60 billion planets in Milky Way may contain life

[AdMan]

The...
<much verbiage snipped>

Are you backing away from your earlier claim that physics works differently in different parts of the universe?

 

[gsmonks]

Are you backing away from your earlier claim that physics works differently in different parts of the universe?

I made no such claim. You misinterpreted what I said.

Let me try to explain it to you in other terms:

A body of knowledge can be analysed taxonomically, and like the taxonomy of a living thing is contingent upon the existing state of the constituents of that body of knowledge, each step of the way.

Environment plays a key role in the chain of events. Here on Earth environment translates as range of temperature, availability of resources, changing conditions over long time scales due to plate tectonics, changing conditions over short time scales due to such factors as the Earth's orbit and the pull of the Moon on the Earth and its oceans.

I might also remind you of the role life played in forming the pretty blue ball we know as Earth today. The planet is liveable today largely because living organisms made it that way over millions of years.

Every body of knowledge shares this in common with evolution- that every step along the way is contingent upon existing conditions. Change the conditions and you change the outcome.

While it is true that the fundamental axiom of physics is the reproducibility of results, results themselves are contingent upon environmental factors. Change the environmental milieu and you change the course taken as the body of knowledge develops.

The things Humanity has discovered, its entire bodies of knowledge, occurred within a very narrow set of conditions. Alter those conditions and you alter the outcome.

For example, consider the body of knowledge concerning art music. The basic underlying principles were first laid out by Pythagoras. He studied the vibration of strings and codified the manner in which harmonics work. Later scholars such as Boetheus wrote theoretical treatises on the mechanics and structure of theoretical music. By the 12th century composers such as Leoninus and Perotinus had built upon their work to the point that counterpoint (from the Latin punctum-contra-punctum) could be developed following their initial efforts at plain chant (aka Gregorian chant). Next came the Invertible Counterpoint of Palestrina in the 15th century, the counterpoint of Bach in the 17th century, bitonality in the early 20th century, and finally the microtonality of Ligeti in the early 1960's.

The point being that if you change the underlying conditions, either none of this would have occurred, or else it would have happened far differently. Why? Because for starters, sound waves travel at very different speeds through different gasses. I'm sure you're familiar with what happens when someone inhales helium and talks. The result is a humorous, high-pitched, cartoonish voice.

When I was studying the social psychology of music in the 1970's, one of the things we did was listen to music through various media such as gasses, solids and liquids. Music as we understand it either sounded very different or in many instances didn't work at all.

Does this apply to the sciences? Of course it does. All Human thought works along taxonomic lines, and the sciences are no exception. Change the environment and you change the content in ways that you can't begin to imagine.

I'm not referring to the Period Table or the Periodic Table of Subatomic Particles here. I'm referring to everything that follows. We live in a world of tremendously complicated strings of proteins and complex biochemical compounds and mountains of other sophisticated stuff that make up both us and everything we think we know, including our consciousness and perception. All of this began with a roll of the dice billions of years ago and countless links in a chain later here we are.

Believing that countless links in a chain over billions of years on other planets is going to result in creatures like ourselves is the very definition of insanity, said definition being the reverse of doing the same thing over and over and over again expecting a different result.

 

[This is The End]

The same is true of an electronic megaphone. It's a tool built by humans to project the human voice. The image on the screen you're looking at right now is an illusion that is geared to the way in which the human eye works. An entity with an optical sense wholly unlike ours would see nothing but an annoying flickering.

These are not good examples. At all.

The megaphone example is especially bad.

Audio and Video are universal physical properties. They have nothing to do with humans or even living creatures.

The electronic megaphone would work on anything that makes noise, even things that are not alive. Even if it only works on certain frequencies it still has nothing to do with humans.

The monitor is perhaps not as flawed as an example because of more vastly different frequency spectrums, but anything that sees the same frequencies as us and as well as us is going to see the video normally.

 

[Dinwar]

gsmonks, do you have a background in any relevant field? Because it seems you don't.

As for "The Gaia Hypothesis", I was (quite obviously) referring to the book, not the moronic nonsense that grew up around it. The concept of feedback loops impacting atmospheric chemistry is now widely accepted, and many of Lovelock's other ideas in the original book hold true. I've actually read the book, and can give a profesional opinion on it. I doubt you've read it, and I seriously doubt you can give any opinion on it worth considering.

Do you understand what Markov Chain Analysis is? What Q and R mode analysis are? Do you grasp why the periodic table is what it is? Are you familiar with the nuclear forces that dictate the characteristics of the elements? If not, you've got nothing to say that's worth listening to.

 

[gsmonks]

gsmonks, do you have a background in any relevant field? Because it seems you don't.

As for "The Gaia Hypothesis", I was (quite obviously) referring to the book, not the moronic nonsense that grew up around it. The concept of feedback loops impacting atmospheric chemistry is now widely accepted, and many of Lovelock's other ideas in the original book hold true. I've actually read the book, and can give a profesional opinion on it. I doubt you've read it, and I seriously doubt you can give any opinion on it worth considering.

Do you understand what Markov Chain Analysis is? What Q and R mode analysis are? Do you grasp why the periodic table is what it is? Are you familiar with the nuclear forces that dictate the characteristics of the elements? If not, you've got nothing to say that's worth listening to.

I'm 70's-vintage physics and psychology and studied privately to become a composer of classical music and professional pianist, brass musician and bassist.

Until you learn to spell, I'm going to take everything you say with a grain of salt. And it's Q and R mode analyses, not analysis.

You come across as professional (which you spelled "profesional") as a self-taught brain surgeon.

 

[R.A.F.]

There were no aliens in Star Trek. There were just humans in silly costumes behaving like humans.

Your knowledge of Star Trek is limited. There were, of course, numerous episodes with non-humanoid aliens.

 

[Dinwar]

I'm 70's-vintage physics and psychology and studied privately to become a composer of classical music and professional pianist, brass musician and bassist.

I'm reminded of Patrick1000. He was another poster that made grandious claims about his background, yet every single post he made disproved it.

Here's a clue: in the 70s, we had a pretty good sense of how atoms worked.

Until you learn to spell, I'm going to take everything you say with a grain of salt.

So you're not going to bother with the actual data because of some petty reason. Gee, what a surprise. This tells me you have no interest in actually discussing anything--you're simply looking for excuses to dismiss anyone who disagrees with you. You obviously know what I'm saying, after all--you're rejecting it not because it's wrong, but because you've found the most petty excuse imaginable.

Sure- when you quit being such a self-righteous little prick.

Odd how often projection happens on this forum.

