal vent at the bottom of an ocean. Every family tree, for any organism, leads to LUCA.

This is a hydrothermal vent on an underwater volcano in the southwest Pacific Ocean. Scientists suspect that LUCA — a microbe and our last universal common ancestor — may have made its home in an extreme environment, such as this one. Coastal and Marine Hazards and Resources Program/USGS

LUCA stands for last universal common ancestor. It wasn’t the first thing that lived on Earth. But LUCA is special, because every living thing on the planet today is related to it through evolution. From people and mushrooms to bacteria and trees — we are all descendants of that one lowly cell.

LUCA’s first offspring were also organisms made of single cells. Some would become the ancestors of bacteria. Others would become archaea, another type of single-celled microbe. Still others, much later, would form clumps, leading to multicellular beings.

Explainer: Cells and their parts

Scientists study LUCA for many reasons. Knowing how life evolved on our world might help them recognize early signs of life on other planets. Or “it may help us prepare for understanding how life may change in the future,” says Edmund Moody. He’s an evolutionary biologist at the University of Barcelona in Spain.

Perhaps the biggest reason scientists want to understand LUCA is to see how evolution unfolds. That’s why Tom Williams studies our great-great-grand germs. A biologist at the University of Bath in England, he recently led a project that described LUCA more precisely than ever before. In 2024, Williams, Moody and others reported when they think LUCA lived, how it ate — even what its insides looked like.

“If you want to understand the early evolution of life, then you’re talking about events that happened to microbes,” says Williams. “I think that is quite important.”

The leadup to LUCA

Earth formed more than 4.5 billion years ago. It likely started as clumps of gas and dirt swirling around the sun. Life on Earth began about half a billion years after our world was born, though the details remain a great mystery.

“We’re probably never going to fully understand all the mysteries of the origin of life,” admits Michael Lynch. A geneticist, he works at Arizona State University in Tempe.

Explainer: How a fossil forms

Scientists have found some clues about the oldest life on Earth. In Australia, they turned up fossils of microbe communities in rocks 3.5 billion years old. Some fossils in Canada may be even older. Scientists aren’t sure, though. “When it comes to fossils of single-celled organisms, they’re very difficult to interpret,” says Williams.

Here’s what we do know: At some point, conditions on Earth were just right for the building blocks of life to come together. Today, some scientists are trying to re-create those conditions in the lab. They hope it will help them figure out how life began.

LUCA was not, however, the planet’s first organism. That’s a misconception Williams often hears. “We often imagine that the origin of life and LUCA would be very close together,” Williams says. But that’s not the case. “They’re really very distinct events, or very distinct organisms.”

Life may have begun when some mix of molecules started making copies of itself. Over time, that first life-form likely gave rise to many different types of descendants. Eventually, simple cells evolved. One of those was LUCA. Researchers like Williams look to organisms alive today for clues about this common ancestor.

In today’s grand tree of life, all organisms fall into one of three domains, suggested by colored limbs above: bacteria (green), archaea (blue) and eukaryotes, including us (orange).VectorMine/Shutterstock

A trio of domains

Everything alive today falls into one of three categories, and they all point back to LUCA. Biologists call these domains.

Bacteria inhabit one domain. Too small to see with the unaided eye, these microbes exist just about everywhere. They’re in soil and water. Many live on your skin and in your intestines. Some of them keep you healthy; others can make you sick. Bacteria can even live in extreme environments, such as inside volcanoes or near vents on the seafloor spewing water and gases at up to 400° Celsius (750° Fahrenheit).

Scientists have identified and named tens of thousands of bacteria, which sounds like a lot. But biologists actually estimate Earth may host more than a trillion types!

Explainer: Prokaryotes and Eukaryotes

Bacteria are all simple. They contain DNA and cell walls that keep their insides from spilling out. Their shapes vary from rods and spheres to spirals. And they’ve been around for a long time. “The first 80 percent of our life’s history is single-celled organisms — exclusively,” says Williams.

Archaea (Ar-KEE-uh) make up the second domain of life. Also single-celled, they’re the size of bacteria and often the same shape. They even look the same under a microscope.

For a long time, scientists thought archaea and bacteria belonged to the same group. That changed in the 1970s, when close studies revealed that the molecules in the cell walls of archaea don’t exactly match the ones in bacterial cell walls. And though archaea also have DNA, their genes differ from those in bacteria.

The third domain includes living things called eukaryotes. More complex than bacteria and archaea, eukaryotes have defined compartments inside their cells. Those are its organelles. Most eukaryotic cells have a nucleus, for example. This holds its genetic material. Neither bacteria nor archaea have nuclei.

One chief difference between bacteria and archaea (prokaryotes) and all other life (eukaryotes) are organelles. Prokaryotes lack these compartments of cellular machinery (such as the nucleus), while eukaryotes have them. But both have such things as ribosomes and DNA (in plasmids and chromosomes). ttsz/iStock/Getty Images Plus

Some eukaryotes are single-celled organisms. Others are complex, many-celled organisms. Plants are eukaryotes, as are mushrooms and animals — including humans.

Living things in the three domains differ in many ways. Still, “all cellular life forms have certain things in common,” says Williams. Those might include genes or cell parts or processes, like how a cell uses energy. “We try to ask what those features mean about what early life could have been like.”

