In astronomy, the word ‘metals’ refers to anything heavier than hydrogen and helium. Stars fuse hydrogen into helium and from there work their way into the higher elements until hitting iron, at which point the end quickly comes, with ‘star stuff,’ as Carl Sagan liked to put it, being flung out into the universe. Through stellar generations we can trace a higher concentration of the heavier elements as stars are born from the materials of their predecessors. And we’ve learned that those metal-rich stars are the most likely to produce gas giants like Jupiter and Saturn.
What’s intriguing is the issue of smaller planets and the conditions for their formation. After all, the content of the disk from which planets are formed parallels the metallicity of the host star. I’m looking at new research from Lars A. Buchhave (Niels Bohr Institute/University of Copenhagen) into planet formation, using data from the Kepler telescope. In Buchhave’s words:
“We have analysed the spectroscopic elemental composition of the stars for 226 exoplanets. Most of the planets are small, i.e. planets corresponding to the solid planets in our solar system or up to four times the Earth’s radius. What we have discovered is that, unlike the gas giants, the occurrence of smaller planets is not strongly dependent on stars with a high content of heavy elements. Planets that are up to four times the size of Earth can form around very different stars – also stars that are poorer in heavy elements.”
Buchhave and team focused on whether small, Earth-like planets needed the same kind of metal-rich environment demanded by the gas giants, at least those with short orbital periods. Given that planets like the Earth are made up of heavier elements — iron, silicon, oxygen, magnesium — you would assume that small planet formation would be much more efficient around metal-rich stars. The new paper, which has been published in Nature, argues that the idea is wrong, and that opens up a lot of territory. Without special requirements for heavy elements in their stars, Earth-like planets could indeed be widespread in the galaxy.
Image: This artist’s conception shows a newly formed star surrounded by a swirling protoplanetary disk of dust and gas. Debris coalesces to create rocky ‘planetesimals’ that collide and grow to eventually form planets. The results of this study show that small planets form around stars with a wide range of heavy element content, suggesting that their existence might be widespread in the galaxy. Credit: University of Copenhagen/Lars Buchhave.
The work also implies that small planets could form earlier in galactic history than has previously been thought. Buchhave’s work examines this through the study of the spectroscopic metallicities of the host stars of the 226 Kepler candidates chosen. The average metallicity for planets smaller than four Earth radii turns out to be close to that of the Sun, but Buchhave says in this NASA news release that stars with just 25 percent of the Sun’s metallicity can also form small planets. Meanwhile the data continue to support the preferential formation of gas giants around higher metallicity stars.
From Natalie Batalha (NASA Ames), a member of the Kepler science team:
“Kepler has identified thousands of planet candidates, making it possible to study big-picture questions like the one posed by Lars. Does nature require special environments to form Earth-size planets? The data suggest that small planets may form around stars with a wide range of metallicities — that nature is opportunistic and prolific, finding pathways we might otherwise have thought difficult.”
Indeed. We are learning that it doesn’t take that many generations of stars to start producing rocky worlds. The work was presented yesterday at the 220th meeting of the American Astronomical Society. “Giant planets prefer metal-rich stars. Little ones don’t,” says David Latham (Harvard-Smithsonian Center for Astrophysics). The CfA’s own news release says the work supports the core accretion view of planet formation, in which steadily accumulating planetesimals combine to form planets, with the largest quickly gathering hydrogen. Higher metallicities make quick formation of large cores more likely, which explains the connection between heavier metals and gas giants.
Are there SETI implications here as well? This from Jill Tarter (SETI Institute):
“The idea that very old stars could also sport habitable planets is encouraging for our searches. In particular, intelligent life has taken a long time to evolve here on Earth. Consequently, it’s reasonable to suppose that older planetary systems are more likely to have technological societies – the kind we might detect with our radio telescopes.”
And that reminds me to note that the SETI Institute is hosting SETIcon II in Mountain View, California from June 22-24, where those in attendance can rub elbows with the likes of Geoff Marcy and Debra Fischer. I see that tickets are still available to the public.
The paper is Buchhave et al., “An abundance of small exoplanets around stars with a wide range of metallicities,” published online in Nature 13 June 2012 (abstract).
that it doesn’t take many generation of stars to create rocky worlds. Does that also mean it doesn’t take many generation for life to form. Maybe rocke world were there already in the early universe, but our planets is one of first generation with life/complex life.
It would also help the fermi peradox.
I hear once that the first planets with life started to form 8 billion years after the bigbang. The first 4 billion years there will be only simple life. After 500 million years maybe the first inteligent life could involve(if the planet is the same as earth) That is 12.5 billion years. That mean the olderst inteligent life could be 1.2 billion years older than us.
This is all just speculation
Henk -it doesn’t help the Fermi Paradox, it makes it worse!
I think this Kepler data simply confirms what has been found by the HARPS people?
I wonder if life could emerge on a planet with greatly reduced amounts of biologically important elements? Of course I’m assuming that the biochemistry would be similar to Earth. Or to put it another way if we could model the reduction of key elements how much could they be reduced before important biological processes would weaken or stop. We may have numerous low metallacity planets but they might remain lifeless.
I’m reminded of the novel Destiny’s Road. Though in that situation the planet did have an indigenous biosphere despite an absence of potassium.
The question is whether these small planets are actually rocky worlds (terrestrial planets) or volatile-rich “failed cores” (water-worlds/mini-Neptunes).
Given the difficulty of getting mass measurements for the Kepler candidates, the nature of these worlds is frustratingly difficult to pin down.
