Revisiting the Drake Equation

[lpetrich]

The  Drake equation is astronomer Frank Drake's attempt to estimate how many communicative civilizations there are in our Galaxy.
Drake Equation | SETI Institute
NOVA | The Drake Equation
Carl Sagan on Drake Equation - YouTube

There have been lots of arguments about this equation and various modifications and extensions of it.

N = R_* * f_p * n_e * f_l * f_i * f_c * L

• N = number
• R_* = rate of star formation in our Galaxy: stars/year
• f_p = fraction of stars with planetary systems
• n_e = number of planets in a planetary system suitable for life
• f_l = fraction of habitable planets where life emerges
• f_i = fraction of planets with life were intelligence emerges
• f_c = fraction of planets with intelligence which have interstellar communicative ability
• L = lifetime as an interstellar communicator

When Frank Drake proposed this equation in 1960, only R_* was well-understood. But I think that we understand the other parameters a little bit better.

With the detection of numerous extrasolar planets, we are starting to get a handle of f_p and n_e. However, we have discovered some complications, like "hot Jupiters", giant planets much closer to their stars than what one would expect from the Solar System and their composition. Also, exoplanet detection has come very close to detecting Earthlike planets around Sunlike stars.

Exploration of the Solar System has also given some hints as to which planets could be habitable. Venus has a runaway greenhouse effect and Mars has evidence of a former ocean of liquid water. Europa, Ganymede, Callisto, Enceladus, Dione, and Titan likely have subsurface oceans, though those oceans may not have a kind of disequilibrium suitable for driving organisms' metabolism and growth.

With the variations in planetary systems that we have discovered, we may have to integrate over different star types -- red dwarfs seem to have different sorts of planetary systems than Sunlike stars.

There are still plenty of uncertainties, like how much water a habitable-zone Earthlike planet might have. Will it have much less than what the Earth has? It will then be a desert world with a very thin atmosphere. Will it have much more than what the Earth has? It will then have a superdeep ocean that covers all of its surface.

So f_p is likely close to 1 while n_e is still uncertain, though likely not much less than 1. Maybe 0.1 or 0.01 or 0.001.

 

[PyramidHead]

So f_p is likely close to 1 while n_e is still uncertain, though likely not much less than 1. Maybe 0.1 or 0.01 or 0.001.

Forgive me, but aren't those numbers several orders of magnitude less than 1? How much less than 1 can you get while still being "not much less than 1"?

 

[lpetrich]

Turning to the origin of life, it must be conceded that that is still an unsolved problem.

The earliest evolution that we can reconstruct with any confidence is the RNA world. In it, RNA serves as both information storage and enzyme. It likely had some biosynthesis capabilities, including electron-transfer biosynthesis, and it must have had some sort of energy metabolism to power it. Coenzymes like ATP and several B vitamins date back to it.

The main criticism I've seen of it is the origin of the RNA. Nucleobases can be produced prebiotically, but it's VERY hard to make ribose prebiotically. One can use the Butlerov formose reaction, but it requires concentrated formaldehyde, and it produces a big mess of sugarlike molecules. Ribose has 4 asymmetric carbon atoms, so the reaction produces ribose as one of 16 asymmetry variants.

For that reason, some people have proposed that the ribose of RNA had a predecessor, like peptides (protein chains) or polycyclic aromatic hydrocarbons.

So f_l is still very uncertain.

-

The road to intelligence from there is a long and winding one. Here are the earlier parts of it.

One can extrapolate backward from present-day organisms to find what their Last Universal Common Ancestor (LUCA) was like. It had a lot of evolution from the RNA world to it:

• Proteins developed as coenzymes that became the primary enzymes
• DNA developed from RNA for being master copies of genetic information
• A cell membrane with chemiosmotic energy metabolism at it
• Respiration-like electron-transfer energy metabolism. It is coupled to chemiosmotic energy metabolism
• Chemoautotrophism likely - complete biosynthesis from inorganic starting materials

However, the first oxidizers were nitrogen oxides and the like, not oxygen.

Chemiosmotic energy metabolism: pumping hydrogen ions out of the cell interior and letting them return through ATP-synthase enzyme complexes. Those complexes take adenosine monophosphate (AMP), a RNA nucleotide, and add phosphates to its phosphate, making adenosine diphosphate (ADP), and then adenosine triphosphate (ATP), with a chain of three phosphates. The phosphate-phosphate bonds are then tapped for energy by a variety of processes.

