A team of Japanese scientists today announces a discovery that could help explain one of the biggest questions in science: how did life on Earth come about?
The answer, according to them, must lie in the oldest material in the solar system that has been accessed: about five grams of soil extracted by the space probe. hayabusa 2 from the surface of Ryugu. This asteroid is a black sphere about 900 meters in diameter that orbits between Mars and Earth at a minimum distance of almost 100,000 kilometers. For scientists, it’s like a time capsule that has wandered intact since the origin of the solar system, 4.5 billion years ago. Its composition reveals how it is the oldest and most basic material from which all planets were later formed, including Earth.
In December 2014, Japan launched the investigation hayabusa 2, which would become the first human spacecraft capable of visiting an asteroid, extracting samples from it and sending them back to Earth. your predecessor hayabusa 1 he had done something similar before, but the amount of land knocked off his asteroid, Itokawa, was very small. Upon reaching Ryugu, the hayabusa 2 He not only collected surface dirt, but fired projectiles at Ryugu, exposing his interior and collecting samples from within.
In December 2020, an armored capsule survived re-entry and crashed in a remote desert location in South Australia. The samples sent by hayabusa 2. Some time later, researchers confirmed that the spacecraft had recovered 5.4 grams of asteroid. It was the oldest and purest material to which humanity had access.
Scientists took this soil and dissolved it in very hot water in search of organic molecules. The latest results, published today in Nature Communications, show that Ryugu contains uracil, one of the four genetic letters from which RNA is made. It is believed that this complementary DNA molecule, although simpler, could have been the first form of life on Earth.
Yasuhiro Oba, biochemist at Hokkaido University and first author of the study, explained to EL PAÍS that “the presence of uracil in Ryugu is conclusive proof that this compound is present in extraterrestrial material [que compone asteroides y otros cuerpos]”. “This discovery further strengthens the hypothesis that organic molecules present in meteorites, asteroids and comets contributed to the prebiotic evolution of the primitive Earth and possibly to the origin of life on this planet”, he says.
The Earth formed 4.5 billion years ago from dust and rock that accumulated around the Sun. Material not used by the giant planets, such as Jupiter and Saturn, was left available to form the four rocky planets: Mars, Venus, Mercury and Earth.
About 4.1 billion years ago, a young Earth suffered intense bombardment by comets and asteroids. Much of the water in today’s oceans, as well as basic organic compounds, could have come aboard these bodies. The first forms of life, microbes that already have the ability to reproduce, were already present about 3.7 billion years ago. While no one knows how they came to be, it’s possible that the DNA and RNA that allowed them to proliferate and evolve formed in an environment with heat, water, and those basic building blocks of life possibly from space, like hydrothermal vents on the ocean floor.
Scientists have also discovered nicotinic acid, which helps living things extract energy from nutrients.
Japanese scientists associated with hayabusa 2, from the Japanese Space Agency, also found other organic compounds, such as nicotinic acid, present in vitamin B3. On our planet, this molecule helps living beings extract energy from nutrients, create cholesterol and fat reserves, and form and preserve DNA.
It is the first time that uracil has been found in an asteroid, points out Oba. One of the most interesting results is that samples taken from the interior of the asteroid contain more uracil than those from the surface, which are much more affected by exposure to radiation and vacuum. This confirms the view of asteroids as chests that protect material intact from the origins of the solar system.
This discovery joins that of other teams, which had already found uracil in meteorites; asteroid fragments that survive entry into Earth’s atmosphere and fall to its surface. The rest of the “letters” that make up RNA and DNA were also found in meteorites: adenine (A), cytosine (C), guanine (G) and thymine (T). In previous studies, Hayabusa’s team had already disclosed that Ryugu also contains amino acids, fundamental building blocks for the formation of proteins from the information stored in DNA. The amino acids found in that body were definitely odd, as they weren’t among the 20 that terrestrial organisms use to form the proteins that keep them alive.
“This discovery further reinforces the hypothesis that organic molecules present in meteorites possibly contributed to the origin of life on this planet.”
It is a scientific and aseptic feat to have managed to collect samples millions of kilometers from Earth, bring them back and analyze them in several laboratories, both in Japan and in the United States, without them being contaminated, a problem that often exists with meteorites found on the surface.
Juli Peretó, a specialist in synthetic biology at the University of Valencia, points out that it is a “technically very good” study. “What this shows us is that the oldest asteroids already contain the bricks and mortar of life, although they are not yet constructions, like walls”, he exemplifies.
The uracil found in Ryugu is composed of four carbon, four hydrogen, two nitrogen and two oxygen atoms (C4H4N202). In nature, this uracil does not walk alone, but needs to be associated with a sugar molecule called ribose, which has five carbons, five oxygens and 10 hydrogens, and also three phosphate molecules, with four hydrogens, 1 phosphorus and four oxygens. And to all this would be added other letters of RNA, with so many other sugars and associated phosphorus compounds, a level of biochemical complexity never before found in asteroids, meteorites or comets. And even so, we would be light years away from the dimensions of the simplest unicellular living beings that could resemble those that appeared on Earth about 4 billion years ago. “A small genome of free-living bacteria can reach a million monomers [letras de ADN], and the total number of atoms would be around 40 million”, explains Peretó. According to the expert, here is the main dilemma of life. On Earth, four genetic letters are enough to form DNA, and only 20 amino acids to form the 200 million different proteins that allow all living beings on the planet to live.
A great diversity of basic compounds could be formed in space, but only on Earth was the enormous complexity necessary to form living beings possible; first, single-celled microbes, and much later, a whole explosion of beings divided into three great domains: bacteria, archaea and eukaryotes, the kind of beings made with complex cells with a nucleus to store their DNA and that includes humans, animals, plants and mushrooms. Another great mystery of the evolution of life is that size does not matter: there are amoebas whose genome is 100 times larger than that of a human being, although both are built with variations and repetitions of the same four genetic letters.
Ester Lázaro, a researcher at the Centro de Astrobiologia (CAB) in Madrid, believes that these new tests “add and reinforce what many of us already had in mind”, referring to the fact that the basic building blocks of life can reach the planet through of asteroids and comets.
Marta Ruiz, prebiotic chemistry researcher at CAB, explains: “The identified compounds are interesting from the point of view of prebiotic chemistry and the hypotheses of chemical evolution that try to explain the appearance of life on Earth from a sample of organic molecules, originating whether from our own planet or traveling to it transported on meteorites and asteroids, which were able to self-assemble, self-organize and finally generate the first protocells.” “Uracil”, he points out, “is part of current nucleic acids ( RNA and DNA), the imidazoles, which were also found in Ryugu, could act as catalysts for the activation of nucleotides and amino acids as condensing agents, and nicotinic acid is part of current cellular metabolism as part of some coenzymes.”
In September of this year, another hermetic capsule coming from space will pass through the atmosphere, open its parachute and land softly somewhere in the Great Salt Lake desert, in Utah, in the United States. It will be NASA’s first attempt to emulate Japan’s feat. If all goes well, inside the chest will be untouched soil samples from Bennu, an asteroid 490 meters in diameter that was dropped into a near-Earth orbit by the gravitational pull of the giant planets. The risk of a collision with Earth is null for at least the next century, according to NASA, but the agency sent the OSIRIS-REx mission to collect samples. The Japanese Oba will be part of the team that analyzes this material, determines its age and, who knows, finds organic compounds that will better clarify how it all began.
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