Dialog between an astrophysicist and a biologist

An interesting and sparkling dialog on the origin of life on Earth between the astronomer Cecilia Ceccarelli of the Institute de planétologie et d’astrophysique (IPAG) de Grenoble and the biochemist, Juan Fontecilla, of the Institut de biologie structurale (IBS). The inerview was published on the French magazine on the science of the Universe Ciel et Espace.
The original version, in French, is in the pdf attached. Below the English translation.

The first experiments on the origin of life date back to the 1950s. But, despite a recent interest, the subject remains marginal in science, both among astronomers and biologists. How did you get interested in these issues and in life elsewhere in general?

Cecilia Ceccarelli: :
As a lot of people, I think, I have always been interested in these subjects.
However, it was when I arrived in Grenoble that I started to be interested in astrochemistry². About twenty years ago, thanks to radio telescopes, interstellar complex molecules were discovered in the direction of the center of the galaxy, in a dark cloud called Sagittarius b2. "Complex" from the point of view of interstellar chemistry, I would stress.
This because with their eleven atoms, they remain extremely simple for a biochemist. With my colleagues we started looking for them in the protostars convinced that these molecules could be the bricks of life.
Today, of course, our aim is not to give an explanation for the origin of life. Our work is upstream: we want to identify the molecules that are produced in the universe and try to understand how they are formed. In particular, those who play a role in living things and that could have sown the earth.

Juan Fontecilla:
My interest in the origin of life dates from the reading of Alexandre Oparine, at the university [this Russian biochemist, auteur of;
origin of life in 1924, is the first to have made the hypothesis of a prebiotic organic chemistry, preceding the appearance of life. n.d.r].
After my thesis in protein crystallography in the United States, i got interested in metalloproteins: proteins that include metals like sulfur, iron, nickel... Nickel is interesting because it is linked to an event important in the history of life, the great oxidation 2.4 billion years ago, when the earth’s atmosphere became rich in oxygen (O₂).
Regarding the iron and sulfur, we find them in a protein called hydrogenase that is capable of supply electrons from molecular hydrogen to fix carbon in CO₂ and make molecules essential for life, such as sugars and proteins.
This brings us back to an even more distant time, when the atmosphere was largely composed of CO₂. The role of metals is so important in biochemistry that I am convinced that life arose on a mineral surface, on Earth, and via simple metabolic processes such as those involving hydrogenase. For me, the sugars and amino acids discovered in the universe have nothing to do with the appearance of life. And I’m not very optimistic about his chances of being present elsewhere.

For what reasons do you think extraterrestrial life is unlikely to exist?

Juan Fontecilla:
The evolution is governed by contingency. His march is a succession of chances, of events which could very well not have happened. If the Pangea hadn’t broken up 200 million years ago, if some meteorite hadn’t wiped out the dinosaurs 65 million years ago, we humans probably would have not been here to talk about it. This argument can be applied to the origin of life itself. Without the moon, which stabilizes the axis of rotation of the Earth, the climate of our planet would be completely irregular. however, the moon was born from the collision of the young earth with a celestial body, Theia. Again, it is the result of chance!

Cecilia Ceccarelli: :
I am optimistic about the chances of life appearing elsewhere. I agree with this sentence of biologist Christian de Duve: "Life is an obligation written in the fabric of the cosmos". From the big bang, the evolution of the universe could only lead to life. Why? Because this evolution is subject to the laws of physics! Life on Earth is organic chemistry. It is based on small molecules, all the same, whose skeleton is made of carbon. It also needs water. Water is an excellent solvent, but it also has an excellent calorific capacity - which can protect the molecules from too drastic environmental variations -, it absorbs ultraviolet rays, and it remains liquid over a wide temperature range.
In short: the three most important elements for life are hydrogen, oxygen and carbon. now what are the three most abundant elements around us? Hydrogen, oxygen and carbon. it means that life has used what was available.
If the most abundant elements in the universe had been totally different from those which life uses, one could defend the idea that it is exceptional, accidental. But this is not the case! We can even go further: life appears on the planets, and the planets are the remains of the formation of stars, which themselves are born by the collapse of molecular clouds into core of dense gas. For these core to form, the gas must cool down to 10 K [-263 °C], through radiation. Which species are responsible for it? carbon and oxygen! In short: if we give it carbon, oxygen and hydrogen the universe, with its laws, makes life. in this sense, I think that life elsewhere is indeed an obligation.

Juan Fontecilla:
But there you defend the idea that the Universe is made so that life can exist. And even, going further, the anthropic principle for which the universe must lead to man! For me, this is teleology: we give an intentionality to a purely physicochemical phenomenon. It is the same point as the Intelligent Design: if we are there, thinking, it is because the evolution was programmed for it. As a biologist this is not acceptable. There is no intention behind our presence in the universe!

Cecilia Ceccarelli: :
But I never said there was an intention, not at all! I am simply saying that, the laws of physics are deterministic, the processes which led to the appearance of life on Earth must have been reproduced elsewhere and often. Besides, I would like to come back to this meteorite story that killed the dinosaurs. It is brandished everywhere to show how much our presence is the result of chance. Very good, except that we must still realize that the fall of meteorites on a planet, it is a direct consequence of the process of formation of the planetary systems. In reality, this is not exceptional.