What part of "environment" do you not understand? All development in the sciences and engineering is environment-driven.

When you demonstrate how strong force, weak force, and electromagnetism are influenced by our environment, we can discuss the validity of this statement. As it stands, this concept is presented as nothing more than an article of faith on your part--you hold to it despite the evidence.

All dogma and no open-mindedness makes Jack a dull SETI scientist.

See, here's the thing: we've actually presented evidence and logic and reason to support our arguments. You've done nothing but insult people.

 

[JoeTheJuggler]

Well the low-end estimate for number of stars is 100 billion. High end is 400 billion.

A hundred billion would be only one planet per star? But our own has 8. Many other stars where we have found exoplanets also have multiple planets. Seems like there's probably more, but maybe 100 billion is the low end of the range and it could actually be 10 times that or even more.

But if we limit our consideration to terrestrial planets (rocky ones as opposed to gas giants), it might be considerably lower.

There are still a lot of unknowns in the Drake Equation. Still, we can say that so far pretty much everywhere we've looked for planets of a given set of characteristics, we've found them in abundance.

I hope the attempts to get Kepler functioning again are successful.
http://www.nasa.gov/content/kepler-mission-manager-update-recovery-begins/#.UelfKt-FGcE

 

[Dinwar]

The sad part is, this had the potential to be a really interesting discussion. And useful--we've got enough people with the right backgrounds here that we actually stand a chance of coming up with some workable concepts for alien life, and methods for looking for them. But for some people, personal validation comes before actually learning anything.

 

[John Jones]

The sad part is, this had the potential to be a really interesting discussion. And useful--we've got enough people with the right backgrounds here that we actually stand a chance of coming up with some workable concepts for alien life, and methods for looking for them. But for some people, personal validation comes before actually learning anything.

Geez. It looks like I got here too late. A pity.

ETA: I went back and read the whole thread. Wow!

It's incredable witnessing someone speak at such length on subjects they don't understand.

And spelling flames just for extra fun!

I had replies to gsmonks' points, but everyone else here handled the situation quite well.

--
NOTE: All spelling errors left intact for the benifit of those having no other rejoinder.

 

[McHrozni]

I never quibbled with whether or not we've yet replicated it in a lab. In fact, though, we have observed the formation and splitting of vessicles formed by simple lipids and we have observed polymers of all sorts forming and we have observed that some polymers are self-replicating.

We soon run to the issue of what is alive and what isn't. Splitting lipid vesicles are a tad short of that.

[ETA: So I'm not sure what needs to be replicated in a lab that hasn't. Getting to this basic, proto-cell stage doesn't take long at all. Getting from there to something that could out-compete an existing bacterium doesn't just take long, it's virtually impossible with the great head start the bacterium has.]

Keeping the whole thing sterile could be a good idea. Certainly outcompeting a bacterium from the start is out of the question, but developing something we could agree was alive would be a good thing.

I was pointing out that we actually have a pretty good idea of how life forms from abiotic molecules.

Assuming that is indeed how life evolved. We have a pretty good idea of how it could have happened. The video effectively called life a thermodynamic necessity under the right conditions.
I'm not saying it's false, I'm saying I need more than a simple video aimed at religious freaks to convince that is indeed how it happened and it is the truth.

McHrozni

 

[McHrozni]

I don't want to sound a silly denialist, so I'll add a few issues with what the video presented.

1. Random rearrangements of the proto-genetic code would generate a code that would exhibit some kind of enzymatic activity that would make the vesicle more likely to replicate itself is certainly possible, but rather unlikely. How unlikely is a bit of guesswork, but under the assumption that you need 20 amino acids, that works out to somewhere in 10^-30 range*.

2. Once it happens, you still have a highly unstable proto-genetic code in an environment that isn't particularly great for it to sustain itself, with no repair mechanism. You'll loose the code quickly, probably within a few cycles through the vent.

3. Furthermore, the translation in this enviroment will be slow, and you'll only get very small amounts of active peptides every time you do get it. Even if you do manage to get a code that gives the vesicle an advantage over the others, the already small advantage will be further diminished by small amounts of peptide produced.

4. Peptides degrade, for example when exposed to heat. You're in a volcanic stream for a while quite regularly. This reduces the effect of whatever advantage that few molecules of a peptide may have brought even further.

5. You're just as if not more likely to obtain a peptide that gives the vesicle a disadvantage over the others. Having a useful code in the past doesn't shield you from that in any way, and may actually make it worse (you would be more likely to get enzymatic activity within a few mutations).

These, along with other concerns (i.e. environmental stability), amount to a question - would described mechanism by itself be strong enough to overcome the many random background effects? It may. It also may not. The "hugely unlikely" issue has not been solved, and I remain rightfully skeptical.

The video presents a compelling theory, which probably is part of the truth. It is useful to show religious kooks that science indeed gets all the answers eventually. However it is sanitized and not to be taken for certainty.

* 20 amino acids with 3 nucleotides per amino acid and 4 nucleotides amounts to 7,5*10^-37, but the genetic code is degenerated (making it more likely) and there are a number of combinations that would be effective is greater than one (again more likely), but in the early world you'll likely have more than just 4 nucleotides (making it less likely). The estimate I used above above should be accurate within a few orders of magnitude.

McHrozni

 

[JoeTheJuggler]

We soon run to the issue of what is alive and what isn't. Splitting lipid vesicles are a tad short of that.

And splitting vesicles that contain self-replicating polymers?

IMO, that's the point where selection begins. In each new "generation" of vesicles, you'll have more of the better self-replicators.

You already at that point have some competition for the things that are strung together to make polymers. Then you start getting other functions. There's really no mystery left.

So what definition of life would you offer that would exclude something as simple as a lipid vesicle that contains a self replicating polymer?

Assuming that is indeed how life evolved. We have a pretty good idea of how it could have happened. The video effectively called life a thermodynamic necessity under the right conditions.
I'm not saying it's false, I'm saying I need more than a simple video aimed at religious freaks to convince that is indeed how it happened and it is the truth.

Again, I was addressing your specific claim that we have little idea how it happened.

In fact, we have observed all of the steps that get us from abiotic molecules to something that interacts with its environment and reproduces with variation. Reproduction with variation is all you need for natural selection to come into play.

 

[JoeTheJuggler]

1. Random rearrangements of the proto-genetic code would generate a code that would exhibit some kind of enzymatic activity that would make the vesicle more likely to replicate itself is certainly possible, but rather unlikely. How unlikely is a bit of guesswork, but under the assumption that you need 20 amino acids, that works out to somewhere in 10^-30 range*.

