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LIFE’S ‘LAST UNIVERSAL COMMON ANCESTOR’ MAY PREDATE LIFE ITSELF
 
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Philip Ball
August 20, 2026
Scientific American
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_ A groundbreaking study hints at a hazy boundary between “life”
and “non-life,” as well as two distinct origins of biology on
Earth _

A venting black smoker formed from deposits of iron sulfide, which is
black. “White smokers” are chimneys formed from deposits of
barium, calcium, and silicon, which are white., credit: NOAA

 

How did life begin? This mystery has tantalized scientists since
Charles Darwin mused on it in 1871, spawning no shortage of competing
theories about the origins of Earth’s biology around four billion
years ago. Yet, even today, a definitive answer remains elusive.
Maybe, a team of scientists now suggests, that’s because we’ve
been asking the wrong question all along.

In a paper [[link removed]] in
_Science Advances_, evolutionary biologist Bill Martin of Heinrich
Heine University Düsseldorf in Germany and his colleagues present
evidence that life might have, in effect, begun twice. They argue that
the two earliest lineages of Darwin’s “tree of life” sprung
independently from a single source that was itself not yet truly alive
[[link removed]].

This source is often called the last universal common ancestor (LUCA)
and is generally regarded as a kind of ur-organism: a primitive
bacteriumlike cell from which all life on Earth has descended. But in
the view of Martin and his colleagues, LUCA was not exactly biological
but rather a chemical system formed in the unique environment created
by hydrothermal vents. At these deep-sea mineral formations, a rich
chemical brew, warmed by volcanic activity, spills out over the ocean
floor. As a source of abundant energy and chemical ingredients, vents
have long been leading candidates for life’s earliest cradles, in
contrast with the “warm little pond” suggested by Darwin.

The Düsseldorf group’s version of LUCA would have possessed many of
the ingredients needed by living systems, including a primitive form
of genetic encoding, as well as metabolic chemical reactions required
to harness energy. The researchers say, however, that some of the
metabolic reactions were catalyzed not by elaborate protein enzymes,
as in all organisms today, but by simple metallic chemical elements
found in the hydrothermal vents’ minerals. LUCA, they say, was part
organic, part rock. The idea “changes the way we view the early
evolution process,” Martin says.

“I think there’s truth in it,” says biochemist Nick Lane of
University College London, an origins-of-life researcher, who was not
involved in the study.

This vision of a “half-alive” LUCA is not totally new. Work from
the Düsseldorf team
[[link removed]] in 2016 “already
pointed to LUCA being reliant on its environment,” says Natalia
Mrnjavac, lead author of the latest paper. But at that stage, she
adds, “we didn’t have much experimental data on the specific
functions the environment could have promoted.”

Mrnjavac, Martin and their colleagues have now performed a
mathematical back-extrapolation from the metabolic enzymes of
modern-day organisms, identifying differences between the metabolic
networks of Earth’s two most ancient single-celled domains of life:
bacteria and archaea. Such differences within the networks of
metabolic reactions seem to extend all the way down to these
domains’ earliest stages: to the last bacterial and archaeal common
ancestors (LBCA and LACA, respectively).

Metabolism involves a complex cycle of chemical reactions. It begins
with an environmental source of chemical energy, which is converted to
energy-rich compounds within cells; these compounds then drive other
enzymatic processes that culminate in a series of reactions resetting
the metabolic network to its original state so that the cycle can
repeat indefinitely.

The researchers identify various “missing links” in the cyclic
metabolic networks of LBCA and LACA, suggesting a serious lack of the
requisite enzymes in LUCA. “LUCA only had genes for about half of
metabolism,” Martin says. What’s more, the enzymes involved in
some reactions are not always shared by LACA and LBCA. “We can see
cases where the ancestors of bacteria and of archaea independently
evolved structurally distinct enzymes to catalyze the same essential
metabolic reaction,” Mrnjavac says.

Rather than reflecting genuinely absent enzymes, these gaps could
simply mirror methodological limitations in the phylogenetic
reconstruction that prevented the identification of all the original
ancient enzymes in bacterial and archaean lineages. Other groups have
previously assumed as much
[[link removed]]. Alternatively,
asks biologist Daniel Segrè of Boston University, “can one rule out
the possibility that LUCA had the enzymes found in LBCA and that LACA
substituted them with different ones, or vice versa?”

But Martin and his colleagues are instead claiming that if an enzyme
can’t be found in the reconstructed metabolic networks, “it was
genuinely missing,” Lane says. Those gaps, the researchers argue,
could have been filled by chemical reactions catalyzed by metals in
the vent systems such as nickel, iron, cobalt and palladium. “We can
see that early biochemical evolution was a hybrid of enzymatic and
metal catalysts,” says co-author Joseph Moran, an organic chemist at
the University of Ottawa.

“What we are beginning to appreciate,” Martin says, “is how
tight the congruence is between these metals and the enzymes of
metabolism.”

Such metals are commonly used as industrial catalysts today—and all
occur in the minerals of hydrothermal vents, where a process called
serpentinization converts igneous rocks from volcanism to metamorphic
rocks. Some previous studies, Mrnjavac says, have suggested that
“serpentinization may have been more widespread and more exposed on
the early Earth.”

