posted
I have an honest question. This is not intended to provoke debate, nor is it a veiled criticism. I just am trying to find some information.
I conceptually understand how natural selection could increase the complexity of single-celled organisms. Conceptually, I get how, given the existence of the DNA machinery, a cell can reproduce itself this lead to more varied traits.
But there is a gap in my mind that I am trying to fill. I don't understand how multicellular reproduction (like asexual budding and later, things like eggs and finally, live births) is believed to have happened. Given the absolutely critical need for a multicellular organism to be able to reproduce before natural selection can began working, it would seem to be an area of great research. Obviously, my google-fu skills are not the best, however, since I haven't found anything.
Can somebody help me out here, either with an abbreviated explanation or a reference to some material on the subject?
posted
I am a little confused on your question so I am not exactly sure how to answer. But what I would suggest is looking at dictostylium (so used to calling it dicty I had to actually go look up spelling, so ashamed). Dicty is slime mold and when there is lots of food it is a unicellular organism. When it gets hungry, it combines with other dicty to form what it called a fruiting body, sporulates and then hopes it gets blown to a new, better food source, where the spores become unicellular again. How it does this and how the individual cells differentiate and decide where on the fruiting body they live and how big the fruiting body is have all been studied.
Posts: 1001 | Registered: Mar 2006
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posted
That's sort of what I'm looking for. I was thinking about the reproductive process, human in particular, and was amazed at the complexity. And tried to think of how it is explained that entire system came into being. As I cast about in my mind, I kept going back and back, to simpler forms of reproduction, and still couldn't remember anything. It wasn't till I got to single-cell reproduction that I could think of an explanation. So I am wondering how you can go from a single-celled reproductive system to more complex ones. The reference you gave me is a place to start. Thanks
Are there more examples/explanations/theories?
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The cape bee I think is a good example of a possible evolutionary link between reproducing asexually and sexually. I believe it is a rare example of a higher level animal species that can reproduce asexually.
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posted
C. elegans can reproduce asexually and sexually as well. Males come about only rarely (they can only produce sexually though). If a male is around, females will use the male over her own genetic material. C. elegans are cool because they have a set number of cells and you can map cell fate all the way through development. Also, they are a model organism which means they have been studied extensively.
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posted
will look into those. Any information on things like egg-laying development/capability and, more complex, mammalian-type reproduction?
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posted
Well, it must be admitted that this is one of the least understood areas of the history of life. One thing you could try is to ask over here, where a bunch of working biologists who do exactly this kind of thing for a living hang out.
For now, though, I'll try to just think out loud. As I write this, I'm not quite sure where I'm going to arrive, so bear with me if I ramble.
I think it's reasonable to assume that multicellular organisms started out as just colonies of single-celled ones. Better still, they start out as colonies of two kinds of single-celled ones, which perhaps have different nutrition requirements (so they don't compete against each other). Now, to get selection for this, you have to have organisms that complement each other in some way; perhaps the one eats the wastes of the other, which in turn benefits from not having the water around it filled with shit.
Now, this is hardly real multi-cellularity. To get that, let's ask whether there's any way we can get advantage from a particular arrangement of the single-celled organisms. Suppose we put the waste-eaters on the inside; that benefits them because they are better protected. Does it benefit the waste-producers? Hmm, maybe not. Okay, put the waste-eaters outside; that benefits the because they are now catching all the produced waste, and they can eject their own wastes into the ocean. And the waste-producers get the protection of a skin, so now both organisms benefit.
We still need to look at whether this can be done by plausible mutations. (Just because trait X would be good, doesn't mean evolution can get to it from here!) However, I don't think this is very difficult. Plenty of single-celled organisms have the ability to respond to chemical cues; you just need to have the waste-eaters go towards a particular concentration of the waste (not too high, not too low) while the waste-producers do the same, but for a different concentration. Those colonies in which even one (you don't need both) of these happens will do better than those in which it doesn't.
