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The molecular framework for protometabolism—chemical reactions in a prebiotic environment preceding modern metabolism—has remained unknown in evolutionary biology. Mono-, di-, and tricarboxylic acids that comprise contemporary metabolism, such as the Krebs cycle, are of particular prebiotic relevance and are theorized to predate life on Earth. Researchers have struggled to unravel the molecular origins of respiration, with theories pointing toward abiotic origins later co-opted by the earliest living organisms; however, the molecular network of these molecules has remained elusive. Recent detections of carboxylic acids linked to the Krebs cycle on the Ryugu asteroid and Murchison meteorite rekindled interest in their extraterrestrial origins. Replicating conditions analogous to the environment of dense molecular clouds in laboratory simulation experiments, our work provides compelling evidence on the abiotic synthesis of the complete suite of biorelevant molecules central to the Krebs cycle. The opportunity for these biomolecules forming in deep space could provide molecular origins of protometabolism on early Earth and also provide the molecular feedstock to worlds beyond our own. 

Abstract For the last century, the source of sulfur in Earth’s very first organisms has remained a fundamental, unsolved enigma. While sulfates and their organic derivatives with sulfur in the S(+VI) oxidation state represent core nutrients in contemporary biochemistry, the limited bioavailability of sulfates during Earth’s early Archean period proposed that more soluble S(+IV) compounds served as the initial source of sulfur for the first terrestrial microorganisms. Here, we reveal via laboratory simulation experiments that the three simplest alkylsulfonic acids—water soluble organic S(+IV) compounds—can be efficiently produced in interstellar, sulfur-doped ices through interaction with galactic cosmic rays. This discovery opens a previously elusive path into the synthesis of vital astrobiological significance and untangles fundamental mechanisms of a facile preparation of sulfur-containing, biorelevant organics in extraterrestrial ices; these molecules can be eventually incorporated into comets and asteroids before their delivery and detection on Earth such as in the Murchison, Tagish Lake, and Allende meteorites along with the carbonaceous asteroid Ryugu.