A Competitor to the Drake Equation

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BillTre
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Summary:

A alternative to the Drake Equation for thinking about how likely intelligent life on other planets has been recently published.

Main Question or Discussion Point

An article in The Astrophysical Journal (not open access, here is an arXiv version): The Astrobiological Copernican Weak and Strong Limits for Intelligent Life proposes a different way to think about the possibility of intelligent life other places than earth.

Abstract:
We present a cosmic perspective on the search for life and examine the likely number of Communicating Extra-Terrestrial Intelligent (CETI) civilizations in our Galaxy by utilizing the latest astrophysical information. Our calculation involves Galactic star formation histories, metallicity distributions, and the likelihood of stars hosting Earth-like planets in their habitable zones, under specific assumptions which we describe as the Astrobiological Copernican Weak and Strong conditions. These assumptions are based on the one situation in which intelligent, communicative life is known to exist—on our own planet. This type of life has developed in a metal-rich environment and has taken roughly 5 Gyr to do so. We investigate the possible number of CETI civilizations based on different scenarios. At one extreme is the Weak Astrobiological Copernican scenario—such that a planet forms intelligent life sometime after 5 Gyr, but not earlier. The other is the Strong Astrobiological Copernican scenario in which life must form between 4.5 and 5.5 Gyr, as on Earth. In the Strong scenario (under the strictest set of assumptions), we find there should be at least
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civilizations within our Galaxy: this is a lower limit, based on the assumption that the average lifetime, L, of a communicating civilization is 100 yr (since we know that our own civilization has had radio communications for this time). If spread uniformly throughout the Galaxy this would imply that the nearest CETI is at most
apjab8225ieqn2.gif
lt-yr away and most likely hosted by a low-mass M-dwarf star, likely far surpassing our ability to detect it for the foreseeable future, and making interstellar communication impossible. Furthermore, the likelihood that the host stars for this life are solar-type stars is extremely small and most would have to be M dwarfs, which may not be stable enough to host life over long timescales. We furthermore explore other scenarios and explain the likely number of CETI there are within the Galaxy based on variations of our assumptions.
Why Copernican?
Our assumption is based on what we call the Principle of Mediocrity: there is no evidence to assert that the Earth should be treated as a special case, and therefore – according to the Copernican Principle – we propose that the likelihood of the development of life, and even intelligent life, should be broadly uniformly distributed amongst any suitable habitats.
This is based on the only case of intelligent life that we know of, on earth.
They are only considering goldilock zone planets and not potential sites of life formation that could be heated by gravitation of large nearby planets (like Europa and Enceledus).

What I find interesting about their argument:
  • Life is assumed to be a result of planetary processes.
  • They emphasis the presence of metals in star forming areas. This means at least second generation stars, after higher molecular weight atoms have been formed in stellar processes, which means life would not be expected to form rapidly after the big bang. Heavier elements are essential for life as we know it (life on earth).
  • They take how long various stages of life took to evolve on earth as rough guidelines for when life could evolve after planets form.
They predicted high and low densities of intelligent communicating civilizations in the galaxy. The densities predicts a lower limit of 36 (+175, -32) civilizations (or between 4 and 211 civilizations, I guess). They use these densities to predict an average distance between civilizations of 17,000 (+33,600, -10,000) light years (between 50,600 and 7,000 light years), which makes communication between communication capable civilization unlikely within the limits of light speed.
I believe that this also addresses parts of the "where are they?" argument.
 
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  • #2
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Isn’t this more like narrowing the Drake equation by making some informed guesstimates?

Anyway, this is a good number to know For sci-fi stories. You could develop 12 extinct cultures, against a backdrop of 12 competing civilizations with room for 12 primitive ones coming into being and current with existing science.
 
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  • #3
phyzguy
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The problem I have with these arguments is that we have no idea what the probability of intelligent life evolving on an Earth-like planet is. Just because it happened once does not mean we can assume that it will happen in a similar length of time on a similar planet. Perhaps the odds of intelligent life evolving are 1 in 10^100 and we just got really lucky. We just don't know, and no amount of pondering or detailed analysis will change "we don't know" into a numeric estimate.
 