Mike raises an important point. I’ve seen suggestions that it’s the availability of carbon (which of course is another aspect of metallicity) that sets the limit on how early our sort of life could have got going. So the existence of low-metallicity planets may not change the odds for the existence of life (at least as we know it).
@kzb- Henk could be right. If most intelligent races were 1.5by older than us, we would likely not be significant to them, so they wouldn’t bother to contact us. Also, they may have spread out across the universe long ago, taking over most of the habitable planets. We could be lucky to be in such a backwater neighborhood that it was overlooked and not colonized, leaving it to us to populate. If this were true, it might behoove us to keep a low profile.
a la Hawking.
Could we not view the icy moons of our solar system as low metal worlds on the whole, there is a possibility of life in their oceans -the heavy minerals although less of them will concentrate at the water/rock interfaces making them available for life chemistry and so should make little difference.
Truly fascinating, more information from Kepler, really my thing.
Such a pity that only the abstract is available, no PDF from ArXiv (yet).
So, I cannot tell t what extent this is (also) base on new data or just re-analysis of the already existing data, but my impression is the latter. Which also means that this analysis still mainly relates to the inner parts of planetary systems, within xx (0.4?) AU.
Ok, I looked at the abstract and the 3 available diagrams.
Basically it confirms what we already suspected and adds a few new interesting things (and kzb, yes, I also think that Kepler and HARPS are largely confirming and complementing each other, see also Wolfgang and Laughlin, 2011, COMBINING KEPLER AND HARPS OCCURRENCE RATES, etc.):
– What we already knew is that there is a strong correlation between metallicity and gas giants (both from HARPS and Kepler): giant planets (defined as > 4 * Re) hardly occur below a (10log) metallicity of -0.1 (80% of solar) and not at all below -0.3 (50% of solar). And giant planets have a high average metallicity host star than smaller ones (1st and 2nd diagrams).
– What we also knew is that smaller planets (but not the smallest ones) are more common than very large ones (1st and 2nd diagrams).
What is really interesting and also new (at least to me), and not so explicitly mentioned in the abstract, is that smaller planets are not evenly distributed against metallicity, i.e. *not* metallicity indifferent at all (the abstract mentions for them ‘a wide range of metallicities’, ‘no special requirement of enhanced metallicity for their formation’). Ok, they also mention between () ‘but on average a metallicity close to that of the Sun’, but I would say the really interesting point about them is much stronger:
– There is a very conspicuous peak of smaller planets around solar metallicity, *dropping off sharply to both sides*, i.e. both to lower and higher metallicity. These planets also get scarce below metallicity -0.3 (half of solar metallicity) and absent below about -0.5 to -0.6 (25- 35% of solar metallicity).
– I find the sharp drop off of smaller planets toward higher metallicity much more surprising: they decrease significantly beyond about 0.1 and they get really scarce beyond about 0.3 (about twice solar). At the same time the giants become more common. I would really think that there has to be a connection between those two trends.
– Diagram 3 confirms and details that picture: smaller and medium-sized planets (less than 4Re) and in particular the small (terrestrial) ones below 2 * Re get scarce beyond about 0.15 to 0.2 metallicity (say about 50% more than solar). Even more telling from this diagram is that even in those rare latter cases they are never the largest planet in a multi-planet system (in other words there is always a giant planet present as well) or they are in a single-planet system, but that may be observational bias, the other planets in wider orbits not having been detected yet.
– Also very telling from diagram 3 is that giant planets (greater than 4Re) are mostly always single-planet or at least the largest in their system: beyond about 5.5 * Re giant planets are either single-planet or the largest in their system, beyond about 10 * Re all giant planets occur in single-planet systems (but for 1 exception).
Of course, this is still mainly the picture for inner systems.
But it looks strongly now as if, particularly beyond a certain metallicity and size, giant planets often spoil the existence of the smaller ones.
What I wanted to add is: I would really like to see a diagram, not just of individual planets, but of all plants in a system, against metallicity.
I think that would be so much more telling, the configuration of planets together, the characterization of entire planetary systems.
We know by now that the systems containing a giant planet in close orbit (Hot Jupiter) are mostly poor and simple: single-planet systems, devoid of smaller planets.
But how about the compact (inner) systems consisting of intermediate planets (super-earths and Neptune-class planets), that appear to be so common, possibly the most common type of planetary system?
I am intrigued, in particular, by the question: what stellar parameters and their values (mass, metallicity, etc.) cause the spectacular distinction between our kind of system (small planets inside, big ones outside, in a rather spacious configuration) and these compact systems?
plants = planets, of course.
Ronald,
Go to http://www.exoplanets.org and click on plots (or go to http://www.exoplanets.org/plots).
For X, pick metallicity (search for [Fe/H]) for Y, pick the mass ( Msin(i) ) Select [Fe/H] in colorscale for a prettier graph.
If you want to see components (# of planets) vs metallicity, pick components for X and metallicity for Y. Make sure to select Msin(i) for the marker scale, to show the mass of the planets as well.
Other than a couple of outliers (if you hover over the planet’s dot, you get its name) all the Big Boys and Big Systems seem to cluster at around an Fe/H of -0.3 or greater… which pretty much agrees with the paper.
Also, high metallicity systems with massive planets tend to have fewer planets overall (almost certainly an observation effect) systems with many planets really picks up at Fe/H of -0.5 or greater.
Greater [Fe/H] = more Jupiter mass and smaller planets.
