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There is still so much we don’t understand about the origins of life on Earth.

The very definition of life is a source of debate among scientists, but most researchers agree on the basic ingredients of a living cell. Water, energy and several basic elements are the prerequisites for the appearance of cells. However, the exact details of how this happens remain a mystery.

Recent research has focused on trying to recreate in the laboratory the chemical reactions that constitute life as we know it, under conditions acceptable to the early Earth (about 4 billion years ago). Experiments have become more complicated thanks to technological advances and a better understanding of what conditions were like on the early Earth.

However, far from uniting scientists and settling the debate, the rise of experimental work led to many conflicting theories. Some scientists believe that life originated in deep-sea hydrothermal vents where the conditions provided the necessary energy. Others argue that hot springs on land would provide a better setting because they are more likely to retain organic molecules from meteorites. These are just two possibilities that are being explored.

Here are five of the most remarkable discoveries of the past five years.

Reactions in early cells

What energy source drove chemical reactions at the beginning of life? This is the mystery that a research team in Germany is trying to solve. The team delved into the feasibility of 402 reactions known to create some of the basic components of life, such as nucleotides (a building block of DNA and RNA). They did this using some of the most common elements that could be found on the early Earth.

These reactions present in modern cells are also thought to be the primary metabolism of LUCA, the last universal common ancestor, a single-celled, bacterium-like organism.

For each reaction, they calculate the changes in free energy, which determines whether the reaction can proceed without other external sources of energy. Fascinatingly, many of these reactions were independent of external influences such as adenosine triphosphate, a universal energy source in living cells.

The synthesis of the basic building blocks of life did not need an external energy boost: it was self-sustaining.

Volcanic glass

Life relies on molecules to store and transmit information. Scientists believe that strands of RNA (ribonucleic acid) were the forerunners of DNA in fulfilling this role because their structure is simpler.

The appearance of RNA on our planet has long puzzled researchers. However, some progress has been made recently. In 2022, a team of collaborators in the US generated stable strands of RNA in the laboratory. They did this by passing nucleotides through volcanic glass. The threads they made were long enough to store and carry information.

Volcanic glass was present on the early Earth, thanks to frequent meteorite impacts combined with high volcanic activity. The nucleotides used in the study are also believed to have been present at that time in Earth’s history. Volcanic rocks could facilitate the chemical reactions that assemble nucleotides into RNA chains.

Hydrothermal vents

Carbon fixation is a process in which CO₂ gains electrons. It is necessary to build the molecules that form the basis of life.

An electron donor is required to drive this reaction. On early Earth H₂ can be an electron donor. In 2020, a team of collaborators showed that this reaction can occur spontaneously and be fueled by environmental conditions similar to deep-sea alkaline hydrothermal vents in the early ocean. They did this using microfluidic technology, devices that manipulate small volumes of fluids to perform experiments by simulating alkaline vents.

This pathway is strikingly similar to how many modern bacterial and archaeal cells (unicellular organisms without nuclei) work.

The Krebs cycle

In modern cells, carbon fixation is followed by a cascade of chemical reactions that assemble or disassemble molecules in complex metabolic networks that are managed by enzymes.

But scientists are still debating how metabolic reactions evolved before the appearance and evolution of these enzymes. In 2019, a team from the University of Strasbourg in France made a breakthrough. They showed that ferrous iron, a type of iron that was abundant in the early Earth’s crust and ocean, can drive nine of the 11 steps of the Krebs cycle. The Krebs cycle is a biological pathway present in many living cells.

Here, the ferrous iron acts as an electron donor to fix the carbon, which drives the cascade of reactions. The reactions produced all five universal metabolic precursors—five molecules that are essential in various metabolic pathways in all living organisms.

Building blocks of ancient cell membranes

Understanding the formation of the building blocks of life and their complex reactions is a major step forward in understanding the emergence of life.

However, whether they evolved in hot springs on land or in the deep sea, these reactions would not have gone far without a cell membrane. Cell membranes play an active role in the biochemistry of the primitive cell and its relationship with the environment.

Modern cell membranes are composed primarily of compounds called phospholipids, which contain a hydrophilic head and two hydrophobic tails. They are structured in two layers, with the hydrophilic heads pointing outwards and the hydrophobic tails pointing inwards.

Research shows that some components of phospholipids, such as the fatty acids that make up the tails, can self-assemble into these bilayer membranes under different environmental conditions. But were these fatty acids present on the early Earth? Recent research from the University of Newcastle, UK provides an interesting answer. The researchers recreated the spontaneous formation of these molecules by combining H₂-rich fluids, likely present in ancient alkaline hydrothermal vents, with CO₂– rich water resembling the early ocean.

This breakthrough is consistent with the hypothesis that stable fatty acid membranes could have originated from alkaline hydrothermal vents, potentially progressing into living cells. The authors speculate that similar chemical reactions may occur in the subsurface oceans of icy moons, which are thought to have hydrothermal vents similar to Earth’s.

Each of these discoveries adds a new piece to the puzzle of the origins of life. Regardless of which are proven correct, opposing theories fuel the search for answers.

As Charles Darwin wrote: “False facts are highly injurious to the progress of science, for they often last a long time: but false opinions, if supported by any evidence, do no great harm, for every one takes a salutary pleasure in proving his falsehood ; and when this is done, one way to error is closed, and the way to truth is often at the same time opened.”