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  • #5
BillTre
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The problem I have with these arguments is that we have no idea what the probability of intelligent life evolving on an Earth-like planet is. Just because it happened once does not mean we can assume that it will happen in a similar length of time on a similar planet. Perhaps the odds of intelligent life evolving are 1 in 10^100 and we just got really lucky. We just don't know, and no amount of pondering or detailed analysis will change "we don't know" into a numeric estimate.
I think their argument is that given the proper kind of planetary setting, life is going to evolve within some time span derived from the time course of what happened on earth.
When dealing with evolving of more highly organized life (more complex like eukaryotes or life that can communicate beyond their planet) by using the times from evolution on earth, this time span approach could lead to under or over estimates.
In this way I think it is different from the Drake equation.
The later transitions, to more complex (eukaryotic) life and to intelligent communicating life are more questionable to me.

Its also not clear how long intelligent life will last.
 
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  • #6
Buzz Bloom
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I think their argument is that given the proper kind of planetary setting, life is going to evolve within some time span derived from the time course of what happened on earth.
Hi Bill:

I agree that this is the approach of the CETI equation which is an alternative to Drake. Drake's fl is taken as an unknown:
fl is the fraction of ne planets that might be able to support life which actually develop life.​
Do you agree that this CETI approach seems to be very naive? There is a plausible theory that the Earth's moon was an essential requirement for life to form on Earth, for example:
The CETI approach just arbitrarily assumes that fl = 1, and then makes calculations of the time it might take to form.
The other is the Strong Condition in which life must form between 4.5 to 5.5 Gyr, as on Earth.​

Regards,
Buzz
 
  • #7
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Gould made the point that for at least 2 billion years, early earth life was only one-celled. That's a LONG time before complex life evolved, much less anything remotely "intelligent" (if we even are as yet).
 
  • #8
BillTre
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Do you agree that this CETI approach seems to be very naive?
I think anything making conclusions about a very complex process (origin of life, origin of complex life, origin of intelligent life) which has only been observed once and has not been replicated in a lab is going to be some what naive.

There is a plausible theory that the Earth's moon was an essential requirement for life to form on Earth, for example: https://phys.org/news/2015-11-moon-life.html .
Most of the results this article claims for origin of life, complex life, and intelligent life are not that impressive to me.
Changing environments to drive life to greater complexity is a fairly common assumption, but not limited to having a moon.
Moons around large planets (Saturn and Jupiter) are also thought to have gravitionally heated interiors. (Substitute for a moon?)

Having a churning geology might be important in that it continuously brings new minerals up to be exposed to the atmosphere and oceans to continue to drive chemical reactions with the newly exposed minerals might be important to maintaining the chemistry thought to nurture early life. Its not clear to me that this depends upon having a moon rather than an internal heat source. In addition, it has been proposed that planets like Pluto (with fairly new looking surface) may also have a churning geology (although not make of rock/metal).

Gould made the point that for at least 2 billion years, early earth life was only one-celled. That's a LONG time before complex life evolved, much less anything remotely "intelligent" (if we even are as yet).
I think this is an important point.
What I am calling complex life is eukaryotic cells, formed through the symbiosis of a bacteria (which became mitochondria) with an archaeal cell. Life on earth has evolved through the series of simple life (prokaryotic(, complex life (eukaryotic), and intelligent life (like humans).
Complex life is (In my mind) probably a prerequisite to developing intelligent life. Prokaryotes (bacteria and archaea) are too limited (energy-wise) to form anything complex enough to achieve intelligence. Eukaryotes, by harnessing the bacterial power source of mitochondria, but losing any excess bacterial baggage, gained a lot of power to build complexity with (see many Nick Lane papers and books). Without that power and complexity, there would have been no intelligent life (brains are energetically expensive).

The 2 billion year delay to develop complex life forms might depend upon a variety of things.
  • Perhaps the development of prokaryotic biochemical diversity so that it could be fruitfully combined in eukaryotes took that long.
  • Perhaps it was such a lucky, unlikely, fluke occurrence that it took 2 billion years to happen.
  • Perhaps it required the unlikely juxtaposition of two different environment where the pre-mitochondrial bacteria and the archaeal cells evolved where they could fruitfully come together.
  • The environments required for complex life may have required an environmental transformation driven by life itself (such as the great oxgenation event) which would have created new kinds of environments. This kind of limitation would require waiting for a the products of life to build up enough to cause geological scale changes, which might take a while.
It might have happened more quickly in other circumstances or not at all. Hard to tell.

