Reconstruction of about 420 metabolic reactions suggests that the common ancestor of bacteria and archaea still relied on metals in its environment. According to the researchers, the two lineages separately evolved the enzymes that allowed them to exist as independent cells – but not all scientists agree that this is equivalent to “two origins of life.”
The headline “Life arose twice” sounds as if scientists have discovered two separate events in which inanimate matter became life. That is not what the new study found. All living things we know still share a genetic code and a basic system for making proteins, indicating a common origin.
The new claim is more subtle and interesting: It is possible that after an ancient system with a shared genetic code already existed, two lineages developed from it – theBacteria Andarchaeons – and each of them separately completed the transition to cells capable of sustaining metabolism On their own.
That is, not two abiogenesiss nor two independent “healths,” but perhaps two independent exits from the same chemical cradle.
The study, published in the journal Science Advances, led by Natalia Mrnjavac and Prof. William Martin of Heinrich Heine University in Düsseldorf. The researchers combined comparisons of genomes and protein structures with mathematical reconstructions of ancient metabolism and chemistry experiments simulating conditions in hydrothermal vents.
LUCA was not the first living creature
At the center of the research is LUCA – An acronym for “Last Universal Common Ancestor.” This does not mean that it was the first creature to live on Earth. LUCA is the name given to the most recent ancestor – or, presumably, the ancestral population – from which all lineages that exist today evolved.
According to the conventional reconstruction, LUCA already possessed a genetic code, ribosomes, and much of the machinery that translates genetic information into proteins. It was not a “first molecule” nor the starting point of pre-biological chemistry. It existed after earlier stages, most of which can no longer be directly reconstructed.
The question at the heart of the new study is how independent LUCA was. Was there already a cell with a complete system of enzymes, which could grow and reproduce independently of a particular geological site, or did it still rely on chemical reactions that occurred on the surface of minerals and metals in its environment?
Restoration of approximately 420 responses
Modern cells use a complex network of chemical reactions to produce amino acids, RNA bases, vitamins, and other essential molecules. The sum of these reactions is metabolism—themetabolism.
The researchers examined a network of about 420 basic reactions that allow cells to make their building blocks from simple materials available on the early Earth, including hydrogen, ammonia, and carbon dioxide. They examined which enzymes currently carry out each reaction in bacteria and archaea, and how similar the enzymes from the two groups are in sequence and three-dimensional structure.
If bacteria and archaea inherited a particular enzyme from a common ancestor, one would expect to find signs of common ancestry between them, even after billions of years of evolution. In contrast, when the two lineages use proteins with completely different structures to carry out the same reaction, the possibility arises that each lineage has developed its own solution.
The reconstruction shows that LUCA only had enzymes for about half of the reactions tested. According to the researchers, the other reactions were not yet carried out using biological enzymes, but relied on inorganic catalysts found in the environment.
When rocks played the role of enzymes
A catalyst is a substance that allows a chemical reaction to occur quickly or under conditions that would not otherwise occur. In modern cells, most of the work of catalysts is done by enzymes – proteins whose structure is tailored to a particular reaction.
However, at the beginning of metabolism, complex systems for producing all the necessary enzymes were not yet available. This is one of the formulations of the chicken-and-egg problem in the study of origin of life: The cell needs enzymes to produce its components, but enzyme production also requires an active cellular system.
The study suggests that geology provided part of the solution. Iron, nickel, cobalt and other transition metals can catalyze reactions that today are carried out by enzymes. Such metals are found in hydrothermal systems that form when water reacts with hot rocks in the Earth's crust.
In such an environment, there was no sharp separation between the “creature” and its “habitat.” The ancient chemical system could have used the minerals and metals in the walls of the spring as part of its metabolism. In this sense, the rocks themselves functioned as part of the metabolic machinery.
Two ways to say goodbye to the rock
According to the scenario the researchers propose, bacteria and archaea split at a stage when metabolism was a combination of enzymes and metal catalysts. After the split, each lineage replaced some of the environmental catalysts with enzymes it developed on its own.
The researchers have identified several essential reactions in which bacteria and archaea use structurally different enzymes to perform the same task. They interpret this as convergent evolution: the two lineages encountered the same chemical problem and separately arrived at different protein solutions.
Completing the enzyme network allowed cells to stop relying on a particular mineral surface. They could carry their metabolism with them, spread to other environments, and live as metabolically independent cells.
This is the basis for the claim of two independent appearances of life: not two genetic codes nor two independent ancestors, but two separate completions of the transition from a system bound to its geological environment to a cell carrying within itself the machinery necessary for its existence.
Where did energy come from before ATP?
The research also addresses the question of how energy was stored before the emergence of the modern ATP system. ATP is a molecule that serves as a kind of universal energy currency in cells, but its production itself depends on enzymes and complex biological systems.