The serpentinization reactions can generate native metals such as iron
and the iron-nickel alloy awaruite (which can contain palladium, too).
“Awaruite is really common,” says Martin, adding that on the early
Earth, which had very little oxygen in the atmosphere, this alloy and
other metals wouldn’t have readily oxidized—rusted—and so
would’ve been even more abundant in rocks on and near the planet’s
surface.

But Lane points out that serpentinization itself happens several
kilometers beneath the seabed and that it’s unclear whether the
metal by-products could have been brought up from such depths in
appreciable amounts. “How much raw metal really is there in these
systems?” he wonders.

The availability of metallic catalysts is only one part of the problem
of how LUCA’s putative metabolic system could have been enabled.
What, ultimately, was the energy source driving it? Modern organisms
use metabolic energy (for example, via burning a candy bar’s sugary
calories) to make the molecule adenosine triphosphate (ATP), a
universal energy store in the biosphere that is derived from
phosphates. But ATP synthesis requires enzymes that LUCA didn’t
possess.

Instead the researchers think that a phosphorus compound called
phosphite, found previously in serpentinizing systems, could have
played the same role. Phosphite is more soluble in water than
phosphate and has been proposed before
[[link removed]]
as a prebiotic source of phosphorus. In their new study, Mrnjavac and
her colleagues report chemical experiments showing that palladium
metal can catalyze the reactions of phosphite in metabolic processes.

Most microbial life at hydrothermal vents today, however, uses
phosphate, not phosphite. “To be convinced that microbes began by
using phosphite, we need a good explanation of why life would switch
to phosphate and not revert to the previous state,” says biologist
Joanne Boden of the University of Bristol in England, who was not part
of the study.

Another quandary concerns how LUCA could have manufactured proteins as
complicated as enzymes, which are made from many amino acids linked
together in a particular sequence. In today’s organisms, proteins
are encoded in the sequences of DNA, which are inherited from one
generation to the next. Martin and his colleagues think that LUCA
already possessed such an encoding system in the form of nucleic acids
much like modern DNA or RNA, as well as the molecular machinery to
translate it to proteins. This system would have used the same genetic
code—the correspondence between nucleic acid sequence and protein
sequence—as that used by all organisms today. “An early
informational system [like this] had to precede a complete enzymatic
metabolism,” Mrnjavac says, “not least because enzymes are
synthesized by the genetic machinery.”

Researchers have long debated whether, at the origin of life, genes or
metabolism came first because each seems dependent on the other. This
new picture makes that question moot: they coevolved, and a complete,
autonomous metabolic network wasn’t needed before a kind of genetic
encoding could arise. Martin and his colleagues say that LUCA only
birthed truly autonomous, free-living systems—LBCA and LACA—when
it evolved a core set of enzymes and assisting compounds called
cofactors that ended its reliance on catalytic metals in the
environment.

And because current working definitions dictate that only
_free-living_ cells can be considered “alive,” Martin says, “we
are looking at one origin of the genetic code but two origins of
life.”

If true, this would imply that the origin of “life” wasn’t as
revolutionary as is often suggested because much of the hard work was
already done incrementally within LUCA. “Traditionally, LUCA has
been associated with a fully functioning modern cell,” Segrè says,
“which I have always felt must have appeared only long after the
problem of life’s origin was solved at a more fundamental level.”

Segrè, however, cautions against regarding this vision as “an
incomplete LUCA inventing new enzymes for previous nonenzymatic
reactions.” Before becoming a firmly established system for making
all the proteins involved in the metabolism of free-living organisms,
he says, LUCA merely “was what it was—not ‘incomplete,’ as
there was no foresight of the next [evolutionary] stage.”

Evolutionary biologist Joanna Masel of the University of Arizona sees
yet another possibility in the differences between the metabolic
enzymes of LACA and LBCA. Maybe, she says, LUCA didn’t precede them
at all. Rather LACA and LBCA might have coexisted with different
genetic codes, and LUCA might then not really have been a kind of
proto-organism at all. Instead it would then represent a stage at
which interactions between LACA and LBCA caused convergence
[[link removed]] that
gave them both a common genetic code.

Such speculations reflect the richness of possibilities that the new
work opens up. Although many aspects of the study remain to be tested,
perhaps its biggest contribution is thus to reframe the whole debate:
to worry less about “when life began” and more about how and when
the various ingredients and pathways arose. “These are great
research questions to be investigating,” Boden says.

At any rate, the emerging picture of life beginning with a
proto-metabolism at hydrothermal vents is a very different scenario to
the spontaneous formation of replicating molecules in Darwin’s
“prebiotic soup.” Before he began touting that notion, Darwin was
pessimistic about the whole issue, writing in 1863
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that speculations on the origin of life were “mere rubbish
thinking,” akin to wondering about the origin of matter itself. Even
if we might never be sure about the true answer, it’s now no longer
a rubbish question to be asking.

_PHILIP BALL_
[[link removed]]_ is a science
writer and author based in London. His latest book is How Life Works
(University of Chicago Press, 2023)._

_Edited by __Lee Billings_
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_Founded 1845, __Scientific American_
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is the oldest continuously published magazine in the United States. It
has published articles by more than 200 Nobel Prize winners._

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