Tag Archives: OriginOfLife

Our Level In the Great Filter

An exchange between Astrophysicist Charles Lineweaver and myself:

In their 2019 paper “The Timing of Evolutionary Transitions Suggests Intelligent Life is Rare”, Snyder-Beattie, Sandberg, Drexler, and Bonsall argue that the expected time for “intelligent life” to appear on Earth “likely exceed the lifetime of Earth, perhaps by many orders of magnitude” which “corroborate[s] the original argument suggested by Brandon Carter that intelligent life in the Universe is exceptionally rare.”

In a Feb. 2022 comment in Inference, “A Lonely Universe”, Charles Lineweaver disagreed:

The Snyder-Beattie et al. result depends on the assumption that … the major transitions that characterize our evolution happen elsewhere. There is little evidence in the history of life on earth to support this assumption. … transition to human-like intelligence or technological intelligence occurred only about 100,000 years ago and is species-specific. The latter trait is strong evidence we should not expect to find it elsewhere.

It [is not] reasonable to argue that … the features of life on earth … most likely to appear in life elsewhere are those that have evolved independently many times, such as complex multicellularity, eyes, wings, and canines. … [because] these … have only occurred within a unique [never-repeated] eukaryotic branch that represents a tiny fraction of the diversity of life on earth. …

Attempting to compute the probability of human-like intelligence elsewhere based on our lineage is akin to analyzing the evolution of the English language on earth and trying to use the timing of the Great Vowel Shift to estimate its timing on other planets

My July 2022 reply, also in Inference, says:

Lineweaver suggests that without good reasons to think “the major transitions that characterize our evolution happen elsewhere,” estimates regarding Earth do not allow us to make estimates regarding other planets.

On the contrary, I see two ways to compare planets so that Earth estimates become relevant for other planets, allowing us to infer a low overall rate at which advanced life appears elsewhere. First, if Earth is a random sample from planets that succeed in making life at our level, the success rate on Earth cannot be too different from the typical success rate on other such planets. Second, if there is a substantial chance that our descendants will soon become very visible in the universe, the fact that no other star in our galaxy has yet done so can set a low upper bound on the fraction of such stars that can have reached our level by now. …

Let R be the chance of life at our current level—i.e., controlling nuclear power and practicing spaceflight—appearing on a particular planet within some fixed planet habitability duration. … chance Q that, within the following ten million years, a planet at our level would give rise to a civilization that becomes permanently visible across its entire galaxy. [I elaborated with math examples for both these approaches.]

In that same place, Lineweaver then responded:

I don’t believe in the general group that he and many others call “advanced life.” … No other life-forms in the universe will be genetically or phenotypically more similar to us than chimps, bonobos, gorillas, naked mole rats, or frogs. Since Hanson and many others exclude our closest relatives from “advanced life,” they are—by their definition—not talking about a generic group with other members. …

On Earth, humans are the only ones who have become humans at our level of technology. To then conclude that among all species, our species had an average chance of becoming humans at our level is meaningless. …

Morris … argues that strong selection pressure leads to convergent evolution which then produces human-like intelligence. Hanson and most physicists subscribe to this view, but most biologists and I don’t. … Hanson refers to … life at our level … I … ask: If we exclude our species from consideration, does this talk of levels make any sense when applied to the rest of life? Are dogs or red oak trees at a higher level?

Reading Lineweaver’s response, I see my reply was off target; his issue is with the very idea of “life at our level”. So let me try again.

A key datapoint is this: we do not now see any big visible civilizations (BVC) in the sky who have greatly changed the natural universe into something more to their liking. In order to explain this fact, we must postulate a “great filter”, i.e., a process whereby simple dead matter might give rise first to simple life, and then to a BVC, or various filter obstacles might end this progress, so that it never produces a BVC. We must conclude that so far, averaging across the universe, this filter process has a very low total pass-through rate to a BVC. After all, no dead matter in the entire universe has yet given rise to a BVC we can see. That is, this great filter is on average very large.

In contrast, Earth today seems to plausibly have a much higher rate for creating BVC. I’d say we have at least a one in a million chance of doing so within the next ten million years. (This isn’t value judgement, just an estimate.) As Earth is now thus much closer to this BVC endpoint than it was originally, there is a sense in which Earth has now passed through part of the great filter, so that a substantially smaller filter lies before us than once lied before a simple dead Earth.

To talk about how much of the great filter we have so far passed, we’d like a way to talk about where we “are now” in this filter process. And this is where we can want to talk about our current “level” along some linear path from dead matter to BVC. But, as Lineweaver points out, evolution is in many ways a tree, instead of a line, and we cannot construct such a level concept merely by creating a conjunct of various random specific features of our species and planet.

Even so, I do think there are useful ways to define “our level” (OL) within the great filter. What we want is an equivalence class OL of alien civilization-moments such that (a) Earth today is in OL, (b) almost all BVC were once in OL at some prior point in their history, and (c) OL covers only a short “time slice” during which few civilizations go extinct. If we have more choices, we’d further like to pick OL so that (d) it minimizes the variance in the (coarse-grained) chance that each civilizations in OL later gives rise to a BVC. The lower this variance, the more it makes sense to talk in terms of the average chance within OL of giving rise later to a BVC.

