On mitochondria and the search for extraterrestrial life
I wrote a bit about exercise physiology, and two different ways that your cells can generate energy. Why do we have these two separate energy systems?
Radiolab describes a big mystery about the evolution of life. Simple cells (prokaryotes, like bacteria) evolved early in the Earth’s history but stayed in that state for over a billion years before evolving into larger, more complicated cells (eukaryotes). Something kept life from progressing beyond simple cells for a billion years.
This evolution happened through symbiosis. A simple prokaryotic cell looks like this:
Eukaryotic cells are much larger, with a partitioned structure, a nucleus, and other organelles.
Mitochondria are unique among these organelles in that they closely resemble bacteria — they have their own membrane, their own DNA. Mitochondria divide asexually, within the cells, the same way that bacteria reproduce.
Mitochondria create power for cells, they oxidize nutrients to create energy. This is a symbiotic relationship, as mitochondria produce energy for complex cells, while they get a stable place to live and thrive.
Radiolab posits that this symbiosis must have started by pure chance, that it took a billion years of dumb luck for the right two cells to combine. One cell (probably an archaeon) enveloped a bacterium, the two somehow managed to work with each other, maybe the larger cell passed food to the smaller one and received energy in return. The combined unit was more successful. All complex life on Earth descends from this one random conjunction of two cells. Complex cells in turn lead to other changes and more complexity — sexual reproduction, multicellular life, complex body plans, intelligence.
Radiolab thinks that this merging of two cells was so rare and unlikely that it means that there’s no intelligent life anywhere else in the universe. If life formed on other planets, it probably never evolved past bacteria.
II.
Let’s zoom out to see the big picture. Why does this question matter?
The universe is a staggeringly large place.
There are an estimated 100 billion stars in our galaxy.
The hubble telescope has found maybe 100 billion galaxies in the visible universe.
It’s hard to even visualize the structure of it, because the space between planets and stars is so vast. If you put the Earth and moon in one scale image together, you can’t make out much detail on each:
You can’t fit the rest of the solar system on the same page. If you make the moon only one pixel large, then viewing the whole solar system becomes an image so large that it takes 20 minutes to scroll through.
One way to visualize the scale differences is with an animation which zooms out exponentially to show the increasing scale of planets, stars, and galaxies:
Another way to try to make sense of it is to visualize it all in one image that is compressed logarithmically, as you go out from the center:
We have only discovered a few thousand other planets, but current searches suggest that maybe 1 out of 5 sun-like stars has an Earth sized planet orbiting in a habitable zone. This suggests there could be maybe 10 billion habitable planets in our galaxy.
If life is common, and intelligent life is the end result of evolution, then there should be other planets with intelligent life on them. A sufficiently advanced civilization should even be able to colonize other planets. Space travel is difficult and energy intensive, but a ship only needs a small, self-replicating payload for success — a small group of humans, or self-replicating machines, should be able to eventually populate an entire world with local resources. Spreading out to fill an entire galaxy might even be possible, given a few million years time. If some other planet in our galaxy had created a super-intelligent civilization a hundred million years before ours, it would already have had plenty of time to colonize the rest of the galaxy.
Our radio searches, via SETI, still haven’t found any other intelligent life, though.
The big question is whether intelligent civilization on Earth is particularly rare and unique. Is there some single reason that prevents it from happening more often? This lack of extraterrestrial life gets described as the Fermi Paradox, and the set of steps that may prevent it from forming are sometimes called the Great Filter.
For life to progress to a galactic civilization, you need a long series of steps:
• Basic elements form into self-replicating organic molecules.
• Simple cells form.
• Complex cells form.
• Multicellular organisms arise.
• Complex body plans evolve.
• Life becomes intelligent.
• Intelligence becomes complex.
• Tool use and language arise.
• Civilization forms.
• Advanced technology is developed.
• Spacefaring civilization spreads from planet to planet.
Is one of those steps far more unlikely than the rest? Are any of these steps inevitable? Are any of these steps nearly impossible? Or is there a moderate probability to each one, such that the combined product is very small? We’re looking for odds of 1 in 10 billion. If only 5 of these steps have a 1% probability, then it makes sense that Earth is the only planet with intelligent life in this galaxy. If this process has 10 hard steps, with a 1% probability of each, then Earth could be the only planet with intelligent life in the entire known universe. You could be the only one in the universe thinking about this question, at this moment.
Some steps in this sequence can be shown to be repeatable, and thus very likely to happen. Simple organic molecules should always be created, because this has been reproduced in a lab — simple gasses mixed with electrical sparks create amino acids. It’s still a very large stretch, though, to see how these form into DNA and enzymes and protein synthesis.