Methanogens are present-day organisms that are likely very LUCA-like.


Photosynthesis is a later invention, and it was invented twice:

• Bacteriorhodopsin - addition to chemiosmotic energy metabolism
• Chlorophyll - addition to electron-transfer energy metabolism

The latter sort is the most familiar sort, and the most familiar sort of that uses water as an electron source, releasing oxygen. The electrons get energized and then get used for electron-transfer biosynthesis. However, chlorophyll photosynthesis can also be run as a closed cycle for energy production, like bacteriorhodopsin photosynthesis.

Oxygen-releasing photosynthesis liberated primary producers from environmental chemical disequilibria, enabling them to proliferate and spread widely. Environmental chemical disequilibria meaning reducers (hydrogen, ferrous iron, and the like) and oxidizers (nitrogen oxides and the like) in one place.

This enabled those producers to support large populations of eaters of them, and those eaters to in turn support eaters of them, and so forth - heterotrophic organisms as opposed to autotrophic ones.

But one has to ask how easily this photoautotrophic metabolism can evolve. The Earth version requires electron-transfer energy metabolism, chemiosmotic energy metabolism, chlorophyll antenna complexes, and water splitting, and each of those evolved only once.

 

[lpetrich]

To make large, complex multicellular organisms, it's necessary to manage large genomes, and prokaryote genomes are not big enough. The step that enabled such genome management was the evolution of eukaryotes.

Their origin continues to be a mystery, because of a lack of prokaryote-eukaryote intermediates -- organisms that have only some but not all of the shared ones, like a cell nucleus, chromosomes, the meiosis-cell-fusion cycle, and mitochondria. Even worse, eukaryotes are a hybrid with several likely contributors.

Turning to multicellularity, it has evolved numerous times, and I once saw an estimate of at least 20 times. Animallike multicellularity has evolved only once, however. All the other origins are either plantlike or funguslike, with the slime-mold habit evolving several times. That one is where animallike one-celled organisms get together and make a funguslike fruiting body that makes spores.

-

Being able to develop radio telescopes means being able to live on land, and let's see how often multicellular organisms have moved to land. Curiously, plantlike organisms have done so only once - there's no land nori or land kelp. However, fungi and oomycetes (funguslike relatives of kelp) have separately done so. Animals have done so several times:

• Arthropods: insects, pillbugs, land crabs, arachnids, myriapods (centipedes, millipedes)
• Mollusks: land snails
• Annelids: earthworms, leeches
• Vertebrates: tetrapods, starting with real-life "Darwin Fish"

Something helpful for being large on land is an internal skeleton, and that's evolved once in animals (vertebrate skeletons), and at least once in plants (wood). Curiously, most animal skeletons are either external (shells), skin-surface (arthropod skins), or just under the skin (echinoderm skeletons). So it may be hard for an animal internal skeleton to evolve.

Grasping limbs and jaws have evolved multiple times, however.

Among land vertebrates, grasping with digits has evolved at least twice, in primates and in perching birds (Passeriformes). Arthropods have made pincer limbs at least twice (scorpions, various crustaceans). Tentacles have evolved at least twice, in cnidarians and in cephalopods.

Turning to jaws, vertebrate ones are modified front gill bars, while arthropod ones are modified limbs. Some polychaete worms also have jaws.

Among sense organisms, vertebrates and cephalopods have independently evolved high-resolution lens-camera eyes, and vertebrates and arthropods have independently evolved color vision.

-

In summary, we have a mixed bag here. Some features suitable for sentient organisms evolved several times, like grasping ability, and some others evolved only once with unsuitable variations evolving several times, like animal-like multicellularity.

 

[lpetrich]

I now come to intelligence proper.

It may not often seem very apparent in our species, but I think that that is because we have rather high standards for ourselves by the standards of most of the animal kingdom. Thus, when we see some shoddy reasoning, we sometimes think that its author must be terribly dumb, but that shoddy reasoning is described using linguistic capabilities that are far in advance of what just about every other species on this planet can do. Like a creationist declaring "Evolution can't be true because dogs don't give birth to cats."

I ran that sentence through the Link Grammar Parse a sentence page, and I got

Found 3 linkages (3 with no P.P. violations)
  Linkage 1, cost vector = (UNUSED=0 DIS=0 AND=0 LEN=14)

    +------------------------------------------Xp-----------------------
    |                                                     +---I*d---+---
    +----Wd----+----Ss---+--Ix-+--Pa-+--MVs-+---Cs--+--Sp-+-N-+     +--O
    |          |         |     |     |      |       |     |   |     |   
LEFT-WALL evolution.n cannot be.v true.a because dogs.n do.v not give.v 


------------------+
--MVp----+        |
s--+     +-Jp-+   |
   |     |    |   |
birth.n to cats.n . 

