The recent discovery of the bases of DNA and RNA, the basic molecules of life, in carbonaceous meteorites evokes big questions: did life reach Earth in meteorites? Was the contribution of these extraterrestrial components necessary for life to originate on Earth?
These questions represent two approaches to the panspermia hypothesis: Perhaps life, directly or through its essential components, was seeded on our planet during the bombardment of meteorites and comets, just over 4.2 billion years ago.
Both hypotheses are not mutually exclusive: it could have happened that the components of outer space were added to the internal processes. The discovery of nucleic acid bases (and other components) in meteorites or asteroids, then reinforces the hypothesis of panspermia? Does it suggest that life could have come from space? The answer today is possibly not, although this is good news.
DNA components in meteorites: how to interpret them
The search for RNA and DNA bases and other simple components of life, such as amino acids, in meteorites that impacted the Earth is a research topic that has not been without controversy. The first question that scientists ask is whether these components are original or the result of contamination on Earth.
In this sense, the discovery of thymine in meteorites, until now considered a biological product, is very tricky. Therefore, it is essential to replicate and confirm these findings. One possibility is to compare them with asteroid samples, that is, samples of rocky bodies orbiting in space, which did not pass through the atmosphere and could not be contaminated after their impact with Earth. Leaving aside the problem of thymine, what does it mean that there are RNA bases in a meteorite?
Analysis of carbonaceous meteorites
The study of the identified compounds and their characteristics the first thing that tells us is that there is no life (as we know it) in the analysis of carbonaceous meteorites. That is, they were not carrying any organisms when they hit Earth. The observed components also suggest that the chemical evolution prior to life was not taking place in the object that gave rise to these meteorites. This process is complex and involves a series of molecular reactions and interactions for which there is, so far, no evidence outside the laboratory.
In our Solar System, apart from on Earth, no firm evidence has been found so far for the molecular processes that could lead to life, beyond the first step: the formation of the building blocks.
Terrestrial life is also a geochemical process, and the study of its evolution helps us to understand its own origin. Hidden in the molecular engine of every cell is evidence of how they came to be. Some striking finds, such as putative fossils in meteorites on Mars, are now considered to be of mineral origin.
We have not found signs of life beyond Earth
Actually, it is good news. There is an extraordinary coincidence between the chemical compounds observed in carbonaceous meteorites and those produced in prebiotic chemistry laboratories, from the pioneering Miller-Urey experiment in 1953, to the most modern experiments on the origin of RNA bases.
This coincidence suggests that the formation processes of simple components of life, such as amino acids and the bases of RNA, are common and robust. They probably occurred and occur in many places in the Solar System and in the universe. Including early Earth. In addition, we can predict and interpret them when we find them. The components found in meteorites confirm that the laboratory work to answer the question of how the simplest chemical components of life are formed was on the right track. However, as for how these components are assembled and evolve, for the moment, the meteorites do not give us any clues.
Of course, the absence of evidence is not evidence of the absence: the fact that we have not found any evidence neither for life, nor for chemical evolution, does not mean that there could not be. Maybe we haven’t found the right samples. Could life originate on Mars and reach Earth aboard fragments detached from the planet by impacts? We have no evidence of it. Furthermore, the composition of all the analyzed meteorites and their coincidence with the fundamental prebiotic chemistry supports the opposite hypothesis: that life did not arrive on Earth from space.
Let’s go with the next question: did we need the RNA bases to come in meteorites or comets, or could they have been formed on Earth?
Possibly, the components of life were generated (also) on Earth
It is a mistake to think that, because there are components of DNA and RNA in meteorites, it was necessary for life that these components have traveled with them. This is a logical fallacy. Just as it is a mistake to think that, because people get off a plane, all the people who live in the city arrived by plane. Or that planes are necessary to create cities. Of course, there could be a place where all the people have arrived by plane. But we need more evidence to confirm this, and at the moment all the evidence, including the composition of meteorites, points just in the opposite direction: that life and its components were generated on Earth itself. Given the robustness of the formation of the building blocks of life, it is likely that the planet had more than enough capacity to generate them.