…how the physics of life could explain why we haven’t met any aliens yet
It is still weird for me to think that humans just happened to wake up here on earth one day without having a clue where we came from.
We find ourselves thrown into this world without being asked to and without much explanation (Heidegger appropriately calls this phenomenon Geworfenheit/thrownness).
Adding to that is that it can be a very challenging task to dig into our past and into the earth to find traces and possible clues that can help us make sense of our situation.
How did we end up here?
Scientists have made much progress in the last couple of hundreds of years in answering this question. Much of our outlook onto our place in biology and nature has dramatically changed, transformed by Copernican, Newtonian and Darwinian revolutions.
The question of how we ended up here is closely intertwined with the question of how life came to exist in a lifeless universe, a question still heatedly discussed and bringing about exciting new theories.
One of them leads us to what might be the most fortunate crash in the history of life on earth.
What is life?
This question is a hard one to answer, and there really is no perfect definition that covers all aspects of what there is to life.
One could look at life from a biological perspective (for instance, as something that has to do with genetics), or as something that showcases Darwinian evolution. One could think about life from a chemistry perspective (carbon-based molecules etc.), or even from a philosophical point of view (a self-realizing principle, something relating to consciousness) and so on and so on.
Adam Rutherford at UCL argues that our ideas about life have been focused too much on what life is and have neglected an important input from physics: we should focus our questioning more on what life does instead.
The influential Miller–Urey experiment (here is the original paper) showed in 1952 that complex organic molecules like amino acids could in principle be created in the chemical environment of the early Earth.
After this undoubtedly important insight, many people jumped to the somewhat premature conclusion that the creation of life out of inanimate matter was only a question of time if there were enough amino acids flowing around in the primordial soup.
If you had enough time and enough amino acids in the vast oceans of the early earth, then surely some of them would team up at one point to form a cell, and from then on life would go its way. Life is, after all, made up of amino acids.
But life is more than just the sum of its (chemical) parts.
What do living systems actually do that makes them alive?
The Physics of Life
Life is doing things that are strange compared to other natural processes on earth. The Thermodynamics of life looks a bit messed up.
In my previous article on the Thermodynamics of Free Will and the Bayesian Brain Hypothesis, I talked about the Free Energy Principle that is an organizing principle behind the behavior of all living systems that is connected to them staying in homeostasis.
In a similar vein, the very foundations of life itself can be thought of from a thermodynamical perspective.
What are the energy flows in living systems? What is life actually doing when it comes to its energy household?
There are valuable lessons to be learned when thinking about the physics of life rather than its biology or chemistry.
Life equals Proton Pumping Across Membranes
Nick Lane’s The Vital Question has been one of my most mind-bending reads in recent years (see him giving an introduction on it here). In the book, Lane gives a detailed account of his theory of how life might have come to, well, life.
What I will focus on here is the following:
All living systems are made up of cells.
All cells have parts where (positively charged) protons are separated by membranes.
The separation of charges causes a charge gradient, which in turn implies an energy difference. Moving protons across this membrane either costs energy or releases energy. This energy can be used by the organism for all kinds of intents and purposes.
This is the working principle behind any battery: if you can reliably separate charges and access the energy inherent in that separation, you have a flexible energy source.
In us humans, the pumping of the protons is used to convert ADP to ATP, which powers all the metabolism in the cell, and in turn the metabolism of the whole living system. This makes the living thing alive.
Proton pumps are at the foundation of all life on earth.
These proton pumps are not easy to set up. They don’t just come out of nothingness. It is hard for amino acids flowing around in the primordial soup to just start pumping protons.
They are something like the powerhouse of the cell. If that expression rings a bell, wonderful! Your public education finally pays off.
The Mitochondrial Crash
I have become a big fan of chaos theory (the theory with the butterfly causing a hurricane somewhere) lately. There is something exciting and a bit nauseating in thinking about how small coincidences can have a huge influence on how things develop in the long run (here I wrote about how a tiny genetic mutation could have changed what happened in Russia during the first world war)
But the coincidence I’m focusing on here could be even larger.
Imagine walking around with your phone in the streets one day.