All of this stuff raise questions about the timeline of the article.
However, I still think its quite interesting and provocative of further thought.
 
  • #9
jim mcnamara
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I think you may be ignoring the fact that eukaryotic cells use oxygen. It took "2 billion years" to get enough free oxygen to allow eukaryotic respiration to out compete procaryotic respiration. - an environmental change thanks to cyanobacteria.

https://en.wikipedia.org/wiki/Great_Oxidation_Event
 
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  • #10
Buzz Bloom
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Its not clear to me that this depends upon having a moon rather than an internal heat source.
Hi Bill:

A role of the moon that I read about somewhere in the past (where not rememebred) is that the combination of solar and lunar tides create four repetitive tidal phases to drive the four cyclic "RNA world" stages of RNA reproduction.
1. Separating the double strand.
2. Having single RNA strands form 3D shapes (ribozymes) that serve as aids to chemical reactions.
3. Opening the 3D shaped strands.
4. Having a single RNA strand attract nucleotides to it to form a double strand.

ADDED
The article cited below discusses a similar idea to the above.

Regards,
Buzz
 
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  • #12
Buzz Bloom
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at the very best, the RNA world is a hypothesis.
Hi jim:

The article you cited has the following quote.
There is now strong evidence indicating that an RNA World did indeed exist before DNA- and protein-based life.​
This is the key point in my post. In the article the next sentence is the following.
However, arguments regarding whether life on Earth began with RNA are more tenuous.​
This does not contradict the point that a lot more pre-biology had to happen before there was something that might be called life.

BTW, this article is dated 2012. I think is was last year that I read something about an experiment with RNA self replicating. What was most interesting was that this experiment involved two different RNA sequences each acting as an enzyme (called a ribozyme) which facilitated the reproduction of the other sequence. Of course this experiment may not have used the same mechanism that actually supported RNA replication in the RNA world. However, it seems to me to represent a lot more progress since 2012 than the hypothesis in the article.
... it is fruitful to consider the alternative possibility that RNA was preceded by some other replicating, evolving molecule, just as DNA and proteins were preceded by RNA.​

ADDED
The following is an article that has some relevance to the concept in my post #10.

Regards,
Buzz
 
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  • #13
BillTre
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Hi Bill:

A role of the moon that I read about somewhere in the past (where not rememebred) is that the combination of solar and lunar tides create four repetitive tidal phases to drive the four cyclic "RNA world" stages of RNA reproduction.
1. Separating the double strand.
2. Having single RNA strands form 3D shapes (ribosymes) that serve as aids to chemical reactions.
3. Opening the 3D shaped strands.
4. Having a single RNA strand attract nucleotides to it to form a double strand.

Regards,
Buzz
Hi Buzz,
Its not clear to me why different tidal phases would have these different causes.
What's the mechanism?
Drying down of a collection of chemicals has been proposed as a mechanism for driving polymerization (by increasing their concentrations) but not for the other actions that I know of.

Also I would like to point out that the "RNA world" and also terms like "RNA first" mean different things to different people, leading to mis-understandings.
I prefer more to the point descriptions of what the proposed situation/mechanisms are intended to be. This helps with discussions.
 
  • #14
Buzz Bloom
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Its not clear to me why different tidal phases would have these different causes.
What's the mechanism?
Hi Bill:
My memory is quite vague, but I will try as best I can to reconstruct the admittedly hypothetical mechanism.

Assume that sea water has a slightly basic pH.
Assume an environment with a steady stream of water with pH of 7 coming from a high natural reservoir that refills from rain water.
Assume the stream passes through a region with some natural chemical that raises the pH of the stream water to become slightly acidic.
Then the stream flows into a basin that fills with sea water at high tides, more so from spring tides, but also some from neap tides.

Phase 1: The basin is fully filled turbulently with the high spring tide sea water slightly basic. The RNA double strands float around freely in the water with the double strands tightly bound with hydrogen bonds.