It looks to me like the plots are showing numbers of planets rather than frequency at a given metallicity. As you move to extremes of metallicity you are going to have fewer stars in the sample which would then naturally cause falloffs at high and low metallicity, it would be interesting to see what happens when you correct for that effect.
One recent paper on the subject of metallicity and planet abundance suggests that the alpha-elements might be worth looking at, particularly magnesium. Adibekyan et al. (2012) “Overabundance of alpha-elements in exoplanet host stars“.
@FranhH: thanks, yes I know that site well, I was limiting myself to the Kepler data here. Also, I did not and would never say that I disagree with the paper, on the contrary (I would not be in any position), I tried to get some interesting (additional) information out of it.
However, you say:
“Also, high metallicity systems with massive planets tend to have fewer planets overall (almost certainly an observation effect)”
“Greater [Fe/H] = more Jupiter mass and smaller planets.”
Well, that I dare doubt on the basis on this and many other publications (e.g. “Kepler constraints on planets near hot Jupiters”, Steffen at al., 2012). I think we can say now that the paucity of planets in combination with very high metallicty and (close-orbit) giant planets is not an observational bias, but a real phenomenon, the giant planets in such systems having spitalled (migrated) inward, growing and absorbing most planetary material in the process. Greedy giants.
Andy: thanks for the ref., this is indeed the fascinating kind of thing that I mean. I will study it in more detail.
Of course, we are still mainly dealing with inner systems and observational constraints (with HARPS, RV method, foremost planet mass relative to orbit), so the ‘stars without planets’ could very well appear to be stars with small terrestrial planets in the inner system and bigger ones toward the outside (like our own).
Also very fascinating in this context is the work of Melendez, Ramirez, Asplund et al., on solar twins. They correllate the ratio of refractory elements (higher condensation temp.), such as Fe, Si, Ca, Mg, and volatiles, such as O, C, with the occurrence of planets and in particular terrestrial planets.
The Fermi Paradox is a paradox because we lack information one way or the other. We can’t make any conclusions.
In all honesty, news like this puts me in mind of Keith Wiley’s speculation that, using even very conservative values for the Drake equation, there plausibly could be vast populations of Bracewell probes in our outer solar system right now. It’s not as if we’ve got spectacularly good surveillance of our solar systems right now.
Of course, the information that we have could as easily indicate that the universe is not full of life, that for some reason it’s quite rare. The exact nature of the filter(s) involve remained to be determined, of course.
The conclusion of this latest work helps us move the bottleneck in the Drake equation further to the right away from the astrophysical factors toward the biological factors– that is, the continued failure of SETI searches is more likely due to a very small value for one or more of the following terms: fl, fi, fc, L. Fp, the fraction of stars with planets is clearly not at issue and, according to this work, ne, the fraction of systems with a potentially habitable planet, will likely be found to be non-negligible (greater than or equal to 0.01).
Recently I read a very interesting article “Searching for a shadow biosphere on Earth as a test of the ‘cosmic imperative'” by Paul Davies that sheds some light on the issue of the biological terms in the Drake equation. For fi, fc, and L Davies makes the point that we at least have something to go on as far as assessing probabilities; for example, biological evolution and history of civilizations. The really hard term of the bunch, according to Davies, is fl– the fraction of habitable planets on which life actually arises. In his view, finding evidence for a second origin of life– not just another branch on the existing tree of life– would imply a high value for fl since it would mean that there is a tendency toward matter complexification from non-living to living systems.
Davies rejects the often espoused notion that our type of life out-competed all other forms of life that may have originated. He points out that Bacteria and Archea have co-existed for billions of years going after similar resources. Also, lifeforms that have entirely different biochemistries would very likely not be at odds with each other; for example, a hypothetical earth-based lifeform that uses biomolecules with a different chirality than conventional life would not even be using the same stuff for sustenance.
I suspect the bottleneck in the Drake equation lies with fl. The fact that all life examined thus far uses the same genetic code and molecular machinary strongly implied that the origin of life on earth was a chemical fluke so rare that there is unlikely to be any other life in the observable Universe. As unattactive as this prospect may be, it would obviously solve the Fermi Paradox.
@spaceman:
“The fact that all life examined thus far uses the same genetic code and molecular machinary strongly implied that the origin of life on earth was a chemical fluke so rare that there is unlikely to be any other life in the observable Universe.”
Seems a somewhat premature conclusion, there may be other reasons: apart from the (I think still, very valid) competition option, life may originate during a certain temporal and conditional window of a planet. i.e. when the conditions are sufficiently right, not before and not after. Or life may originate several times but go extinct again.
I would still favor the competition option: early precursors to life must be very, very vulnerable, surviving and thriving only in very, very ‘quiet’ environments, quiet meaning biologically quiet, no competition, no predation, lots of resources.
“As unattactive as this prospect may be, it would obviously solve the Fermi Paradox.”
That would definitely be true. logically.
Though unattractive?: lonely and disappointing in a way, but at the same time it means there would be a LOT of potentially suitable real estate out there.
I am an optimist and usually say about that: either there is life or we will bring it.
With ref. to the paper by Adibekyan, mentioned by andy, I would really like to get hold of the raw abundance data for all (1111) stars that they examined. I could not find those anywhere, I mean not even on Vizier. Maybe too recent (2012)?
Does anybody have an idea?