Constituent tree:

(S (NP Evolution)
   (VP cannot
       (VP be
           (ADJP true)))
   (SBAR because
         (S (NP dogs)
            (VP do not
                (VP give
                    (NP birth)
                    (PP to
                        (NP cats))))))
   .)

This syntactical complexity is far ahead of what (nonhuman) great apes have been able to do. They can learn lots of sign-language signs, but the most syntax that they have used is two-sign phrases, and even that is doubtful ( Great ape language). Most animal language is even simpler, the equivalent of single words or canned phrases in our languages.

So that leaves dolphins and other toothed cetaceans as our only plausible competition on this planet. Dolphin language has been difficult to decode, but some parts of it are now understood, like imitations of echoes of objects. In human terms, that's like calling a dog a woof-woof or a cat a meow-meow.

I went into this digression into language because that's necessary for describing how to build a radiotelescope for interstellar communication -- and for describing why it might be worth building.

 

[lpetrich]

Another criterion is self-awareness, like being able to recognize oneself in a mirror. This would indicate an ability to make a mental model of oneself. Human children become able to do that at about 18 - 24 months old, and a few other species seem to have this ability ( Mirror test): (other) great apes, bottlenose dolphins and orcas, elephants, and European magpies.

Most others don't, and to a dog or a cat, the dog or cat in the mirror is another one.

The next question is what would drive the evolution of intelligence, since big brains can cost a lot of energy to keep going.

Visual perception or echolocation interpretation can require a lot of brainpower to discover a lot of details, and the larger brains are indeed of users of these abilities ( Cetacean intelligence, etc.).

Another hypothesis is Robin Dunbar's social-brain hypothesis. According to it, large brains evolved for keeping track of other members of one's species, and among various simian species, there is indeed a correlation between brain size and social-group size. Extrapolating to our species, he finds  Dunbar's number, about 150.

Large social groups and full-scale language can combine for transmitting complex information down the generations.

Might that also be true of cetaceans? Or might it be a side effect of large brains elaborated for interpreting echolocation? In any case, it's evident from a lot of research that bottlenose dolphins, the best-studied species, have very complex societies.

So we have one example of human-scale intelligence, ourselves, and some species that come close - some of the toothed cetaceans.

Thus, the final part of f_i may be relatively high, even if some of the earlier parts have a risk of being relatively low, like the evolution of animal-like multicellularity. But we need all the parts of f_i, and the uncertainties add up. So it's still very much up in the air.

 

[lpetrich]

Now to consider what would go into f_c, the fraction that can communicate over interstellar distances.

Living in water is out, and dolphins don't even have tentacles or pincers.

Turning to our species, we took a long time before we discovered agriculture. For some reason, it was only invented after the end of the last Ice Age, in the Holocene, though it was invented independently in several places. Why was it invented only then and not in the 100,000 - 200,000 previous years of our species? The best theory that I've seen is that the Holocene's climate was stable enough to enable populations of farmers to become well-established with their cultivated plants. Before the Holocene, however, the climate was not stable enough, and the would-be farmers would have to give up after a while.

As to why Eurasia got ahead of the rest of the world, I like Jared Diamond's theory of a combination of east-west extension and convenient domesticatable animals.

A crucial invention for storing information was writing, but it was invented in only a few places: Sumeria, Central America, and perhaps also China. But once it was invented, it slowly spread to the rest of the world, in some cases by stimulus diffusion: learning about writing inspiring people to invent writing systems for their languages.

The next step was development of theoretical science, but that was difficult and slow. It started in classical Greece and continued into the early Roman Empire, where it was interrupted by the third-century strife and civil war. It did not restart until over a millennium later in western and central Europe, and even then it was slow going at first.

-

The social-brain theory of intelligence suggests that sentient species may prefer to be considered with social relations and gossip and the like; that sometimes seems rather evident in our species. That extends to anthropomorphizing nonhuman species like pet species, like making LOLcat pictures.

However, some of us have Asperger's syndrome, which may help its sufferers understand impersonal things and features and relations. Not many, but enough to be useful for the rest of us. So might other sentient species have Asperger-like variants?

Considering all these factors together, f_c is very uncertain.