One option would be to just define OL as the class that meets criteria (a,b,c) and actually minimizes (d). But while this might be well defined, it seems unwieldy. Which is why I tried above to define OL above in terms of a civilization having just mastered the basics of both nuclear power and spaceflight. It might be reasonable to add a few other techs to this list, such as computers.
Sure, we’d define somewhat different OL sets if we added or cut techs from this list. But the key point is that any civilization that had mastered all of them would be well on its way to being able to start a BVC soon. And most likely the chance of extinction is low between the point of having mastered half of these techs and mastering all of them. Thus the exact list of techs in our OL definition probably doesn’t make that much difference.

Yes, this way to define OL can let humans pass through OL, while chimps never do. But I just don’t see why that’s a problem. There is in fact a big important difference between what humans and chimps have accomplished, and I’m fine with our OL definition reflecting that.

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Beware Cosmic Errors

Imagine that you came across an enormous dry grassland, continuously covered with dense grass. It seems to go on for thousands of miles in all directions, and historical records suggest that it has been in this same dry state for millions of years. You conclude that if a spark had touched it anywhere anytime during that period, a fire would have begun that would eventually spread across the entire grassland. 

In this situation you either have to believe that sparks are extremely unlikely, so that for example lightning is just a very rare thing in this world. Or you have to conclude that appearances are deceiving; there are many wide barriers that limit the spread of fire in space, or there are serious defects in your historical record. Either sparks almost never happen, fire starting in one place does not spread to the entire grasslands, or fires do periodically spread everywhere but quickly burn out and then their historical records are quickly erased. 

Now imagine that you came across an enormous pleasantly-wet mildly-windy barren land; it seems to be a millions-of-years stable continuum of sand that goes on for thousands of miles. You can tell from lab tests that this wet sand could serve as fertile soil. That it is, it has sufficient nutrients, water, sunlight, temperature, pressure, etc. to enable some kinds of grass seed to grow into grass plants that send out more seeds. And yet this land has apparently remained empty and barren for millions of years; it holds neither grass nor other life that might evolve from grass. 

In this situation, you either have to believe that almost no grass seed has ever fallen on this land for millions of years, or that the appearance of a stable continuum of sand is seriously misleading. Perhaps that are wide strong hidden barriers to the dispersal of seeds, such as wide barrier regions of no wind. Or perhaps some big disaster happens periodically to kill basically all seeds across this entire connected land, and then later all historical records of both the prior seeds and the event that killed them are erased. 

Imagine further in these situations that we the observers making these observations and drawing these conclusions are in fact made out of, or closely connected to, fire in the first case, and seeds in the second. We would then have to believe that our origins are extremely crazy rare, that we will either remain permanently isolated behind travel barriers, or that soon we will suffer a quite thorough death that erases most all records of our existence. 

These imaginary scenarios seem close analogues to humanity’s actual situation in the cosmos today. Except that now we are talking a period of fourteen billion years and a scope even more billions of lightyears wide. We seem to be close to becoming part of a fire or seed that would be capable of spreading across the cosmos, burning most all or turning most all to grass, or to some descendant life. And yet our historical records seem to be good enough to tell us that no such fire has yet happened, or that almost none of it has been turned to grass or descendant life. 

We must then conclude that either (A) we not remotely as close to these expansion abilities as we think, (B) the appearance of life like us is extremely rare, or we are seriously mistaken about either (C) the feasibility of long-distance travel, or (D) the absence of frequent cosmos-wide disasters that kill everything. Yes we do know of substantial obstacles to our future evolution and long-distance travel, and of periodic large disasters that would kill many things. But our best understanding is that these evolution and travel obstacles can be plausibly overcome, and that these large disasters have a quite limited scope. 

Our grabby aliens analysis suggests aliens who spread across the cosmos as would a fire or grass are in fact quite rare. They appear roughly once per million galaxies, and appear in time according to a power law that emphasizes later times which we are less able to see from here now; we’ll meet them in roughly a billion years if we expand. But in this post I want to remind us of other possibilities; maybe our future evolution or long-distance travel are much harder than they seem, or maybe there are hidden disasters much more severe and frequent than we suspect. Beware. 

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Great Filter With Set-Backs, Dead-Ends

A biological cell becomes cancerous if a certain set of rare mutations all happen in that same cell before its organism dies. This is quite unlikely to happen in any one cell, but a large organism has enough cells to create a substantial chance of cancer appearing somewhere in it before it dies. If the chances of mutations are independent across time, then the durations between the timing of mutations should be roughly equal, and the chance of cancer in an organism rises as a power law in time, with the power equal to the number of required mutations, usually around six.

A similar process may describe how an advanced civilization like ours arises from a once lifeless planet. Life may need to advance through a number of “hard step” transitions, each of which has a very low chance per unit time of happening. Like evolving photosynthesis or sexual reproduction. But even if the chance of advanced life appearing on any one planet before it becomes inhabitable is quite low, there can be enough planets in the universe to make the chance of life appearing somewhere high.

As with cancer, we can predict that on a planet lucky enough to birth advanced life, the time durations between its step transitions should be roughly equal, and the overall chance of success should rise with time as the power of the number of steps. Looking at the history of life on Earth, many observers have estimated that we went through roughly six (range ~3-12) hard steps.

