These meteorites contain all the building blocks of DNA -

Three meteorites contain the molecular building blocks of DNA and its cousin RNA, scientists have recently discovered. A subset of these building blocks had previously been detected in meteorites, but the rest of the collection seemed mysteriously absent from space rocks – until now.

The new discovery supports the idea that around four billion years ago, a barrage of meteorites could have provided the molecular ingredients needed to jump-start the emergence of the oldest life on Earth, the researchers say.

However, not everyone is convinced that all new DNA the components are of extraterrestrial origin; Rather, some may have ended up in the meteorites after the rocks landed on Earth, said Michael Callahan, an analytical chemist, astrobiologist and associate professor at Boise State University who was not involved in the study. . “Further studies are needed” to rule out this possibility, Callahan told Live Science in an email.

Assuming all the compounds come from space, a subset of the building blocks – a class of compounds known as pyrimidines – appeared at “extremely low concentrations” in meteorites, he said. added. This discovery suggests that the world’s first genetic molecules emerged not due to an influx of DNA components from space, but rather due to geochemical processes taking place on early Earth, he said. he declares.

For now, however, “it’s hard to say” what concentration of DNA building blocks the meteorites should have contained to help foster the emergence of life on Earth, said Jim Cleaves, a geochemist and president of the International Society for the Study of the Earth. Origin of Life, which did not participate in the study. This question remains under study.

Related: 7 theories about the origin of life

The building blocks of life in space rocks

(Image credit: KATERYNA KON/SCIENTIFIC PHOTO LIBRARY via Getty Images)

Components of DNA and RNA have already been found in meteorites, Previously reported Live Science. Specifically, these space rocks have been found to contain nucleobases, the nitrogen-containing compounds that serve as “letters” in the genetic code of our DNA and RNA. Nucleobases come in five main flavors – adenine (A), thymine (T), guanine (G), cytosine (C) and uracil (U) – but previously only A, G and U had been identified in meteorites.

However, in a study published Tuesday, April 26 in the journal Nature Communicationscientists reported finding all five nucleobases inside carbon– rich in meteorites. This included traces of the three pyrimidines: cytosine, uracil and thymine. “In particular, the detection of cytosine is surprising,” because cytosine is relatively unstable and susceptible to reacting with water, said Yasuhiro Oba, associate professor at the Institute of Low Temperature Sciences at the University of Hokkaido in Japan and first author of the study. .

Although thymine and cytosine have not been found in meteorites before, laboratory studies have indicated that these nucleobases may be hiding, undetected, in space rocks that have collided with Earth.

For example, in the lab, scientists have recreated the chemical conditions of interstellar space – the space between stars – where huge clouds of gas and dust measure about 10 kelvins (minus 441.67 degrees Fahrenheit or minus 263.15 degrees Celsius) and the parent asteroids meteorites can be found. Through these experiments, the researchers synthesized thymine, cytosine and the other primary nucleobases, suggesting that all of these compounds could theoretically be detectable in meteorites, the study authors note in their report.

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The team therefore went in search of these nucleobases in three well-known meteorites. “The Murchison, Murray and Tagish Lake meteorites belong to a class of meteorites called carbonaceous chondrites, which are known to contain a lot of organic compounds,” Callahan said.

For example, hydrocarbons and protein building blocks (amino acids) have been identified in all three meteorites, Oba said. addition, in previous worksOba and his colleagues detected an elusive molecule called hexamethylenetetramine (HMT), which is considered an important precursor to organic molecules, in space rocks, according to NASA.

In their latest study, the researchers used a technique called high-performance liquid chromatography, which involved using pressurized water to separate meteorite samples into their component parts. In this way, the team extracted the nucleobases from each sample and then analyzed the bases using mass spectrometry, a technique that revealed the chemical composition of the material in great detail. This method “allowed us to detect nucleobases with very low concentrations, as low as parts per trillion,” Oba told Live Science.

Analysis revealed that all meteorites carried adenine and guanine. The Murchison samples also contained uracil, while the other meteorites carried at least one isomer of uracil, that is, a compound that contains the same number and types of atoms as uracil. but in a different spatial arrangement. Additionally, the Murchison and Tagish Lake samples contained thymine, and the Murray meteorite contained thymine isomers. All of the meteorites contained cytosine, as well as various isomers of the compound.

Still uncertain

(Image credit: University of Glasgow)

To verify that the nucleobases were of extraterrestrial origin rather than the result of terrestrial contamination, the team repeated the experimental procedures without any meteorite material in the test chambers. No nucleobase was detected during these so-called blank experiments.

The team also had access to soil samples from the site where the Murchison meteorite first fell to Earth. They detected some nucleobases in the soil, but “their distribution and concentrations are clearly different from those found in meteorites,” Oba said. Moreover, some specific isomers appear only in meteorites and not in the soil sample; These “strange isomers” are rarely observed on Earth and are therefore unlikely to be contaminants from the planet’s surface, Cleaves said.

By comparing the diversity of nucleobases found in the meteorite to those found in the soil, the team concluded that the space rock compounds formed in space, Oba said. And because of that, they expect nucleobases “contributed to the emergence of genetic properties for the earliest life on Earth,” the authors wrote.

However, on these points there is still some uncertainty, Callahan said.

Related: DNA is just one of more than a million possible ‘genetic molecules’, scientists say

The soil sample analyzed by the researchers contained higher concentrations of cytosine, uracil and thymine than those found in the Murchison meteorite, “so it is difficult to determine the amount of extraterrestrial compared to terrestrial in the meteorite,” Callahan said. Additionally, the team did not identify a specific chemical process that would produce C, U, T and their various isomers; such an analysis might have supported the idea that all compounds formed in interstellar space.

Another way to determine if nucleobases really came from outer space is to look at what forms of carbon and nitrogen they contain, Cleaves told Live Science. These elements come in different flavors, called isotopes, which contain the same number of protons but different numbers of neutrons. Earth matter contains different ratios of carbon and nitrogen isotopes than matter from space, so such analyzes could help distinguish terrestrial nucleobases from extraterrestrials, Callahan said. Unfortunately, such experiments require a fair amount of meteorite material to run and can therefore be difficult to run, Cleaves said.

In any case, even if the C, T and U detected are extraterrestrial, their low presence in meteorites casts doubt on the theory that the first life on Earth was seeded by DNA components from space, Callahan said. “If these results are representative of typical pyrimidine concentrations in meteorites, then geochemical synthesis on early Earth would likely have been responsible for the emergence of genetic material, rather than inputs from extraterrestrial inputs,” he said. he declares.

In the future, Oba and his colleagues plan to search for nucleobases in materials collected directly from asteroids, rather than meteorites on Earth, Oba told Live Science; this could minimize the problem of land-based contaminants. For example, the Japanese spacecraft Hayabusa2 recently brought the asteroid Ryugu back to Earth, Previously reported Live Scienceand NASA’s OSIRIS-REx probe is due to land with samples from near-Earth asteroid Bennu in 2023, according to Space.com.

Originally posted on Live Science.

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