So that leaves us with continuing uncertainty in f_l, f_i, and f_c.

 

[lpetrich]

For the final parameter, the lifetime L, I don't have much of a clue. In principle, L could be as long as as the time that the Universe has a usable disequilibrium, and that is MUCH longer than its age so far.

I will confine myself to listing some possible causes of shorter lifetime.

• Big natural disasters like impacts of large asteroids
• Wars
• Environmental problems
• Resource depletion
• Malfunctioning technology (like "gray goo" nanotechnology)
• Loss of interest
  • Partial low-tech lifestyle (Ba'ku of Star Trek: Insurrection)
  • Turning inward (permanent residence in virtual-reality simulations)
  • Belief that ET's are not worth searching for
  • Discouragement from searching and failing to find ET's

 

[barbos]

I think we need to add one more parameter - "willingness to communicate".
Also I think we can be reasonably sure that fl =1. If conditions are suitable, then life is unavoidable.
Intelligence is unavoidable too, once you managed to multicellular stage. What is highly unlikely is technological civilization. There is probably a bunch of planets out there with monkeys running around but no human-like level.

 

[James Brown]

I think we need to add one more parameter - "willingness to communicate".
Also I think we can be reasonably sure that fl =1. If conditions are suitable, then life is unavoidable.
Intelligence is unavoidable too, once you managed to multicellular stage. What is highly unlikely is technological civilization. There is probably a bunch of planets out there with monkeys running around but no human-like level.

Ah, but why would monkeys evolve to a human-like level on this world but not others?

 

[lpetrich]

I think we need to add one more parameter - "willingness to communicate".

That's usually folded into fc and L, but if one wants to expand Drake's equation, one can separate ability and willingness.

Also I think we can be reasonably sure that fl =1. If conditions are suitable, then life is unavoidable.

I don't find that very convincing -- the origin of life is still a big unsolved problem.

Intelligence is unavoidable too, once you managed to multicellular stage.

If our planet's biota is any guide, then there is a problem. Animal-like multicellularity evolved only once, if present-day organisms are any guide. However, plantlike, funguslike, and slime-mold-like multicellularity each evolved several times, including once each among prokaryotes (cyanobacteria, actinobacteria, myxobacteria). So there could be a planet with lots of plants, fungi, and slime molds, but no multicelled animals -- only animal-like one-celled organisms. Imagine landing on a planet with a huge forest. You decide to land on a shoreline, and you explore the nearby forest. Big trees towering overhead, with lots of mushrooms on the forest floor. But after a while, you realize that something is missing. No animals. Not even insectlike ones. No footprints either, except yours.

What is highly unlikely is technological civilization. There is probably a bunch of planets out there with monkeys running around but no human-like level.

Could well be.

 

[lpetrich]

On our planet, animallike multicellularity evolved only once, as far as we can tell. Our closest one-celled relatives are the choanoflagellates (collar flagellates), and some of them create colonies. There are various other sorts of protists that could conceivably give us multicelled animals, like ciliates, alveolates, rhizarians, and excavates. There are no animals descended from organisms like some amoeba or Paramecium or the malaria bug (plasmodium) or the red-tide bug (dinoflagellate) or the sleeping-sickness bug (trypanosome). Not even tiny worms, though we might find one some day.

 

[Jimmy Higgins]

To me n_e, f_l, f_i are all extremely hard to determine as we have no basis to derive how common life is on other planets. However, I think that f_c is the game killer. The question with f_c is if such communication is even possible. The universe could be saturated with life so far apart that it can't communicate with one another. In order to communicate, you need to figure out how to do so at a rate not just "faster" than light, but "much faster".

So f_p is likely close to 1 while n_e is still uncertain, though likely not much less than 1. Maybe 0.1 or 0.01 or 0.001.

Forgive me, but aren't those numbers several orders of magnitude less than 1? How much less than 1 can you get while still being "not much less than 1"?

1 in 1000 is much higher than 1 in 1,000,000,000. Think of it as the difference between having an actual chance of winning the Powerball.

 

[lpetrich]

So f_p is likely close to 1 while n_e is still uncertain, though likely not much less than 1. Maybe 0.1 or 0.01 or 0.001.

Forgive me, but aren't those numbers several orders of magnitude less than 1? How much less than 1 can you get while still being "not much less than 1"?

I concede that 0.01 and especially 0.001 is stretching it. But I think that it is likely that our Galaxy has a large number of Earthlike planets, meaning that our homeworld is far from alone.

