Biologists have been debating the correct definition of a living organism for more than a century. To decide, for example, whether it should also include microscopic entities such as viruses, equipped with genetic material, but incapable of reproducing autonomously. Carelessly, in the meantime, research continues to encounter and describe new oddities, which force us to rethink notions that are now taken for granted. The latest example comes from Stanford laboratories, where a new class of biological entities has just been identified that could force us to review the taxonomy of the natural world: the obelisks, or obelisks, microscopic rings of RNA related to viruses, which inhabit inside the bacteria of the human microbiota.
Traditionally, science considers all biological entities ranging from unicellular creatures such as bacteria, to the multicellular kingdoms where we find animals and plants, to be living organisms. Below bacteria, the existence of viruses, entities composed of DNA or RNA filaments enclosed in a protein envelope, has been known for more than a century, and more or less since the 1970s that of viroids, even more infectious agents. microscopic, composed of small RNA molecules without a capsid (i.e. envelope).
Viruses and viroids are the smallest known biological entities present in nature, and are not considered life forms by a predominant part of the scientific community because they do not possess one of its most fundamental characteristics: that of reproducing, an activity which in both cases is entrusted , in effect, to the structures of the infected cells. This, in broad terms, is the taxonomy of our planet’s biosphere currently accepted by science. If the Stanford researchers are right, however, it will soon be updated to introduce a new class of biological entities, which are positioned right between viruses and viroids.
The discovery came as researchers investigated the presence of viroids – or similar entities – inside bacteria and animal cells, where to date they have not yet been identified with certainty, unlike what has happened in the plant world. To do this they used an innovative method, analyzing existing catalogs of RNA sequenced in the human microbiome, and asking software to predict which sequences within these huge genetic databases were highly likely to take on a circular shape similar to that of viroids.
The first screening done by the software highlighted approximately 30 thousand RNA sequences with the desired characteristics. By analyzing the sequences identified in this way, the researchers realized that they were faced with something completely new: they were RNA molecules composed of around a thousand nitrogenous bases (the “letters” with which the genes in DNA and in RNA), too short to be viruses, and too long to be viroids, whose length is usually around 300 bases. As in the case of viroids, these RNA molecules do not include the genes necessary to produce a viral capsid, but they differ from these because in many cases they encode the genetic information necessary to produce a single protein, renamed “oblin” by researchers, which with every probability plays a role in their replication.
A clue to the origin of life
These characteristics pushed the authors of the discovery to classify the obelisks as a new, previously unknown biological entity, which lies between viruses and viroids due to its complexity. The obelisks were identified by the authors of the research in approximately 7% of the bacteria of the intestinal microbiome studied, and in approximately 50% of that of the mouth. The sequences identified in different microbiomes also have different characteristics, and this confirms the hypothesis that they are biological entities in all respects.
These are preliminary results, which have not yet passed the careful scrutiny of the scientific community. And at the moment, it is impossible to say if and what effects these obelisks might have on the bacteria they infect to reproduce. However, the discovery was welcomed with enthusiasm by many experts, because if confirmed it would provide precious clues for understanding the evolution of viruses and viroids towards more complex forms. A field of study that has to do with the very origin of life on our planet, given that one of the most current hypotheses is that the first biological molecules that appeared on Earth were RNA entities such as viroids and obelisks, from which subsequent evolution towards increasingly complex forms would then give rise to all the incredible biodiversity that inhabits our planet today.