Nathalie Cabrol Wants the Nature of Life, Not Just a Signal from Mars

Lex Fridman opens with the résumé that sounds like fiction. Nathalie Cabrol directs the Carl Sagan Center for the Study of Life in the Universe at the SETI Institute. She explores some of Earth’s harshest places, including free diving in volcanic lakes, in pursuit of organisms that live in conditions analogous to Mars. For that work she holds the women’s world record for diving at altitude, both scuba and free diving. He asks whether the job posting looks like Ernest Shackleton’s 1900 advertisement: hazardous journey, small wages, bitter cold, months of darkness, constant danger — and where does one apply?

Cabrol laughs because there was not really a job application. When you are a scientist, she says, you have questions and hypotheses, you list what you would need to answer them, and then you know the places you have to go. Her path started in Mars-mission data. She wrote a PhD on water on Mars, first looking at channels and the history of water, then, in her postdoc, at where that water was ponding. At the time everybody was talking about channels, catastrophic floods, and seepage. Lakes and oceans made people “wade their arms a little bit.” It was a bit of a battle.

Working with her husband, she developed the idea of lakes in impact craters. Why craters? Because they were using Viking data from 1976 — two landers and two orbiters, humanity’s first feet on Mars, but not rovers — and in the 1990s, before Mars Global Surveyor, the resolution and topography were so poor there was no way to tell a real topographic low. The only hole in the ground you could trust was an impact crater. She did her master’s and PhD on Viking and worked that dataset for twenty years. The whole analog program started with a $40,000 discretionary grant.

The through-line is curiosity about life in the universe and how we got here. After twenty-five years in the field, she says the bigger question is the origin and nature of life, not merely whether there is life on Mars. Mars was a stepping stone. Reconstruct the environment 3.5 to nearly 4 billion years ago, find an analog on Earth, see what still survives there, and then ask what instrument and what resolution you would need to detect it. That is how a crater on a map becomes a dive in a thin-air lake.

Fridman asks her to recover the mindset of first looking at a planet. The first mission that successfully snapped pictures of Mars was Mariner 4. A lot of people, she says, were still under the spell of H.G. Wells and of a Mars that, through a telescope, looked so similar to Earth: polar caps, seasons, a tilt much like ours. When Mariner 4 left, many expected crystal cities and domes, another civilization evolving in parallel in the solar system. The first images came back, and at that resolution Mars looked like the Moon. It was a huge disappointment.

Then Mariner 9 changed everything, after a little drama. Mars kicked off one of the biggest dust storms it had ever experienced, and for three months an orbiter circled and saw nothing. When the dust cleared, volcanoes, valleys, ancient channels, dune fields, and polar caps appeared. Cabrol’s line is the one scientists still quote in spirit: when she describes Mars to Fridman, she does not need to invent any words. The myth of an extraterrestrial civilization was gone. The imagination of the scientists picked up, because they were seeing something familiar they could describe.

Viking was put on the fast track. If Mars looked so much like Earth, arid and thin-atmosphered as it was, could there be life? Behind the mission were people like Klein and Carl Sagan, thinking about how to test for biology. The landers arrived without geological context. The data they sent back was very confusing. Some people still think Viking discovered life, because one experiment showed a strange signal; most of the community thinks it can be explained by chemical reactions we see today.

It was so confusing that NASA decided that if it wanted to be serious about looking for life on Mars, it had to understand the environment, because life and environment coevolve. A planet supplies the physical-chemical boundaries; once life is here, it changes everything. One of the biggest impacts of life on Earth was injecting oxygen into the atmosphere 3.2 billion years ago, which changed the planet and its signature in space. You cannot solve for life without solving for environment. It is a two-unknown equation, and Viking had tried to skip the first unknown.

Fridman asks the practical question: if life had existed on Mars, what signs would we look for now, even a billion years after it died? Cabrol draws the parallel with Earth. It took 82 percent of Earth’s geological history to go from very simple microbial life to complexity, and by complexity she does not even mean us — she means animals. Mars is smaller. It lost its magnetic field very fast and lost its atmosphere very fast. Life also appeared on Earth very fast. Conditions were quite similar at that time, so if life appeared on Mars it would still have been simple life when conditions started to degrade, less than a billion years after the planet formed.