 

[bilby]

To me n_e, f_l, f_i are all extremely hard to determine as we have no basis to derive how common life is on other planets. However, I think that f_c is the game killer. The question with f_c is if such communication is even possible. The universe could be saturated with life so far apart that it can't communicate with one another. In order to communicate, you need to figure out how to do so at a rate not just "faster" than light, but "much faster".

Communication is not really about speed. Two-way communication, sure; But if we were to discover the alien equivalent of a Voyager probe, or to detect the alien radio signals from their DEW RADAR arrays, then I think that would qualify as 'communication' as long as it left us in no doubt that the signal or artifact was the product of an intelligent civilization.

The real problem is the sheer size of space, regardless of any speed limits. If aliens send out a Voyager style probe, then the chances of it getting close enough to Earth to be detected are approximately nil, even if it was lucky enough to pass inside the orbit of the moon; If such probes were aimed at the solar system, and were coming from every star within a thousand lightyears, they still have to pass close enough to earth to be detected (highly unlikely) and they need to do so within the window of time that humans are able to detect them at all (so basically the last few decades, unless they are broadcasting some kind of signal, and the last century at most in that case). Half of the objects in the asteroid belt could be alien space-probes, with plaques showing pictures of hydrogen atoms, other solar systems, and nude aliens, and we would be none the wiser.

Things are not much better for radio; The inverse square law is the killer here. Just about the only signals we have broadcast into space so far with sufficient power to reach even 'nearby' solar systems at a strength that we could detect above the background noise of the universe (using our currrent technology) are the RADAR pulses from the DEW network in the cold war. These broadcasts were only active for 31 years (1957 - 1988). That's a pretty tiny window of opportunity. If the Cold War on Proxima Centauri b lasted from 1625 to 1685 (by our calendar) - twice as long as our own - then the chances of our spotting those signals was nil, even if they happened to be sometimes pointing directly towards us. In fact, the chances of us detecting them are not much greater than nil, even if they were broadcasting four years ago, and the signals were passing Earth right now.

The famous 'WOW!' signal could well have been just such a detection - and we missed the chance to confirm it because our technology at the time required days to process the data and alert a person to its existence, by which time it had stopped (or was no longer pointing at us, which amounts to the same thing).

The nearest star systems could be teeming with intelligent life, and we would be very unlikely to be able to detect it, even if it was putting quite a lot of effort into trying to be detected.

Even with a really sensitive receiver and a very large collector/antenna area, the power needed from a transmitter to have a hope of being detectable several lightyears away is immense.

 

[Loren Pechtel]

I think we need to add one more parameter - "willingness to communicate".
Also I think we can be reasonably sure that fl =1. If conditions are suitable, then life is unavoidable.
Intelligence is unavoidable too, once you managed to multicellular stage. What is highly unlikely is technological civilization. There is probably a bunch of planets out there with monkeys running around but no human-like level.

The geologic record disagrees with you.

Life arose as soon as it was possible to within our ability to measure. I do agree that means the probability of it arising is quite high.

There are two hurdles beyond that, though:

1) Multi-cellular life. That took billions of years. Were we fast at doing so? Then a good number of life-bearing planets never develop multi-cellular life. We we slow at doing so? Then virtually all do.

2) Intelligence. Again, it took hundreds of millions of years. We we fast? Then intelligence is rare. We we slow? Then it's very common.

Something to keep in mind: We just squeaked in under the wire. Earth will only remain suitable for intelligent life to arise for about 1% more time than has already elapsed. If our evolution is typical half of life-bearing worlds never reach intelligence and a world that started even a little farther in has basically no chance at all.

Even if the star doesn't put a stop to it other cosmic debris might. Looking around our solar system the time between cosmic reset buttons is projected to be around 2 billion years--stuff that makes the dinosaur killer look like a toy and blasts the Earth back to deep-buried single-cell life or even wipes it out entirely.

 

[Loren Pechtel]

To me n_e, f_l, f_i are all extremely hard to determine as we have no basis to derive how common life is on other planets. However, I think that f_c is the game killer. The question with f_c is if such communication is even possible. The universe could be saturated with life so far apart that it can't communicate with one another. In order to communicate, you need to figure out how to do so at a rate not just "faster" than light, but "much faster".

I disagree. I don't think you need FTL comms.