Is it not the case that Earth-sized planets and smaller are below the current detection limit? So really there is little evidence of a peak around solar metallicity. It could be that lower metallicity= smaller planets, but lots of them. It’s just that they are not seen yet, so are missing from the diagrams. So rather than the somewhat Guassian distribution you see now, really it is more like a Boltzman distribution.
@spaceman:
“The fact that all life examined thus far uses the same genetic code and molecular machinary strongly implied that the origin of life on earth was a chemical fluke so rare that there is unlikely to be any other life in the observable Universe.”
The PRIONs responsible for mad cow disease , seems close to beeing an alternative system for selfreplication , even if too specific in present form to be capable of comteting with DNA..
Would these rocky worlds still have enough heavy metals to support complex life?
e.g. . Zinc, molybdenum and copper
see http://www.newscientist.com/article/dn21947-sex-born-from-hard-rock-and-heavy-metal.html?DCMP=OTC-rss&nsref=online-news
This part of Davies’ argument does not hold up to scrutiny. The fact that multiple branches still exist does not contradict the fact that almost all branches have gone extinct. From the time before the last universal common ancestor (LUCA), only a single branch survives, even though there were countless others in existence at the time.
This is incorrect. Ultimately, all life competes for water, carbon dioxide, and energy. None of these are related with molecular chirality, or even any other specifics of biochemistry except the most basic ones of using carbon and water.
Your conclusion is correct, but the justification is not. The true reason we must conclude life is rare is because we have not observed it outside of Earth. Earth itself is hopelessly contaminated by the life we know, both mechanistically (existing life precluding new life) and statistically (the existence of life being a precondition of observation).
@Eniac: I feel rather awkward to correct you, because I know that you know your things very well, but your statement: “The true reason we must conclude life is rare is because we have not observed it outside of Earth” is simply incorrect.
Our observation of life in the universe, logically, means little or nothing but only has to do with our own technological (in)abilities. The universe may be teeming with life without us knowing it (yet).
Unless you equate ‘life’ with very advanced (KIII level) intelligent life, but that is a tremendous leap to conclusion.
Only after we have spectroscopically and thoroughly examined a representative sample (xxx?) of earthlike planets in the habitable zones of their (sunlike and other) stars, without finding any biosignatures, we can carefully conclude that life may be rare.
We are often being extremely premature in our conclusions in discussions here.
Ronald:
While you are certainly correct that we have not looked very closely, if the galaxy were really “teeming” with life, SETI would be like looking for trees in a forest.
This would be true if what we were looking for were sure to stay confined to wherever it arises. Life is not like that, especially as it concerns ETI.
The combination of life’s tendency to spread and the enormous amounts of time available for it to do so, come here, and spoil it for us rules out anything other than some improbably stealthy or lethargic type of life. Unlike the life we know, it would have to limit itself in range and keep a low profile to avoid detection. The theory that we are intentionally being left alone does not cut it, because we were not here most of the time. These considerations more than balance our currently limited powers of observation.
Eniac, I understand our difference of opinion and it is as I suspected. You make two important assumptions:
1) If life is common, there inevitably has to be intelligent life somewhere as well.
2) If there is intelligent life, then sooner or later this will also give rise to interstellar communication capabilities.
I (still) strongly disagree here and consider both assumptions way too optimistic:
1) Even if life is very common, there is absolutely no inevitability of (advanced) intelligence.
2) Even if some form of intelligence arises, there is absolutely no guarantee that it will advance far enough and survive long enough to ever reach interstellar capabilities or even advanced technology of some sort.
I am often puzzled that some people (I do not say you) consider life per se a fluke, but at the same time intelligence as inevitable if life arises.
I, however, consider intelligence the greatest fluke, a very expensive anomaly, even here on earth.
And the odds are not as favorable, even in our MW galaxy full of stars, as often thought: as has been treated here on several occasions, the number of habitable planets in our MW galaxy is probably not greater than a few hundred million (min. tens of million, max. a few billion). That may sound like a lot but it is not, as a starting point for life, which then has to take many hurdles, each hurdle probably reducing the next subset to a minority percentage.
I and others, much more knowledgeable, have made the guesstimates several times, with a few variations in parameters, starting from several hundred million habitable planets, and always ending up with only a few (1 to a handful) contemporaneous technological civilizations.
There is a temptation, and a tendency, to view our MW galaxy as infinite in time and space. However, no matter how humongous it is, even a galaxy has its limits, particularly when considering a chain of required and interdependent (that is: in one progressive direction) events that are, however, not logically and inevitably related.
From what we know about our own planet and its life’s history, it is only very logical that advanced intelligence and technological civilization are exceedingly rare: even in our MW galaxy it has hardly had time, space and chance to arise. And I am convinced (for what that is worth) that this very simple answer, rarity of (persisting) technlogical civilizations, i.e. not rarity of suitable planets and not even rarity of (primitive) life, is also the answer to the Fermi paradox.
A bit further to my previous:
I would still agree with the school of thought of ‘life inevitably leads to intelligence’ if we saw, in life’s history on earth, one or both of two phenomena:
– A consistent progress, increase.
– A frequent and independent repetition.
We can see this with many biological phenomena and trends, such as body size, flight, swimming capability, lungs, gills, even sight. In fact, that is what convergence and analogy are about.
However, (higher) intelligence and (increased) brain size do not seem to be *the* big thing or winning formula in life’s hostory. By far most organisms and the most successful lineages in number and survival period (micro-organisms, insects) have done very well without.
