How did life come about? The answer to this question affects the very core of our existence on planet Earth.
Did life arise from chemical reactions between organic compounds from a primordial soup left over after Earth was clumped together from space debris? If so, where do the organic compounds come from?
Some of the so-called “building blocks of life” may have been very common in the early Solar System.
A team of Japanese and American scientists led by Yasuhiro Oba has analyzed samples taken from the Ryugu asteroid in 2018 by the Hayabusa2 mission and has found uracil, one of the five key bases of RNA and DNA molecules that are crucial for life as such. as we know it. Their study has just been published in Nature Communications.
At the most basic level, the development of life is a matter of combining simple organic molecules into increasingly complex compounds that can participate in the myriad reactions associated with a living organism.
Simple amino acids are believed to act as building blocks in the formation of these more complex molecules. But this is not a simple shuffling exercise.
The largest “chunk” of the human genome, chromosome 1, is made up of 249 million base pairs (the rungs of the twisted ladder of the DNA molecule). Each base pair is made up of two bases: guanine and cytosine, or adenine and thymine.
Building from simple chemical base pairs to a complete strand of DNA is a daunting task. A DNA strand also has a complex structure, which varies from individual to individual. Life on Earth uses the structure of DNA to memorize the construction of the life form in question.
In addition to DNA, life uses a molecule called RNA to make proteins and perform other tasks inside cells. RNA is also made up of a long chain of bases: guanine, cytosine, and adenine (like DNA), but instead of thymine it has uracil, which is what turned up in Ryugu’s sample.
Ryugu is what is called a C-type or carbonaceous asteroid. These asteroids are the most common in the asteroid belt, making up about 75% of what we can see.
The Hayabusa2 mission established that C-type asteroids like Ryugu are the source of a rare type of meteorite sometimes found on Earth, called a carbonaceous chondrite.
Uracil and other organic molecules had previously been found in these meteorites, but the possibility that some of the molecules had a terrestrial origin could not be ruled out. The meteorite samples could have been contaminated here on Earth, or their chemistry could have been changed by heating as they fell through the atmosphere.
However, since the Ryugu sample was taken from the surface of an asteroid and brought back in a hermetically sealed container, scientists are confident that it is free of contamination or any effects of coming to Earth.
Furthermore, the presence of these amino acids in Ryugu demonstrates that even on asteroid surfaces, exposed to solar wind, micrometeorites, and cosmic rays, organic molecules can survive transport through the solar system.
An enormous variety of different organic compounds have already been found in the Ryugu samples.
Many organic molecules, such as amino acids, have two forms: left-handed and right-handed. Life on Earth depends on left-handed amino acids, but both forms are equally common in Ryugu samples, indicating that the molecules found in Ryugu are not signs of life.
The Solar System formed about 4.57 billion years ago from a cloud of molecular dust that was exposed to ultraviolet radiation and bombardment by proton particles.
The molecular cloud contained simple molecules such as methane (CH₄), water (H₂O), and ammonia (NH₃). These would have been fragmented by radiation, and the fragments would have been reassembled into more complex molecules such as amino acids.
C-type asteroids like Ryugu are thought to have formed so far from the Sun that the water and carbon dioxide they contain would have remained frozen. However, as the asteroids heated up and the ice melted, liquid water could have reacted with rocks and minerals.
Whether these conditions led to the creation of more complex organic molecules is an open question, but these conditions would certainly be conducive to new reactions. In addition, these conditions could affect the survival of the different compounds.
The Ryugu samples obtained by Hayabusa2 provide a new context for understanding the origin of organic compounds that could have given rise to life on Earth. There is still a long way to go before these organic compounds are available for the primitive Earth and the formation of life itself.