Scientists have found, in three meteorites that have fallen to Earth in recent decades, the entire molecular basis of our DNA and RNA — that is, the recipe for life.
This new discovery supports the theory that, about 4 billion years ago, it was space rocks and cosmic dust that brought the necessary ingredients for the emergence of living beings on Earth.
However, it is still not possible to say with certainty that all the substances found are of extraterrestrial origin. Some may have penetrated the rocks after colliding with Earth. Additional studies will attempt to rule out this possibility.
The researchers analyzed three famous meteorites: Murchison, which fell in Australia in 1969; Murray, in the United States in 1950; and Tagish Lake, Canada in 2000. They are of a rare type, called carbonaceous chondrites, which contain many organic compounds and date back to the early days of the Solar System.
In them, traces of the five nitrogenous bases of life (the popular “letters”) were found: adenine (A), cytosine (C), guanine (G), thymine (T; present only in DNA) and uracil (U; in RNA ).
Three of these substances (A, G and U) had been found in meteorites before, but cytosine and thymine did not appear to exist in objects coming from outside Earth — until now. “In particular, the detection of cytosine is a surprise because it is relatively unstable and tends to react with water,” said Yasuhiro Oba, a professor at Hokkaido University in Japan and the study’s lead author.
To analyze the rocks, the scientists turned to high performance liquid chromatography (CLEA or HPLC), which uses pressurized water to separate components from meteorite samples. The nitrogenous bases were then extracted and analyzed by mass spectrometry, which revealed the detailed chemical composition of the materials.
To verify that the substances were indeed extraterrestrial, the team made sure there was no contamination in the laboratory and equipment, and also analyzed samples from the site where the Murchison meteorite fell. Some bases were found in soil, but “their distribution, concentrations and isomers are clearly different from those found in meteorites,” Oba said.
The authors concluded that the components formed in space and “contributed to the emergence of genetic properties for the earliest life on Earth.” The study was published this week in the journal Nature Communications.
The origin of life on Earth is one of humanity’s greatest mysteries. The phenomenon could have happened by the internal geochemical processes of our planet, or, as some believe, it could have had a little push from outer space. The Earth went through a period of intense bombardment by runaway rocks in its “youth”.
But even assuming that all essential substances did indeed originate in space, it’s hard to say what concentration of DNA bases meteorites would have had to have, or how many would have hit us, to actually have triggered the emergence of life.
In the future, Oba and his team plan to look for nitrogenous bases in material collected directly from asteroids — not from meteorites that have fallen to Earth. This minimizes the concern of contaminants.
And that may not be far off. Recently, the Japanese probe Hayabusa2 brought back fragments of the asteroid Ryugu; and NASA’s OSIRIS-REx is expected to arrive with pieces of Bennu in 2023.