I admit that there is a certain degree of analogy: a few different lineages have given rise to larger brains and higher intelligence, such as some mammals, some birds, a few molluscs (squids/octopusses). And some of these possibly already long time ago (squids, octopusses, maybe some small predatory dinosaurs?).
However, even that does not bode well: in each case it concerns only a few members of the lineage and relatively late in history. And even the early-comers are not a comfort, on the contrary: apparently in by far most cases intelligence will stop at a modest, or at least non-technological level, without advancing further.
Again: life is not necessarily a fluke within organic chemistry, but intelligence is most probably a fluke within life.
Ronald, I understand your point of view. Let me just briefly restate why I do not share it:
Of all the hurdles that you mention, all but one are hurdles to Darwinian evolution. We understand evolution very well, and in front of it, all these hurdles might as well be dams trying to keep water from reaching the sea.
There is one and only one hurdle not facing the power of evolution: abiogenesis itself. The spontaneous creation of that first entity complex enough to self-replicate and thereby kick-start evolution.
This simple consideration makes it easy for me to pick favorites among the hurdles.
I would argue with that. Sure, we do not have the highest numbers, and we have not been around the longest, but we are firmly entrenched at the very pinnacle of the food chain. Not only that, but we have ended evolution as we know it, to the point where the most successful organisms other than ourselves are those whose fate it is to end up on our dinner plates.
The fact that this happened only once does not mean it is not inevitable. It is easy to explain otherwise: Advanced intelligence can only happen once, because it ends the very process that drives its formation. From now on, we decide which lineages survive, and it is more likely to be the tastiest rather than the fittest.
Ronald
We had a similar discussion on the inevitability of the rise of intelligence long ago. Then I pointed to strong fossil evidence that showed less data processing was involved in predator-prey relationships in bilaterally symmetric animals with eyes than at any time ever since.
Cambrian trilobites fossils show bite damage that inevitably occurs on the same side. Since game theory dictates that the best (= most intelligent) strategy for attack or evasion is the one that is least predictable, this indicates an exceeding low level of data processing was used in this relationship.
Note how that suggests that all sighted animals extant today are more intelligent than their typical Cambrian equivalents – whether they be mammal or insect.
We have also well covered before the evidence that average brain size increases in many independent groups.
Eniac, if there once existed a terrific variety of freeliving organisms on Earth whose genetic instruction set was at least an order of magnitude lower than those today, I have a problem.
Evolution does not just select for efficiency, it also selects for simplicity. Thus the displacement of all these organisms by modern forms implies that this one line had a truly incredible advantage. This is particularly so in niches where complex styles of living confer little advantage, and fundamentally different building materials for life forms alleviates competition.
I understand that there is real competition for some resources. To me the best such material recourse that you gave was carbon dioxide, but even this (now just 350ppm of our atmosphere) does not provide much problem from a chemical potential point of view – only one from a kinetic maximise-the-rate-of-incorporation view.
You also give (to my eyes) some nefarious reasons that eukaryotes dominate Earth, despite prokaryotes having far great numbers, a much more diverse gene pool, and possibly a greater biomass. I think that their only real domination (pre-planetary colonisation phase) is your aforementioned “competition” for energy. Most of the carbon fixation and degradation on land is by eukaryotes. But even here, I must note that their never seemed to be much potential for bacterial life on land, and that most would posit that there is far more terrestrial prokaryote life now, than their would be if this eukaryote “competition” was suddenly removed.
This takes me to an assumption in the interpretation of those Viking life experiments. All three of them proved positive for life by preselected criteria – and the level of carbon compounds that might be expected to be incorporated into bacterial-type life to give that level of activity indicated in the best 2 of these experiments was about three orders of magnitude less than Vikings ability to detect organic matter. From Earth we learned to expect that the total amount of organic matter in soil should be >> 1000 times the level of living bacteria, and so the result were reinterpreted to be null.
And that brings me to a strange thought. Could it possibly be that this is a mistake, and that most Earths of Earth’s biomass (about 99.99%!) is, in fact, part of a shadow biosphere, that lives in life’s slow-lane.
Ronald,
Fossil evidence that true intelligence is on the rise is hard to find, and if it were its nature would be hard to evaluate.
So I will just put it to you that there are several elements in today’s fauna that suggest development towards true intelligence, and that these would have likely been less common 100 million years ago.
Many modern animals seem to be tool users. Many cetaceans use personal names in interactions with others. Some elephants seem to have a propensity for the spontaneous creation of art, and definitely treat the remains of dead differently according to whether they have known them in life. African Grey parrots have been verified as able to use abstract ideas, such as shape and colour. Chimpanzees, gorillas, and at least one cetacean species have been shown to have theory of mind by their use of their reflection in mirrors.
I can’t say which of the above are significant, but I can’t help feeling that to say that there is NO evidence (and remember that it was only ever likely to be of the anecdotal sort anyway) is to be in denial.
Eniac, by admitting that advanced intelligence only occurred once on Earth, you are leaving behind the question of how independent the rise of Neanderthal and our own intelligence was about 50,000 years ago.
I also continue to be unsettled by my inability to say that Sperm Wales are likely to be less intelligent than us on current scientific data. To me that question is still far too open.
Do you have an alternative to suggest? Of course there were once organisms with genomes an order of magnitude less complex than those today. More so, it is 100% sure that organisms of the entire continuum of complexity between dead chemistry and today’s life must have existed at least once in history.
Are you saying then, that those existed, but were not diverse? On what grounds? Common sense tells us that more primitive organisms are more diverse, because they do not replicate as faithfully. This goes all the way down to quasispecies and hypercycles, where there is not much doubt that diversity must have existed, if only to explore the “fitness landscape” efficiently enough.
