What bottlenecks would you consider for the drake equation that prevent the development of intelligent, technologically advanced life

Great Debates
1

As a spinoff of this thread, what would you consider to be bottlenecks that prevent intelligent, technologically advanced life forms from evolving?

There are roughly 10^24 planets in the universe. But if you keep adding bottlenecks (only 10% pass this bottleneck, only 1% pass this bottleneck, only 15% pass this bottleneck) then after you have a couple dozen bottlenecks, you end up with only one planet in the known universe that can support intelligent life.

So my understanding of various bottlenecks include the following:

A solar system has to be in the right part of a galaxy. Too close to the center and it gets sterilized by radiation. Too far on the edges and there aren’t enough metals.

The star has to be stable enough for life to evolve

Life has to spontaneously arise

The planet has to have easily accessible phosphorus. Supposedly this is a major bottleneck, since most planets have phosphorus locked up deep in the planet and life can’t get to it.

There needs to be a very narrow range of water on the planets surface. Too much water and you end up with a water world where the most intelligent life is fish (who can’t make tools) since there is no land. Too little and life doesn’t evolve. Whales can be intelligent, but because they don’t live on land, they can’t build technology with their intelligence.

The planet has to have abundant metals which can be used to make tools

Fossil fuels have to be easily accessible. This one isn’t guaranteed as a bottleneck, but a civilization that doesn’t have access to endless energy in the form of oil, natural gas, coal, etc that was made by previous life forms may not be able to build their way to advanced technology. You need abundant energy for society to advance enough that you can develop PV solar, hydroelectric, nuclear, wind, etc. I have no idea if a society could transition from an agrarian society to a developed technology w/o easily accessed fossil fuels.

The planet has to be geologically active. Its core needs to be active so it can create pockets of minerals that can be mined, and its core helps protect the planet from dangers like solar storms. A planet w/o an active core may not have pockets of easily mined raw materials.

Life has to transition from prokaryotes to eukaryotes, which may be far harder than people realize.

The atmosphere has to have a negative feedback mechanism, not a positive feedback mechanism. Earth’s atmosphere has a negative feedback mechanism. Supposedly venus had apositive feedback mechanism, which meant the atmosphere eventually blew off into space. Mars was so small that its atmosphere drifted off into space.

A gas giant like Jupiter has to be close enough to divert asteroids, but not so close that it causes constant bombardment

Stars have to be stable enough that they aren’t constantly emitting gamma rays (supposedly this only happened in the last 5 billion years, before that land based life probably wasn’t possible).

But with the evolution of homo sapiens, there are also multiple bottlenecks to consider. Granted, life could’ve evolved in a different direction and still led to intelligent, technologically capable life. But ours required things like the following:

Humans split from chimpanzees about 6 million years ago. For the first 3 million years our brains didn’t grow. Then starting 3 million years ago, our brains started growing and went from 450cc to maybe 1400cc, and our cortex grew by about 600%. Supposedly a major factor in this was Panama formed a land bridge between North America and South America about 3 million years ago, changing global weather patterns. This resulted in less rainfall in Africa, and turned the forests into grasslands. This climate change led to our higher brains, and w/o the land bridge in Panama we would probably still be as intelligent as chimpanzees.

Humans self domesticated. Starting roughly 1 million years ago, we became intelligent enough that we could gang up on and murder asshole alpha males (by killing them in their sleep or in hunting accidents). As a result of this, humans are one of the more egalitarian and cooperative primate species since we killed off people with dark triad traits over hundreds of thousands of years. It may not feel like it, but we engage in much less reactive violence (emotion driven violence) than other primates. We engage in a lot of proactive violence though, which is pre-meditated and organized violence. However, if humans never self domesticated we probably couldn’t build a global trade economy of 8 billion people since we would treat each other far worse.

The asteroid that killed the dinosaurs led to the emergence of mammals on land. Without that the world would probably still be run by dinosaurs.

The climate needs to be stable enough for agriculture. Humans have existed for 300k years but we only started engaging in large scale agriculture about 10k years ago. Supposedly this is because the climate was too unstable due to the ice age until about 10-20k years ago, so people couldn’t plant crops year after year. W/o a stable climate, you can’t have agriculture.

