Other potential systems may exist and simply never got the foothold necessary to evolve into an efficient competitor. I would guess that it's quite likely we're at a local maximum, not the single and only possible way to organize cellular life.
OK, what are they?
It isn't necessary to know what they are. GavinB is just saying that there could be others.
For it to happen twice the 'new' trail would have to be totally non-nutritious to the established kind.
I wished I remembered where I read it.
I'm sorry, but this doesn't make any sense. Just being called "food" is not a sufficient reason for extinction. In fact, organisms at the bottom of the food chain constitute the majority of biomass in all known ecosystems.
It's a debatable argument, with obvious parallels in the startup world.
In ecology, the competitive exclusion principle is a proposition which states that two species competing for the same resources cannot coexist if other ecological factors are constant. When one species has even the slightest advantage or edge over another, then the one with the advantage will dominate in the long term. One of the two competitors will always overcome the other, leading to either the extinction of this competitor or an evolutionary or behavioral shift towards a different ecological niche. The principle has been paraphrased into the maxim "complete competitors cannot coexist".
Any animal from the 'other' branch that observes this 'complex' life from made of two cells and another one nearby made of only one would have the choice between two bites for the price of one and one bite.
So new 'complex' life would not be around for very long because the ohter branch has presumably already reached a higher level of evolution.
It would have be a very successful mutation to stick around long enough to have enough offspring to take on higher organisms than itself by having some survive in spite of being predated on.
The lowest level organisms best defense against extinction is their enormous numbers or symbiotic relationships by evolving in tandem with higher order forms, a brand new attempt at complexity would not have that advantage yet.
The same goes for another carbon based evolution scenario, after all, if there could be two independent complex branches of life it would not be too much of a stretch to think that over the last billions of years a second life form would have come in to existence. But it would have found each and every niche already occupied by 'our' kind of life and likely not survive long, and likely not leave any trace of its existence.
to make (a technical or abstruse work) easier to understand and more widely known; popularize. (from dictionary.com)
EDIT: I see that someone beat me to the punch ...
In fact, healthier ecosystems promote prey more than predators.
Ignoring the inconceivably remote chance of life forming from non-life, it's very very rare for simple life to evolve into more complicated life. Also, we just realized that cells need to get bigger to get more complex, and that somehow isn't just 'another' mutation.
And then the title? There is nothing about how or why complex life only evolved once. Just that it was unlikely. Chance of life forming from non life * age of the universe * number of stars * the chance a star has an earth like planet, is far from zero.
I wouldn't even bother ranting but this was number 3 on the front page.
The article is behind a paywall, so I can't read the details.
Anecdotically, there exist anaerobic eukariot cells that live on hydrogenosomes . I don't know if they were originally aerobic and lost their mitochondria, or if they evolved in parallel.
There also exist sea slugs that live in symbiosis with the chloroplasts of the algae they eat , so this kind of symbiosis has happened more than once.
The main calculation in the Nature paper is the energy budget of a cell. They give the example of E. coli: "to raise gene number tenfold, E. coli must also increase its energy budget by close to tenfold; and therein lies the problem." The reason so much more energy is required is due to protein synthesis (DNA codes to RNA which codes proteins, if you're unfamiliar with genetics). So obviously with a larger genome, there's more proteins to synthesize, and this cannot be accomplished by simply making fewer copies of the old proteins. If anything, you need more of some of those original proteins as infrastructure support for this new Genome 2.0.
So (and this is my wording) prokaryotes are at a local maximum for genome and cell size. Chucking mitochondria into the picture provides the wattage (literally) for them to reach a much greater genome and cell size, and therefore organismic complexity.
But I'm no biologist. Am I missing something obvious, or is there a particular paper explaining why this mitochondrial upgrade is considered such an unlikely event?
Even on content I would bury this if I had the button for it, it makes pretty heavy claims without any evidence and necessarily brushing aside fundamental problems with the overall conclusion.