Phase 2: The low tide comes and it no longer fills the basin. The streams flows into basin. The basin water in it becomes slightly acidic. This weakens the hydrogen bonds, and the double strands separate.

Phase 3. The tide becomes the neap tide. The water calms down and the single RNA strands sink to the bottom where their "backbones" attach to some crystals. (I forget what the crystals are, maybe with aluminum, but their crystal periodicity is close to the size of the distance between RNA nucleotides. I also do not remember what the chemistry is that makes a fairly good bond between the crystal and the single strand RNA "backbone".)

Phase 4. The pH of the basin water has returned to basic. This allows the nucleotides to be attracted to the active part of the RNA which is bound to the crystal. During this phase the double helix is formed. (Note that the shape is not yet a helix because the RNA backbone is straight.)

Phase 1 (again). The spring tide turbulence separates the double strands of RNA from the crystals. The basic pH causes these double helix strands to roam freely in the basin with their strong hydrogen bonds.

I hope you find this to be reasonably clear as a possible plausible example of the mechanism.

Regards,
Buzz
 
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  • #15
Dr. Courtney
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Seems like this paper is on the bounds of testability.

But there do seem to be circumstances and future observations under which we could say the predictions have been falsified.

So I guess it is better than string theory, which is not even wrong.

I hope they're not getting too much grant money for this stuff, and I hope they pick an easier to read font next time.
 
  • #16
Vanadium 50
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I spent a little time with this paper, and what I read doesn't ,make me want to invest any more.

The claim that there are terms in the Drake Equation that are poorly known and likely to remain so, but the product of these terms is better known. Sure. This happens all the time in lots of contexts: anyone interested in orbintal mechanics knows that GM is often better known than G or M.

The error analysis is innumerate and their significant figure usage would not pass muster in a 5th grade science fair: for example, they say "we find there will be a minimum of 928+1980-818 civilizations". Ignoring the question of what an error bar means on a lower bound, they are saying they don't know the central value to within a factor of 30 (could be between 100 and 3000) but they know the lower error bar to almost 0.1%.

The central argument is that the Earth is typical. It's not a very good argument, because even if the Earth were atypical, we would use the same data to conclude it's typical.
 
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  • #18
BillTre
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I felt I should clarify my reservations about the "RNA World" term.

Here is an article from The Scientist on the place of the RNA world concept in biology today.

The common view of the RNA world seems to be that RNA was the first biological molecule and the enzymed the rest of biochemical molecules into existence with its ribozyme powers.
The ribozyme powers combined with RNA being able to store and replicate information make it a good candidate for the first genetic molecule (as we know genetic today). RNA's ability to catalyze reactions, would make possible an inherited (genetically programed) direction to metabolism as well as other functions.
These are clever and innovative ideas that could lead to the origin of life, while putting off the issues of how translation can about, for a later evolutionary time. The translation mechanism, which would have to evolve later, involves a genetic source of protein encoding sequence, the ribosome, 20 different tRNAs, and 20 enzymes that match the right amino acids with tRNAs (with the correct anti-codons). Metabolic energy is used in the translation process to join the amino acids together in a string.

This is now considered too simplistic as other considerations are considered prerequisits for various reasons.
One reason is that a slowly developing (evolving) pre-organism could probably not depend upon using the molecules just found lying around in the ancient earth environment (whether they arise from sources like comets and meteorites (too dilute), or were formed by abiotic sources in other locations (lightening or ...). The chemicals would be too dilute.

Therefore, abiotic sources of organics based on a localized production sources have been proposed as sites for life arising: alkinaline hydrothermal vents (synthetic reactions driven by the redox differences between sea water and the hydrothemal vent fluid), terrestrial hydrothermal fields (intermittent ponds where chemicals can be collected and dried down (causing polymerization) and perhaps others.

Solid surfaces are often involved in these proposed processes (such as: alkaline hydrothermal vent walls (microporous), mica surfaces, nickle iron sulfate compounds in various forms, pumice, clays).
They often have metallic components that can act as natural catalysts. This combined with some energy source (like chemicals ready to react) then would provide a localized source of organic chemical synthesis.
This provides a higher local concentration of organics. Doing this in a contained space (like a foam cell in the wall of an alkaline hydrothermal vent) would better retain the chemicals (rather than them drifting away).