In our grabby aliens analysis, we say that a power of this magnitude suggests that Earth life has arrived very early in the history of the universe, compared to when it would arrive if the universe would wait empty for it to arrive. Which suggests that grabby aliens are out there, have now filled roughly half the universe, and will soon fill all of it, creating a deadline soon that explains why we are so early. And this power lets us estimate how soon we would meet them: in roughly a billion years.

According to this simple model, the short durations of the periods associated with the first appearance of life, and with the last half billion years of complex life, suggest that at most one hard step was associated with each of these periods. (The steady progress over the last half billion years also suggests this, though our paper describes a “multi-step” process by which the equivalent of many hard steps might be associated with somewhat steady progress.)

In an excellent new paper in the Proceedings of the Royal Society, “Catastrophe risk can accelerate unlikely evolutionary transitions”, Andrew Snyder-Beattie and Michael Bonsall extend this standard model to include set-backs and dead-ends.

Here, we generalize the [standard] model and explore this hypothesis by including catastrophes that can ‘undo’ an evolutionary transition. Introducing catastrophes or evolutionary dead ends can create situations in which critical steps occur rapidly or in clusters, suggesting that past estimates of the number of critical steps could be underestimated. (more)

Their analysis looks solid to me. They consider scenarios where, relative to the transition rate at which a hard step would be achieved, there is a higher rate of a planet “undoing” its last hard step, or of that planet instead switching to a stable “stuck” state from which no further transitions are possible. In this case, advanced life is achieved mainly in scenarios where the hard steps that are vulnerable to these problems are achieved in a shorter time than it takes to undo or stuck them.

As a result, the hard steps which are vulnerable to these set-back or dead-end problems tend to happen together much faster than would other sorts of hard steps. So if life on early Earth was especially fragile amid especially frequent large asteroid impacts, many hard steps might have been achieved then in a short period. And if in the last half billion years advanced life has been especially fragile and vulnerable to astronomical disasters, there might have been more hard steps within that period as well.

Their paper only looks at the durations between steps, and doesn’t ask if these model modifications change the overall power law formula for the chance of success as a function of time. But my math intuition is telling me it feels pretty sure that the power law dependence will remain, where the power now goes as the number of all these steps, including the ones that happen fast. Thus as these scenarios introduce more hard steps into Earth history, the overall power law dependence of our grabby aliens model should remain but become associated with a higher power. Maybe more like twelve instead of six.

With a higher power, we will meet grabby aliens sooner, and each such civilization will control fewer (but still many) galaxies. Many graphs showing how our predictions vary with this power parameter can be found in our grabby aliens paper.

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Biologists Taboo Artificial Life

Recently I’ve reviewed three new books by academic biologists on the future of life in the universe. All three books have gained high profile and prestigious reviews in major media and academia. (Which is how I heard of them.) And all of these books, and all of these prestigious reviews, seem to share and enforce a taboo against seriously considering the possibility that artificial life will make a big difference to the cosmos.

For example:

Arthur admits the possibility of intelligent life spreading across planets, … and Arthur admits the possibility of artificial life. … But somehow these admissions make little difference to his forecasts, which ignore the possibility of artificial life at places other than planets, or made out of stuff other than carbon. And which ignore the possibility of intelligent artificial life spreading very far and wide, to become even more common than non-artificial life.

Similarly:

I recently reviewed The Zoologist’s Guide to the Galaxy, wherein a [Cambridge] zoologist says that aliens we meet would be much like us, even though they’d be many millions of years more advanced than us, apparently assuming that our descendants will not noticeably change in million of years.

And in a new book The Next 500 Years, a geneticist [and computational biologist] recommends that we take the next few centuries to genetically engineer humans to live in on other planets, apparently unaware that our descendants will most likely be artificial (like ems), who won’t need planets in particular except as a source of raw materials.

I actually just did a written debate with this last author, who wouldn’t even admit that I disagreed with him:

You write a long book mostly on the details of genetic engineering, saying we should use it to slowly change humans and their allied plants and animals, so that in 500 years we could launch them out to the cosmos, to arrive at other stars in a few thousand years.

I say, no, long before then artificial minds and life should have thoroughly replaced biology. A new kind of life, far more robust, able to grow far faster, able to travel out into space much sooner and faster, all made in factories out of stuff dug up in mines, and not at all based on biological cells, so that genetic engineering has little to offer them.

This all suggests more than just a few biologists with a mental block; it suggests an overall taboo within their shared intellectual culture, of biology academics who study astrobiology and our future. A taboo that has likely discouraged and distorted related research and analysis.

Added 30May: This post is discussed at Hacker News.

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The Biological Universe

In his new book The Biological Universe: Life in the Milky Way and Beyond, evolutionary biologist Wallace Arthur predicts the life we will find in the galaxy and universe:

Life forms are to be found across the Milky Way and beyond. They will be thinly spread, to be sure. … we can anticipate what life elsewhere will be like by examining the ecology and evolution of life on Earth.