Simplicity is not much of a virtue in biology. It is usually enforced by constraints like mutation rate or energy availability. Constraints that can be overcome by complex mechanisms. Complexity is there to permit doing things that could not be done with simpler systems, such as photosynthesis, multicellularity, sexual reproduction, packing a large genome, etc. etc.
Intelligence is the ultimate such capability, it is the one that allows us to transcend the bounds of Darwinian evolution entirely. It replaces natural selection by intelligent design, which is many orders of magnitude faster.
Eniac, with all due respect and appreciation, but the observation that we humans do or do not understand something well, is no argument for or against its rarity.
Concrete: the fact that biological evoluton is rather well understood does not make certain phenomena and occurrences any more likely, on the contrary, it even emphasizes that the observed rarity is most likely not an observational bias, but very real, no place to hide.
Your second comment outright disappoints me (selection only):
“Sure, we do not have the highest numbers, and we have not been around the longest, but we are firmly entrenched at the very pinnacle of the food chain. Not only that, but we have ended evolution as we know it, to the point where the most successful organisms other than ourselves are those whose fate it is to end up on our dinner plates.”
You know much better than that! By far the most succesfull organisms in number are bacteria and the like and in diversity the insects.
We still do NOT dictate life or its course and only a minute fraction of species are ever used by us.
“The fact that this happened only once does not mean it is not inevitable.
It is easy to explain otherwise: Advanced intelligence can only happen once, because it ends the very process that drives its formation. From now on, we decide which lineages survive, and it is more likely to be the tastiest rather than the fittest.”
I sense a degree of wishful thinking here? First and foremost, the rise of high intelligence did not happen early in history and then stop evolution as we know it, you know that both are not so at all: it arose very late and even now it is so recent that we cannot say at all that it will end or even determine the course of evolution.
You may be right BTW about the rarity of abiogenesis, we simply cannot tell. But again I stress, that even if it is common, there may be countless planets with only bacteria and similar organisms. After some 4 gy without intelligence and another 0.5 without higher (self-aware) intelligence it seems wishful thinking to me to state that it is inevitable, and anyway it is obvious that even if and when it comes, it comes late and rarely.
Rob: to you my rebuttal is similar as to Eniac (and with similar respect and appreciation for both of you gentlemen’s excellent comments).
Regardless, it still stands firm that higher life and intelligence are very late appearances on planet earth. Apparently a planet and its life can do well with just bacteria and the like for gy’s, for whatever reasons.
And even if you are right that there is an inevitable increase in brain size and intelligence among different lineages, once it appears (I will not even contest that), this is hardly a comfort. First, because, as you are stating yourself, this is a very recent appearance. Secondly, apparently in virtually all of those cases this increase did not give rise to sufficiently high intelligence to enable anything like a technological civilization.
I would tend to agree with you both, if there was common fossil evidence of various organisms approaching our (relative) brain size in various lineages in various times and in achieved in various ways (analogy). There isn’t, contrary to flight, sight, smell, lungs, gills, fins, legs, etc. Everything rather seems to indicate that we are a fluke among flukes.
May I conclude in a reconciliatory way by stating that we agree on one essential thing: whichever cause, intelligence is most probably exceedingly rare.
Quite sobering, but also quite an opportunity and quite a responsibility.
I apologize if I sound patronizing or if I overexplain (ok, I have been teaching in the past), but I realize that maybe I should have used more correct biological terms and arguments.
Hereby:
The issue is ‘inevitability’ (of higher intelligence). In biological terms I would rephrase that as ‘analogy’ or ‘convergent evolution’: similar environmental conditions (i.e. selective pressures) will lead to similar forms and functions. This is shown by the repeated *independent* (i.e. phylogenetically unrelated) appearance of a form or function in different places and/or times.
In other words: something is invented independently several times.
For example, we see this very nicely for flight ablity, legs, gills, even eyes.
And large brains, intelligence?
Ok, I have to be reasonable myself here. I admit that higher (self-aware) and increasing intelligence can be found in a few different lineages. How unrelated these are depends on the level at which we consider them: a few different mammals (elephants, primates, cetaceans, maybe bears), even fewer birds (parrots, crows). And even squids and octopusses possess rather high intelligence it seems, that would indeed be quite exceptional, outside the vertebrates.
So, yes there is a certain degree of convergent evolution. But not much.
Most of the ‘high-intelligence’ lineages are mammals and maybe birds, relatively very small taxa. And even among these, most of them, even those that have been around for (tens) of millions of years (e.g. elephants) do not seem to show a persisting increase in brain size or intelligence, rather a levelling-off?
Are primates really an exception in this repect?
Please prove me wrong here, if I am.
Eniac, unlike you, I see no hint that freeliving forms on Earth were once simpler than any today. I remember you once emphasising LUCA’s simplest reconstruction. But other (and equally controversial) reconstructions imply that it was far more complex than the most complex modern forms. To me the decider of where it is on the spectrum, is that of the 23 proteinogenic amino acids, at least 21, and more probably 22 (N-formylmethionine being the exception) were used by LUCA. The universal set of modern forms is a simpler one of 20. Such a reconstruction for LUCA’s deep ancestors would also solve another mystery. Using today’s universal code, a new random sequence of length 100 (that represents the shortest proteins that are likely to be useful) have only about a 4% chance of not being broken by a stop. This new reconstruction would boost that above 20%, and explain how new proteins could have arisen then.