Someone had to invent the printing press. The ability to cheaply record knowledge was a big part of what allowed science and technology to progress. Going from hand written scribes to mass available books allowed knowledge to compound. Intellectual revolutions before the 15th century didn’t gain as much traction as they did after the 15th century since the knowledge could more easily be stored and spread. Chinese printing presses before this weren’t as effective since their alphabets had so many more letters.

The point is, if you end up with a few dozen bottlenecks, its easy to see how there are very few planets in the universe capable of supporting intelligent life that engages in technology.

However the universe is young. Its under 14 billion years old, and the universe will supposedly be at its peak ‘life forming’ years around 1-10 trillion years from now. This is supposedly because stars will be more stable and planets will have more heavy elements.

So the fact that an intelligent, technological species has evolved within the first 14 billion years, considering that the stellar era of the universe will last 100 trillion years, kind of implies to me that we are among the first but won’t be the last.

Have we reached the point when it is kooky to not believe in massive amounts of intelligent life having evolved throughout the universe?
2

I am no biologist but as I understand it, abiogenesis is one of the most impossible to predict or create things and you could have all the necessary conditions for abiogenesis but it still just stubbornly refuses to happen. But you already basically alluded to it.

3

A couple of nitpicks…

Actually the opposite. Venus had a runaway greenhouse effect, which boiled off its ocean, and liberated all of the carbon dioxide in the former ocean and rock, resulting in a a crushing atmosphere >90 times the sea level atmospheric pressure on Earth.

I don’t believe this is true. One trillion years from now, dark energy will have driven all of the galaxies apart beyond the visible horizon, and star formation will have ceased as all the gas needed to create them up will have dispersed or ended up in black holes. The only stars that will be left will be tiny, dim red dwarfs.

4

Another nitpick. As noted above, Venus did not lose its atmosphere, but on the contrary has a very thick atmosphere primarily composed of CO2. But also, a positive feedback is not the same thing as a self-reinforcing runaway feedback. A positive feedback can be self-limiting. In the Earth’s atmosphere, for example, water vapor is a positive feedback. But the heat radiated to space for each degree of temperature rise is approximately twice as much as the flux reduction due to water vapor, so water vapor feedback is self-limiting and stable. Not that water vapor amplification of GHG forcing isn’t a huge problem, and it interacts with other feedbacks like ice albedo reduction, but we aren’t yet demonstrably experiencing self-reinforcing runaway climate feedback, though we may at some point.

On the bigger question, I think you’re being much too overly pessimistic on the odds of extraterrestrial life. You’ve postulated a large set of “bottlenecks” – more commonly referred to as “filters” – but in reality there are probably only a few – the obstacles to the abiogenesis of life itself, the obstacles to the emergence of multicellular life, and the potential limit to the lifespan of a civilization once certain technology has been developed, particularly massively destructive technology like nuclear weapons.

The chances that we’ll annihilate ourselves in a nuclear apocalypse are likely far higher than the chances of being hit with a massive asteroid. If the same is true of extraterrestrial civilizations – who may have invented even more destructive technologies – and if they have human-like mentalities of primitive reptilian origin, that would explain a lot.

5

I don’t think there’s any rule that intelligent life has to evolve in the same manner, and under the same conditions, that it did on Earth.

That said, going with the theme, I’ve heard physicists mention the presence of a very large moon was an important variable. It creates a relatively stable climate and also tides. The tides caused sea organisms come into contact with nutrients from the land.

Additionally, our metal-based large core creating a magnetic shield from all the harmful rays that the sun bombards us with.

6

Personally, I suspect that there were two Great Filters, and that they were both the Great Oxygenation Event. On most planets with life, the Great Oxygenation Event, or something like it, never happened. On those planets, life just goes on and keeps living, but it’s slow: Without the readily-available energy from a highly-reactive atmosphere, all biological processes, including evolution, are much slower, such that life never has a chance to reach our level of development. On a small fraction of planets with life, there was a Great Oxygenation Event or something like it (maybe with fluorine or chlorine or some other reactive element instead of oxygen), but even that small fraction isn’t on our path yet, either, because in most cases where it happened, it resulted in the extinction of all life on that planet, not just almost all like it did for us. Only if both the oxygenation happens, and if life survives it, can sapience happen.

