Kind of bad news for human kind if we find out the life also started in Mars. One Great Filter down.
Put simply, I expect the universe is littered with single-celled life. I think multicellular life, on the other hand, is rare.
Which in turn depends on existing enough atmospheric oxygen for mitochondria to make sense in first place. I believe the filter is atmospheric oxygen. If Earth had more iron in the crust, perhaps the cyanobacteria would never finish oxidizing all of it, and we would be doomed to only host microscopic life forever. Macroscopic life requires high-energy metabolism molecular oxygen allows.
We have lots of iron , but most of it sank to the core during the "iron catastrophe" . And Mars is red because of theres so much iron.
If your thesis is specifically that oxygen-powered metabolism fueling multicellularity is rare... that wasn't clear. And I'll still say that endosymbiosis being the shortest path to scaling oxygen metabolism is probably Earth happenstance (with a reminder that cellular endosymbiosis is also common). But some critter had to originally evolve oxygen metabolism before being turned into mitochondria. What if that had evolved in a cell prepared to directly take advantage of it?
But no multicellular prokaryotes. You're right that eukaryotes can be single celled. But my hypothesis is that multicellular life is rare because eukaryotes are rare.
The much more reasonable explanation is that life emerged on one planet and transferred over. Earth and Mars aren't particularly close, but they're close enough for material to transfer between them, particularly early in the solar system when there were far more asteroid impacts kicking rocks and dust out into space.
Two planets in the "habitable zone" both receiving "ingredients" from the same stream of comets/meteors/cosmic dust, both with what we believe to be environments conducive to life (liquid water, surface temperatures/atmospheric pressures that aren't like Venus..:)
In fact, I'm more surprised that life didn't start independently on both planets at roughly the same time (it could turn out that life did indeed start at the same time on both planets too, just it didn't "take hold" or "last as long" on Mars)
I do think the "galactic seeding" theory is more likely.
Beyond panspermia, I lean toward another perspective:
Life exists because it serves as a natural means of accelerating "entropy" production. In an otherwise relatively stable system, life provides a shortcut, catalyzing processes that dissipate energy gradients far more efficiently than non-living chemistry alone.
At the microbial level, metabolism drives the dissipation of redox gradients, pushing chemical systems toward lower free-energy states. While cells locally maintain order and complexity, their activity increases the overall entropy of their environment. In this sense, life is not a violation of thermodynamic laws but a direct expression of them.
If this is true, life may be extremely common in the universe, arising naturally wherever the right conditions exist to favor energy dissipation.
What may be rare, however, is complex life. Complexity requires not only a lucky balance of stability and change, but also the ability to endure, or even be forced forward, by catastrophic events such as mass extinctions, without wiping out all of it. Earth's active techntonic plates provided yet another means to enable evolution.
It should be observable. Planets with life as entropy generators should absorb light from their star and reemit it as thermal radiation. More so than a lifeless planet of the same type. It is the case for Earth.
I have seen this idea somewhere, though I don't know how serious it is.
Of course, to notice something like this from such vast distances, the energy transformations would have to be happening on a very large scale. So, it's possible life is out there, but just not at a scale on those planets that our current instruments could pick up.
By any chances, do you have any good reading references for this: "I have seen this idea somewhere" .. I am genuinely curious to read more on this.
It's like a bioreactor for creating life. One could argue, it's highly unlikely given all those experiments and all that time, that life would avoid being created.
And Mars? It has a moon too, two of them. Tides as well, lunar and solar. Pretty much the same deal.
And of course nothing is ruling out life in the nooks and crannies of Mars.
The big problem is that the solar wind strips away the atmosphere and water, but that's (probably) not what killed all Martian life. As the magnetic field decreases, more and more harmful radiation reaches the surface. The planet was probably sterilized by radiation long before the atmosphere was lost and the oceans evaporated.
We're pretty sure this is what happened. We've been studying Mars's geology for a long time and we can see evidence for most of this process.
If this isn’t true, and life is actually common throughout the galaxy ... then the great filter might still be ahead of us — such as not surviving technological adolescence. Meaning we’re not special, we just haven’t died off yet.
Great filters start with the observation that we have detected no signs of alien technological civilization. The assumption is this means they’re rare.
In other words, even if the average technological civilisation lasts 1000 years, the odds of those civilisations overlapping are nearly zero if the great filter is ahead of us. Unless civilisations manage to last much, much longer than 1,000 years (millions of years), the chance that two blips overlap in time and space closely enough to detect each other is basically negligible.
That is why the “life on Mars” point feels ominous:
- If abiogenesis is easy, the filter isn’t there.
- If the filter is later (like surviving technological adolescence), then most civilisations blink out quickly.
Which means overlapping, detectable civilisations would be vanishingly rare, explaining the silence, but also suggesting our future may be short.
We aren’t special. We will die off.
Personally I think the great filter is a dumb idea for precisely the reason you posit. The universe is (probably) infinite, which means there's an infinite probability that we aren't special or alone. Maybe we're the first; the universe is (relatively) pretty young from what we can tell. I doubt that too, but I think it's one of the most plausible explanations.
But really what it comes down to is that in an infinite universe, the probability of anything happening exaxtlt once is infinitely small. It is infinitely more probable that there is or will be other life out there.
Really, out of uncountable trillions of planets in trillions of galaxies across tens of billions of years, how could it be that exactly one planet can produce life? I think it's egotistical navel-gazing in the extreme to assume we're alone.
The more common that life is, the more likely it is in front of us.