I am not bashing the scientific article that this editorial is 'about'; but the article itself, besides a tiny nugget about the necessity of mitochondria being perhaps otherwise overlooked, it is just drivel.
You have no proof for this, and things can well be the other way round. Let
n = age of the universe * number of stars * the chance a star has an Earth like planet
n is a number that we currently have a pretty good guess at. Let p = chance of life forming from non life on Earth-like planet within a given year. We are basically clueless as to the value of p. p may well be much less than 1/n, which would make our biosphere very special indeed.
Life certainly does exist so p > 0. Since our understanding (or I should say, my under standing of 'our' understanding) of n is approaching infinity, it stands to reason that p x n is > 0.
Our understanding of n is not a constant.
Because when we look out into the universe, it looks like we expect it to look when it is dead, not full of life, some significant percentage of which becomes intelligent. Nothing that looks like Dyson spheres of any form (recalling that the "solid shell" is actually only a special case), nothing that looks like curated stellar evolution (very high-end stuff but a truly advanced civilization will not want to leave stars just randomly flying around a galaxy and exploding), very very little that could even conceivably be a ship traveling between the stars (which turns out with any physically-plausible propulsion mechanism to be visible from a very long ways away and would show very clear blue/red shift changes), nothing that doesn't appear entirely natural.
This isn't the whole story (maybe they're hiding, maybe our idea of the top end of tech is wrong, maybe they're all virtual, maybe they escaped into another, friendlier dimension, maybe they all kill themselves, maybe the universe is soaked with simple life but this article is essentially correct and complex intelligent life really is a 10^-1000 event, etc.), but it is not true that the claim that complex life must be hard is some sort of stupid idea in light of the size of the universe. Arguably it's actually the claim best supported by the evidence at this time.
People are really bad at thinking about small probabilities even on the scale of one out of a million, and it's not impossible that complex intelligent life turns out to be one out of 10^1000 or worse; we simply don't know. It is not hard to come up with calculations that plausibly set the odds of advanced life occurring even once in the universe as less than 50/50 without having to make stuff up. Our ignorance dominates our knowledge here.
Our reach is virtually zero in the context of the universe. It seems from our vantage point, I would expect the universe to look exactly the same whether it was bustling with life or we were the only ones.
I had never thought about searching for Dyson spheres (it made me replay a part from The Big Lebowski where the dude says, "We'll that's fucking interesting man."), but they strike me as a incredibly hard thing to search for. Don't we see into the universe with the energy escaping from stars, galaxies, etc..? If some species were able to actually harvest the output of a star to anything approaching 100%, wouldn't it be exactly the type of thing we would never see?
> Arguably it's actually the best claim supported by evidence
Unless I am fundamentally wrong about the resolution of our map of the universe, I don't think there is any evidence either way unless you believe in any 'encounters'
You'd still have residual heat. Some have suggested that what we'd see is awfully similar to red giants though I rather expect we'd still see something odd about them.
Besides, the one you really should have focused on is star-traversing ships. They're visible from a really long ways away.
"I would expect the universe to look exactly the same whether it was bustling with life or we were the only ones."
Allow me to re-add in the word "intelligent" to your "life". The universe looks the way it would be expected to work if absolutely nobody ever makes it off their planet to any reasonable degree, or there is no other intelligent life out there. Or we just happen to be the "precursors".
Part of the problem with this debate is that most people are still participating in it with very 1960s ideas about the limits of technology, very Star Trek ideas about what technologically sophisticated aliens will be like. Of course the Federation could be floating around out there without us knowing, with its starships powered by magic, its technologies powered by magic, and humanoid aliens scattered about everywhere due to magic. (Yes, I know about the episode in question.) But that's not what it looks like. In reality, a modern conception of what a technologically-sophisticated culture would do results in a lot of things visible from a very great distance, the ones I previously mentioned before. And we see 0 of them.