I note your calculation is limited to protein formation (polymers composed of amino acids). Even so I would point out that if you are talking about events that happen trillions of times in the world every second, even extremely low probability events are virtually certain to happen, given enough time. And again, once you have something more likely to replicate itself, that thing will begin appearing with greater frequency in subsequent generations (affecting your "random" probability calculation already).

I'm not sure why your standard of evidence is that something must happen in a volume of liquid in a test tube [ETA: and in a matter of hours to months rather than over hundreds, thousands, tens of thousands, hundreds of thousands or millions of years] for you to accept that it is likely to have happened somewhere on the planet oEarth.

2. Once it happens, you still have a highly unstable proto-genetic code in an environment that isn't particularly great for it to sustain itself, with no repair mechanism. You'll loose the code quickly, probably within a few cycles through the vent.

Yet we'll "loose" those replicators less than we'd lose non-replicators; and we'd lose poorer replicators less than we'd lose better replicators. [ETA: Or more accurately, those losses will happen to both sets. The result will still be more replicators than non-replicators and more of the better replicators than the poorer replicators even after losses. And if a trait that gives resistance to whatever causes those losses arises, molecules with it will be more abundant than molecules without it in environments with that cause of loss in successive generations.] That is selection.

3. Furthermore, the translation in this enviroment [sic] will be slow, and you'll only get very small amounts of active peptides every time you do get it. Even if you do manage to get a code that gives the vesicle an advantage over the others, the already small advantage will be further diminished by small amounts of peptide produced.

At this simple level, I'm not really talking about a polymer that gives an advantage to the vesicle. I'd be willing to assume that vesicles elongate and break off at the same rate no matter what molecules large enough to be trapped inside are trapped inside. At this stage, there will be a higher frequency of replicating molecules inside the daughter vesicles than non replicating ones. And then there will be a greater number of better replicating molecules than poorer replicating molecules.

4. Peptides degrade, for example when exposed to heat. You're in a volcanic stream for a while quite regularly. This reduces the effect of whatever advantage that few molecules of a peptide may have brought even further.

Heat would be an environmental factor in selection. Molecules destroyed by heat will be selected against in a hot environment. OTOH, molecules that thrive in heat will be selected for.

5. You're just as if not more likely to obtain a peptide that gives the vesicle a disadvantage over the others. Having a useful code in the past doesn't shield you from that in any way, and may actually make it worse (you would be more likely to get enzymatic activity within a few mutations).

Again, the first level of selection goes on within these vesicles. Molecules that reproduce will appear more frequently than those that don't. By the time we get to molecules that somehow benefit the vesicle as a whole, you've already had selection at work.

Any changes in a molecule that make it replicate less successfully will be selected against (there will be fewer with that change in subsequent generations). So you don't start with a blank slate each generation. Selection ratchets these traits (or proto-traits, if you like) upward toward greater ability to reproduce.

The "hugely unlikely" issue has not been solved, and I remain rightfully skeptical.

The "hugely unlikely" issue has in fact long been solved.

Given enough chances, highly unlikely events are virtually certain to happen.
http://www.skepdic.com/lawofnumbers.html

And once you have selection at work, you don't start from a "random" place with each successive generation. No matter how unlikely, any trait that appears that is advantageous to replication will result in more of that type of molecule and any trait that appears that is less advantageous to replication will result in fewer of that molecule.

The video presents a compelling theory, which probably is part of the truth. It is useful to show religious kooks that science indeed gets all the answers eventually.

And it sufficiently debunks your claim that we have little idea how it happened.

 

[McHrozni]

I note your calculation is limited to protein formation (polymers composed of amino acids). Even so I would point out that if you are talking about events that happen trillions of times in the world every second, even extremely low probability events are virtually certain to happen, given enough time. And again, once you have something more likely to replicate itself, that thing will begin appearing with greater frequency in subsequent generations (affecting your "random" probability calculation already).

Sure. If could happen, say a billion trillion of times every second, it would happen a lot. About once every 31 years. Most wouldn't result in anything. See the problem? The effect is rather weak. Sure, it could eventually lead to something, just not within the lifetime of a universe.

I'm not sure why your standard of evidence is that something must happen in a volume of liquid in a test tube [ETA: and in a matter of hours to months rather than over hundreds, thousands, tens of thousands, hundreds of thousands or millions of years] for you to accept that it is likely to have happened somewhere on the planet oEarth.

It doesn't have to, but that would be the best thing to have.

Conditions in a test tube can be made optimal and perfectly stable and unstable as required. It can take substantially less than in real world. A computer simulation would be good too, those can be much faster still.

Yet we'll "loose" those replicators less than we'd lose non-replicators and we'd lose poorer replicators less than we'd lose better replicators. [ETA: Or more accurately, those losses will happen to both sets. The result will still be more replicators than non-replicators and more of the better replicators than the poorer replicators even after losses. And if a trait that gives resistance to whatever causes those losses arises, molecules with it will be more abundant than molecules without it in environments with that cause of loss in successive generations.] That is selection.

It's not the issue that the whole thing can't happen as described. It certainly can. But it takes a lot of time, where conditions have to be right. How long, and how likely are conditions to remain stable for that amount of time?

Those are all factors we must consider if we're to debate how common life is.

At this simple level, I'm not really talking about a polymer that gives an advantage to the vesicle. I'd be willing to assume that vesicles elongate and break off at the same rate no matter what molecules large enough to be trapped inside are trapped inside. At this stage, there will be a higher frequency of replicating molecules inside the daughter vesicles than non replicating ones. And then there will be a greater number of better replicating molecules than poorer replicating molecules.

Sure, but there will be no selection within the vesicle for molecules that benefit the vesicle. Rather, it would be towards longer living molecules alone.

Heat would be an environmental factor in selection. Molecules destroyed by heat will be selected against in a hot environment. OTOH, molecules that thrive in heat will be selected for.

That's not really the problem. The problem is that this slows down the process further, making individual steps less likely to work in the first place.

Given enough chances, highly unlikely events are virtually certain to happen.
http://www.skepdic.com/lawofnumbers.html

I'm not arguing against this at all. Unlikely events may happen, regardless how unlikely they are. There is nothing in the video to convince me that life doesn't require several unlikely (though possible) steps, that happened on Earth in much closer temporal proximity as one would normally expect.
Without that we can't use it to predict how common life is in the universe.

Naturally the whole thing is complex, and depends heavily on the amount of material you have to work with. Fifty vesicles will need an ethernity, 50 quadrillion vesicles probably much less.

And it sufficiently debunks your claim that we have little idea how it happened.