Everything at the surface would have disappeared except, maybe, morphological traces of the interaction between life and its environment. On Earth the best examples are stromatolites, rock formations built by microbes, which we know how to recognize. There could be chemical traces as well. Gale Crater has an intriguing question mark: an abundance of carbon-12, which life on Earth normally uses, but which other processes can also produce. It is not a real biosignature by itself, but it is intriguing, especially next to methane.

That era on Mars, 3.5 billion years ago, was full of destruction and impact cratering, yet some very old rocks survive. That is why rovers go to ancient lakes, impact craters, and other places with very old rocks. An impact is both bad and good for life. The building blocks — carbon, hydrogen, oxygen, nitrogen, phosphorus — were built into the planet by accretion of asteroids and comets. When an asteroid hits there is destruction, but those rocks also bring the bricks of life and a lot of energy. If the surroundings are favorable, you might get some cooking going on.

That cooking is one aspect of panspermia: comets and asteroids have the building blocks of life embedded in them, and given favorable conditions they might seed planets. Fridman asks what percent of her, looking up at the stars, thinks that is what happened on Mars or Earth. Cabrol splits the question. Panspermia is a potential vector. It distributes the stuff of life left and right. It does not explain the origin of life. It is not the environment itself. The stuff we are made of is very abundant, including in the interstellar medium. Maybe it just waits for the proper environment. On Earth we know what that needs: water, energy, shelter, and nutrients.

Fridman offers a backup plan. If civilization survives another few hundred years, take a big gun and shoot life out there — a life gun. Copy brains and complex information, or just send bacteria and the rugged organisms of Earth. Cabrol notes that those are not building blocks; they are actual organisms. The bricks of life were produced by the death of previous stars. Stars like the Sun started to form ten billion years ago. What he is talking about is a little different, and people are already working on versions of it: putting DNA on some substrate and preserving it in vaults on Earth, or on the Moon. Some people are already thinking about putting DNA on the Moon. Brains point toward transhumanism, enhancing who we are through AI and machine learning.

Backups are a good thing. For her, taking care of the planet and going back to equilibrium with the environment would be the best backup possible, and then letting evolution do its thing. Right now, she says, we are like teenagers with enough brain to create cool tools, but not enough brain to understand the consequences, and we are paying for it. The question is whether we can learn from the mistakes and become a mature civilization. That is the L at the end of the Drake Equation: the length of time a civilization remains detectable. It can disappear by destroying itself, by being destroyed from outside, or by becoming so in tune with the universe that it melts into the background. There are wild theories that an advanced civilization might be indistinguishable from physical processes — that dark matter, for example, is some sort of biological process. She does not say that is the case. She puts it on the table.

Preserving information, she says, is kind of what life does. We should not look at ourselves as different from the little cell that started off. That is a clue not to the origin of life but to the nature of life, which interests her more, because the nature of life is what will give you a universal signature — not only around ponds of water for life as we know it. Life wants the most information possible about its surroundings. Complexity is the ability to gather, exchange, and preserve the most information possible. We do not know what life is. We have 123 definitions, and some people say we do not have any definition, only descriptions. We are looking for something we do not know, with a few clues about its nature.

There are theories. Schrödinger wrote in the 1940s. Jeremy England has another biophysical theory: life is the inevitable result of thermodynamics, the best way to beat entropy. If you want to know the nature of life, Cabrol says, look at our languages. They can be very different, but they all have the same purpose: exchange information, store it, understand it, whether with somebody outside or in thought. That is the same thing the cell was doing.

Then she maps the scaffold. Life started with an atom; atoms made inorganic molecules, then more complex ones, then organic molecules, then RNA and DNA. Languages created alphabets — those are the atoms — then syllables, then words. Words tell you something but are nothing without a verb that gives the direction; that is RNA and DNA, and then you can add all the complements you want. “Our languages are built exactly as life is built. We are repeating patterns.” She calls this the Mandelbrot universe, the fractal universe. As much as she believes in sending probes, she says we should also look inward, because we are expressing life. The day we understand the nature of life, we have a universal biosignature, and it does not matter whether that life uses the same biochemistry we do.