The thing is, it's hard to picture humanity of a 1000 years in the future not being capable of interstellar colonization. I find it very unlikely that we won't have major life extension technology by then and boosting a craft to 1% of lightspeed isn't that big a deal with fusion power. Beyond that you need some pretty darn reliable equipment and very good recycling, neither of which strikes me as anything like a showstopper. Put those together and you have slowboats.

Sooner or later the colonies that result will send out their own colonies. Taking very pessimistic values for the rate of spread still leaves colonizing the galaxy in a eyeblink of galactic time unless there are areas where the colony ships can't reach. Thus one colonizing species in the galaxy means ET is either here, or if they leave life-bearing worlds alone, at some nearby star.

 

[Jimmy Higgins]

Mankind will be lucky to be around in 1,000 years.


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[bilby]

Mankind will be lucky to be around in 1,000 years.


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I dunno; It's going to be pretty hard to eradicate us.

What do you see as the extinction mechanism?

War won't do it - we probably had just barely enough nukes in the 1980s to engineer a total extinction event, but even then, it would have required a deliberate strategy of extinction by at least one of the two superpowers - and I suspect that both would tend to avoid nuking remote non-combatants (Why would the Soviets or the Americans have wanted to fire a warhead at a few hundred Aborigines on the fringes of the Great Sandy Desert, or the tribes of the upper Amazon, or the Tuareg villages on the edge of the Sahara, or the Kalahari bushmen?). I reckon even a worldwide, all-out thermonuclear exchange at the peak of the cold war would struggle to kill every last pocket of humans, and history shows that we can bounce back within a few tens of thousands of years.

A 'dinosaur killer' style meteorite is something we are on the cusp of being able to detect and deflect; and within mere hundreds of years, we might have colonies elsewhere in the Solar System that could survive for long enough to re-populate the Earth once the dust settles.

The Sun seems to be sufficiently stable as to give us a pretty good chance of at least many thousands, probably a few million, years before it eradicates us.

Pandemic diseases might kill billions; But even the most deadly plagues in history only kill about 95 - 97% of the host population, which would leave over 200 million people alive to rebuild.

Humans are like cockroaches; Easy enough to kill in large numbers, but very hard to eradicate completely, because we are numerous, adaptable, and widespread.

Kill 99% of humanity, and then have 90% of the survivors succumb over time due to lack of survival skills and/or resources in the post apocalyptic wasteland, and you still have 7 or 8 million surviving humans. 10,000 years is more than enough time for such numbers to recover to the current level of population and technology, particularly as they need not re-invent the wheel.

 

[Jimmy Higgins]

Mankind will be lucky to be around in 1,000 years.


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I dunno; It's going to be pretty hard to eradicate us.

What do you see as the extinction mechanism?

Likely mankind or mankind because of the Sun.

War won't do it - we probably had just barely enough nukes in the 1980s to engineer a total extinction event, but even then, it would have required a deliberate strategy of extinction by at least one of the two superpowers - and I suspect that both would tend to avoid nuking remote non-combatants (Why would the Soviets or the Americans have wanted to fire a warhead at a few hundred Aborigines on the fringes of the Great Sandy Desert, or the tribes of the upper Amazon, or the Tuareg villages on the edge of the Sahara, or the Kalahari bushmen?). I reckon even a worldwide, all-out thermonuclear exchange at the peak of the cold war would struggle to kill every last pocket of humans, and history shows that we can bounce back within a few tens of thousands of years.

You don't need extinction to bring progress to a halt.

A 'dinosaur killer' style meteorite is something we are on the cusp of being able to detect and deflect; and within mere hundreds of years, we might have colonies elsewhere in the Solar System that could survive for long enough to re-populate the Earth once the dust settles.

I don't know if we are psychologically strong enough to handle space travel and space colonies. We are talking Joe Schmo who throws the remote control at the TV because of a bad call in a baseball game handling the depths of space?

Pandemic diseases might kill billions; But even the most deadly plagues in history only kill about 95 - 97% of the host population, which would leave over 200 million people alive to rebuild.

Humans are like cockroaches; Easy enough to kill in large numbers, but very hard to eradicate completely, because we are numerous, adaptable, and widespread.

Kill 99% of humanity, and then have 90% of the survivors succumb over time due to lack of survival skills and/or resources in the post apocalyptic wasteland, and you still have 7 or 8 million surviving humans. 10,000 years is more than enough time for such numbers to recover to the current level of population and technology, particularly as they need not re-invent the wheel.

Science and mankind are on a great track. Every time it discovers something that'll benefit mankind, it gets repurposed to try and destroy it.