We have several good ideas of how it might have happened. Probably all are to some extent true. This one probably is part of the story. I sincierly doubt it's the whole story.

McHrozni

 

[McHrozni]

And splitting vesicles that contain self-replicating polymers?

That too, yes. Life has about 9 defining characteristics, including metabolism, response to stimuli and others.

So what definition of life would you offer that would exclude something as simple as a lipid vesicle that contains a self replicating polymer?

WP has 7 conditions life must fulfill: homeostasis, organization, metabolism, growth, adaptation, response to stimuli and reproduction.

The vesicle with a self-replicating polymer has maybe organization and growth. Adaptation and reproduction are questionable.

Again, I was addressing your specific claim that we have little idea how it happened.

Maybe "little" wasn't the best word. Limited would be indeed better.

McHrozni

 

[JoeTheJuggler]

Sure. If could happen, say a billion trillion of times every second, it would happen a lot. About once every 31 years. Most wouldn't result in anything. See the problem? The effect is rather weak.

I don't see the problem. The events that you're calculating the probability of are chemical interactions that happen in liquid water on the Earth trillions of times every second not once every 31 years. That means something of extremely low probability should happen virtually certainly at all, and repeatedly over thousands of years.

Conditions in a test tube can be made optimal and perfectly stable and unstable as required. It can take substantially less than in real world.

A test tube isn't the real world?

It's not the issue that the whole thing can't happen as described. It certainly can. But it takes a lot of time, where conditions have to be right. How long, and how likely are conditions to remain stable for that amount of time?

Good questions, but the answers to them do not change the fact that your claim that we have little idea how it happened is false.

Those are all factors we must consider if we're to debate how common life is.

I agree. And with the limited information we have (it happened on Earth in a relatively short time), and there is pretty strong evidence that it happened on Mars even though conditions were only right for liquid water to exist for a relatively short time.

That's not really the problem. The problem is that this slows down the process further, making individual steps less likely to work in the first place.

Selection follows no pre-ordained track. I'm pointing out that selection begins as soon as you have self-replicating molecules. Then the individual steps "work" because "work" is defined simply as the observation that variants that are better at replicating will be more abundant in subsequent generations.

I'm not arguing against this at all. Unlikely events may happen, regardless how unlikely they are.

My point is that given enough chances, an extremely unlikely event (extremely low probability for every one chance) becomes and extremely likely event. The odds of winning a lottery are extremely low for every single ticket, but if you buy a billion tickets, your odds of winning are quite high.

There is nothing in the video to convince me that life doesn't require several unlikely (though possible) steps, that happened on Earth in much closer temporal proximity as one would normally expect.

Again, given an extremely large number of chances, even low probability events are expected to happen with near certainty.

Without that we can't use it to predict how common life is in the universe.

Even so, this does nothing to defend your claim that we have little idea how life began from abiotic molecules.

Naturally the whole thing is complex, and depends heavily on the amount of material you have to work with. Fifty vesicles will need an ethernity, 50 quadrillion vesicles probably much less.

And 50 quadrillion is a much more likely guesstimate than 50, since we're talking about a planet with a lot of liquid water on it, and we know that lipids will spontaneously form vesicles. While I have no idea if 50 quadrillion is correct even within several orders of magnitude either way, I'm confident to the point of certainty that 50 is wrong.

And again realize that the relatively "short" time it took for life to start on Earth was still geologic time.

We have several good ideas of how it might have happened. Probably all are to some extent true. This one probably is part of the story. I sincierly doubt it's the whole story.

It may not be the whole story, but it's general enough to be in fact the only idea that fits with the evidence at hand. What other good ideas are you thinking of? That life arrived from outer space actually isn't an alternate explanation for abiogenesis. It still had to start somewhere.

 

[JoeTheJuggler]

Maybe "little" wasn't the best word. Limited would be indeed better.

Since we don't have unlimited knowledge about any topic, observing that our knowledge of abiogenesis is "limited" isn't at all the same as claiming we have little idea of how it happened. In fact, it's such a trivial observation as to be virtually meaningless.

If you're only claiming we don't know enough to fill in all the values in the Drake Equation--including f_l--in order to have an estimate as to how prevalent life (or ETI) is in the galaxy, I agree, and have said as much earlier in this thread.

But the little we do know seems to indicate high numbers. Virtually anytime we used a technique to find extrasolar planets with certain characteristics, we have found them in abundance.

Of the two planets we're aware of that have had liquid water, one is teeming with life (and has been since relatively early in its history), and there is evidence that the other one developed life, even though it no longer has much, if any, liquid water.

 

[McHrozni]

Of the two planets we're aware of that have had liquid water, one is teeming with life (and has been since relatively early in its history), and there is evidence that the other one developed life, even though it no longer has much, if any, liquid water.

If Mars is proven to have (had) life, then the scenario becomes much more likely. But we can't judge from one event, Earth could easily be a fluke of statistics - either that life isn't likely at all but only happened quickly on Earth by chance, or that it requires a specific set of conditions for specific times with correctly timed changes to occur, and it happened on Earth.

I don't see the problem. The events that you're calculating the probability of are chemical interactions that happen in liquid water on the Earth trillions of times every second not once every 31 years. That means something of extremely low probability should happen virtually certainly at all, and repeatedly over thousands of years.

No, once in 31 years is for a correct molecule to form, if the chance is 10-30 and a new molecule that could or could not have the desired form forms a billion trillion times a second, and the entire thing is random. Clearly the whole thing was either much more likely through some mechanism (likely: set of mechanisms), or the number of takes was larger.

The proposed mechanisms seem too weak to me. I'd think more of them if there were some estimates as to what is needed.

A test tube isn't the real world?

You know what I meant.

My point is that given enough chances, an extremely unlikely event (extremely low probability for every one chance) becomes and extremely likely event. The odds of winning a lottery are extremely low for every single ticket, but if you buy a billion tickets, your odds of winning are quite high.

Again, I'm not disputing that. I'm saying that as presented, the whole thing is not necessarily compatible with life being common.

And again realize that the relatively "short" time it took for life to start on Earth was still geologic time.

Which is why I'm mentioning enviromental stability all the time.

It may not be the whole story, but it's general enough to be in fact the only idea that fits with the evidence at hand. What other good ideas are you thinking of? That life arrived from outer space actually isn't an alternate explanation for abiogenesis. It still had to start somewhere.

As I told you before, I don't find the theory unappealing, but insufficient. The topic is not whether life arose from simple molecules (it clearly did), but rather if it's common or not. There are too many holes in it to show life is common.