The researchers examined phosphite – a phosphorus compound that may be found in hydrothermal systems – and showed that under certain conditions it is capable, with the help of metal catalysts, of transferring phosphorus groups to organic molecules. Among other things, the experiment demonstrated a transition from AMP to ADP, a substance directly related to the cell's modern energy system.
The experiment does not recreate an ancient cell and does not prove that this is how the first energy mechanism was created. It shows that at least some of the reactions needed to store energy can occur in water using geological materials, without a full enzymatic system.
As evolution progressed, cofactors—small molecules that help enzymes carry out reactions—appeared. According to the model, the cofactors and enzymes gradually replaced the solid metals, allowing metabolism to detach from the walls of the hydrothermal vent.
Was LUCA alive?
This is where a question of definition comes into play. Prof. Martin argues that only a system capable of independently maintaining its own metabolism deserves to be called a living cell. By this definition, the LUCA reconstructed in the study still lies on the border between geochemistry and biology, and independent life did indeed appear twice – once in the bacterial lineage and once in the archaeal lineage.
But this is not the only possible definition. Many modern creatures are completely dependent on their environment or on other creatures. Intracellular parasites, for example, have lost large parts of their metabolic systems, but no one claims that they are inanimate matter. The ancient cell could also be alive, although it needed the metals and compounds around it.
It is also important to distinguish between the scientific article and the way it was presented to the public. The title of the article is “Intermediate stages in the origin of metabolism in a phosphorescent hydrothermal vent.” The claim about “two origins of life” appears mainly inUniversity announcement And in the words of the researchers about the significance of the findings.
A convincing reconstruction – not a photograph of the past
The researchers didn't look at cells that were four billion years old. They inferred the history of metabolism from the genomes and proteins of organisms alive today, and combined it with responses demonstrated in the laboratory.
This is an important scientific method, but it is subject to limitations. Genes can disappear, change beyond recognition, or pass between lineages through horizontal transfer. It is also difficult to know whether a reaction that occurred in an experiment actually played the same role in the ancient environment.
Researchers who were not involved in the study told theSmithsonian Magazine Because the reconstruction presents a strong and thought-provoking scenario, it is not the only possible way to interpret the data. Other models suggest that LUCA was already a relatively complex cell, perhaps with a membrane that incorporated features that were later preserved separately in bacteria and archaea.
Even if the two lineages evolved different enzymes, it does not necessarily follow that the transition to life occurred twice. The safer conclusion is that the completion of enzymatic metabolism occurred at least partly independently in the two lineages.
Why is the research important?
The important finding is not the sensational number of “two healths,” but the continuous picture it offers of the transition from geology to biology. Instead of a sharp moment in which a collection of molecules suddenly becomes a living cell, one obtains a series of intermediate stages: first reactions on the surface of metals, then an integrated system of metals and enzymes, and finally cells with their metabolism packaged within them.
This picture could also have implications for the search for extraterrestrial life. If hydrothermal systems can provide not only heat and raw materials but also catalysts and a source of energy storage, worlds with water and active rocks could allow for the first stages of chemical evolution. However, the distance between a network of chemical reactions and a reproducing cell is still large, and much of it remains unexplained.
The study does not rule out the common origin of known life. On the contrary: the shared genetic code and ribosomes remain strong evidence for such an origin. What is novel is the possibility that after that shared heritage emerged, bacteria and archaea had to complete the task separately – and build two different ways to break away from the rock.
The article and sources
The original article: Intermediate stages in the origin of metabolism at a phosphorylating hydrothermal vent - Science Advances
Research message: Early Evolution of Life: Two origins of life – Heinrich Heine University Düsseldorf
A review that includes responses from external researchers: Smithsonian Magazine
More on the subject on the science website
- Cooperation between simple molecules may explain how primitive cells formed
- Asteroids didn't just destroy worlds, they may have also helped ignite life
- How were the first cells formed? with protein
- Following the first proteins
- Life on Earth originated in volcanic hot springs, not in the sea?
Questions and Answers
Does the research prove that life originated twice?
No. It does not point to two independent events in which inanimate matter became alive. The study suggests that bacteria and archaea, which share a common ancestor and genetic code, separately completed the transition to metabolically independent cells.
What is LUCA?
LUCA is The last common ancestor For all living things known today. It was not necessarily the first living thing, and it may not have been a single individual but a population of ancient systems or cells.
How could metabolism exist without all the modern enzymes?
According to the study, metals and minerals in hydrothermal vents could have catalyzed some of the reactions. Later, enzymes and cofactors replaced the environmental catalysts, allowing cells to survive away from the mineral surfaces.
Is the scenario agreed upon by all researchers?
No. The reconstruction is based on comparisons of modern creatures and laboratory experiments simulating ancient conditions. It presents a possible scenario and is supported by evidence, but there are other models for the structure of LUCA and where life emerged.
For the original publication: Opening the original publication