Personally I do not put much stock in the idea that intelligent life is abundant and easy and not a single one of them ever decides to go to another star. For such a society it just isn't that hard and the payoff of being the first in a star system is incomprehensibly enormous, well out of proportion to the difficulty of getting there. Something about standard story of easy, abundant intelligent life is very very wrong, and the most parsimonious explanation is that intelligent life is in fact not easy or abundant.
What? No. If there is residual heat you aren't capturing all the energy.
> In reality, a modern conception of what a technologically-sophisticated culture would do results in a lot of things visible from a very great distance, the ones I previously mentioned before. And we see 0 of them.
This appears to be the crux of your argument. Can you go into or point to a place where I can find out more about what these ships would supposedly look like?
I think the fact that we will almost certainly need a paradigm change before we can really conceive of interstellar travel combined with the radical shift in conceived ideas following a paradigm shift, current guesses about what a super advanced culture's vehicles act like doesn't really hold much weight for me.
I argue on the basis of real science that we know, not because we know it all but because we can't argue on the basis of science we don't know about. Given that, we damn well can make some guesses about what a space ship will look like, which inevitably includes some form of propulsion involving either the expulsion of very, very hot matter (to get optimum mass efficiency, propellant will be at a premium and no point dumping it out the back cold) or light directly (unlikely but at least plausible) in a directional manner.
The topic came up a few months ago during the discussions on whether it is rational for an alien civilization to summarily execute any other civilizations it discovers with relativistic planet-killers, but unfortunately trying to Google up a specific discussion about spaceships and relativistic projectiles is an exercise in futility. Still, work the math on what it takes to get a decent payload (at least several thousand tons would be nice) up to ~.1c, and then recognize that thanks to Newton's laws, the resulting mass-energy number you find must actually be coming out the back of your spacecraft in as close to a single direction as possible. Possibly literally as a laser, though as I said I'm pretty skeptical about the utility of a pure light-drive. It's pretty damned bright.
Look, I hate to be offensive, but if you don't fail basic thermodynamics and you actually take seriously the real science we already know, we don't have to retreat into romantic cliches about how we can't possibly understand advanced cultures. We can in fact put bounds on things if our understandings are basically correct, and if our understandings are basically incorrect then frankly we have bigger problems than the question of alien life.
The upshot of this, going back to your first post, is that if either of us is retreating into "religious dogma", romantic conceptions of reality held onto despite rational examinations of the evidence and science, taking shelter in the possibility of who-knows-what magic science may produce in the future and refusing to use what we already know, it's you, not me.
Lets take a step back into the 1500s and talk about what an advanced culture able to move thousands of tons of goods from one side of the country to the otherside in a matter of days would look like. or what being able to search all of the worlds knowledge instantly would look like, etc etc. Any of their guesses are necessarily limited to their 'limited' perspectives.
We have had numerous paradigm shifts since then, each giving fundamentally new ways to conceive of problems, and I think it's incredibly likely that we will undergo another paradigm shift with regard to physics/space travel before our society is leaving the solar system, let alone the galaxy. If that's the case it's not an unnecessary romantic conception, it's a necessary repercussion of a historically verifiable phenomenon, paradigms change, and so do ways of thinking about problems.
Besides, I still don't understand how we are seeing a several-ton payload going .1c in a galaxy we just discovered.
I guess I just don't see a compelling argument that we would, from our vantage point and with our tools be able to make any type of reasonable observation against a hypothesis about other intelligent life.
Intuition is often wrong, especially when it comes to probability.
We have n planets, which we can rewrite as 10^x. There's a probability to evolve complex life of 10^-y on any given planet. Which is bigger, x or y? By how much? What are your error bounds? I've seen people throw out numbers with thirty zeros after them and think they've made a point, but really, either x or y could be much bigger than 30. There could be complex life on 10^many planets, or it could be a total fluke that it exists even on this one.