Arthur defines life broadly:

If an entity is metabolically alive and membrane-bound, and groups of individual entities of this kind are characterized by variation, reproduction, and inheritance, then we describe the situation as ‘life’. … And regarding extraterrestrial life we should try to keep as open a mind as possible (p.13)

He says life is only near the surface of planets:

There are no macromolecules in [interstellar] clouds. There is thus no basis for life even approximately as we know it. So in the end we rule out all of the interstellar medium as a home for life. And that means in spatial terms that we have ruled out more than 99% of the galaxy. … Next we rule out suns. This means all suns and all parts them. No metabolizing, reproducing life, whether simple like bacteria, or more complex, like mammals, could exist in such a hellish environment. … By ruling out suns as possible homes for life, we rule out more than 99% of the matter of the galaxy. … Here’s a selection of other objects that seem likely to be barren. First, dead stars, including white dwarfs, neutron stars, and black holes. Second those entities somewhere in between a small star and a large planet that we call brown dwarfs. … Third, pulsars. (pp.42-44)

Arthur says most life is enclosed, made of carbon, and of long molecules with sequence specificity:

Carbon based life is the most probable, and hence more common, form of life in the Milky Wa, and indeed in the universe. … Life requires a type of macromolecule that exhibits sequence specificity that is that is similar in general, though not necessarily in detail, to the specificity that is found in nucleic acid and proteins. … Membrane-enclosed cellular life is the norm. (p.203)

Life is almost everywhere that it can be:

The fraction of habitable planets that actually become inhabited. My personal view is that it is close to 100%. (p.191)

And here is how many planets of each type:

Number of planets in Milky Way: 1 trillion
Number of planets with microbial life: 1 billion
Number of planets with animal life: 10 million
Number of planets with broadcasting life: between 0 and 1 million

Arthur even predicts more intelligent life is rarer:

Lets define four thresholds levels of intelligence. … animals with a small brain … crossed the first threshold. … Animals that can use tools, and indeed plan their use of tools, … cross the second threshold. … Animals that have begun to investigate the abstract nature of things, and to keep written records of their investigations, have cross the third threshold., … fourth threshold the achieving of a civilization with a technology that includes the use of radio signals and other means of interstellar communication, such as lasers. … It’s hard to believe that the number of planets whose evolutionary processes have crossed these four respective thresholds would go upward rather than downward. (p.328??)

How does Arthur make all these predictions? By assuming that that the distribution of stuff in the universe is much like the distribution of stuff across our solar system and across the history of Earth:

On the basis of Earth’s history to date, the fraction of microbial inhabited planets that also have animals can be estimated by the relative durations of these two types of life here, which is 630 million compared to 4 billion years. (p.200)

The fact that [intelligence] and the physical basis for it – the brain – can be downplayed or even lost altogether in some lineages [in Earth history] should temper our hopes for the discovery of extraterrestrial intelligence. … Natural selection is not on a long-term quest for the ultimate brainy animals. (p.134)

With regard to possible life, the vast majority of the solar system, like the vast majority of the galaxy, is of little interest to us. For the most part, our system looks barren. (p..139)

But doesn’t all this neglect the possibility of that intelligent life on some planet will develop a more robust and powerful artificial life, which then spreads widely across the cosmos? Arthur admits the possibility of intelligent life spreading across planets:

Between two and three billion years from now … if new make it that far, we might have the technology to colonize the closest suitable exoplanets. (p.160)

Intelligent life may have colonized nearby planets, as may the the case in the mid-term future wit humans on Mars. (p.315)

Planets on which radio-level intelligence has evolved constitute only a tiny fraction of those on which life in general has evolved. Yet because of the vastness of the universe, and perhaps also because of planetary colonization, there are many planets with such life-forms in the universe right now. (p.328)

And Arthur admits the possibility of artificial life:

But there is a caveat here. What about AI (artificial intelligence)? It’s a moot point whether any of our machines are yet intelligent enough to truly merit that label, though no doubt they will get there eventually. Perhaps the machines associated with ultra-intelligent aliens are already there. In this case, the intelligent universe and the biological universe … are overlapping sets. Having made this point, let’s focus on intelligent living beings across the universe, not intelligent machines. And let’s ignore the advanced organism-machine hybrids of science fiction, even though entities of this type probably exist somewhere. (p.318)

But somehow these admissions make little difference to his forecasts, which ignore the possibility of artificial life at places other planets, or made out of stuff other than carbon. And which ignore the possibility of intelligent artificial life spreading very far and wide, to become even more common than non-artificial life.

Arthur instead assumes that advanced intelligence and artificial life will just not spread much, perhaps due to self-destruction:

Intelligent life may have a tendency to self-exterminate within a few centuries of its inception. (p.221)

Wallace Arthur seems to be yet another biologists who just can’t imagine our descendants being that different from us, or artificial life making much of a difference to the cosmos.

Out of a great many reviews of this book I read, I only found one other reviewer, David Studhalter, a non-academic, making a similar complaint:

Arthur … blithely assumes that humans and their descendants will simply become extinct before advancing to a stage where they are spreading terriform life elsewhere in the Galaxy, and that we will never exceed the bounds of our own Solar system. … Arthur mentions virtually nothing discussed in this last paragraph. But they are crucial to his subject, which does purport to discuss the future of life. (More)

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Panspermia Siblings

The UFOs as aliens hypothesis is only as believable as the most a priori believable story for how it could be true. When I tried to find a story like that, I ended up relying heavily on the idea of panspermia siblings. And now that I’ve given that idea a bit more thought, I’ve realized that it is somewhat harder to arrange than I’d realized, and thus somewhat less believable. Making UFOs as aliens less likely, though still quite possible.