7

I didn’t read the original thread so don’t know if this was mentioned but Harvard biologist E. O. Wilson wrote a whole book about this: "The Social Conquest of the Earth."In it, he goes through all the unlikely things that have to happen for simple life to evolve into intelligent, problem-solving and tool-building creatures like us. Among the factors are the right environment, use of fire, helpful anatomy (i.e hands with opposable thumbs) and a eusocial organization. It’s a fascinating read.

8

I was basing that off of papers like this:

https://arxiv.org/pdf/1606.08448v2

Relative Likelihood for Life as a Function of Cosmic Time

We conservatively restrict our attention to the context of “life as we know it” and the standard cosmological model, ΛCDM. We find that unless habitability around low mass stars is suppressed, life is most likely to exist near ∼ 0.1M stars ten trillion years from now

9

My issue with that is once the population becomes large enough, it becomes hard to drive us extinct. Humans were driven down to only a few thousand people due to the toba explosion about 70,000 years ago affecting climate, but we survived.

Even if 99.9% of humanity died tomorrow, that still leaves 8 million people. That is enough people to restart.

Pus as a society advances they not only gain the tools to destroy themselves, they also gain the tools to survive hardships and rebuild. They gain better tools to kill but they also gain better medicine, better agriculture, interplanetary civilizations, larger populations, etc which makes driving them extinct harder.

10

Interesting. I guess that prediction comes from the fact that low-mass stars are so much more long-lived.

The paper seems to confirm that, along with their predicted greater abundance:

Life around a cold, dim red dwarf would likely be much different than life supported by a star like our Sun, though. Although I suppose this could also be mitigated if the planet in question is much closer to its host star.

11

This is the critical bit for me, that makes Drake’s Paradox not a paradox. It would be perfectly reasonable for the odds of life spontaneously arising to be, say, the same odds of any particular arrangement of cards in a deck (8 × 10⁶⁷). And that would mean the result of the equation is exactly one. No need for any outlandish scifi “filter”.

That’s not to say that 8 × 10⁶⁷ is definitely, or even likely, the odds of life arising, but until we discover other life (that definitely does not share the same origin as life on earth) it’s a perfectly plausible number, so no paradox.

12

On the contrary, I pessimistically think it is relatively easy to drive humanity extinct with something like a genetically engineered virus or rogue AI. (Not to mention a large enough asteroid.) And if this can happen, it likely will happen given enough time.

Also, even if a small fraction of the survivors are left, it would be supremely difficult to restart civilization without fossil fuels.

As for interplanetary civilizations, I think the difficulty of getting even one self-sufficient colony established within our solar system is so great that it will almost certainly never happen. Because of the distances involved, it is even more unlikely that an interstellar colony could ever be established.

At least not by humans. Maybe our AI progeny will spread across the galaxy, though.

13

The answer to the OP is of course that we don’t know, but they are all plausible enough. There are many steps between organic soup and spacefaring species, and it’s hard to say how likely most of them are, given we only have one example to go off.

All of the steps collectively could get us down to a very low chance of intelligent life, or it could be a single one that’s far more significant than the others.

But while it’s plausible enough that, say, phosphorus is a critical block, it’s plausible that another molecule could serve the function of a phospholipid or a nucleotide. So the speculation on most of these factors goes both ways. We shouldn’t start this with the assumption that we are the only kind of life that can exist.

They may be tiny and dim, comparatively, but they still have a habitable zone, and we already know that a high proportion have planets in that zone.
Now of course, there are issues with the habitality of those worlds; mainly tidal locking and huge magnetic storms in most (not all) red dwarfs’ youth. I’m just saying that ruling them out as tiny and dim seemed a bit dismissive.

“Paradox” can be a misleading term here. In a scientific context, it usually just means a surprising result.

It is somewhat surprising to see an apparently sterile sky. We know intelligent life is possible because we’re an example of it. And the upper bound for how many ETIs we should be able to see from earth, or indeed, meet, is right now, in the tens of millions, maybe more.
So we need something(s) to bring that upper bound down. Where our models of abiogenesis, evolution, intelligence and development would specifically predict a very low, but non-zero, number of technological species. That’s all it’s alluding to.

