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13 maltatoday | WEDNESDAY • 24 MAY 2023 OPINION Jordi Paps is Senior lecturer, School of Biological Sciences, University of Bristol, University of Bristol FROM its humble origin(s), life has infected the entire planet with endless beautiful forms. The genesis of life is the oldest biological event, so old that no clear evidence was left behind other than the existence of life it- self. This leaves many questions open, and one of the most tan- talising is how many times life magically emerged from non-liv- ing elements. Has all of life on Earth evolved only once, or are different living beings cut from different cloths? The question of how difficult it is for life to emerge is interesting – not least because it can shed some light on the likelihood of finding life on other planets. The origin of life is a central question in modern biology, and probably the hardest to study. This event took place four billion years ago, and it happened at a molecular level – meaning little fossil evidence remains. Many lively beginnings have been suggested, from unsavoury primordial soups to outer space. But the current scientific con- sensus is that life emerged from non-living molecules in a natural process called abiogenesis, most likely in the darkness of deep-sea hydrothermal vents. But if life emerged once, why not more times? What is abiogenesis? Scientists have proposed var- ious consecutive steps for abio- genesis. We know that Earth was rich in several chemicals, such as amino acids, a type of molecules called nucleotides or sugars, which are the building blocks of life. Laboratory experiments, such as the iconic Miller-Urey experiment, have shown how these compounds can be natu- rally formed under conditions similar to early Earth. Some of these compounds could also have come to Earth riding mete- orites. Next, these simple molecules combined to form more com- plex ones, such as fats, proteins or nucleic acids. Importantly, nucleic acids — such as dou- ble-stranded DNA or its sin- gle-stranded cousin RNA — can store the information needed to build other molecules. DNA is more stable than RNA, but in contrast, RNA can be part of chemical reactions in which a compound makes copies of itself – self-replication. The "RNA world" hypothesis suggests that early life may have used RNA as material for both genes and replication before the emergence of DNA and proteins. Once an information system can make copies of itself, natu- ral selection kicks in. Some of the new copies of these mole- cules (which some would call "genes") will have errors, or mu- tations, and some of these new mutations will improve the rep- lication ability of the molecules. Therefore, over time, there will be more copies of these mutants than other molecules, some of which will accumulate further new mutations making them even faster and more abundant, and so on. Eventually, these molecules probably evolved a lipid (fatty) boundary separating the inter- nal environment of the organ- ism from the exterior, forming protocells. Protocells could con- centrate and organise better the molecules needed in biochem- ical reactions, providing a con- tained and efficient metabolism. Life on repeat? Abiogenesis could have hap- pened more than once. Earth could have birthed self-replicat- ing molecules several times, and maybe early life for thousands or millions of years just consisted of a bunch of different self-rep- licating RNA molecules, with independent origins, competing for the same building blocks. Alas, due to the ancient and mi- croscopic nature of this process, we may never know. Many lab experiments have successfully reproduced differ- ent stages of abiogenesis, prov- ing they could happen more than once, but we have no certainty of these occurring in the past. A related question could be whether new life is emerg- ing by abiogenesis as you are reading this. This is very un- likely though. Early Earth was sterile of life and the physical and chemical conditions were very different. Nowadays, if somewhere on the planet there were ideal conditions for new self-replicating molecules to appear, they would be promptly chomped by existing life. What we do know is that all extant life beings descend from a single shared last universal common ancestor of life (also known as LUCA). If there were other ancestors, they left no de- scendants behind. Key pieces of evidence support the existence of LUCA. All life on Earth uses the same genetic code, namely the correspondence between nucleotides in DNA known as A, T, C, and G – and the ami- no acid they encode in proteins. For example, the sequence of the three nucleotides ATG al- ways corresponds to the amino acid methionine. Theoretically, however, there could have been more genetic code variants between species. But all life on Earth uses the same code with a few minor changes in some lineages. Biochemical pathways, such as the ones used to metabolise food, also support the existence of LUCA; many in- dependent pathways could have evolved in different ancestors, yet some (such as the ones used to metabolise sugars) are shared across all living organisms. Simi- larly, hundreds of identical genes are present in disparate live be- ings which can only be explained by being inherited from LUCA. My favourite support for LU- CA comes from the Tree of Life. Independent analyses, some us- ing anatomy, metabolism or ge- netic sequences, have revealed a hierarchical pattern of related- ness that can be represented as a tree. This shows we are more related to chimps than to any other living organisms on Earth. Chimps and we are more related to gorillas, and together to oran- gutans, and so on. You can pick any random or- ganism, from the lettuce in your salad to the bacteria in your bi- oactive yogurt and, if you travel back in time far enough, you will share an actual common ances- tor. This is not a metaphor, but a scientific fact. This is one of the most mind boggling concepts in science, Darwin's unity of life. If you are reading this text, you are here thanks to an uninterrupt- ed chain of reproductive events going back billions of years. As exciting as it is to think about life repeatedly emerging on our planet, or elsewhere, it is even more exciting to know that we are related to all the life beings in the planet. Did life evolve more than once? Researchers are closing in on an answer Jordi Paps Just like us… (Photo: Sharon Morris/Shutterstock)