Did life on Earth arise from chemical reactions among organic compounds in a primordial soup left after our planet clumped together from space rubble? If so, where did these organic compounds come from? Recent findings by a team of Japanese and American scientists led by Yasuhiro Oba have provided fascinating insights into the possibility that some of the building blocks of life were surprisingly common in the early Solar System.
The scientists analyzed samples taken from the asteroid Ryugu in 2018 by the Hayabusa2 mission and found uracil, one of the five key bases of the RNA and DNA molecules crucial to life as we know it. The study, published in Nature Communications, challenges our understanding of the development of life and its basic building blocks.
Life’s development at its most basic level involves 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 construction of these more complex molecules. The discovery of uracil in a sample from Ryugu, a C-type or carbonaceous asteroid, is a significant development in our understanding of the origins of life’s building blocks.
C-type asteroids like Ryugu make up about 75% of the asteroids in the asteroid belt and are the source of a kind of rare meteorite sometimes found on Earth, called a carbonaceous chondrite. Uracil and other organic molecules have previously been found in these meteorites, but it was impossible to rule out the possibility that some of the molecules had a terrestrial origin. However, since the Ryugu sample was taken from the surface of an asteroid and brought back in a tightly sealed container, scientists are confident it is free of contamination or any effects of coming to Earth.
The presence of amino acids on Ryugu, despite being exposed to the solar wind, micrometeorites, and cosmic rays, shows that organic molecules can survive transportation through the solar system. A huge variety of different organic compounds have already been found in Ryugu samples, although they are not considered signs of life.
The Solar System formed around 4.57 billion years ago from a molecular dust cloud that was exposed to UV radiation and particle bombardment from protons. This molecular cloud contained simple molecules such as methane, water, and ammonia. These would have been fragmented by the radiation, and the fragments would have reassembled into more complex molecules such as amino acids.
C-type asteroids like Ryugu are believed to have formed far from the Sun, where water and carbon dioxide would have remained frozen. However, as the asteroids warmed up and the ice melted, liquid water would have been able to react with rocks and minerals, potentially creating more complex organic molecules. These conditions would have been conducive to further reactions and could affect the survival of different compounds.
The Hayabusa2 samples from Ryugu offer a new context for understanding the origin of organic compounds that may have been the start of life on Earth. Although it is still a big step from having these organic compounds available to early Earth and the formation of life itself, these findings open up new avenues for exploration and understanding of the complex story of life’s origins.