Either way, your intuition isn't going to tell you anything meaningful.
10^-y could be anywhere from 0 to 1, but you have to be extremely close to 0 for y to be bigger than x. Since we are here pondering this, it makes intuitive sense that it's likely we 10^-y isn't infinitesimally close to zero.
They're saying that on Earth, complex life only happened once:
"It suggests that complex alien life-forms could only evolve if an event that happened just once in Earth's history was repeated somewhere else."
All of them are eating. Some of them are mutating.
Let them do this for a few billions of years and what seems to be a very unlikely event suddenly becomes almost certain to occur. Complex life FTW!
I don't care how big your bignum is compared to everyday experience. Bignum * Smallnum does not automatically equal certainty, impossibility, or feasibility. It means do the math.
In re: 1,
"Mitochondria must respond quickly to changes in membrane potential and the penalty for any failure to do so is serious. The electron and proton transfers of chemiosmotic energy coupling generate a transmembrane potential of 150–200 mV over the membrane (~5 nm across), giving a field strength of about 30 million volt per metre, equal to that discharged by a bolt of lightning. This high membrane potential sets the inner membrane of bioenergetic organelles (mitochondria and chloroplasts) apart from all other eukaryotic membrane systems. Failure to maintain the mitochondrial membrane potential is penalized by a collapse in energy charge, blocking active transport across the cell membrane, and a rise in free-radical leak, which in eukaryotes and many prokaryotes leads directly to programmed cell death.
"This requirement for physical association of genes with bioenergetic membranes to maintain ATP synthesis constrains both the genomes and the complexity of prokaryotes. If some genes for oxidative phosphorylation must be physically associated with a certain unit area of bioenergetic membranes, then beyond that threshold prokaryotes could not maintain membrane potential homeostasis unless additional genomes are co-localized with the membranes."
In re: 2,
"The main difference between endosymbiosis and polyploidy relates to the size and distribution of genomes over evolutionary time. In endosymbiosis, surplus organelle genes are lost or transferred to the host’s chromosomes, streamlining endosymbiont replication via cytoplasmic inheritance. The outcome is a massive reduction in genome size, both in prokaryotic endosymbionts and organelles, with a reciprocal relocation of genes in low copy number to nuclear chromosomes in the latter. By contrast, in giant polyploid prokaryotes, all genomes are essentially the same. Without cytoplasmic inheritance, no genomic specialization ensues.
"In principle, prokaryotes could control respiration using specialized, membrane-associated plasmids that emulate organelle genomes in gene content and function. In practice, such plasmids are not found. Bacteria usually have small, high-copy-number plasmids that segregate randomly at cell division, or very few giant plasmids that co-segregate with chromosomes on filaments from midpoint. For plasmids in a prokaryote to support electron flux as organelle genomes do, high-copy-number giant plasmids encoding components of the electron-transport chain would need to associate with the plasma membrane, and evolve counter to the tendency to segregate with size rather than function. That no mtDNA-like plasmids are known indicates that high energetic barriers preclude their evolution: unlike organelles, which pay back energetically from the start, substantial energetic costs must be paid up front (high copy number of the correct plasmids, and the machinery to associate them with the membrane at regular intervals) before any energetic advantage can accrue.
"Thus, being large and having masses of DNA is not enough to attain complexity: cells need to control energy coupling across a wide area of membranes using small, high copy, bioenergetically specialized genomes like mtDNA. Segregating the genes relinquished by the endosymbiont (mtDNA) into low copy number in the host’s chromosomes, specialization of the endosymbiont into an ATP-generating organelle and increasing organelle copy number provides sufficient energy per gene to support the evolution, maintenance and expression of some 10^5 more host genes, affording the cell the chance—but not the necessity—of becoming complex."