Ok, so far so good - we now have, at least, cellular specialisation, which is a good first step to true multicellularity. Next, it seems pretty clear that it would be a good thing if the waste-producers weren't always being eaten. So if the waste-eaters clump together, maybe grow a bit in size, generally speaking form a relatively impenetrable outer layer instead of a permeable membrane, that would be a fine thing. Now we are beginning to look at a real multicellular organism, with a genuine division between inside and outside. So, how does it breed?
Hmm, I should be working. I'll get back to this.
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posted
Not a lot of online stuff on dictostylium. The Cape bee article was short. Going to look at c. elegans.
Does anyone know of any studies or ideas into how multicellular complex reproduction came about? Specifically, the things I mentioned above? Just curious, since, as I said, explaining how the particular reproductive system of a species came into existence seems to be a necessary precursor to showing how that that species was then modified through natural selection. And by the system, I'm not talking about the DNA machinery, though that is where such changes specifically are made, since that already existed. I'm talking about the reproductive means of the species itself that USES the DNA machinery.
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posted
Most borderline multi/single cellular organisms we know about organize along chemical gradients. Just as some single-celled organisms have different forms depending on surrounding environment, by different forms of a conglomerate organism generating and conveying different chemicals in different ways, they can enforce a particular 'structure' for the various component forms. These structures are emergent properties of the single-celled organisms being in close proximity.
Those multicellular organisms we understand well. Similarly, its a bit of a silly question to ask 'how did the egg happen', because its so far along. We can see how the amniotic egg might have from the amphibian egg. We can see how that might have come from the fish egg. The origins of the earliest eggs are very hard to come up with good explanations for, because the hurdle is very far in the past and likely very rarely overcome.
Here's a possible hypothesis: at some point, one of these conglomerate organisms that's not really single-celled or multi-celled, instead of just splitting and having the two parts go their merry ways, had a small part split off inside and not become reabsorbed, for whatever reason. This small part was sheltered from the environment, while the large part didn't give up many resources to the small part, so everyone was better off than previous splitting scenarios. Gradually individuals better able to protect these smaller offspring until relatively grown came to dominate the population. Their offspring were better off if during this period of protection they spent as little energy as possible and absorbed as much energy as possible; the shape for them to adopt to fit those requirements is ovoid (maybe even spherical). Ta-da, eggs.
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posted
A reply to someone's post by the moderator of that forum KOM linked to:
quote:Supersport, I strongly urge you to concentrate on the threads you have already started. Banging out new posts may indicate a decent typing speed, but it is also disruptive, since you will not possibly be able to reply to everything.
Any further new threads will be temporarily closed until you have reasonably followed up what you already have on your plate.
-- Oolon, E/C Moderator
Just thought that was really cool. Posts: 8741 | Registered: Apr 2001
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posted
Thanks, KoM. That's what I am looking for, though you had to break away just at the crux of the issue. Will have to wait for it, I suppose.
Right now I am reading about morphogenesis. Not exactly what I am looking for, but still interesting.
Why, do you think, it is the least understood area? Because other areas were being studied? Or has it been studied for a long time and progress has been slow, compared to progress in, say, determining the ancestral tree of homo sapiens (still in work, I know, but much more fully explained.)
I just keep thinking how critical it is to explain with evolution through natural selection the multicellular reproductive mechanism since that is then where a species is modified into all the forms of life we see today.
And a side point, to preempt any criticism that might come up (as I know it has repeatedly in the past). I am not trying to make an argument or point out flaws, logical fallacies or anything like that. I honestly am interested in the question. That is not to say, though, that it does not inform my own opinion about things. But that's neither here no there. I am really interested in, at least a conceptual model for how these complex mechanisms formed, given how important they are.
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posted
Thanks Fugu. As I said, I was reading about morphongenesis and so could better understand your explanation of chemical gradients.