The scenario, if you recall, is that there are aliens visiting Earth today who have not expanded much to colonize and remake the universe, aliens who were born at a planet around a star that is a sibling to our sun. That is, this alien’s star was born in the same stellar nursery as our sun. This scenario requires three key elements:

Old Non-Expansionist Aliens – A substantial fraction of advanced civilizations choose not to expand and visibly remake the universe, but do choose to go visit their sibling stars that develop advanced life, and these civilizations last for longer than the typical differences between when advanced life would appear when grown from simpler life at the same level four billion years before. Thus a substantial fraction of alien civilizations must last for several hundred million years. (Oh and they choose do all these apparently-useless glow-buzzings of our treetops.)

Easy Earth Filter – In order for there to be at least two advanced civilizations both born from the same stellar nursery, it can’t be too hard to evolve advanced life from the sort of life that Earth starts with. The time of the origin of life on Earth and the time now remaining suggest 3-9 hard steps happened on Earth, if this whole time was take up by hard try-try steps. So we need some combination of a large nursery, fewer such hard steps, much of Earth history being taken up with delay steps instead of hard try-try steps, and the “hard” try-try steps not being that hard. So, for example, in a nursery of ten thousand stars, there might be just three try-try steps each only a factor of ten hard, and perhaps half of Earth history was taken up with delay steps.

Panspermia or Huge Try-Once Step – In order for life to spread across a large fraction of a stellar nursery, that life would have to appear within roughly a hundred million years after that nursery formed. So either life appeared from nothing very fast, mainly via some very hard try-once steps, or our nursery was seeded by life from an Eden at some other passing star, either just as our nursery was forming, or via a prior seeding of the molecular cloud which collapsed to form our nursery. (Which requires life to survive a long time in a molecular cloud.) On average stars pass within 5 parsecs of  such clouds every 50-100Myr.

While this prior Eden would have had a similar number of hard steps as Earth, those steps would on average be much harder, so that most of the total great filter would have happened at Eden. Very hard steps might include the very first life, and the transfer from Eden to a stellar nursery.

A 2012 paper in Astrobiology works out details of this scenario for life moving between star systems in a stellar nursery, where many stars are crammed together and many rocks are flying between them.

We don’t know when life first appear on Earth, but current best guess is 400Myr, with a range 200-800Myr, after the Earth and Sun formed together. They were formed together with ~1K-10K other stars, all packed close together.

Earth had water to support life within ~160–290 Myr, while our cluster took ~135–535 Myr for sibling stars to drift away from each other (the largest value is for the largest star clusters). During this early period there were a lot of rocks smacking into Earth kicking up a lot more rocks. Maybe the top kilometer of rock across Earth was kicked up.

About ~1% of these rocks were ejected from Earth with a weak enough impact shock to let life survive, and rocks of >10 kg seem like they could protect life from radiation and impact over the 3-5 million years it would take to drift to the closest star system in this cluster during this period. Some kinds of life could last that long.

About 2 * 10^11 such rocks would escape our solar system at a slow enough velocity to be captured by a neighboring star. Given such assumptions, if the nearest star were also Sun-like, then the number of such rocks ejected from Earth in this period that would land on an Earth-like planet around that nearest star is about 3*10^4. If that star had half the sun’s mass, this number falls to just 10^4.

Thus if our Sun’s stellar nursery were big enough, and if life appeared early enough in this cluster, then life might have spread to many stars in this cluster. And thus aliens could have evolved before us at one of those stars, and then came here to be the UFOs we see. But this is a lot of ifs, and so the a priori unlikeliness of this scenario has to be weighed against the a priori unlikeliness of: secret Earth orgs with really advanced tech, a vast conspiracy to create the false appearance of UFO encounters,  or mass delusions widespread enough to create the same.

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Schulze-Makuch & Bains on The Great Filter

In their 2016 journal article “The Cosmic Zoo: The (Near) Inevitability of the Evolution of Complex, Macroscopic Life“, Dirk Schulze-Makuch and William Bains write:

An important question is … whether there exists what Robin Hanson calls “The Great Filter” somewhere between the formation of planets and the rise of technological civilizations. …

Our argument … is that the evolution of complex life [from simple life] is likely … [because] functions found in complex organisms have evolved multiple times, an argument we will elaborate in the bulk of this paper … [and] life started as a simple organism, close to [a] “wall” of minimum complexity … With time, the most complex life is therefore likely to become more complex. … If the Great Filter is at the origin of life, we live in a relatively empty universe, but if the origin of life is common, we live in a Cosmic Zoo where such complex life is abundant.

Here they seem to say that the great filter must lie at the origin of life, and seem unclear on if it could also lie in our future.