It is not possible to exactly reproduce the state we have with life on Earth, like it isn't possible to get precisely the same weather in 2 different days - both are results of unimaginably many values behaving and interacting in unique way and even minor change will change the resulting outcome . Yet there are a lot of days that look and feel very similar and there is 99.9999999% chances that tomorrow, for example, we wouldn't have 150 F in San-Francisco while we can't be sure what precisely it will be - 60 or 70 or 50?
Does the life evolution system have such bound-ness and attractor-ness properties - well, as of now we can only guess. My guess is "yes".
Btw. Uniqueness because of complexity, in the sense of "evolved only once", stinks like Creationism/ID
If evolution on a habitable planet has a 10^-4 chance of happening, then the universe is almost certainly teeming with life. If it's 10^-18, the size of the universe is the difference between believing and doubting the idea. If it's 10^-200, the universe barely helps at all, and you need to start talking about billions upon billions of barren universes and we just happen to live in this one.
Do you know what the number is? Do you know where it lies between 0.9 and 10^-(10^10^10)? Do you know whether the 20 orders of magnitude you get from the stars is overwhelming, decisive, or not even enough to make a dent?
[big bang --(we're here-ish)------------------------------------------------------- end of universe]
So the facts that we're that far to the left of that line, and that the universe(s) is/are really really big, suggest that life isn't that rare.
"whenever simple cells start to become more complex"
The definition of complexity here is irrigorous to the point of laughability. At one point the definition makes a call about "size" - " if a bacterium grew to the size of a complex cell, it would run out of juice"
Nonsense. There was recently discovered a macroscopic bacterium - http://en.wikipedia.org/wiki/Thiomargarita_namibiensis Yes, you can see them with your own eyes.
"If the energy-producing machinery straddling the membrane is not constantly fine-tuned, it produces highly reactive molecules that can destroy cells. Yet fine-tuning a larger membrane is problematic because detecting and fixing problems takes longer."
The solution, then is to create short-length response pathways. Presumably the "reactive molecule" they're talking about is superoxide - Life has evolved an incredibly awesome enzyme that takes care of it - superoxide dismutase - and even a slightly defective SOD doesn't lead to inviability. Certain types of photosynthetic bacteria, which need to "carefully manage reactivity" have solved this problem by creating 'hydrogenases' which bleed off excess energy in the form of hydrogen molecules (these may become useful in the alternative fuel industry).
"Acquiring mitochondria, it seems, was a one-off event."
Well, no. Plants later acquired chloroplasts. Some organisms have hydrogenosomes (which is inetresting, because we now have discovered vertebrates that seem to be able to survive off their hydrogenosomes, mitochondria be damned). Some bacteria have evolved anammoxasomes.
"cells capable of complex functions – such as communicating with each other and having specialised jobs – could evolve"
Clearly the author has not heard of quorum sensing. The notion that bacteria are hyperindividualistic robinson crusoes adrift in their environment is generally not believed anymore.
Then the author redefines complexity to be a 'large genome' thing. Of course there really isn't anything stopping bacteria from having ploidy. Deincococcus Radiotolerans, for example, has 4-10 copies of its genome inside itself (for reasons that you might be able to guess from its name).
And the whole complexity thing is bunk anyways. The actinomyces are such complex bacteria that for many many years scientists thought they were fungi.
It's also worth noting, that there certainly are eukarya that are plenty 'complex' that simply don't have mitochondria.
Better explanation: Eukarya were enabled because the Great Oxygen Catastrophe suddenly made being a paired oxidizer (mitochondrion)-reducer (host cell) beneficial. Prior to the oxygen catastrophe, mitochondrial cells would have only been able to survive in homeostasis with a cell that was happy to have its poop (oxygen) taken care of locally... Not to mention that eukarya are generally considered to have branched off of archae... The specialness of eukarya is that they were a unification of two highly divergent branches of the phylogenetic tree.
Right, midichlorians are in all living things of sufficient complexity.
Episode 1 was onto something?
Kill me quickly.