Perhaps the lack of simpler forms on Earth today is due to transference by panspermia – or even directed panspermia.
Perhaps the first forms have to be part of a knifes-edge arrangement of hypercycles or gain some of minimal necessary complexity by complex interactions with other parts. For freeliving forms to develop from these ancestral nets, they may have to add many extra instructions to ensure that their ability to replicate is truly independent. When those original knife-edge conditions go that net would soon die, unless it had produced that said freeliving form.
Perhaps we are surrounded by much simpler forms that we are yet to identify.
Perhaps life really is a millions of orders of magnitude to one against possibility, as I occasionally, and half-heartedly posit.
I may not be able to put my finger on the answer, but I recognise the problem is significant.
Eniac, it also appears that we may have very different ideas about modern knowledge over quasispecies. True, if we treated them as a species, we would find that their mutation rate is amazingly high, but think of this. Their evolutionary pressures is defined by reversion, such that if we inoculated a small proportion of a population with an attenuated strain of a deadly virus, we stand the chance of the reconstructed original emerging from that group.
True, qusispecies and thus hypercycles would have a superlative ability to explore the fitness landscape, but this does not tell us how ongoing the process is. They might typically find three different optima but seldom more than that.
I think that the question of how far and how much qusispecies evolve in the typical case is very much open as our knowledge of them stands today. I suspect that extending what little knowledge we have to hypercycles is getting into wild guess territory.
On my first post dated 25 June I should have stated the problem I was addressing rather than just implying it.
The problem was the interpretation of our knowledge that *every freeliving organism amenable to study is related through one seemingly complex ancestor* in regards as to its implication for the prehistory of ALL early terrestrial life.
Ronald, your arguments are good and I have the highest respect for them. Nevertheless, I detect some misunderstandings and would like to try one more time to elucidate them:
Sorry for not expressing myself clearly. My argument was supposed to go: We know evolution well. We know it is like a steamroller when it comes to obstacles. It tries, and when it fails, it tries again, if needed for billions of years, until it succeeds. Your “hurdles” might as well be dams trying to keep water from flowing into the sea.
You are of course right that the fact that we know something does not prove anything. I was using it as an entry to state what we actually know, which is that evolution is persistent and progressive.
Popular misconceptions to the contrary notwithstanding :-)
I was trying to point out that measures of population size or diversity are poor quantifiers for evolutionary success. Instead I proposed elevation in the food chain as an alternative (and I think much better) quantifier. I think that my food-chain arguments, facetious as they are (a little), apply very well in this light.
Certainly the uncounted masses of bacteria do not matter to the subject at hand, as they have no chance of becoming intelligent.
Here you get very close to the crux of the matter. I think you are almost right, but you are missing one thing that I mentioned before: It is not possible for us to observe other lineages becoming as intelligent as us, because if that had happened, they would have done what we are doing now, and we would not exist. Or we would be them, which is merely a slightly different perspective.
The reason we are not observing other lineages of our intelligence is not that they are unlikely, but that we are the first. And being the first in this case is not an unlikely coincidence. It is inevitable, because one is all there can be.
I do not follow this at all. Who is to say that 4 billion years is late? If it has to happen sometime, 4 billion years is as good a time as any. Remember, there were a lot of hurdles to clear, which takes time.
Your characterization of the hurdles as single probabilities that multiply together to make intelligence very rare is flawed. Evolution never stops trying, and all these hurdles are characterized by rates rather than probabilities. The probability of overcoming them will approach one if sufficient time is given, which is the statistical equivalent of inevitability.
Let me finally make clear that I am not saying inevitability can be proven. It cannot, because after all the available time is not unlimited. All I am saying is that your arguments for rarity do not convince me, and there is a whole lot of time available to come real close to inevitability even for very formidable hurdles.
Rob,
It is not a hint, it is a 100% certain consequence from the assumption that life evolved gradually from dead chemistry. If you do not grant this assumption, you get into fantasy-land. Panspermia is not an answer, because it simply moves the problem somewhere else, with less time available.
Unfortunately, there can be no reconstruction of LUCAs ancestors. The only way we know how to reconstruct past genomes is by comparative genomics. By definition, only one branch remains from LUCA and its ancestry. There is nothing to compare, and therefore no chance of reconstructing ancestry. This central part of the tree of life will forever be unknown.
I am sure you appreciate that this does not mean LUCA was alone, it had many siblings, probably quite diverse, with many amino acids now lost and exotic chemistries completely unknown to us. The only thing distinguishing LUCA from its many siblings is from our posterior perspective, as the only organism from that time that left descendants all the way down to us. This may have something to do with LUCAs particular model being effective, but there is likely also a good dose of luck (aka arbitrariness) involved.
Ronald, I disagree that mammals show the greatest example of a broadly based increase in intelligence over the last hundred million years. Dinosaurs showed a clear increasing trend of intelligence late in the Cretaceous, and I think that you would find that nearly ever modern dinosaur has a higher intelligence that its Cretaceous equivalent of similar-ish niche. I certainly doubt that there is a single case where the EQ is lower (though, admittedly most birds fly, and most non-avian dinosaurs do not, that this does muddy the waters a little)
Eniac,
thanks for your clarification, makes it very clear indeed.
The first part, about our understanding of evolution etc., I can agree with.
About your alternative definition of evolutionary success, well, I suppose that would make you right by definition :-)
The rest, the whole essential part about inevitability of intelligence: I suppose we agree to differ, this will simply be very hard to prove or disprove at this stage.