In the introduction to in their longer 2017 book, The Cosmic Zoo: Complex Life on Many Worlds, Schulze-Makuch and Bains write:

We see no examples of intelligent, radio-transmitting, spaceship-making life in the sky. So there must be what Robin Hanson calls ‘The Great Filter’ between the existence of planets and the occurrence of a technological civilisation. That filter could, in principle, be any of the many steps that have led to modern humanity over roughly the last 4 billion years. So which of those major steps or transitions are highly likely and which are unlikely? …

if the origin of life is common and habitable rocky planets are abundant then life is common, and we live in a Cosmic Zoo. … Our hypothesis is that all major transitions or key innovations of life toward higher complexity will be achieved by a sufficient large biosphere in a semi-stable habitat given enough time. There are only two transitions of which we have little insight and much speculation—the origin of life itself, and the origin (or survival) of technological intelligence. Either one of these could explain the Fermi Paradox – why we have not discovered (yet) any sign of technologically advanced life in the Universe.

So now they add that (part of) the filter could lie at the origin of human-level language & tech. In the conclusion of their book they say:

There is strong evidence that most of the key innovations that we discussed in… this book follow the Many Paths model. … There are, however, two prominent exceptions to our assessment. The first exception is the origin of life itself. … The second exception … is the rise of technologically advanced life itself. …The third and least attractive option is that the Great Filter still lies ahead of us. Maybe technological advanced species arise often, but are then almost immediately snuffed out.

So now they make clear that (part of) the filter could also lie in humanity’s future. (Though they don’t make it clear to me if they accept that we know the great filter is huge and must lie somewhere; the only question is where it lies.)

In the conclusion of their paper, Schulze-Makuch and Bains say:

We find that, with the exception of the origin of life and the origin of technological intelligence, we can favour the Critical Path [= fixed time delay] model or the Many Paths [= independent origins] model in most cases. The origin of oxygenesis, may be a Many Paths process, and we favour that interpretation, but may also be Random Walk [= long expected time] events.

So now they seem to also add the ability to use oxygen as a candidate filter step. And earlier in the paper they also say:

We postulate that the evolution of a genome in which the default expression status was “off” was the key, and unique, transition that allowed eukaryotes to evolve the complex systems that they show today, not the evolution of any of those control systems per se. Whether the evolution of a “default off” logic was a uniquely unlikely, Random Walk event or a probable, Many Paths, event is unclear at this point.

(They also discuss this in their book.) Which adds one more candidate: the origin of the eukaryote “default off” gene logic.

In their detailed analyses, Schulze-Makuch and Bains look at two key indicators: whether a step was plausibly essential for the eventual rise of advanced tech, and whether we can find multiple independent origins of that step in Earth’s fossil record. These seem to me to both be excellent criteria, and Schulze-Makuch and Bains seem to expertly apply them in their detailed discussion. They are a great read and I recommend them.

My complaint is with Schulze-Makuch and Bains’ titles, abstracts, and other summaries, which seem to arbitrarily drop many viable options. By their analysis criteria, Schulze-Makuch and Bains find five plausible candidates for great filter steps along our timeline: (1) life origin ~3.7Gya, (2) oxygen processing ~3.1Gya (3) Eukaryote default-off genetic control ~1.8Gya, (4) human-level language/tech ~0.01Gya, and (5) future obstacles to our becoming grabby. With five plausible hard steps, it seems unreasonable to claim that “if the origin of life is common, we live in a Cosmic Zoo where such complex life is abundant”.

Schulze-Makuch and Bains seem to justify dropping some of these options because they don’t “favour” them. But I can find no explicit arguments or analysis in their article or book for why these are less viable candidates. Yes, a step being essential and only having been seen once in our history only suggests, but hardly assures, that this is a hard step. Maybe other independent origins happened, but have not yet been seen in our fossil record. Or maybe this did only happen once, but that was just random luck and they could easily have happened a bit later. But these caveats are just as true of all of Schulze-Makuch and Bains’ candidate steps.

I thus conclude that we know of four plausible and concrete candidates for great filter steps before our current state. Now I’m not entirely comfortable with postulating a step very recently, given the consistent trend in increasing brain sizes over the last half billion years. But Schulze-Makuch and Bains do offer plausible arguments for why this might in fact have been an unlikely step. So I accept that they have found four plausible hard great filter steps in our past.

The total number of hard steps in the great filter sets the power in our power law model for the origin of grabby aliens. This number includes not only the hard filter steps that we’ve found in the fossil record of Earth until now, but also any future steps that we may yet encounter, any steps on Earth that we haven’t yet noticed in our fossil record, and any steps that may have occurred on a prior “Eden” which seeded Earth via panspermia. Six steps isn’t a crazy middle estimate, given all these considerations.

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Humans Are Early

Imagine that advanced life like us is terribly rare in the universe. So damn rare that if we had not shown up, then our region of the universe would almost surely have forever remained dead, for eons and eons. In this case, we should still be able to predict when we humans showed up, which happens to be now at 13.8 billion years after the universe began. Because we showed up on a planet near a star, and we know the rate at which our universe has and will make stars, how long those stars will last, and which stars where lived far enough away from frequent sterilizing explosions to have at least a chance at birthing advanced life.