And of course we’re free to guess at any step being improbable but that doesn’t end the paradox until we know: until we have a concrete model making predictions.

14

Right, that paper that @Wesley_Clark provided is pretty convincing. Any disadvantages for life on a planet orbiting a red dwarf is likely to be more than outweighed by their prevalence in the universe and how long-lasting they are—potentially as long as ten trillion years, which is unfathomably long.

15

At the time of the Chicxulub asteroid strike, there were a minimum of, at the very least, around 1 billion non-avian dinosaurs on Earth, and possibly as many as 10 billion, which is more than the present human population. Yet they all perished, Admittedly they were not tool-makers and lacked human intelligence, but I’m not sure how much difference that would have made given the scope of the disaster.

True, but no matter how good you are at building stuff, it’s pretty hard to survive on a planet glowing with radioactivity. And “interplanetary civiliastions” is really a stretch. There are no habitable planets in our solar system, and potential habitable planets in other star systems are completely out of reach with any technology that we can realistically even imagine.

Not trying to be a Debbie Downer here, but even a tiny chance of global nuclear war multiplied by a sufficiently long time scale is practically a guarantee it will happen. In the relatively short time that we’ve had nuclear weapons, it’s already nearly happened at least three times.

16

Why single out the core with regards to ore genesis? The contributions it makes to the heat budget of tectonic processes are mostly primordial heat and the latent heat of inner core crystallization, which would happen with any planet. Mantle processes and mantle heat are what differentiate Earth from the other, tectonically inactive, rocky planets in our system and are responsible for metallogenesis.

17

Except that the red dwarfs won’t be dim any more. As the fraction of helium inside a red dwarf builds up, they get denser and brighter; a trillion years from now red dwarfs will be comparable in brightness to the Sun, and will remain in that state for a lot longer.

18

Here’s my take on just the bottlenecks we might have passed so far:

  1. How many stars there are in “The Universe” is mostly irrelevant. Barring some fantastic revolution in our understanding of physics that would make jumping to distant points in the universe possible, almost all of it will remain inaccessible to us, and vice-versa. As far as the Fermi paradox goes (why wasn’t our development long ago forestalled by interstellar colonization), very few people think that sublight travel could cross intergalactic distances. Unless they created very conspicuous technosignatures, civilizations in galaxies hundreds of millions or billions of light-years away would remain unobservable and effectively non-existent. Take only the stars in our galaxy and the number of zeros to consider drops dramatically.
  2. For various astrophysical reasons, almost all planets will be inhospitable to life. Whether it’s that entire regions of our galaxy are poor choices for life, the stars they orbit, their position in their solar system, their mass and composition, etc. etc. I can easily suppose that fewer than one solar system in a thousand has a planet that even could develop life; unknown factors could add one or two orders of magnitude to the odds against life. And for all the following steps, the possibility that life, intelligence or civilization could arise only to get strangled in the cradle by cataclysms cannot be discounted.
  3. The abiogenesis enigma. It’s an article almost of faith that organic molecules plus liquid water plus energy plus enough time creates life; we only know that it somehow happened here. But the jump from polymers of organic molecules to self-reproducing Von Newman machines made of protein is so stupendous that we cannot currently meaningfully estimate how frequently life arises even when the starting ingredients are there. We know exactly zero about what special conditions might be required to facilitate the intermediate steps because we have no idea whatsoever what those intermediate steps are.
  4. Life when it arises might remain locked in the unicellular stage for its entire existence. We know from our only example that life remained at the unicellular level for billions of years, multicellularity only taking hold within the last billion. Multicellularity might require special conditions favorable to the survival of more complex and therefor more fragile organisms.
  5. It’s unclear how strong the selection bias for high intelligence is. When multicellularity did arise, the result was mostly organisms programed by selection to robotically follow instinct. Even mammals didn’t start developing larger brains until the Cenozoic was well underway.
  6. Intelligence ≠ tool using. What we would call intelligent life is life that uses brain power to manipulate its environment in a purposeful way. If e.g. dolphins or elephants are an example, an intelligent species might arise that simply never has any use for designing and making tools.
  7. Environmental limitations on how advanced tool using could become. Resources such as fossil fuels or ores might conceivably not be available in all circumstances. A nascent civilization could remain trapped at a pre-industrial level forever.