McHrozni

 

[Dinwar]

And splitting vesicles that contain self-replicating polymers?

IMO, that's the point where selection begins. In each new "generation" of vesicles, you'll have more of the better self-replicators.

Depends on the definition. Natural selection as we know it today doesn't start operating until horizontal gene transfer stops--cladistics, phylogeny, and every other way we know of for dealing with evolution simply don't work with horizontal gene transfer. It violates some fundamental assumptions of those models.

That said, other types of selection are in operation before even that point. Vesicles lacking self-replicating polymers, for example, don't survive as well as those with self-replicating polymers. Chemicals that self-replicate survive better than random chemicals. All that evolution requires is heritable variation and time; it works with ANYTHING that has those two components.

There are three other things to remember here:

1) We're not dealing with the modern world. The Hadean was an alien world, just as alien as Venus or Europa. Assumptions that hold true for today may not necessarily be true in the Hadean. Limited oxygen plays merry havoc with chemical equations, particularly in o-chem.

2) We almost certainly didn't start from scratch. Space is full of organic molecules. There are nebula that taste like raspberry vodka, and others that are sweet. Proteins have been found in space. So calculating the odds is premature--first you have to demonstrate the starting point, something no one has done with any rigor.

3) The chemicals weren't evenly distributed. Certain minerals concentrate certain organic chemicals, for example (it's one idea for why life only uses certain types of certain molecules). Black smokers are another option--lots of nutrients, lots of energy, no nasty UV radiation to shatter the newly-formed DNA. In order to calculate the odds of a certain reaction happening, you need to know the concentration where that reaction is occurring. Otherwise, you're just guessing.

 

[Soapy Sam]

I worry a bit when I see absolute statements about life such as "life could not evolve on a star." I have little idea what happens on stars, except a lot of gravity, heat and radiation. Clearly life like us is not going to evolve there, but who ever said life has to be like us, at all?
Does it have to be organic (as in organic chemistry)? Must it be chemical at all? Can there be nuclear life, or purely electromagnetic life? Can gravitating dust clouds be alive in any way? Can spacetime itself? Such would require a redefinition of life - but words are meant to be tools, not straitjackets. We define them according to the evidence and so far the evidence is for bacteria, bugs, beetles, grass and people- organic life. But it's a big universe.

As for intelligent life... well we don't seem too good at recognising that when it's right in front of us, so identifying it at long distances is likely to be hard.Any technology indistinguishable from magic may be undetectable as well. Stealthed. Encrypted by it's complexity and alienness. Is this not what the "Strong" anthropic principle is getting at- that the nature of physical laws look...artificial?(Personally I see that as a failure of human perception, but that's another story.)

I think when we try to think about truly alien life we must think so far outside our normal comfort zone that much of what we think will seem like nonsense- and of course, much of it will be wildly wrong, but perhaps it's worth taking that chance. It might suggest other things to look for.
Life on Earth varies hugely, but at a molecular level is very uniform. Alien life may be so different we can't even think of how different.

None of which is much help, sadly.

I have a pet fantasy that intelligent life possibly has evolved before on Earth - and reached at least a simple tool using level before being erased by climate change, disease, falling rocks, whatever.
It's an amusing conceit, but the reason Dinwar and folk like him are not looking for stone arrowheads in the Eocene is because it has never occurred to anyone to do so- and because if they found one, it would be dismissed as a human artifact accidentally transported in, like mouse bones in Carboniferous limestone, that fell through the cracks in a later land surface. We are not looking for intelligent toolmakers in the Eocene, because we think it's not there and anyone who said it was is going to need rock solid evidence and balls of brass.

Would we know alien life if we saw it?

Not always, I suspect.

But which is more likely? Microbes on Mars? High tech super civilisations in space? Or tool using proto-lemurs on the Earth of 40 million years ago?

Sorry. Tired and rambling because I daren't go to sleep and miss a bus.

 

[JoeTheJuggler]

That said, other types of selection are in operation before even that point. Vesicles lacking self-replicating polymers, for example, don't survive as well as those with self-replicating polymers. Chemicals that self-replicate survive better than random chemicals. All that evolution requires is heritable variation and time; it works with ANYTHING that has those two components.

And that's the point where calculating the odds of random molecule formation is misleading. From one "generation" (which I've been considering as vesicles calving by the process of elongating into tubules and breaking off as described in the video I posted earlier) to the next, this sort of selection skews the results from random.

And I think all of this points out that while the line between what is and isn't life may be fuzzy, it's not an unfathomable mystery how we got from non-life to life on Earth.

And even though, as I said earlier, we don't know many of the values to plug into something like the Drake Equation, there's also no reason to think that the Earth is unique or even as highly improbable as some would have it.

 

[JoeTheJuggler]

No, once in 31 years is for a correct molecule to form, if the chance is 10-30 and a new molecule that could or could not have the desired form forms a billion trillion times a second, and the entire thing is random. Clearly the whole thing was either much more likely through some mechanism (likely: set of mechanisms), or the number of takes was larger.

The number of takes is much much larger than x in the 1:x probability. Again, the number of takes is molecule formation, right? And how many such takes are there in an environment with liquid water over say a span of 100 years? 1000 years? 10,000 years?

As I told you before, I don't find the theory unappealing, but insufficient. The topic is not whether life arose from simple molecules (it clearly did), but rather if it's common or not. There are too many holes in it to show life is common.

Again, I agree with you on the point that we don't know enough to know how common life might be in the galaxy. I've been taking issue with your earlier claim (that you seem to have retracted) that we have little idea how life formed from non-life.

But here in particular I was responding to your claim that there are other good ideas as to how life originates (than the one I used to refute your claim that we have little idea how it happened). I asked you what those other good ideas were.

 

[Dinwar]

I worry a bit when I see absolute statements about life such as "life could not evolve on a star." I have little idea what happens on stars, except a lot of gravity, heat and radiation. Clearly life like us is not going to evolve there, but who ever said life has to be like us, at all?

If life is composed of chemicals it can't form in a star. Stars rip apart the very atoms that chemicals are composed of. Hard to have life when that happens.

I think when we try to think about truly alien life we must think so far outside our normal comfort zone that much of what we think will seem like nonsense- and of course, much of it will be wildly wrong, but perhaps it's worth taking that chance. It might suggest other things to look for.

You want to waste your time looking for that sort of thing, go for it. I mean that with all sincerity--I wish you luck. But until you get a coherent definition of "life" that includes things like energy, gas clouds, etc., you haven't got a shot. Even if you're right, you'll never know it. So step 1 is to develop that definition.

