César Menor Salván
Astrobiologist and lecturer of Biochemistry at the University of Alcalá
The first thing to note is that this work does not solve the problem of the origin of life or the origin of molecules such as DNA or RNA. We must be very careful about drawing conclusions, but it is a novel, highly significant and extremely interesting discovery.
It is an extraordinary piece of work that combines radio astronomy, molecular spectroscopy and computational chemistry, confirming the detection of a sugar molecule, erythrulose. The origin of sugars is one of the problems to be resolved in the origin of life, and their detection in interstellar space shows us two things: firstly, that they can form under natural conditions in space. Secondly, that it may support the hypothesis that these molecules, accumulated and preserved in ice, could have released these sugars into environments conducive to the origin of life, such as the early Earth.
Furthermore, erythrulose forms part of a family of sugars, known as ketoses, which are particularly stable; we believe they played a key role in the process of chemical evolution leading to life and are, in my opinion, more significant than the better-known ribose or glucose. One thing that saddened me on reading the paper is that they have not found glycolaldehyde, glyceraldehyde or dihydroxyacetone. These molecules are predicted by various models, including the computational model used in the study itself; they are extremely significant in the origin of life and, had their detection been confirmed, it would have been a far more revolutionary result.
The problem with this study is that, whilst it is technically extraordinary and presents a highly significant result, it risks generating headlines in the press that do not reflect reality.
The study raises questions that could be misinterpreted as non-sequitur fallacies: they detect erythrulose, so does that mean the building blocks for the first nucleic acids existed? No. All the study demonstrates is that erythrulose exists in a molecular cloud in space; it does not prove that it reaches Earth or any other place where life might emerge; nor that it survives that journey; nor that it reaches significant concentrations; nor that it participates in prebiotic synthesis.
The second non-sequitur fallacy that might be inferred is: erythrulose isomerises into tetroses, therefore it could lead to TNA, an alternative to RNA. This is a vast chain of inferences, lacking at least five experimental steps.
The third possible non-sequitur fallacy is: there are sugars on Bennu, therefore interstellar sugars arrive intact. No. Bennu demonstrates their final existence. It does not demonstrate where they were synthesised. They may have formed in ice, during the evolution of the protoplanetary disc, within the parent body, or through aqueous alteration. In general, it suggests that interstellar erythrulose exists; therefore, it supports an exogenous origin for the sugars.
In reality, it merely adds one piece to the jigsaw. The existence of interstellar erythrulose merely tells us that these molecules can form in that environment, not that the sugars on the early Earth arrived via that route – just as seeing people getting off a plane at an airport does not imply that everyone in the city arrived by plane.
The fourth non-sequitur fallacy is the claim that this supports the origin of homochirality. This is probably the weakest point in the entire discussion. The authors write that the discovery of a chiral molecule supports the emergence of extraterrestrial enantiomeric excesses. But the radioastronomical detection does not measure any enantiomeric excess. There is absolutely no observational evidence of preferential chirality. The discussion of homochirality here is out of place.
My final assessment: it is an extraordinary, thought-provoking and robust piece of work with a highly significant result in terms of astrochemistry. It has important implications for prebiotic chemistry and the origin of life, but one must be wary of extrapolations and exaggerated, speculative interpretations.