In fact, this makes the preoccupation with humans escaping a Great Filter all the more childish. Even on a single planet the species that will evolve from humans by the time Earth is swallowed by the sun will have less in common with humans than we do now with single cell organisms. Internalize that fact a little bit. Once you realize it is absurd to talk about the human species being preserved as is to the end of time, you will understand the silliness of this obsession. Cause after that point you might as well believe in a deity.
If it makes you feel warm and fuzzy inside, it suffices to hope that only single cell organisms survive the Great Filter, since given enough time it might lead to something that is as intelligent or more intelligent and kind than humans.
Embrace the silliness. The answer to "Why should we humans spread to other planets?" need only be "Why the fuck not?"
That is, unless you want to fund your rocket company. In which case you have to make people believe in a deity.
We can't know or even begin to guess at what an alien civilization may do or think or how they evolved. Best we can do is assume it's probably somewhat similar to our experience. At least it's based on something factual. Anything else is really just wild speculation.
We pretty much have to assume aliens will be sort of similar to us because we haven't met any. Our experience is the only one we've got, so it's the most reasonable baseline we have. We know that aliens will probably be wildly different from us, but it's so unknowable as to be moot. Do we base our assumptions on Heinlein's writing? Asimov? Douglas Adams? Anything other than what we know from our own experience is just fanciful fiction.
But also you're not supposed to take as read the Fermi paradox, Kardashev scale, or any other ways of thinking about aliens. It's implied that they won't be anything like us. You're not supposed to take it as a literal statement that alien species will be hairless bipeds with a warlike society who think and look like us. You're supposed to follow the assumptions that statistically, we're probably not special as a species and probably any aliens we meet will have evolved along similar lines and probably will be relatable to us. Implicitly we understand that this likely is not true. We just don't know and there aren't really any options that are more reasonable or reliable than basing assumptions on the one and only planet we know that has intelligent life.
As in, different stars may encourage slightly different chemical reactions and interactions on their planets.
Like, for example, could it be possible for a planet to form with a naturally highly magnetized mineral in its crust? Would it affect the rest of the chemistry on the planet? Would it cause the ground to interact with its star's magnetic envelope, like our atmosphere within the Auroras does?
Early on I would expect a whole lot of "horizontal gene transfer" sort of things to have taken place. So for example in addition to actual horizontal gene transfer, there are mechanisms like one organism enveloping another to eventually become organelles, co-opting products from each other, etc. All of which would act to homogenize life and make certain process ubiquitous.
Finally, there's an outside chance that "there's only one way to do it".
Diversity could exist in harmony and the lack of any diversity is a pretty strong signal that the only extant version is either very rare or the only to ever emerge.
Everything in nature is diverse except RNA/DNA and this fact alone is a sort of evidence.
Or RNA was just a winning virus that infected all other life or killed all competition to make it seem like there was only one origin.
Absence of evidence is not evidence of absence.
When you should see evidence but don't, "absence of evidence is not evidence of absence" doesn't apply. Otherwise absence becomes unprovable à la Russell's teapot.
By comparison, we’ve looked nowhere (Earth is big and mostly inaccessible to humans) and don’t know what we should be looking for, since we don’t have any testable models for the origin of life yet.
And re RNA I was just giving one example of why all life might have it (ie RNA gave an evolutionary advantage so anything without RNA died). There’s all sorts of reasons why we don’t have evidence of multiple origins, not least of which is that we just haven’t looked in the right place and there could easily be life being created from scratch on this planet without even knowing about it. That’s what the entire field of synthetic biology is even about; just doing it in a lab instead of out in nature so that we can understand the conditions better.
This is about a quarter of the lifetime of the universe ago, and we don’t have any evidence at all that life has ever occurred in any other way. We’ve only really been looking for a hundred years or so, but we’ve not found any “fountain of life” where life is being created, we’ve not found evidence of any type of life that isn’t broadly related.
I absolutely agree that it’s not evidence, but I believe that on balance, it makes more sense to take our working hypothesis to be something that fits the evidence we do have, rather than believing the evidence must exist we just don’t have it.
To be clear, I’m not advocating that we don’t investigate both possibilities, and I wouldn’t put much weight behind my own guess here.
And you’re right, assuming those fossils even managed to survive in the first place and not get destroyed. And that’s ignoring that early life would have been microscopic and we wouldn’t really see fossils of that.
But even if it did, one clear argument could be that the conditions on Earth when it happened would have been very very different than the earth today. Lots more volcanic activity, lightning storms, and UV radiation. We don’t know the exact conditions needed to create life from inorganic precursors so we don’t have a solid hypothesis of was there multiple origins or a single or is it still happening today.
But given we have the beginnings of evidence of life on Mars, Titan, and Triton, and that it would make sense since we know life must arise naturally out of non-life origins (since there was no life at the Big Bang), I would venture to guess that life is both rare and common. Intelligent life also seems common although less common than life overall (other primates, dolphins, Elephants, ravens/crows, and cephalopods are all quite intelligent). Intelligence + tool use is also not uncommon but rarer (we’ve caught animals on tape using tools). Advanced industrial civilization is the only thing so far that we only have a single existence evidence for - is that unique, can there ever only be one on a planet at a time, or can there only be one ever when conditions are right? Eg we wouldn’t have gotten very far if we were on the planet earlier and didn’t have dinosaur bones to power our industry with - the jump from whale oil to nuclear/photovoltaics/wind turbines seems unlikely but maybe it would happen anyway, just longer.
Life increases entropy and doesn't break 2nd law of thermodynamics.