I have a pet fantasy that intelligent life possibly has evolved before on Earth - and reached at least a simple tool using level before being erased by climate change, disease, falling rocks, whatever.

Tools are fairly obvious in the rock record. Even non-experts can identify the more impressive ones, and while others are more obscure (I still can't tell what fire-cracked rock is, despite seeing three examples of rings of the stuff) experts can easily identify it. None have been found prior to our lineage. So there is no evidence of intelligent life on Earth prior to our immediate ancestors and cousins.

but the reason Dinwar and folk like him are not looking for stone arrowheads in the Eocene is because it has never occurred to anyone to do so- and because if they found one, it would be dismissed as a human artifact accidentally transported in,

Hold on there. You are now telling me what I think, something you have no evidence for. And you're dead wrong. The reason I don't look for them is that they've never been found. We haven't even found chips. Nor have we found bone tools, which is far more damning. We know bones make good spears (we have a few bone spear tips in mammoths), and we know bone is preserved (that's what I spend my time looking for). We have found NO evidence of bone being made into tools, or of any other postmortem damage to bones outside of predation and fracturing associated with trampling and being crushed by stuff. You also have to remember that a fair number of archaeologists and paleontologists have worked on the other field's sites. I've done archaeo work as well as paleo; my boss has done far more.

But I'll give you a chance to support this notion. Please present a paper where a paleontologist says ANYTHING that you've said we're thinking. Or talk to one of us about this subject. But please don't tell me what I'm thinking without bothering to learn what I'm thinking.

 

[McHrozni]

The number of takes is much much larger than x in the 1:x probability. Again, the number of takes is molecule formation, right? And how many such takes are there in an environment with liquid water over say a span of 100 years? 1000 years? 10,000 years?

Assuming the 10^-30 number and 10²⁰ vesicles (and way more molecules), where each vesicle has one try per second, about one success per 320,000 years. Most will be useless.

All these numbers could be BS of course. I'm using them not to claim the model is practically impossible, but to show the whole thing is a lot less certain than the video shows. As I was saying it's severely sanitized, and I think it's up to the claimant to show these concerns are unfounded and most importantly why that is.

Again, I agree with you on the point that we don't know enough to know how common life might be in the galaxy. I've been taking issue with your earlier claim (that you seem to have retracted) that we have little idea how life formed from non-life.

There is no proof. There are good theories, better than what I thought earlier, but insufficient for the topic at hand.

McHrozni

 

[JoeTheJuggler]

Assuming the 10^-30 number and 10²⁰ vesicles (and way more molecules),

You're assuming only 10²⁰ vesicles in all the liquid water on the planet Earth?

ETA:

All these numbers could be BS of course.

I agree. It smacks of the Creatonist argument about assembling an airliner by wind blowing through a scrap yard.

I'm using them not to claim the model is practically impossible, but to show the whole thing is a lot less certain than the video shows.

Well you've failed to do that.

Using your figures: let's say you have a 10 ³⁰ sided die, and you can roll it 10 ²⁰ times every second. It means any particular face you "call" is nearly certain to come up in a matter of seconds. And you'll get a great many such longshots repeatedly if you roll that die at that rate for geological time periods. And since selection begins operating as soon as you have replication with variation (the better replicators will be more prevalent over time, for example), it's not even purely random across time--and you get a ratcheting effect toward something more and more like "life".

 

[JoeTheJuggler]

There is no proof. There are good theories, better than what I thought earlier, but insufficient for the topic at hand.

Insufficient for what?

The topic at hand is how many planets there are in the galaxy that "may contain life"--which, for purposes of the thread topic means estimating how many planets there are within the "Goldilocks" zone where liquid water could exist.

In fact, the theory for abiogenesis that I pointed to is pretty much a tacit assumption for the estimates made in the study described in the article in the OP. If it weren't, then we'd have to come up with an entirely different definition of potentially habitable planets. As it is, the question is the same as asking how many planets might there be in the galaxy where liquid water is possible.

So in this context, your comment that we know little about how abiogenesis happened is simply wrong. The best definition we have for a "habitable planet" is one where liquid water can exist and the kinds of chemical reactions this very conventional theory of abiogenesis relies on can happen.

 

[Dinwar]

All these numbers could be BS of course.

They are. Again, you are assuming that we're dealing with a uniformly distributed. You haven't addressed the fact that organic chemicals are concentrated. We're not talking one or two reactions per second--we're talking huge numbers in a comparatively tiny volume. And then there's the whole geologic time thing--the Earth is aproximately 4.6 billion years old, the oldest evidence for life is 3.9 billion year old (from Australia, I believe). That gives us nearly 700 million years to work with. The entire Cenozoic is less than 100 million years long. Even if we assume that most of that time was inhospitable to life (lava tends to be), even if we only had 1/7th of that time to work with we're talking a tremendous, literally incomprehensible length of time. Even geologists can't really wrap our minds around it; we simply know how to use short-cuts to deal with the scale.

There is no proof. There are good theories, better than what I thought earlier, but insufficient for the topic at hand.

Okay. What would convince you? And what background do you have in the field of biogeochemistry and Hadean geochemistry? I ask because I know I've got a background in biology and geology, and abiogenesis papers make my eyes glaze over. I can push through them, but I'm not even close to being up on what's going on in the field. I do, however, know what it takes to be knoweldgeable on the topic (no one knows better than the one who's gone up to the abyse and gone "No way!").

 

[JoeTheJuggler]

Using your figures: let's say you have a 10 ³⁰ sided die, and you can roll it 10 ²⁰ times every second. It means any particular face you "call" is nearly certain to come up in a matter of seconds.

Sorry--that's obviously not right. But I don't agree with your 10 ²⁰ value (for molecule formation events in all the liquid water on the planet Earth). If it were something within an order of magnitude of the probability value, it would come up in a matter of seconds.

And then consider it over geological time rather than seconds.

 

[JoeTheJuggler]

They are. Again, you are assuming that we're dealing with a uniformly distributed. You haven't addressed the fact that organic chemicals are concentrated. We're not talking one or two reactions per second--we're talking huge numbers in a comparatively tiny volume. And then there's the whole geologic time thing--the Earth is aproximately 4.6 billion years old, the oldest evidence for life is 3.9 billion year old (from Australia, I believe). That gives us nearly 700 million years to work with. The entire Cenozoic is less than 100 million years long. Even if we assume that most of that time was inhospitable to life (lava tends to be), even if we only had 1/7th of that time to work with we're talking a tremendous, literally incomprehensible length of time. Even geologists can't really wrap our minds around it; we simply know how to use short-cuts to deal with the scale.

