News from the cosmic origin of life
Nitriles, a class of organic molecules with a cyano group, i.e. a carbon atom bonded to a nitrogen atom via an unsaturated triple bond, are usually toxic. Yet, paradoxically, they are also an important precursor for molecules that are essential for life – namely, ribonucleic acid (RNA). Astrobiologists already knew that complex molecules are surprisingly common even in space, which is hostile to life. Now, a team of researchers from Spain, Japan, Chile, Italy and the United States has shown that a wide range of nitriles occur in interstellar space in the molecular cloud G+0.693-0.027, near the center of the Milky Way.
Dr. Víctor M. Rivilla, astrobiologist and first author of the new study, says, “Here we show that the chemistry that takes place in the interstellar medium is capable of efficiently forming multiple nitriles, which are important molecular precursors for the ‘RNA world’ scenario.” According to this scenario, life on Earth was originally based only on RNA, and DNA and protein enzymes evolved later. RNA can perform both functions: Storing and copying information like DNA and catalyzing reactions like enzymes. According to the “RNA world” theory, nitriles and other building blocks of life need not have all originated on Earth itself: they could also have come from outer space, hitchhiking to the young Earth in meteorites and comets during the “late heavy bombardment” 4.1 to 3.8 billion years ago. Nitriles and other precursor molecules for nucleotides, lipids and amino acids have already been found in comets and meteorites.
But where in space could these molecules come from? The first candidates are molecular clouds, dense and especially cold regions of the interstellar medium that are suitable for the formation of complex molecules. The molecular cloud G+0.693-0.027, for example, has a temperature of about 100 Kelvin, a diameter of about three light-years, and a mass about a thousand times that of our Sun. There is no evidence that stars are currently forming in G+0.693-0.027, although scientists suspect it could become a stellar nursery in the future.
“The chemical content of G+0.693-0.027 is similar to that of other star-forming regions in our galaxy, as well as solar system objects such as comets. This means that its study can give us important insights into the chemical constituents that were present in the nebula from which our planetary system formed,” Rivilla explains. He and his colleagues studied the electromagnetic spectra of G+0.693-0.027 with two telescopes in Spain: the IRAM telescope in Granada and the Yebes telescope in Guadalajara. They discovered the nitriles cyanoallene (CH2CCHCN), propargyl cyanide (HCCCH2CN), and cyanopropyne, which had not previously been found in G+0.693-0.027, although they were detected in 2019 in the TMC-1 dark cloud in the constellations Taurus and Auriga, a molecular cloud with conditions quite different from G+0.693-0.027. In addition, the researchers found evidence for the presence of cyanoformaldehyde (HCOCN) and glycolonitrile (HOCH2CN) in G+0.693-0.027. Cyanoformaldehyde was first detected in the molecular clouds TMC-1 and Sgr B2 in the constellation Sagittarius, and glycolonitrile was detected in the solar-like protostar IRAS16293-2422 B in the constellation Ophiuchus.
Another author of the study, Dr. Miguel A. Requena-Torres, concludes, “Thanks to our observations in recent years, including the present results, we now know that nitriles are among the most abundant molecules in the universe. We have found them in molecular clouds at the center of our galaxy, in protostars of various masses, in meteorites and comets, and also in the atmosphere of Titan, Saturn’s largest moon.” Second author Dr. Izaskun Jiménez-Serra looks ahead, “We have so far detected several simple precursors of ribonucleotides, the building blocks of RNA. But there are still important missing molecules that are difficult to detect. We know, for example, that the origin of life on Earth probably required other molecules, such as lipids, that were responsible for the formation of the first cells. Therefore, we should also focus on understanding how lipids could have formed from simpler precursors present in the interstellar medium.”