Other steps can be ruled out because they have happened multiple times in Earth’s history. For instance, life has transitioned from single celled to multi-cellular dozens of times in the past.
Convergent evolution has also produced moderate intelligence multiple times, so this is unlikely to be the bottleneck. Octopuses are intelligent creatures, capable of solving puzzles. Our last common ancestor with the octopus existed maybe 500 million years ago, it was likely an aquatic worm with a very simple nervous system. Any common intelligence evolved completely separately. Even our eyes may have evolved separately. Crows are intelligent, tool-using birds. Our last common ancestor was a reptile, over 100 million years ago, with only a small lizard brain. A variety of mammals such as elephants, dolphins, and primates have all developed a reasonable level of intelligence. Only humans have reached the point of building radio telescopes, though, so it’s possible that it’s very rare for life to find the right combination of intelligent, land-dwelling creatures with language ability and dextrous hands and enough creativity and drive to create advanced technology.
For other steps, it’s not clear how large the difficulty is. Maybe life rarely evolves at all. Maybe planets are often sterilized or destroyed by significant external events, like gamma ray bursts or asteroids or catastrophic climate changes. Maybe life rarely reaches a human or greater level of intelligence. Maybe technology has limits that we don’t understand, and spacefaring is impossible. Maybe intelligent civilizations don’t last very long without destroying themselves.
Radiolab picks out the transition to eukaryotes as the critical step, because it seems to have only happened once, by pure chance, and it took a billion years for it to happen at all.
III.
I think there’s a better explanation for the billion year time lag. First, let’s ask: why were proto-mitochondria successful? They used oxygen to process food more efficiently. However, there was no oxygen in the Earth’s early atmosphere.
Life first had to evolve photosynthesis, which might also have taken millions of years. Photosynthesis makes oxygen as a byproduct. Any oxygen that was initially produced left the atmosphere through weathering with other materials, particularly Iron. After hundreds of millions of years of weathering, these sites became saturated, and oxygen was able to build up in the atmosphere.
At that point, other cells would rapidly need to adapt, to survive in this new oxygen rich environment, or to find productive ways to use the gas. Oxidizing bacteria would then be at a huge advantage, and other cells would benefit from borrowing the ability to oxidize.
Mitochondria create energy through a number of complicated steps, via the krebs cycle, taking food as input and ultimately producing water, carbon dioxide, and energy as ATP.
Ignoring all the intermediate steps, the full result of this oxidation is the same as burning the same input:
Mitochondria gave cells the gift of fire, and this surplus of energy allowed them to become much larger and more complicated.
The same surplus of energy can explain other big transitions in history. Traditionally, it was thought that man became intelligent, tamed fire, and then ate better. One theory holds that this happened in reverse, though, that early man tamed fire, cooked food, ate better, and was able to evolve a larger brain as a result.
Likewise, industrial civilization has been able to rapidly grow over the last few hundred years because of a vast surplus of energy from burning fossil fuels.
After oxygen lead to eukaryotic life, it may have also lead to oxidative stress and DNA damage in cells, which in turn might have given an advantage to sexual reproduction, which offers an opportunity to combine and repair DNA but also allowed evolution to speed up, by creating more variants for selection to choose from.
Radiolab makes it sound incredibly unlikely for such a symbiotic relationship to occur. However, both symbiosis and ingestion happen somewhat often. Symbiosis occurs between cells of some photosynthetic bacteria.
Scientists have found modern archaea cells which have many eukaryotic features, but no nucleus or mitochondria. These are a likely candidate for the type of cell which could ingest bacteria to form an early eukaryote.
Eukaryotes are still able to envelop other cells and form symbiotic relationships. Researcher Jeong found that amoebas that he studied sometimes got infected by bacteria. The amoebas that survived the infection actually became dependent on the bacteria — when they were treated with antibiotics, killing the bacteria also killed the host cells.
Infectious bacteria are also able to cross cell walls and hide inside human cells to evade the immune system.
Endosymbiosis isn’t uncommon for modern life, so it doesn’t seem incredibly rare for primitive life. It’s also clear that it must have happened multiple times in history. Plant cells also enveloped photosynthetic bacteria and turned them into chloroplasts. Some cells seem to have even swallowed other complex cells and discarded everything but the useful chloroplasts, the remaining evidence being that the organelle has extra walls around it.
IV.
If Eukaryotic life isn’t the key step, what are the likely candidates for the great filter? Life forming at all might be unlikely. Human level intelligence and civilization might be unlikely. Or the great filter might still be in the future.