Exactly.

 

[McHrozni]

They are. Again, you are assuming that we're dealing with a uniformly distributed. You haven't addressed the fact that organic chemicals are concentrated. We're not talking one or two reactions per second--we're talking huge numbers in a comparatively tiny volume.

Certainly not. I was talking about 10²⁰ reactions per second.
A much less unreasonable number, certainly, than one or two reactions per second.

And then there's the whole geologic time thing--the Earth is aproximately 4.6 billion years old, the oldest evidence for life is 3.9 billion year old (from Australia, I believe). That gives us nearly 700 million years to work with. The entire Cenozoic is less than 100 million years long. Even if we assume that most of that time was inhospitable to life (lava tends to be), even if we only had 1/7th of that time to work with we're talking a tremendous, literally incomprehensible length of time. Even geologists can't really wrap our minds around it; we simply know how to use short-cuts to deal with the scale.

Yes. What else can I say

The issue is that you should present an estimate (guesstimate?) as to how long the whole thing needs to be considered life (7 conditions), and whether it's reasonable to expect the correct conditions to endure for that long. I made a few calculations, and they came up out of bounds completely. Either the numbers are wrong, the model is wrong or just incomplete, or life is a statistical anomaly par excellence.

With one example that we have, any combination of the above* is possible.

*except the model being both wrong an incomplete at the same time, obviously

Okay. What would convince you? And what background do you have in the field of biogeochemistry and Hadean geochemistry? I ask because I know I've got a background in biology and geology, and abiogenesis papers make my eyes glaze over. I can push through them, but I'm not even close to being up on what's going on in the field. I do, however, know what it takes to be knoweldgeable on the topic (no one knows better than the one who's gone up to the abyse and gone "No way!").

Convince me of what? That this model is completely accurate and there is all there is to it? Quite a bit.

That life is common in the universe? Finding life on Mars would essentially seal this one. If there is or ever was life on Mars, then we can reasonably expect life is fairly common, and this would also lead credence to all models that show life being a thermodynamical necessity, like this one.

That the model possible? That the model is a part of the story? That life arose abiotically? I already agree to all of those, and have written so in this very thread.

Using your figures: let's say you have a 10³⁰ sided die, and you can roll it 10²⁰ times every second. It means any particular face you "call" is nearly certain to come up in a matter of seconds.

Not for any "a matter of seconds" I'm familiar with. You can express the time in seconds of course.

McHrozni

 

[lpetrich]

At Puppycow's request, an improved link: 60 billion planets in Milky Way could hold life - Computerworld
noting
Cloud modeling expands estimate of life-supporting planets | UChicago News
noting
Stabilizing Cloud Feedback Dramatically Expands the Habitable Zone of Tidally Locked Planets - Abstract - The Astrophysical Journal Letters - IOPscience

From the abstract, "Previous estimates of the inner edge of the HZ were based on one-dimensional radiative-convective models. The most serious limitation of these models is the inability to predict cloud behavior."
One-dimensional???

Simulating an atmosphere requires a LOT of number-crunching, and it's not surprising that various people try to take shortcuts. The article is behind a paywall, so I can't tell how much they simulated the atmospheric circulation. However, several other recent simulations have done that, so this simulation may also have done that.

I've found [1103.3101] Equatorial superrotation on tidally locked exoplanets

Under strongly irradiated conditions, models of tidally locked, short-period planets (both hot Jupiters and terrestrial planets) tend to exhibit a circulation dominated by a fast eastward, or "superrotating," jet stream at the equator. Under appropriate conditions, this phenomenon can cause the hottest regions to be displaced eastward from the substellar point by tens of degrees longitude.

Superrotation is well-known inside our Solar System: Venus's atmosphere superrotates. A new theory to explain superrotation on Venus

The next thing to model is likely ocean circulation. It also transports heat from hot areas to cold areas, but it is both slower and more capacious.

Here are some attempts to simulate Pangaea's climate: Paleoclimate data and model comparisons Low and mid latitudes are more-or-less OK, while high latitudes are warmer than what one calculates. That's likely because the oceans were calculated with the swamp approximation, making their water stationary. If the oceans' water can move, then one gets better results: Simulated warm polar currents during the middle Permian

 

[lpetrich]

Around 1960, Frank Drake proposed his famous equation:

N: number of civilizations that we could communicate with
= product of
Rs: rate of star formation
fp: fraction of stars with planets
ne: number of planets that can support life
fl: fraction of planets were life emerges
fi: fraction of biotas where intelligence evolves
fc: fraction of intelligent species that develops interstellar-communication technology
L: how long they stay capable of interstellar communication

When he proposed it, we only had a good value of Rs, but we are now getting clues about fp and ne. In particular, fp seems close to 1 and ne does not seem far from 1.

 

[McHrozni]

Around 1960, Frank Drake proposed his famous equation:

N: number of civilizations that we could communicate with
= product of
Rs: rate of star formation
fp: fraction of stars with planets
ne: number of planets that can support life
fl: fraction of planets were life emerges
fi: fraction of biotas where intelligence evolves
fc: fraction of intelligent species that develops interstellar-communication technology
L: how long they stay capable of interstellar communication

When he proposed it, we only had a good value of Rs, but we are now getting clues about fp and ne. In particular, fp seems close to 1 and ne does not seem far from 1.

Yup. We're discussing the fl here, which could be big or small. Example of Earth suggests big.

If we use Earth as a model, fi (life develops intelligence) and fc (intelligent life develops interstellar communication) will be both small and L (length of time life remains so) will remain controversial for all eternity

McHrozni

 

[Dinwar]

Certainly not. I was talking about 1020 reactions per second.
A much less unreasonable number, certainly, than one or two reactions per second.

These are still derived from thin air, and still equally arbitrary. They cannot be otherwise--you've yet to discuss the geochemical conditions of the Hadean oceans, which is what would dictate what these numbers would actually be. Remember, Earth didn't start out from scratch--many of the necessary molecules almost certainly arived on the planet from space. When the molecules were concentrated, reaction speed sped up.

Also, remember that 10²⁰ isn't really that impressive in chemistry. 6.022x10²³ is a standard unit of measure in chemistry, and is three orders of magnitude greaterr than your estimate. You are saying that less than one mol of these chemicals interacted over an entire planet. 10⁵⁰ would not be an unreasonable estimate. And it could be a lot higher, particularly when you consider concentration of organic molecules--basic chemistry, concentrate the reactants and rate of reaction goes up.