This may look like a bacterium or archaeon. In fact, this one-celled paramecium actually is part of the same domain — the eukaryotes — as all plants and animals, including us.Namscience/Creatas Video+/Getty Images Plus

Portraits of LUCA

The idea that everything on Earth has a common ancestor has been around for a long time. Charles Darwin, who championed the theory of evolution, argued all life came from a single ancestor. “Probably all the organic beings which have ever lived on this Earth have descended from some one primordial form,” he wrote in 1859.

Scientists have been thinking about that primordial form ever since. But today’s ideas about LUCA only started to take shape in 1977. That’s when biologist Carl Woese found that archaea and bacteria represented two different kinds of organisms. He proposed organizing life into the three domains.

These scribbles from one of Charles Darwin’s notebooks show how he thought all life might have branched out from a common ancestor (labeled “1”). Charles Darwin/Wikimedia Commons (Public Domain)

Woese also argued that the universal ancestor was not even a cell. He proposed it was simpler: a “progenote.” His idea was that progenotes mixed genetic material with each other until, over time, some organized organism emerged.

Two decades later, microbiologist Patrick Forterre coined the acronym LUCA.

The origin of life on Earth always interested Williams, at Bath. He began thinking about common organisms — and their evolution — while he was a college student. After he became a scientist, he turned his focus to LUCA.

“The starting point for trying to reconstruct something like the last universal common ancestor,” he says, “is just to look at … the things that everything or almost everything has.”

All living things have DNA, he notes. So early life would have had some genetic code as well. Inside all living cells, too, are ribosomes. These tiny machines use DNA to build proteins. LUCA must have had something like that. All cells today have some type of outer wall or membrane, he says, “to keep the insides in and keep them separate from the outside world.”

Then there’s the cellular energy source called ATP. “Almost all modern life-forms use that [molecule] … in some way,” Williams says. So LUCA probably had some way to use ATP as well.

“There’s a bunch of features,” Williams says, “that we can say are common in everything.” He made lists of such shared features. Then his team analyzed the genes linked to these. His group also identified features that LUCA likely lacked. For instance, since only eukaryotes have organelles, LUCA probably didn’t have them.

Who was LUCA? Starting 1:59 minutes into this video you’ll learn more, and how Carl Woese’s discovery of archaea launched a genetic treasure hunt to track down our earliest common ancestor.

Next, the scientists had to consider what they knew about evolution. It’s messy.

“We know features can be invented, and they can be lost” over time, Williams says. Some features of life today do not trace back to LUCA. And LUCA may have had some that disappeared as organisms evolved over billions of years.

Scientists who study evolution often look for “ancient” genes — ones that have been in an organism’s genome for a long time. Scientists can find hints of these by comparing genes in living things.

These comparisons turned up evidence that LUCA was likely a rod-shaped cell that lived some 4.2 billion years ago. It probably wasn’t as simple as earlier biologists had suggested, Williams says. “LUCA was actually already a rather complex organism.”

LUCA’s age and other questions

When it comes to studying LUCA, Williams is just getting started. He still has questions about LUCA’s metabolism — the chemical reactions that would have provided it with energy. “By trying to model those sorts of processes, we can come up with perhaps a more complete reconstruction of LUCA,” he says. Moody, too, studies metabolism and how it’s evolved.

Questions also remain about LUCA’s age. The new study suggests it lived around 4.2 billion years ago. But there are no fossils that old. Even if there were, Williams says, they couldn’t confirm if scientists are right. Recognizing any LUCA fossil would be difficult, he says. “It would probably just look like a little blob or something.”

Not all scientists agree that LUCA could have lived 4.2 billion years ago. Their reasoning has to do with alien asteroids.

Big craters and scars riddle surfaces of the moon, Mercury and Venus. Many date back some 3.9 billion years. Scientists have pointed to these as evidence of something called the “late heavy bombardment.” The idea is that asteroids streamed into the inner solar system for some 20 million to 200 million years. As they did, they would have smashed into anything in their path. That includes Earth.

Some scientists argue life could never have survived such a cataclysm. That would mean LUCA can’t be as old as Williams predicts.

“If the late heavy bombardment was real, either it wasn’t this kind of planet-sterilizing event,” says Moody, “or it didn’t happen.” But if more evidence suggests that it did occur, he says, scientists will have to recalculate LUCA’s age.

This illustration shows what the late heavy bombardment might have been like. Some scientists think that if such a catastrophic rain of asteroids occurred on Earth, life would not have been able to survive. NASA’s Goddard Space Flight Center Conceptual Image Lab

Evolutionary what-ifs

What might be most interesting about studying LUCA, Williams says, isn’t learning about how life first emerged on Earth. Rather, it’s how evolution happens in general, leading to more complex life over time.

Life on Earth today represents one way that evolution played out. But maybe it could have gone in a different direction. In its time, LUCA was likely just one of many organisms that had descended from the first living thing. What if one of its neighbors had evolved instead? 

Let’s learn about the hunt for alien life

In the future, scientists might be able to explore this question, says Julia Schwartzman. She is a biologist at the University of Southern California–Dornsife in Los Angeles. At a recent meeting of the American Society for Microbiology, she raised the possibility of re-creating those other organisms from around the same time as LUCA.

“Can we use synthetic biology to re-synthesize old organisms and see if [they] can re-evolve?” she asked. “There’s a lot of potential.”

Research on LUCA could also aid the search for life on other worlds. If scientists understand the rules of how microbes evolve on Earth, says Williams, they can run experiments to test how that might play out on other planets.

“What are the conditions under which life can occur?” asks Williams. We only have one example, he notes: life on Earth. “Having multiple cases,” he says, might help scientists learn which biological stages can only happen here — and which might unfold somewhere else in the cosmos.