I like your hypothesis about the egg, though obviously I know it's a rough one missing many crucial details. Did you come up with that or is that a synopsis of one working model?
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posted
That's just something I came up with while thinking about your question; something along those lines is probably extremely likely (that is, fission transforming into egg-bearing), the specifics are much more up in the air. Part of the problem is, of course, that the line between egg and fission offspring almost certainly isn't clear.
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posted
I see your point about the difference between fission and egg offspring being more fuzzy. What about the formation of a live birth infrastructure? And thoughts there?
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posted
Well, actually, that's pretty trivial once you have eggs. You just need to keep the eggs inside the mother until they are ready to hatch, as some snakes do today; from there, increasing weakness of the eggshell, until it is more of an amniotic sac (if that's the word I want) is an obvious advantage.
I'm still thinking about where I want to go from my scenario above. It seems to me that the first step is to reproduce simply by building up the internal population, the waste-producers, until the pressure is so high that the organism explodes , spreading both kinds of cells everywhere. Spores! The next step would be that the organism takes steps ensure that both kinds of cells land together, so that you can form proper colonies. The obvious way to do this would be that cells respond slightly differently to the chemistry when the pressure increases. When the pressure is near the breaking point, those cells that are close to the surface should bond themselves into tightly packed little spheres that won't be disrupted by the explosion - which won't actually be that explosive, after all. The obvious way to do this is to make use of the membrane-forming capacity that the skin cells (the waste-eaters, in my previous terminology) already have. That would be fairly egg-like, yes? Actually it's more like a live birth, since we are forming basically small copies of the organism. But it becomes egglike if you also add deposits of whatever they eat, to live on until they find a place to settle.
I think that's a good starting place from which we can evolve sex; I'm still not quite sure how I'm going to do that, I must admit. Please note, I'm not a biologist and I'm just thinking as I go along.
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posted
One thing you might want to look into is mitochondria. These are the the engines that drive the production of energy in your cells. Mitochondria are an interesting topic because, like few other parts of the cell, they have their own DNA and can produce teir own proteins. What this means is that, mitochondria have their own means of reproduction WITHIN YOUR CELLS. This bacterial ability of the mitochondria suggests that it used to be a separate organism from your average eukaryotic cell. It could've been that these bacteria were taken into the eukaryotic cells and adapted a symbiotic relationship which eventually led to what we have today.
Anyway, this relationship could've been one of the earlier occurences of one organism living inside of another; a sign that germination could also be achieved internally in the future.
On a side note, I find it fascinating that a woman's autoimmune system does not attack the growing fetus. Brilliant I tell you.
posted
We should also look to understand simple multi-cellular organisms and ancient forms with specialization of cells. One interesting example would be sponges. Another example would be primitive jelly fish.
(sorry, I'm doing this off the top of my head). The deal with sponges is, IIRC, they are among the simplest of organisms with an inside and an outside -- some specialization for digestion, for example.
the jellies are weirder.
But I'm starting to run out of memory for why.
I think they are worth looking into, though.
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posted
We have examples of live birth transition, actually. Most fish lay eggs in the water either before or after fertilization (this is far more advanced than our previous scenario because it is sexual reproduction, of course). Some fish, however, have internal eggs that hatch internally, then leave the parent. Even more interesting, a small number of fish sometimes let the eggs out and sometimes have the eggs hatch internally.
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posted
I was thinking about the live birth transition, however, and the problem seems more complicated than simply having an organism hatch inside the mother. The entire support structure would eventually have to be there, not missing the important step kaminari mentioned, preventing the autoimmune system from attacking the foreign material.
But still, how does one one explain the gradual development of sexual implantation/fertilization. Seems to me that you would now need two actors to be modified- a proto-male and a proto-female whose changes would allow them to be just compatible enough that they would produce offspring with a slight advantage.