The issue is that you should present an estimate (guesstimate?) as to how long the whole thing needs to be considered life (7 conditions), and whether it's reasonable to expect the correct conditions to endure for that long. I made a few calculations, and they came up out of bounds completely. Either the numbers are wrong, the model is wrong or just incomplete, or life is a statistical anomaly par excellence.

Actually, you've got this completely backwards. Life arose fast--the oldest life is in the oldest sedimentary rocks. We know that life arose fast, in the same way we know the dinosaurs existed and that some trilobites were nectic. If your calculations came up out of bounds YOUR CALCULATIONS are wrong, not the data. We know it happened fast; the question is WHY it happened fast. Anything that says it didn't happen fast can be dismissed as contradicting the data.

I'd start by re-examining those seven conditions you mentioned. Peter Ward's "Life As We Do Not Know It" is a good place to start. You're using a modern definition of life which may or may not be applicable to the Hadean organisms. They were truly alien life forms, on a truly alien world.

Convince me of what? That this model is completely accurate and there is all there is to it? Quite a bit.

"Quite a bit" is not a scientific statement. What evidence would you require to be convinced of a model of abiogenesis?

As for the Drake Equation, it's greate for wraping your head around the problem. It's horrible for actually answering anything. It's a tool to direct one's thinking in a more rigorous manner. If you run the calculation, however, it becomes nothing more than an expression of what you want to see. The numbers all have such wide ranges (except a few that we can start to maybe constrain a bit) that any answer to running this equation has nothing to do with reality, and everything to do with the person plugging in the numbers.

 

[lpetrich]

My take on fl. Molecular biology has advanced hugely since 1960, and we now have a much clearer picture of early evolution.

Of Carl Woese's three domains, the ancestor of Eukarya was a chimera of at least one each of Bacteria and Archaea. So it's rather far from the origin of life.

The other two have a split between them, a split that's the oldest known split in the family tree of organisms. One can deduce a fair amount about that ancestral organism. It had a DNA or DNA/RNA genome; its DNA-replication system was elaborated separately in Bacteria and Archaea. It made proteins with messenger RNA, transfer RNA, and ribosomes, also with RNA. It had electron-transfer metabolism and chemiosmotic (hydrogen-ion-pump) metabolism. It was almost certainly chemoautotrophic, getting energy from chemical reactions between inorganic compounds and using that energy to build all its biological molecules.

A very complicated organism, and likely one that had a lot of evolution behind it. In fact, there's an emerging paradigm, the RNA world, of exactly that. Some ancestral organism had RNA as both information storage and enzyme. Its enzymes used lots of coenzymes, and some of them were amino acids. Elaborating the amino-acid coenzymes led to proteins, which eventually became the main enzymes. A variant of RNA got developed for master-copy duty: DNA.

The main problem with the RNA world is the origin of the RNA, since it's rather hard to make it prebiotically. It's possible to make ribose with the Butlerov formose reaction, but it requires a concentration of formaldehyde, and it produces sugarlike molecules with a variety of sizes and asymmetries.

I've seen speculations about ribose alternatives like amino acids: peptide nucleic acids.


A side note about viruses. They are cellular information-system parasites, so I don't think that they qualify as a separate origin of life.


But the most important result is that all known cellular organisms are descended from one origination of life, at least all those whose biochemistry and molecular biology have been studied in any detail. None are known that have a separate origin. How might it be possible to find one? One way is if it has a molecule of heredity other than DNA or RNA, especially one unable to base-pair with either. So one would grow some organism in the lab, and then completely fail to discover genes often used as phylogeny probes, like ribosomal-RNA genes. Then one discovers that the organism has no measurable quantity of either DNA or RNA -- much less than what one usually finds in an organism. Then one finds various other biochemical oddities.

That would be a huge story if it ever happened, but the arsenic-bug fiasco is likely to make many biologists *very* skeptical.


That leaves us in a quandary.

The single origin of surviving organisms suggest that life either originated only once on Earth, or else that only one organism out of several had descendants that took over the ecospace for early organisms.

However, many of the simpler building blocks of Earth organisms can easily be made in prebiotic-chemistry experiments.

How typical was the Earth? It's hard to say.

Prebiotic-chemistry syntheses can happen just about anywhere with appropriate conditions, and we've detected numerous extraterrestrial organic molecules that were likely assembled in this sort of fashion.

 

[Dinwar]

The single origin of surviving organisms suggest that life either originated only once on Earth, or else that only one organism out of several had descendants that took over the ecospace for early organisms.

However, many of the simpler building blocks of Earth organisms can easily be made in prebiotic-chemistry experiments.

I don't think there's a quandry. Our models for evolution break down in early life--simply put, there was too much horizontal gene transfer. My guess is that there were multiple origins of life, but that that life was more fluid than today's. The earliest organisms were able to exchange information more freely, which resulted in an eventual homogenization of life. Not necessarily an average, but not necessarily one form becoming dominant, either; it's likely to have been a vastly complex process. So we're really the offspring of multiple origins of life. LUCA likely represents both the last common ancestor and the end result of a period of quite different evolutionary styles.

 

[JoeTheJuggler]

Yup. We're discussing the fl here, which could be big or small.

Actually the topic is ne. The study was estimating how many planets could support life. In fact, it was about ne and all the factors before it--which are necessary to estimate how many planets there are in the galaxy that could support life (that is, terrestrial planets in the zone where liquid water is possible).

 

[JoeTheJuggler]

I don't think there's a quandry. Our models for evolution break down in early life--simply put, there was too much horizontal gene transfer. My guess is that there were multiple origins of life, but that that life was more fluid than today's. The earliest organisms were able to exchange information more freely, which resulted in an eventual homogenization of life. Not necessarily an average, but not necessarily one form becoming dominant, either; it's likely to have been a vastly complex process. So we're really the offspring of multiple origins of life. LUCA likely represents both the last common ancestor and the end result of a period of quite different evolutionary styles.

Similarly, the notion of mitochondrial Eve doesn't mean there was only one first human mother.

For us lay people, I find this diagram (hotlinked from Wiki Commons) helpful in understanding the point:

There's also the fact that fully-developed arachaebacteria (which are now ubiquitous in, on, under and above the Earth) will easily outcompete newly originating self-replicating molecules.

 

[JoeTheJuggler]

The main problem with the RNA world is the origin of the RNA, since it's rather hard to make it prebiotically.

Are you sure? I'm no chemist or biologist, but I'm pretty sure I've read that this has happened in vitro.

http://www.sciencedirect.com/science/article/pii/002228369190729P

http://www.nytimes.com/2009/05/14/science/14rna.html?pagewanted=all&_r=0