Did life come to Earth from space? Possibly not, and that’s good news

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.

The alternative hypothesis is that life is our own, that it emerged on our planet Earth thanks to the chemistry of the atmosphere, geochemistry, the environment and the available energy sources.

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 have 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 difficult. Therefore, it is essential to replicate and confirm these findings. One possibility is to compare them with samples of asteroids , that is, samples of rocky bodies orbiting in space, which have not passed through the atmosphere and have not been contaminated after their impact on 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 so far been found 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. Probably, they have 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 laboratory work to answer the question of how the simplest chemical components of life are formed has been 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 have 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.

Atmospheric and geological processes occurred on the early Earth that suggest that huge amounts of organic precursor materials for life were formed. It is also possible that the combination of both was relevant: meteorite impacts plus generation on the planet. A meteorite impact, apart from its composition, provides enormous energy and local conditions that could be favorable for the origin of life.

We must explore more. Perhaps places like Mars or worlds rich in liquid water, like Europa or Enceladus, are the key. At the moment there is no evidence of current or past life in these places. It would be good news if, in the end, there was no life on them: paradoxically, a barren solar system may help us understand how life originated on our planet.The Conversation