However, this chart (taken from our new paper) calculates the percentile rank of our current date within this larger distribution. And it finds that we are surprisingly early, unless you assume both that there are very few hard steps in the evolution of advanced life (the “power n”), and also that the cutoff in lifetime above which planets simply cannot birth advanced life is very low. While most stars have much longer lives, none of those have any chance whatsoever to birth advanced life. (The x-axis shown extends from Earth’s lifetime up to the max known star lifetime.)

In the paper (in figures 2,17), we also show how this percentile varies with three other parameters: the timescale on which star formation decays, the peak date for habitable star formation, and a “mass favoring power” which says bu how much more are larger mass stars favored in habitability. We find that these parameters mostly make only modest differences; the key puzzle of humans earliness remains.

Yes, whether a planet gives rise to advanced life might depend on a great many other parameters not included in our calculations. But as we are only trying to estimate the date of arrival, not many other details, we only need to include factors that correlate greatly with arrival date.

Why have others not reported the puzzle previously? Because they neglected to include the key hard-steps power law effect in how chances vary with time. This effect is not at all controversial, though it often seems counter-intuitive to those who have not worked through its derivation (and who are unwilling to accept a well-established literature they have not worked out for themselves).

This key fact that humans look early is one that seems best explained by a grabby aliens model. If grabby aliens come and take all the volume, that sets a deadline for when we could arrive, if we were to have a chance of becoming grabby. We are not early relative to that deadline.

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Try-Menu-Combo Filter Steps

A great filter stands between simple dead matter and a visible expanding lasting civilization. Many hard steps (and also easy ones) must be passed to make it through this filter. But what kind of steps are these hard steps?

The first kind of steps that most people imagine are try-try steps. The local system must keep trying random variations at a constant rate until a successful one is found. Here the chance per unit time is a constant, and the chance of success by a time is linear, at least for small times. When a system must go through many hard steps by time t, but that success is quite unlikely, then for n hard steps the chance of that unlikely success by time t goes as tn.

I recently pointed out that there’s another kind of hard step: try-once. Here the local system has only one chance; if it fails then, it fails forever. For these sort of steps, the chance of success doesn’t increase with time trying.

In this post, I want to point out that there are worse kinds of steps than try-try steps. Such as try-menu-combo steps.

Imagine that to pass some important step, evolution needed to create a species with a particular combination of eyes, hands, feet, stomach, ears, etc. Except that the available menu for each of these parts increased linearly with time.

For example, at first there is only one kind of stomach available. All species must use that kind of stomach. Then there are two kinds, and then three. Which kind of stomach is the next to be added to the stomach menu is pretty random. But there is zero chance of achieving his menu-combo next step until the right kind of stomach is added to the menu.

In this scenario, having the right kind of stomach on the menu is far from enough. The system also needs to add the right kind of eyes to the eye menu, and so on. Once all of the right kinds of items are on the right menus, then the last thing needed is a try-try step, to create a specific species that includes all the right parts via randomly combining menu items.

If there were just one kind of part needed, the chance of success by some date would increase linearly with time, making this an ordinary try-try step. But if there were two kinds of parts needed, chosen from two menus, then the chance would go as t2. With three menus, it is t3. And so on.

So now we can see that the tn rule for the chance of many hard steps by time t can be generalized. Now instead of n being the number of hard steps, n becomes the sum of powers m for each of the hard steps. Step power m is zero for a try-once step, is near one for a try-try step, and is greater than one for try-menu-combo steps.

In terms of its contribution to the tn power law for completing all the hard steps, a try-menu-combo step is the equivalent of several try-try steps all happening at the same time. That is, great filter hard steps can in some sense happen in parallel, as well as in sequence.

With ordinary try-try steps, one only sees progress in the history record when steps are passed. So looking at the many forms of progress we’ve seen in the past half billion years through the lens of try-try steps, one concludes that these were many easy try-try steps, and so contained no hard steps.

But what if some sort of combo step has been happening instead? During a menu-combo step, one should see the progress of increasingly long menus for each of the parts. And yet it could still be a very hard step, the equivalent of many hard try-try steps happening in parallel. Maybe something about humans was a hard step after all?

Can anyone think of other plausible mechanisms by which hard steps could have a tm dependence, for m > 1?

Added 10a: I expect that an m power step will be completed on average in m/(n+1) of the available window for life on Earth, where n is the total power of the steps done on Earth. So that’s still a problem for having a lot happen in the last half billion years.

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How Far To Grabby Aliens? Part 2.

In my last post, I recommended these assumptions:

  1. It is worth knowing how far to grabby alien civs (GCs), even if that doesn’t tell about other alien types.
  2. Try-try parts of the great filter alone make it unlikely for any one small volume to birth an GC in 14 billion years.
  3. We can roughly estimate GC expansion speed, and the number of hard try-try steps in the great filter.
  4. Earth is not now within the sphere of control of an GC.
  5. Earth is at risk of birthing an GC soon, making today’s date a sample from GC time origin distribution.

I tried to explain how these assumptions can allow us to estimate how far away are GC. And I promised to give more math details in my next post post. This is that next post.

