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Thursday, 21 November 2019

First detection of sugars necessary for life found in meteorites



The origin of life on Earth is an open question and represents a field of active research. Although prebiotic models of the appearance of life have been proposed, the process of appearance of building blocks of life (amino acids, nucleic acids, sugars, etc.) is still very little constrained. One of the hypotheses currently proposed proposes an extraterrestrial origin of these components within the framework of a model called panspermia. In this model, the constituents of life would have been brought to the primitive Earth through the incessant bombardment of comets and meteorites. And recently, researchers have for the first time discovered ribose and other sugars necessary for life in two meteorites, thus reinforcing this hypothesis.

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An international team of researchers has found sugar molecules essential for life in meteorites. This new discovery adds to the growing list of biologically important compounds found in meteorites, furthering the hypothesis that chemical reactions in asteroids - the parent bodies of many meteorites - can make certain life ingredients. If this is correct, the bombardment of meteorites on the primitive Earth may have helped the appearance of life by providing the basic elements.


First direct evidence of the existence of extraterrestrial ribose

The team discovered ribose and other bio-essential sugars, including arabinose and xylose, in two carbon-rich meteorites, NWA 801 (CR2 type) and Murchison (CM2 type).

Ribose is a crucial component of RNA (ribonucleic acid). RNA serves as a messenger molecule, copying the genetic instructions of the DNA molecule (deoxyribonucleic acid) and transmitting them to molecular factories within the cell, called ribosomes, which read RNA to construct specific proteins.

Structures of sugars (pentoses) discovered in the two meteorites. Credits: Yoshihiro Furukawa et al. 2019

" Other important elements in life have already been discovered in meteorites, including amino acids (protein components) and nucleic bases (components of DNA and RNA), but sugars have been an element missing among the main building blocks of life, "said Yoshihiro Furukawa of Tohoku University, Japan. " The research provides the first direct evidence of ribose in space and the contribution of sugar to Earth. The extraterrestrial sugar could have contributed to the formation of RNA on the prebiotic Earth, which probably led to the origin of life .

" It's remarkable that a molecule as fragile as ribose can be detected in such an old material, " says Jason Dworkin, an astrobiologist at NASA's Goddard Center. " These results will help guide our sample analyzes of the original Ryugu and Bennu asteroids, which will be returned by Hayabusa 2 of the Japan Aerospace Exploration Agency and NASA's OSIRIS-REx space probe ."


Ribose: a sugar used in the composition of RNA

A persistent mystery about the origin of life is how biology could have come from non-biological chemical processes. DNA contains the genetic instructions necessary for the functioning of a living organism. However, RNA also contains information and many researchers believe that it evolved first and was later replaced by DNA. This is because RNA molecules have abilities that are lacking in DNA.

Structure of the RNA. RNA uses ribose as sugar, unlike DNA that uses deoxyribose. Credits: BioC

RNA can reproduce without the need for other molecules, and it can also initiate or accelerate chemical reactions as a catalyst. These results support the possibility that RNA has coordinated the mechanism of life before DNA.

" The sugar contained in DNA (2-deoxyribose) was not detected in any of the meteorites analyzed in this study. This is important because there may be a lack of extraterrestrial ribose delivery to the early Earth, which is consistent with the hypothesis that RNA evolved first, "explains Danny Glavin.

Sugars brought by meteorites from space

The team discovered sugars by analyzing powdered meteorite samples using gas chromatography mass spectrometry, which sorts and identifies molecules based on their mass and electrical charge. They found that the abundances of ribose and other sugars ranged from 2.3 to 11 parts per billion in NWA 801, and from 6.7 to 180 parts per billion in the Murchison meteorite.

With the Earth now full of life, the team had to take into account the possibility that meteorite sugars could have simply come from a terrestrial contamination. Several sources of data indicate that contamination is unlikely, including isotopic analysis. Isotopes are versions of a different mass element because of the number of neutrons in the nucleus of the atom.

The isotopic analysis of the sugars found in the meteorites confirmed that they came from space, not from the Earth. Credits: Yoshihiro Furukawa et al. 2019

Carbon chemistry on Earth uses carbon 12 compared to the heavier version (carbon 13). However, the carbon contained in meteorite sugars was significantly enriched in carbon 13, beyond the amount observed in terrestrial biology, which corroborates the conclusion that it comes from space.

Towards a better understanding of the emergence of life on Earth

The team plans to analyze more meteorites to get a better idea of ​​the abundance of extraterrestrial sugars. They also plan to determine whether extraterrestrial sugar molecules have a preferred left or right orientation. Some molecules come in two varieties that are inverted images of each other. On Earth, life uses left amino acids and straight sugars.

Since it is possible that the opposite works perfectly - right amino acids and left sugars - scientists want to know where this preference comes from. If some processes in asteroids favor the production of one variety over another, then perhaps the influx from space via meteorite impacts has made this variety more abundant on the ancient Earth.

Bibliography:

 Extraterrestrial ribose and other sugars in primitive meteorites

 Yoshihiro Furukawa, Yoshito Chikaraishi, Naohiko Ohkouchi,  View ORCID ProfileNanako O. Ogawa, Daniel P. Glavin,  View ORCID ProfileJason P. Dworkin, Chiaki Abe, and Tomoki Nakamur

PNAS first published November 18, 2019

https://doi.org/10.1073/pnas.1907169116


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