That sort of leads me to wonder what kind of time-frame we are looking at. From first cell to the first sexually produced eggs. Then from those eggs to first live birth. And, in the case of land mammals, particularly genus homo, from that first live birth to our earliest ancestor. That is to say, this reproductive system must be in place to explain the first mammals and their descendents, right. Because it is sort of dividing line between reptiles and mammals.
hmmm... Now I am really curious to see some actual concepts or studies on this subject, as opposed to the (quite good) ideas that you have come up with off the top of your heads. It's importance is even more obvious to me. This has to have been looked at, right?
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posted
Remember, evolution isn't necessarily a straight line... I wonder how many proto-"sexually produced eggs" schemes arose and were selected against, until the proper environment/mechanism took hold among certain segments of life?
posted
For me, Bok, that only compounds the complexity. Because now we're postulating that these modifications occurred a number of times, in slightly different forms, until one took.
I am interested to know if a conceptual model of this entire necessary structure has been proposed and/or studied and tested/observed.
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posted
That is the actual definition of evolutionary theory though. It leaps and starts, not always going in a straight line, and far more things fail than than are passed on to offspring.
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posted
I get that. It just seems amazing that the rudimentary mechanisms basically need to have evolved more than once before a viable support structure (for lack of a better word) came with it, for example.
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posted
well it'd be just too much of a coincidence if it evolved perfectly on the first try! Might make me question my atheism with that kind of purposeful evolution(some might even say guided!)... Posts: 8741 | Registered: Apr 2001
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posted
It seems to me that you would start off with hermaphrodites, actually. In my scenario above, I think the next step is that different organisms begin to exchange internal cells. The advantage is resistance against parasites; you get slightly different cell chemistry from one organism to the next, and thus slightly different tradeoffs. As yet there's no gender involved, though; everybody exchanges cells with everybody, or at least with all their neighbours.
Now, I think at first this would occur more or less at random; indeed, before the evolution of the outer shell, the proto-organisms can't help exchanging genetic material.
Now, as the shell evolves, it is an advantage if it lets through internal cells from other places, but it's a disadvantage if it lets through other things. The obvious solution, I think, is that the shell responds to the 'mating pressure' that I've already got forming the eggs, by becoming slightly more permeable - preferably just to the other cells, but that can come later. So now we've got a 'fertile period' and an exchange of genetic material, which is at least a start on sex.
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posted
Oh, hey, this is cool! At the beginning of the previous post I had no idea how I was going to get gender, but now I see it! With the exchange of cells, above, there are now two viable strategies for my organisms to follow : One, they can concentrate on making really good eggs, or two, they can put a lot of resources into pumping out individual exchange cells, to get a free ride from the other organisms. (Yes, I am indeed proposing the first males as parasites on the first females! And hey, I like knowing I'm descended from the smart ones, too. ) Presumably, at first, the organisms do both, so near to 'mating time' (and it is clearly good if groups of organisms synchronise this, so the ones that do it in response to, say, seasons or tides have an advantage over the ones that just breed at random) the outer portions are filled with both 'eggs' and free-floating inner cells. But there are clear advantages to specialisation. Those who produce only 'sperm', if I can call them that, are using a lot less energy than those who produce both 'sperm' and 'eggs'. (Incidentally, this would be the situation where females reproduce sexually in the presence of males, but asexually otherwise.)
Now, it seems to me that the 'males' don't actually need to be producing the whole internal cells. All they want is that their genes get to ride along with the female (it's not complete parasitism; the female gets extra parasite resistance, remember). So those males with mutations making them produce less complete sperm cells - that is, less of the cellular infrastructure needed for self-sufficiency, but the same number of genes - have an advantage, since they can produce more sperm and fertilise more females. (Of course, they only do this in response to the 'mating time' signal, whatever it is - at other times, they do need the self-sufficiency.)
Ok, I do believe I've found a plausible path by small steps, from single cells to gendered multi-cellular organisms. Does anyone disagree?