Not your normal phone, but a different one. It’s hard to even call this thing a phone. To be honest, it’s quite useless. That’s because its battery only lasts for ten seconds. You can turn it on, put in your PIN, and see it turning off again straight away. In a way, the phone works, but the tasks it can accomplish are very limited.
But now you are walking through the streets and someone runs into you. Coincidentally, that person is carrying around a battery with him. In the momentum of your collision, the battery flies out of his hands straight into your phone, where it gets stuck.
You are angry because you think your useless phone is now fully broken, but much to your surprise, when you turn it on, not only does it still work but suddenly, the battery lasts for a full day. What a coincidence, you think!
You discover that there is a camera on your phone that you can take pictures with, that you can listen to music and navigate through the city with maps. An ocean of possibilities opens up in front of you. Because suddenly, you have much more energy at your disposition. What an unlikely turn of events.
What does this construed metaphor I just made up teach us about the origin of life?
Bacteria, Archaea and Eucaryotes
There are only three main types of cellular life on earth that we know of: Bacteria, Archae, and Eucaryotes.
Bacteria are well known to us from everyday life: estimates say that you have as many bacterial cells in your body as your own cells. You have 100 to 200 different species of Bacteria living in your mouth alone.
Archaea are kind of like Bacteria: both of them tend to be relatively primitive life forms. Both of these life forms are distinguished from Eucaryotes in that they don’t have a cell nucleus. They are sometimes also grouped together as Prokaryotes. T
Bacteria and Archaea have been around earth for much longer than Eucaryotes, which only came to life around 1.6–2.1 billion years ago.
While Eucaryotes make up only a small fraction of all life forms on earth, the group features all the coolest ones: dogs and cats and trees and mushrooms and humans are all a part of it.
They all have proton pumps in their cells, but Eucaryotes have special proton pumps. Their cells are significantly more complex than Procaryote cells, and the organisms built up by them are also more complex by orders of magnitude.
We don’t fully understand how Eucaryotes came into existence.
But one of the most promising theories we have is that the first Eucaryotic cell came into existence when an Archaea crashed into a Bacteria, and, for some reason, that crash did not destroy both organisms (as it usually does), but laid the foundation for an extremely fruitful collaboration.
As Nick Lane and Adam Rutherford argue, it could be that this crash happened just once in the history of the earth and that there is one mother Eucaryote cell from which all other Eucaryotic cells descended.
The Mitochondria we see in today’s Eucaryotes are the remnants of the initial Bacteria that collided with an Archaea.
After their collision, a new cell was formed out of the leftovers of the two. This started off a symbiosis that allowed them to have a much more efficient energy household: the Mitochondria pumps protons while the rest of the cell has more energy per cell volume to spend by several orders of magnitude, allowing to build more complex structures with it.
It’s like with the phone battery. When you only have a small amount of energy at your disposition, you can’t do much besides unlocking your phone. But once you get that new energy source, you unlock its full potential.
This surplus in energy allowed the Eucaryotes to grow and, eventually, conquer the world.
The Crash and the Fermi Paradoxon
In all fairness, I have to admit that this is not the only theory we have for the genesis of Eucaryotes. There are also gradual models (f.e. by David and Buzz Baum), or autogenous models that say that Eucaryotic cells developed the cell nucleus before acquiring Mitochondria, and no collision was necessary.
But as Nick Lane describes in The Vital Question, there are a couple of good reasons for believing the crash hypothesis, and some of the implications are very interesting to think about.
One is that it could shed some light on a big question relating to the origin of life: the Fermi Paradoxon, also known as the question “Why haven’t we had contact with any aliens yet?”
If really this crash was necessary for starting off a new evolutionary phase in the history of life on earth, who knows what the odds were for it to succeed?
It could be that bacteria-like life evolves in the universe frequently, but energy constraints usually keep the evolution of more complex life forms limited, as they could only be overcome by the dramatic merging of two species.
This, in turn, would make the genesis of life forms with our level complexity in other places of the universe more unlikely to a degree that could help solve Fermi’s paradox.
While this is speculative, acknowledging that there is something special to Eucaryotes and their evolution certainly adds a new parameter to be tuned when estimating the probability of complex life forms in the universe.
…and can help us feel even more overwhelmed by the unlikeliness of our existence.