Juan Fontecilla:
We have nevertheless passed several times very close to a total extinction of life on earth. For a "cosmic imperative", sorry!

If life appeared several times on earth, one could perhaps more easily defend the idea that this is an inevitable consequence of cosmic evolution. Do we have any indications on this?

Cecilia Ceccarelli: :
I am not a specialist, but this is a hypothesis which is considered by a hundred researchers.

Juan Fontecilla:
In any case, from the virus to the elephant, all living beings work in the same way. We all have the same genetic code, which shows that we all come from a single ancestor. It is unique because is it the only one to have appeared, or because is it the only one to have survived? No one can answer that. But until proven otherwise, life is extremely rare. Only one example is known.

Around a hundred of amino acids have been discovered in the famous Murchinson meteorite, indicating a synthesis in space, outside the Earth. For you Cecilia Ceccarelli, this is a strong argument in favor of the universality of life. For you, on the contrary, Juan Fontecilla, that tells us nothing about the origin of life. Why then?

Juan Fontecilla:
To answer you, we have to go back to the year 1953 and to the experiment carried out at that time by the chemist Stanley Miller. By subjecting a mixture of methane, ammonia, water and hydrogen to electric discharges for a few days, he manages to synthesize organic molecules, including amino acids. The experiment makes a lot of noise, because its mixture is supposed to reproduce the primitive atmosphere of the Earth. Everyone believes that we have taken a giant step towards explaining the origin of life! But we know today that the primitive atmosphere did not have this composition at all. In short, these amino acids have nothing to do with life. The experiment was often reproduced and the conclusion is clear: the chemistry used is not at all specific, it has no direct relationship with biochemistry. For Murchinson’s meteorite, it is the same thing: there are 90 different amino acids, but only five or six have a biological interest. And even! In Murchinson, like in Miller’s experiment, they are found in two different stereochemical forms, while living beings only use one. Whether amino acids or sugars can be made outside of Earth is fine with me. But what is the relationship with life? Imagine that a meteorite with its hundreds, its thousands of molecules, falls into water, How are the reactions selected which will lead to the appearance of a primordial metabolism?

Cecilia Ceccarelli: :
There are chemical reactions that are more likely, or that occur faster than others. Not all species will react!

Juan Fontecilla:
Maybe, but in my opinion, there are anyway too many people so that a fine chemistry, like the living beings one, can get its pin out of the game. For me, the chemistry of living beings must start from a much more basic stage, on Earth, with molecules simpler than those observed in space. The complexification took place on Earth, by the very early appearance of an simple metabolism. Again, I believe that the presence of relatively complex organic molecules in space has no direct connection with terrestrial life.

You have insisted on the role of chance in evolution, Juan Fontecilla, while you, Cecilia Ceccarelli, have emphasized the determinism of the laws of physics. Isn’t your disagreement a reflection of your background?

Juan Fontecilla:
As Jacques Monod biologist and Nobel Prize for Medicine in 1965, n.d.r wrote, what completely defines evolution is chance and necessity. Chance is the mechanism of mutation. The need is to be able to survive. I completely agree with that. Evolution has no direction. It is, and that’s all. My problem with determinism is that it involves an ability to see the future. However, I do not believe that one can determine anything concerning life. Life is much more complicated than the laws of physics where, if I kick a ball, I will be able to determine its trajectory!

Cecilia Ceccarelli: :
It is true that physicists are intellectually trained to find laws. Their goal is to free themselves from details to identify deep structures. When you kick a ball, in reality it does not always go to the same place. it depends on the way you hit it, the relief of the terrain, the wind, etc. And yet, we can determine a general law which governs its movement! For life, in my opinion, it is the same. The fall of a meteorite, the fact that there is the Moon or not to stabilize the axis of rotation of the Earth, etc. are details which should not obscure the profound law from us: life is a natural consequence of the evolution of the Universe. So, it is not mysterious that life appeared on Earth and there is no reason that it cannot be found elsewhere. I would add that there is a difference between physics and biology: it is that in physics, we have done a lot of modeling, for a long time, whereas in biology - which deals with more complex systems, we do not. We are only at the beginning. When we will be able to simulate the evolution of complex chemical systems, prebiotic type, we will realize that the most likely solution is the appearance of life. In my mind, when Christian de Duve speaks of cosmic obligation (note that he is a biologist!), this is what he means: given the laws of the universe, the appearance of life is very likely everywhere.

Do you think we’ll find out about alien life first, or how life came to be on Earth?

Juan Fontecilla:
For a biologist, it would be a dream to land on another planet and be able to study another form of life there. How different would it be? Would it also rely on nuclear acids? Unfortunately, the chain of events in the appearance of life seems so complex that it seems very difficult to me that it has happened often. We will have a hard time discovering it in the vastness of the cosmos.

Cecilia Ceccarelli: :
I am of the opposite opinion. Be careful, I am not talking about extraterrestrial civilization, I’m talking about microorganism!. But we will find it, and probably well before having understood how life appeared on Earth.

¹ The discovery of a protein in a meteorite was announced on February 22, 2020 by three American researchers. However, it remains to be confirmed.
² Grenoble is the headquarters of the Institute of Millimetric Radioastronomy (IRAM), which manages radio telescopes with which we observe, in particular, interstellar molecules.

See the 10 podcast on the origin of life on ciel et espace (in French)

Updated on 16 December 2021