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posted
Interesting. Quick question: Are we assuming the existence of a multicellular organism? Or a collection of the same cells. If the former, to be more explicit, are we saying that some of the cells in that organism have specialized and they are working together as a community (which is how I'd define a multi-cellular organism.) Because if that is the case, there'd have to be mechanism for this multi-cellular organism to reproduce it's community-working-advantage through each generation. Unless, perhaps, it is a clump of cells working together and reproducing individually so that the proto-organism as a whole is relatively unchanging (or always existing) even as each of it's cell's offspring are reproducing. Except, now that I think about it, for true variation, you'd need to have a bunch of these proto-multi-cellular organisms, right? Is that a viable scenario?
Now, if we're talking the latter (the proto-organism is simply a collection of the same cells and specialization has not yet occurred), then what, exactly are they exchanging? You said 'internal cells'. Perhaps you meant cellular structures? Still wondering, though, how this interchange helps without involving a change in the genetic structure of the cell that would prompt that cell to incorporate those items into their reproductive process (actually making them.) Ok, with mitochondria, I can see it, since mitochondria is responsible for it's own replication. So excluding mitochondria, what items would be exchanged and how would that help? Let's say one cell develops a proto-type for another cellular structure (I'm pulling proto-endoplasmic reticulum out of my nether regions, for an example). And let's say that cell sends some proto-ER to another cell. Well, the other cell may now have an advantage in having that particular structure. But since that cell with the advantage does not actually have the DNA instructions for creating proto-ER, what good does it do, genetically? And what good does it do for the proto-ER creator cell to keep making it? So the exchange has to be of genetic material, period, right?
But that is the definition of sex, isn't it. Am I missing something? It seems circular. To come up with cellular sexual reproduction, you need genetic exchange, which is cellular sexual reproduction, other wise the exchanges are akin to somatic mutations rather than germ. Maybe I'm missing something.
posted
Some support structure would evolve before live birth, actually. Remember how in my scenario the fission offspring started out small? Well, the better able the host is able to keep it alive without endangering itself, the greater the evolutionary success. There's the start of a support structure.
Genetic exchange is pretty common in single-celled organisms; they sidle up close to one another and let their DNA mingle, then take a full (but different) set back with them.
Also, your definition of a multi-cellular organism is hazy. There are a number of 'organisms' that are made up of differentiated single cells . . . that can go off and survive on their own. There are some that only exist for a short time (such as during resource shortage), for instance. Getting from a single-celled organism to a multi-celled organism seems to actually be a pretty simple jump (for a loose definition of multi-celled; getting to more cohesive organisms probably is a bit harder and takes longer).
Posts: 15770 | Registered: Dec 2001
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quote:Quick question: Are we assuming the existence of a multicellular organism? Or a collection of the same cells.
We are assuming the multi-cellular things I built up earlier in the thread. So yes, there is specialisation.
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I guess, then, a better definition of a multicellular organism would be one where the genetic structure describes the entire organism, as opposed to just single cells. And this is part of my question: the transition between single-celled and multicellular organisms, genetically. How would it occur. King of Men provided an interesting scenario. Which leads me to...
King of Men,
Even assuming specialization, the same questions I raised about non-specialized proto-organisms are still there.
And I wonder about time frame: from the first sexual reproduction to the first live birth. How much actual time is that, roughly speaking? 1 Billion years? 500 million?
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posted
Hmm, indeed, that is a very interesting question. I can't answer it right off, I'll have to think a bit.
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posted
Um, my previous post referred to the multicellular definition, not the timescale. The timescale, I can answer right off : Insufficient data, does not compute. I have no idea.
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posted
I'm not sure that's a good definition of a multicellular organism. For instance, if the genetic structure just defines the production of certain chemicals depending on environment, resulting in the emergent shape being the organism, is that such a case? If so, the organisms I identify qualify. If they don't, then I don't think humans do, either. The blueprint analogy doesn't hold that far. Some of our parts grow because they work out that way based on emergent properties of our cells (stem cells on the ends of young bones, anyone?)
