Asteroid samples provide new insights into the spatial origin of the main components of life on Earth

How did life originate? The answer to this question touches the very essence of our existence on planet Earth.

Did life arise as a result of chemical reactions between organic compounds from the primordial soup left after the Earth was glued together from space debris? If so, where do 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 analyzed samples taken from the asteroid Ryugu in 2018 by the Hayabusa2 mission and found uracil, one of the five key bases of RNA and DNA molecules that are critical to life itself. we know it. Their study has just been published in Nature Communications.

Building blocks

At its most basic level, the evolution of life is a matter of assembling simple organic molecules into ever more complex compounds that can participate in the myriad reactions associated with a living organism.

Simple amino acids are thought to act as building blocks in the formation of these more complex molecules. But this is not just a shuffling exercise.

The largest “piece” of the human genome, chromosome 1, consists of 249 million base pairs (rungs of the DNA molecule’s twisted ladder). Each base pair consists of two bases: guanine and cytosine or adenine and thymine.

Building from simple chemical base pairs to a complete strand of DNA is no easy task. The DNA strand also has a complex structure that varies from person to person. Life on Earth uses the structure of DNA to remember the design of the life form in question.

In addition to DNA, life uses a molecule called RNA to make proteins and perform other tasks within cells. RNA also consists of a long chain of bases: guanine, cytosine and adenine (like DNA), but instead of thymine it has uracil, which turned out to be in the Ryugu sample.

ryugu

Ryugu is what is known as a C-type carbonaceous asteroid. These asteroids are the most common in the asteroid belt, making up about 75% of what we can see.

The Hayabusa2 mission has determined that Type C asteroids such as Ryugu are the source of a rare type of meteorite occasionally found on Earth called carbonaceous chondrite.

Uracil and other organic molecules had previously been found in these meteorites, but the possibility that some of the molecules were of terrestrial origin could not be ruled out. The meteorite samples could have been contaminated here on Earth, or their chemistry could have changed when heated as they fell through the atmosphere.

However, since the Ryugu sample was taken from the surface of the asteroid and brought back in a sealed container, scientists are confident that it is free from contamination or any consequences of hitting Earth.

In addition, the presence of these amino acids in Ryugu demonstrates that even on an asteroid surface exposed to the solar wind, micrometeorites and cosmic rays, organic molecules can survive transport through the solar system.

A huge number of different organic compounds have already been found in Ryugu samples.

Many organic molecules, such as amino acids, have two forms: left-handed and right-handed. Life on Earth depends on left amino acids, but both forms are equally common in Ryugu samples, indicating that the molecules found in Ryugu are not signs of life.

General form

The solar system formed about 4.57 billion years ago from a cloud of molecular dust exposed to ultraviolet radiation and bombarded by proton particles.

The molecular cloud contained simple molecules such as methane (CH₄), water (H₂O) and ammonia (NH3). They would be fragmented by radiation and those fragments would be reassembled into more complex molecules like amino acids.

Type C asteroids such as Ryugu are thought to have formed so far from the Sun that their water and carbon dioxide would remain frozen. However, as asteroids warmed up and ice melted, liquid water could react with rocks and minerals.

Whether these conditions led to the creation of more complex organic molecules is an open question, but no doubt these conditions would facilitate new reactions. In addition, these conditions may affect the survival of various compounds.

The Ryugu samples obtained by Hayabusa2 provide new context for understanding the origin of organic compounds that may have given rise to life on Earth. There is still a long way to go before these organic compounds become available to the primitive Earth and the formation of life itself.

Trevor Ireland, Professor, School of Earth and Environmental Sciences, University of Queensland

This article was originally published on The Conversation. Read the original.