Looking for something like us is not stupid, she argues, because of stellar generations. The Sun is part of the youngest generation of stars. The first two generations did not produce the elements we are made of; they were helium and hydrogen, or poor in metals. Those stars died, stars like the Sun were born, and that is why we have carbon, hydrogen, oxygen. Kepler, TESS, and now James Webb show many types of planets that may be habitable. There are countless Sun-like stars, and more interestingly other types with habitable zones whose lifetimes are sometimes a thousand times that of our Sun. Simple life started really quickly on Earth. Her take: the universe is full of cyanobacteria. Intelligent life takes more time.

We do not know what triggered the evolution to complexity on Earth. The Cambrian is what took us toward what we are; in the meantime there were dinosaurs, and the dinosaurs were wiped out, so evolution could have taken a completely different turn. Mass extinctions drive the endgame. You take two planets and change the asteroid impacts or the geological events that wipe out 90 percent of life, and the path changes. Location in the galaxy matters. We are in the habitable zone of the Milky Way. Too close to the center, it is denser; gravitational interactions perturb the Oort Cloud and fling more comets into the inner solar system, on top of the radiation. There is a bad neighborhood in the galaxy. You would not be able to have life there.

Climate history matters too. There is a theory, still needing more observation, of a correlation between big jumps in evolution and Snowball Earth episodes. Intuitively you would think a planet covered in ice means a decrease in life; for some reason there were big jumps right after each of those episodes. Mutations that seem responsible for jumps are also unclear. She cannot tell you that a planet habitable much longer than Earth will have the same evolutionary path. It depends on extinction, climate, and whatnot. We are biased on the origin of life because Earth is the only model we know, but it is a good bias: it has the numbers, the principle of mediocrity. Earth is probably representative of an abundance of other worlds. The endgame need not resemble us. Other biochemistries exist even here — Titan, with hydrocarbon lakes and rains, is a candidate, and we are going there.

The nature of life, if it is the universe’s best way to fight entropy, has no such bias, because physics is the same across the universe we know. Then the question becomes flavors of ice cream: what flavors of complexity does that process lead to? We do not know what life is, what intelligence is, or what consciousness is. A cyanobacterium that has survived, in one incarnation or another, for billions of years has some intelligence about its environment. Fridman and Cabrol end up in a connected universe: photons and atoms linked, life as a permeation, Sagan right that we are the universe trying to understand itself. Her greatest hoped-for achievement is finding the process of life, at which point the universe “illuminates itself with actually its living force.”

Fridman admits it makes him a little sad, watching breakthroughs in AI applied to natural sciences and physics, that the creatures who solve the nature of life might be AI systems. Cabrol does not think so. Remember who was behind AI, she says. She is not buying the singularity yet. AI is not aware. AI is built by humans, so it is a tool, an extremely smart tool. As long as we build it and use it as a tool, it remains a tool. There is a lot of brouhaha, and science fiction movies do not help.

Fridman pushes back with DeepMind. AlphaZero, playing chess against Stockfish, humbles the best human players not only by being better but by coming up with ideas humans do not understand: sacrifice a pawn, a queen, a bishop. You start to intuit why, but you do not deeply understand. You can say the AI is not conscious and does not understand the way humans do, and there is still a wisdom and a depth of knowledge in that program that humans do not have. The same with AlphaFold and protein folding, and with simulating nuclear reactions. We may intuit the nature of life poetically; the true understanding may come from a system that is much more computational.

Cabrol pushes back in turn. Humans gave themselves the ability to do that by building the tool. She still sees a great tool, and still thinks we are far from the sci-fi scenario. If the tool gives her a new perspective, that is just a deeper cognition of what the universe is, whether through our brain or through a different way of gathering information. That is what we do: build tools and integrate them into our way of thinking. Maybe another generation has to be born that is raised with those tools. People growing up now have minds integrated with the internet. You reconfigure memory; you no longer have to memorize a lot of facts because you can look them up, and you reallocate resources toward thinking.

She wrote about that relationship with technology in a book due in January, first in French. She wrote it in English and translated it; the English version is ready if they find a U.S. publisher. The point is a complete change that is, to her, the singularity more than anything else: the coevolution of humans with technology, not anymore with their environment. Why are we messing up the environment? Why don’t we respond to pandemics the way we should? Because we are disconnected. We used to take information the planet was sending. Now we take it from the web and from phones, with no filter, and no way of knowing if the information is correct. Fridman starts to push back again as the available conversation breaks off — but the claim is already on the table. If life is information about surroundings, a species that has unplugged from the planet has a problem that no DNA vault on the Moon will fix.