A team of Japanese scientists today announces a discovery that may help explain one of the biggest questions in science: how did life arise on Earth?The answer, according to them, must be sought in the oldest material from the solar system to which access has been had: about 5 grams of soil extracted by the Hayabusa 2 space probe from the surface of Ryugu.
This asteroid is a blackish sphere about 900 meters in diameter that orbits between Mars and Earth at a minimum distance of almost 100,000 kilometers. For scientists, it is like a time capsule that has been roaming and intact since the origins of the solar system, 4.5 billion years ago. Its composition reveals how it is the oldest and most basic material with which all the planets were formed later, including the Earth.
In December 2014, Japan launched the Hayabusa 2 probe, which was to become the first human spacecraft capable of visiting an asteroid, taking samples from it, and sending them back to Earth. Its predecessor Hayabusa 1 had already done something similar, but the amount of land torn from its asteroid, Itokawa, was very little. Arriving at Ryugu, the Hayabusa 2 not only collected dirt from the surface, but also fired projectiles at Ryugu, exposing its innards and taking samples from within.
In December 2020, an armored capsule survived re-entry and crashed in a remote desert spot in South Australia. They were the samples sent by Hayabusa 2. Later, the researchers confirmed that the ship had recovered 5.4 pristine grams of asteroid. It was the oldest and purest material humanity had had access to.
Scientists have taken that 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 that RNA is made of. It is thought that this complementary DNA molecule, although simpler, could have been the first form of life on Earth.
Yasuhiro Oba, a biochemist at Hokkaido University and first author of the study, explains that “the presence of uracil in Ryugu is conclusive evidence that this compound is present in extraterrestrial material [that makes up asteroids and other bodies]”. “This finding further strengthens the hypothesis that organic molecules present in meteorites, asteroids and comets contributed to the prebiotic evolution of early Earth and possibly to the origin of life on this planet”, he says.
The Earth was formed 4.5 billion years ago from dust and rocks that had accumulated around the Sun. Material not used by the giant planets, such as Jupiter and Saturn, was left available to form the 4 rocky planets: Mars, Venus, Mercury and Earth.
About 4.1 billion years ago, a young Earth suffered intense bombardment from comets and asteroids. A good part of the water of today’s oceans and also basic organic compounds could have arrived on board these bodies. The first forms of life, microbes that already have the ability to reproduce, were already present about 3.7 billion years ago. Although no one knows how they arose, it is 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, such as hydrothermal vents on the ocean floor.
Japanese scientists associated with Hayabusa 2, from the Japanese Space Agency, have also found other organic compounds, such as nicotinic acid, present in vitamin B3. On our planet, this molecule helps living things extract energy from nutrients, create cholesterol and fat stores, as well as form and preserve DNA.
It is the first time that uracil has been found on an asteroid, Oba points out. One of the most interesting results is that the samples taken from the interior of the asteroid contain more uracil than the surface ones, which are much more affected by radiation and vacuum exposure. This confirms the vision of asteroids as chests that protect material intact from the origins of the solar system.
This finding joins that of other teams, which had previously found uracil in meteorites; fragments of asteroids that survive entry into the Earth’s atmosphere and fall on its surface. The rest of the “letters” that make up RNA and DNA have also been found in meteorites: adenine (A), cytosine (C), guanine (G), and thymine (T). In previous studies, Hayabusa’s team had already announced that in Ryugu there are also amino acids, fundamental building blocks for forming proteins from the information stored in DNA. The amino acids found in that body were definitely alien, since they were not among the 20 that terrestrial organisms use to form the proteins that keep them alive.
“This finding further strengthens the hypothesis that organic molecules present in meteorites possibly contributed to the origin of life on this planet”. It is quite a scientific and aseptic feat to have managed to take samples millions of kilometers from Earth, bring them back and analyze them in various laboratories, both in Japan and in the United States, without them having been contaminated at all, a problem that does exist throughout often with meteorites found on the surface.
JuliPeretó, an expert in synthetic biology at the University of Valencia, points out that it is a “technically very good” study. “What it shows us is that the oldest asteroids already contain the bricks and mortar of life, although not yet constructions, such as walls”, he exemplifies.
Uracil, found in Ryugu, is composed of 4 carbon, 4 hydrogen, 2 nitrogen, and 2 oxygen atoms (C4H4N202). In nature, this uracil does not go alone, but needs to be associated with a sugar molecule called ribose, which has 5 carbons, 5 oxygens, and 10 hydrogens; in addition, 3 phosphate molecules, with 4 hydrogens, 1 phosphorus and 4 oxygens. And to all this would be added other letters of RNA, with so many other sugars and phosphorus compounds associated, a level of biochemical complexity that has never been 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 the first ones that appeared on Earth some 4 billion years ago. “A small genome of free-living bacteria can reach a million monomers [DNA letters], and the total number of atoms would be about 40 million”, explains Peretó. According to the expert, here is the key dilemma of life; on Earth, 4 genetic letters are enough to form DNA, and barely 20 amino acids to form 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, unicellular microbes, and a long time later, a whole explosion of beings divided into 3 large domains: bacteria, archaea and eukaryotes, the type 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 4 genetic letters.
Ester Lázaro, a researcher at the Center for Astrobiology (CAB) in Madrid, believes that these new tests “add to and reinforce what many of us already had in mind”, referring to the fact that the basic building blocks of life could reach the planet through aboard asteroids and comets. Marta Ruiz, a prebiotic chemistry researcher at the CAB, explains: “The compounds identified 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 collection of organic molecules, either originating from our own planet or traveling to it transported in meteorites and asteroids, which were capable of self-assembling, self-organizing and finally generating the first protocells”. “Uracil”, she points out, “is part of current nucleic acids (RNA and DNA), imidazoles, which have also been found in Ryugu, could act as catalysts for the activation of nucleotides and amino acids as condensation agents, and the Nicotinic acid is part of the current cellular metabolism forming part of some coenzymes”.
In September of the current year, another airtight capsule from space will pass through the atmosphere, open its parachute, and land softly somewhere in the Great Salt Lake Desert of Utah, in the United States. It will be NASA’s first attempt to emulate the feat accomplished by Japan. If all goes well, inside the chest will be the pristine soil samples from Bennu, a 490-meter-diameter asteroid that has fallen into a near-Earth orbit by the gravitational pull of the giant planets. The risk of a collision with Earth is nil -at least for the next century-, according to NASA, but the agency sent the OSIRIS-REx mission there to collect samples. The Japanese Oba will be part of the team that analyzes this material, determines its age and, perhaps, finds organic compounds that allow us to further clarify how everything began on our planet.