The un-directed abiotic production of these chemicals would generate a variety of simple organic molecules. The more complex nucleotides would not be very common initially.
Once some primitive, rather undirected (no enzymes) metabolism develops, more complex chemicals are made.
Lipids are thought to be among the first of those organic molecules synthesized.
In all the recent origin scenarios, the lipids self assemble into bilayers encasing the molecular production source (either in the foam-like holes in the walls of alkaline hydrothermal vents or as bubbles on the surfaces of other solids with catalytic surfaces).
This benefits the eventual ascendance of life in that it:
  1. creates a permeability barrier so that any molecules being generated can't just diffuse away
  2. provides a container, making a unity from a population of evolving molecules (a unity of inheritance, an individual at a higher level, or a unit of selection fro evolution to act upon)
  3. the membranes become the site of metabolic electron transfer reactions (in the alkaline hydrothermal vents scenario).
After this stage, is where RNAs and their abilities could take these pre-biological vesicles of chemicals to their next step where Darwinian evolution can start shaping the RNA genome.

The RNA would be an important component of all this, but there would be a lot of other molecules around.
It would not be a pure RNA, but instead wold be a dirty RNA world.
 
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  • #19
Buzz Bloom
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The RNA would be an important component of all this, but there would be a lot of other molecules around.
It would not be a pure RNA, but instead wold be a dirty RNA world.
Hi Bill:

I have not yet read the Scientist article, but it does look interesting, and I defintely do plan to read it. Thanks for posting the reference.

About the above quote: In this dirty RDA world, were there any other molecules than RNA that reproduced?

Regards,
Buzz
 
  • #20
BillTre
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About the above quote: In this dirty RDA world, were there any other molecules than RNA that reproduced?
Probably not unless you want to consider possible RNA-like molecules (forget their names right now) which have been propose to precede RNA. I think their synthesis is supposed to have been easier.
The other molecules would be generated by whatever proess is generating organic molecules, or through the function of a primitive metabolism that some claim could run with out enzymes.
 
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  • #21
Buzz Bloom
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The other molecules would be generated by whatever process is generating organic molecules, or through the function of a primitive metabolism that some claim could run with out enzymes.
H i Bill:

Thanks much for your reply.

It does seem that as time passes speculation continues to grow with respect to more and more aspects of science topics. Until there is some significant new evidence supporting a speculation, I generally prefer my own biased choices, and I see no reason why I shouldn't.

Regards,
Buzz
 
  • #22
BillTre
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It does seem that as time passes speculation continues to grow with respect to more and more aspects of science topics. Until there is some significant new evidence supporting a speculation, I generally prefer my own biased choices, and I see no reason why I shouldn't.
There is a lot of science, but in this field, it is scattered in a lot of different and difficult to reference.
I would recommend doing a lot of reading of recent books on the subject, plus papers.
There is a lot of people, trying to replicate aspects of pre-biotic chemistry in labs, who are finding interesting results. Finding what chemicals can be created under what conditions.

The origin of life field is very complex. There are many ideas, some of which cover what I would consider relevant aspects of things and others not.
Not every scenario covers all interesting aspects of the abiological to biological transition.
I often feel like its a kind of pick things that seem plausible and string them together to make a somewhat reasonable whole.
Not the most inspiring, but hard to do better right now.
 
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  • #23
bob012345
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Its also not clear how long intelligent life will last.
:eek: It's not clear that intelligent life exists at all.
 
  • #25
Buzz Bloom
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Isn't most of this discussion based on the presumption of carbon-based life, RNA/DNA, and H2O? For one thing, the "goldilocks" zone depends on the chemistry.
Hi anorlunda:

There are distinctions concerning what is not known. Life exists on Earth, and it is a relatively simple extrapolation that maybe it could exist elsewhere in our galaxy. This is a relatively modest speculation, and there are methods that can be used to search for clues about whether it has happened.

Other forms of life not based on the Earth life chemistry (or something very similar) is complete speculation with no current or past speculations that I have ever seen (including Asimov) about possible methods for seeking it.

Regards,
Buzz
 
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