Have we already beaten the odds, just by getting here, by evolving into complex, intelligent lifeforms on a stable planet? Having reached this point, is it inevitable we’ll become a much more advanced and successful civilization?
I worry that the great filter is still ahead, that our complex technological civilization won’t last for millions of years, or even for hundreds.
I don’t pretend to know how the world will end. We could get hit by an asteroid or wiped out by a virus. We could invent robots that kill us all or change the climate until the biosphere collapses. The closest we’ve come to destruction, in recent history, is with nuclear weapons.
The largest bomb the soviet union ever produced could destroy every building in Los Angeles and deliver 3rd degree burns to people across the entire extended LA area.
A full scale nuclear war between the US and Russia would kill hundreds of millions of people, but would not be the immediate end of humanity. Radioactive fallout has a shorter half-life than is commonly expected — every 7 days, 90% of the radioactivity dissipates. After 2 weeks sheltered in a bunker, only 1% of the radioactivity would remain. Anyone who could take shelter for several weeks could certainly survive. Global wind patterns would also have left some countries, like New Zealand, largely unaffected by any fallout. Nuclear winter would likely kill even more people through starvation, as agriculture collapsed (although, one scientist thinks we could feed billions with fungus alone, if we prepared for this). Enough people would survive to perpetuate the species, but progress with advanced technology would be stopped for generations.
We have avoided nuclear war in the seven decades since the bomb was invented, with a few very close calls. This is only one human lifespan, though. What are the odds that we can continue to survive for hundreds or even millions of years without an accidental or deliberate war?
The concept of mutually assured destruction suggests that neither Russia nor the United States would be willing to launch an attack, because they logically understand that their own country will be devastated by the response. This logic has kept us safe from devastation so far, but it can break down in several cases. First, it is not the country that makes the decision to launch, but a single leader or submarine commander. Any leader starting a conventional war also knows they will face combat losses. Leaders starting the first world war knew they were facing millions of casualties, but they were undeterred, in part because the leaders knew they were unlikely to personally face risk. (i.e. millions of Germans died in WWI, but Kaiser Wilhelm survived unharmed, as did leading generals like Hindenburg and Ludendorff). In theory, I could imagine a world leader with a good bunker being willing to start a nuclear war because their personal survival was insured.
Second, game theory suggests that it’s safest to attack a potentially dangerous opponent before they can retaliate. John Von Neumann, Manhattan project scientist and inventor of game theory, tried to persuade the US government to nuke Russia at the beginning of the cold war, before they could invent nuclear weapons of their own.
“With the Russians it is not a question of whether but of when,” he would say. An oft-quoted remark of his is, “If you say why not bomb them tomorrow, I say why not today? If you say today at 5 o’clock, I say why not one o’clock?”
The same logic might apply to emerging nuclear powers. Would Israel launch a first strike on Iran, before the latter could develop a bomb? If North Korea was on the verge of developing an ICBM that could hit the US mainland, would it be best to launch a first strike to destroy North Korea? As other countries get wealthier and proliferation continues, we’ll revisit this decision again and again.
This same logic would also apply to any kind of extraterrestrial contact. Suppose that a more advanced civilization intercepts radio transmissions from Earth, but has no idea how advanced or hostile humanity is. The logical response would be to destroy life on Earth before Earth becomes a threat. One resolution to the Fermi Paradox is that other planets are intentionally, cautiously silent. There could also be a single, predatory civilization that’s already colonized much of the galaxy. We’ve been transmitting radio signals into space for 100 years now. A hostile response could already be on route.
V.
Will we ever know the answer to these questions?
It’s not fair to say, yet, that the universe is empty, as we’re still developing the means to search. SETI focuses on the search for radio waves from space, but to date we would only be able to detect another Earth a few light years away. Future experiments might extend this range to 100 light years or more.
Rather than SETI’s search for radio transmissions, I think that a different search might give us some answers first.
An extrasolar planet may only be a single pixel in an advanced space telescope, but we can measure the spectrum of that pixel, or look at the spectrum change as a planet transits in front of a star, and deduce the atmosphere of that planet. Oxygen in that atmosphere should indicate the presence of life. The first person to discover extraterrestrial life may be an astronomer looking at that curve, and finding signs of photosynthesis on another world.
What we find might give us some clues about our future. If we don’t find any other planets with oxygen, then it’s possible that the rare event is life beginning at all. In that case, we may have already passed the bottleneck and have a good chance at a sustained future. If we find many planets with oxygen but none emitting radio waves, then that implies that life is common, but that highly intelligent life is rare or that civilizations don’t last very long, and this gives us less hope for our own future.