But we do agree on the fact that we are (obviously) the first on earth. We may very well be the first in our MW galaxy as well, by whatever cause.
My assumption and expectation, that there are many planets with relatively simple life (single-celled, Prokaryote, simple cell-colonies, etc.) we will be able to test in the foreseeable future, when we spectro-analyze planets for biosignatures. Well, we probably won’t be able to distinguish simple life from more advanced life that way, but at least the prevalence of living planets can be tested.
Eniac, we can still use clever ways to try and reconstruct LUCA, and might largely get it right. It is just that we would have no good way of testing a confidence level in that correctness. That is not just me being pedantic, since several people have tried. LUCA would have had lots of contemporaries- but I posit that they were almost certainly of reasonably similar complexity and construction.
Of cause there must have been a continuums of complexity between the inorganic and organic somewhere. I was just saying;
1) it does not have to have been on Earth
2) no simpler form than LUCA has to be freeliving
3) if simpler freeliving forms once did exist, there is no need for us to say that their existence was ever stable, rather than just metastable
And one more point. If the Steady State theory is ever resurrected, then panspermia no longer just needs to displace the problem – it then need have no origin.
Ronald:
We do agree on that. The important thing to realize is that the first is also the last, and that that makes us the only.
Rob: We can reconstruct LUCA, to a certain extent, but not its progenitors or contemporaries. There is simply no way. I am not sure what you mean by your “freeliving” and “stable” qualifications. Depending on their meaning you may well be right. It is unlikely, though, that the transitions you postulate (non-freeliving to freeliving or metastable to stable) coincide in time with LUCA, considering the arbitrary and posterior definition of the latter.
Eniac:
“We do agree on that. The important thing to realize is that the first is also the last, and that that makes us the only.”
Only higher intelligence and technological civilization, that is.
Not necessarily the only life, even according to your own way of reasoning:
even if such high intelligence is indeed an inevitable outcome of the evolution of life (which I contest, as you may understand by now), it is very well possible that a planet is still in an earlier stage of evolutionary development.
Which brings me to the next thought, you state:
“4 billion years is as good a time as any. Remember, there were a lot of hurdles to clear, which takes time”.
Theoretically then, it is even possible that a certain planet remains stuck in an environmental state which is not favorable for ‘the next level’, like our own planet was (according to some theories) in a state which only allowed for Prokaryotes for a few gys.
For geological, atmospheric, astronomical, or other reasons. This also depends on whether the evolutionary development ‘success’ and rate primarily depend on the planetary (abiotic) conditions, or on intrinsic biological factors (mutation rates, predation, competition, etc.).
So, summarizing, even following your line of thought that ‘intelligence is the inevitable outcom of life’ given enough time and the right conditions, merely based on this premise one cannot conclude that life perse is rare.
Eniac says “It is unlikely, though, that the transitions you postulate (non-freeliving to freeliving or metastable to stable) coincide in time with LUCA, considering the arbitrary and posterior definition of the latter.” and I agree. We seem to disagree on the following two points, the second of which I implied much earlier.
1) That if the LUCA of currently known modern forms was the only survivor from its time, this implies that their were never any very much simpler Earthly forms, or forms built of dramatically different materials that could ever have left descendants under any likely conditions. So I certainly do not postulate that that transition occurred at that exact time, only that that form could only have been slightly simpler and its biochemistry not radically different than LUCA’s
I must admit though, if LUCA appeared on Earth before the Late Heavy Bombardment (whereby just one group may be expected to cling on in a deep endolithic refuge), I might have to modify my thinking.
2) This debate is fairly likely to be irrelevant, because we are likely to be embedded in a large shadow biosphere (my reasoning is that LUCA feels to complex to be the sole ancestor of our biosphere). The problem of studying life is now so specialised at multiplying signal of known life that I believe that we would not be able to recognise it if we could find it. If we found such a form, we would just use PCR till we identified an associated LUCA relative – and its metabolism likely too slow.
Ronald:
True, for my most recent reasoning. For chosing between rare and common life, given the assumption of rare intelligent life, I reach back to my older arguments:
Rob:
I disagree here. There certainly were such simpler forms, and they did leave descendants, one lineage of which includes LUCA and survives until today.
As an example of simpler lifeforms that certainly existed were those that used neither DNA nor proteins. One of the very few things we can reasonably deduce about pre-LUCA evolution is that both DNA and proteins were (probably successive) disruptive innovations that contributed to the demise of contemporary organisms lacking them.
I disagree here, too. The discomfort you feel about the complexity of LUCA is misplaced. In any pedigree, most lineages always die out. This leads to the phenomenon of LUCA, and also the phenomenon of the “mitochondrial Eve”. It may ease your discomfort if you think about this imperfect, but relevant analogy.
A shadow biosphere would definitely be known to us if it had any significant presence. If not in PCR, it would show as complex molecules in mass spectra, GC, and HPLC readings or simply chemically via its metabolic activity. To remain hidden, it would have to be located well beyond our reach, certainly we could not be embedded in it.
Lastly, even if there were a shadow biosphere, it would not change much. Its discovery would move LUCA back in time, to the point of common ancestry between ours and the shadow biosphere. Even if that shadow biosphere were radically different from ours, we would not be able to exclude a common ancestry, going far enough back in time. The only reasonable way to exclude common ancestry would be to find life off Earth, but even there we have to consider the possibility of cross-contamination.