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There is still so much we don't understand about the origins of life on Earth.
The definition of life itself is a source of debate among scientists, but most researchers agree on the basic ingredients of a living cell. Water, energy and a few essential elements are the conditions for the creation 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 form life as we know it, in conditions that were plausible for the early Earth (about 4 billion years ago). Experiments have become increasingly complex thanks to technological advances and a better understanding of Earth's early conditions.
But instead of bringing scientists together and settling the debate, the rise of experimental work has led to many conflicting theories. Some scientists think that life originated in deep-sea hydrothermal vents, where conditions provided the necessary energy. Others argue that hot springs on land would have provided a better environment because they are more likely to retain organic molecules from meteorites. These are just two possibilities being explored.
Here are five of the most notable discoveries from the past five years.
Responses in early cells
What energy source powered the chemical reactions at the origin of life? This is the mystery that a research team in Germany has been trying to unravel. The team delved into the feasibility of 402 reactions known to create some of the essential components of life, such as nucleotides (a building block of DNA and RNA). They did this using some of the most common elements found on the early Earth.
It is believed that these reactions, present in modern cells, also constitute the core metabolism of LUCA, the last universal common ancestor, a single-celled, bacteria-like organism.
For each reaction, they calculated the changes in free energy, which determine whether a reaction can proceed without other external energy sources. The fascinating thing is that 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 required no external energy boost: it was self-sustaining.
Volcanic glass
Life depends on molecules to store and transmit information. Scientists think that RNA strands (ribonucleic acid) were the precursors of DNA in fulfilling this role because their structure is simpler.
The emergence 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 RNA strands in the laboratory. They did this by passing nucleotides through volcanic glass. The strands they made were long enough to store and transmit information.
Volcanic glass was present on the early Earth, thanks to frequent meteorite impacts combined with high volcanic activity. It is also believed that the nucleotides used in the study were present at the time in Earth's history. Volcanic rock could have facilitated the chemical reactions that assembled nucleotides into RNA chains.
Hot water craters
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 achieve this reaction. On the early Earth, H₂ could have been the electron donor. In 2020, a team of collaborators showed that this response could occur spontaneously and could 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 amounts of liquids to conduct experiments by simulating alkaline vents.
This pathway is strikingly similar to the functioning of many modern bacterial and archaeal cells (single-celled organisms without a nucleus).
The Krebs cycle
In modern cells, carbon fixation is followed by a cascade of chemical reactions that assemble or break down molecules, in complex metabolic networks powered by enzymes.
But scientists still debate how metabolic reactions unfolded before the emergence and evolution of those 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 Earth's early crust and ocean, could power nine of the eleven steps of the Krebs cycle. The Krebs cycle is a biological pathway present in many living cells.
Here, ferrous iron acted as an electron donor for carbon fixation, driving the cascade of reactions. The reactions produced all five universal metabolic precursors - five molecules that are fundamental to different metabolic pathways in all living organisms.
Building blocks of old cell membranes
Understanding the formation of life's building blocks and their complicated reactions is a major step forward in understanding the origins of life.
But whether they unfolded 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 a primitive cell and its connection to the environment.
Modern cell membranes are usually composed of compounds called phospholipids, which contain a hydrophilic head and two hydrophobic tails. They are structured in bilayers, with the hydrophilic heads pointing outward and the hydrophobic tails pointing inward.
Research has shown that some components of phospholipids, such as the fatty acids that make up the tails, can self-assemble into 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. Researchers have recreated the spontaneous formation of these molecules by combining H₂-rich fluids, which were likely present in ancient alkaline hydrothermal vents, with CO₂-rich water similar to the early ocean.
This breakthrough is consistent with the hypothesis that stable fatty acid membranes may originate in alkaline hydrothermal vents and potentially develop into living cells. The authors speculated that similar chemical reactions could unfold in the subsurface oceans of icy moons, which are thought to have hydrothermal vents similar to those on Earth.
Each of these discoveries adds a new piece to the puzzle of the origin of life. Regardless of which proves to be correct, conflicting theories fuel the search for answers. As Charles Darwin wrote:
False facts are very injurious to the progress of science, because they often persist for a long time: but false opinions, if supported by any evidence, do little harm, for every one takes a salutary pleasure in proving their falsity; and when this is done, one path to error is closed, and often at the same time the path to truth is opened.
This article is republished from The Conversation under a Creative Commons license. Read the original article.