First, I promised to elaborate on how well tn works as the chance that a small volume will birth a GC at time t. The simplest model is that eternal oases like Earth are all born at some t=0, and last forever. Each oasis must pass through a great filter, i.e., a sequence of hard steps, from simple dead matter to simple life to complex life, etc., ending at a GC birth. For each hard step, there’s a (different) constant chance per unit time to make it to the next step, a chance so low that the expected time for each step is much less than t.

In this case, the chance of GC birth per unit time in a small volume is tn, with n = h-1, where h is the number of hard steps. If there are many oases in a small volume with varying difficulty, their chances still add up to the same tn dependence as long as they all have the same number of hard steps between dead matter an a GC.

If there are try-once steps in the great filter, steps where an oasis can fail but which don’t take much time, that just reduces the constant in front of tn, without changing the tdependence. If there are also easy steps in this filter, steps that take expected time much less than t, these just add a constant delay, moving the t=0 point in time. We can accommodate other fixed delays in the same way.

We have so far assumed that, one the prior steps have happened, the chance of each step happening is constant per unit time. But we can also generalize to the case where this step chance per time is a power law tm , with t the time since the last step was achieved, and with a different mi for each step i. In this case, h = Σi (1+mi). These step powers m can be negative, or fractional.

Instead of having the oases all turn on at some t=0, oases like Earth with a chance tn can instead be born at a constant rate per unit time after some t=0. It turns out that the integrated chance across all such oases of birthing a GC at time t is again proportional to tn, with again n = h-1.

A more elaborate model would consider the actually distribution of star masses, which have a CDF that goes as m-1.5, and the actual distribution of stellar lifetime L per mass m, which has a CDF that goes as m-3. Assuming that stars of all masses are created at the same constant rate, but that each star drops out of the distribution when it reaches its lifetime, we still get that the chance of GC birth per unit time goes as tn, except that now n = h-1.5.

Thus the tn time dependence seems a decent approximation in more complex cases, even if the exact value of n varies with details. Okay, now lets get back to this diagram I showed in my last post:

If the GC expansion speed is constant in conformal time (a reasonable approximation for small civ spatial separations), and if the civ origin time x that shapes the diagram has rank r in this civ origin time distribution, then x,r should satisfy:

((1-r)/r) ∫0x tn dt = ∫x1 tn (1 – ((t-x)D/(1-x))) dt.
Here D is the space dimension. D = 3 is appropriate on the largest and the small many-star scales, but D = 2 across galaxy disks, and D = 1 in filaments of galaxies. This equation can be solved numerically. The ratio of the time from an GC origin til that GC directly meets aliens, relative to universe age at civ origin, is (1-x)/x, and is shown in this table:

The x-axis here is the power n in tn, and the y-axis is shown logarithmically. As you can see, aliens can be close in the sense that the time to reach aliens is much smaller than is the time it takes to birth the GC. This time til meet is also smaller for higher powers and for more spatial dimensions.

Note that these meet-to-origin time ratios don’t depend on the GC expansion speed. As I discussed in my last post, this model suggests that spatial distances between GC origins double if either the median GC origin time doubles, or if the expansion speed doubles. The lower is the expansion speed relative to the speed of light, the better a chance a civ has of seeing an approaching GC before meeting them directly. (Note that we only need a GC expansion speed estimate to get distributions over how many GCs each can see at its origin, and how easy they are to see. We don’t need speeds to estimate how long til meet aliens.)

To get more realistic estimates, I also made a quick Excel-based sim for a one dimensional universe. (And I am happy to get help making better sims, such as in higher dimensions.) I randomly picked 1000 candidate GC origins (x,t), with x drawn uniformly in [0,1], and t drawn proportional to tn in [0,1]. I then deleted any origin from this list if, before its slated origin time, it could be colonized from some other origin in the list at speed 1/4. What remained were the actual GC origin points.

Here is a table with key stats for 4 different powers n:

I also did a version with 4000 candidate GCs, speed 1/8, and power n = 10, in which there were 75 C origins. This diagram shows the resulting space-time history (time vertical, space horizontal):

In the lower part, we see Vs where an GC starts and grows outward to the left and right. In the upper part, we see Λs where two adjacent GC meet. As you can see, for high powers GC origins have a relatively narrow range of times, but a pretty wide range of spatial separations from adjacent GC.

Scaling these results to our 13.8 billion year origin date, we get a median time to meet aliens of  roughly 1.0 billion years, though the tenth percentile is about 250 million years. If the results of our prior math model are a guide, average times to meet aliens in D=3 would be about a factor two smaller. But the variance of these meet times should also be smaller, so I’m not sure which way the tenth percentile might change.

A more general way to sim this model is to:

  • A) set a power n in tn and estimate 1) a density in space-time of origins of oases which might birth GCs, 2) a distribution over oasis durations, and 3) a distribution over GC expansion speeds,
  • B) randomly sample 1) oasis spacetime origins, 2) durations to produce a candidate GC origin after its oasis origin times, using tn , and 3) expansion speed for each candidate GC,
  • C) delete candidate GCs if their birth happens after its oasis ends or after a colony from another GC colony could reach there before then at its expansion speed.
  • D) The GC origins that remain give a distribution over space-time of such GC origins. Projecting the expansion speed forward in time gives the later spheres of control of each GC until they meet.

I’ll put an added to this post if I ever make or find more elaborate sims of this model.

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