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posted
Bacteria have sex by the definition above (exchange of genetic material). Plasmids travel between bacteria- hence why antibiotic resistance is so prevelant. Also, hfr cells tranfer genomic DNA. Interestingly, it looks like syphilis does not do this, hence why you can still take ampicillin and kill it.
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Yeah, I see your point. So how would you define a multicellular organism. Keep in mind, my whole question has to the with the natural formation of multicellular organisms that reproduce themselves as a whole, genetically.
Scholar, that's interesting. I wonder how'd that fit into KoM's scenario. But even then, it's not multicellular organism sexual reproduction, which is my ultimate question.
I love this, guys. Thanks for the input. Keep it coming.
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posted
I think perhaps the question could best be put as 'how does the transition from decentralised to centralised DNA transfer occur?' In my scenario, although there are beginning to be specialised cells that transfer DNA from males to females, the skin cells and internal cells still reproduce separately. I think my next step should be to figure out how we go from there to having all the necessary DNA in the transfer cells, and then having both kinds of cell develop from a single fertilised egg.
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posted
Well, you also have explain what is being transferred so as to make this a trait to select for, no?
edit: by that I mean, your scenario described an exchange of 'internal cells', which I took to mean internal cellular structures. But that, as I mentioned, is not an exchange of genetic material and it would have to be for the exchange to be selected for (as in the case with the proto-Er, in my example.) The exchange has to be more than the equivalent of a somatic mutation. It has to be that of a germ mutation.
And that seems like circular reasoning. But, adding in the fact that some bacteria exchange genetic material, that is a viable explanation...to a point. Because now you have to rework what is being exchanged. In your example, it was cells being exchanged that embark down two paths that lead to gender. But it's way too eary for cellular exchange, yet, isn't it? Cellular exchange, to me, can only work for a multicellular organism and where the genetic material in that cell is for the whole organism- as you put it, a centralized genetic source. But that's putting the cart before the horse, cause that's what we're trying to explain. So you have to rework your exchange methodology to begin with plasmids, for example, and explain for centralization.
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posted
The corner cases of multicellular organisms aren't that clear . Most things aren't, in biology, most of the labels we use being useful heuristics rather than dividing lines.
One thing that is clear is that we have very good scenarios (and examples, in some cases) for the transitions between single-celled and multi-celled, fission and egg formation, egg birth and live birth, and between various varieties of reproduction.
Several of these transitions, especially those we see borderline species for, likely happened more than one time. Others may have only happened once.
How and when these changes occurred in relation to each other is a much more open question, especially as they could have occurred in several combinations.
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posted
Actually, I don't see any real reason for the individual parts to evolve in any particular order. It seems to me that the path of least resistance is to begin with the exchange of DNA packaged in whole cells, and gradually strip away the cells until you're left with pure DNA exchange. Now, it's entirely possible you could build it up the other way too, as you are suggesting, since we do know that bacteria exchange material that way. But I don't see any reason to say that either one is the only way it could happen.
It's still possible I'll hit a dead end where I don't see how to get the next step; if so, I'll retrace and try it your way. Evolution is like that. Posts: 10645 | Registered: Jul 2004
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So, if I envision this properly, you're saying that the 'organism' your describing (decentralized reproduction, some specialization for mutual advantage) exchanges DNA packaged in whole cells, which then is gradually stripped away to being nearly pure genetic material. The advantage to this is that two viable strategies become possible, which lead to the eventual formation of gender.
So my question is, how, exactly is this 'organism' reproducing right now? Are each individual cells, even the specialized ones, merely reproducing themselves? Ok, I think I can see that. The 'organism' is a loose confederation, working together for mutual advantage. The individual cells are reproducing and thus the confederation continues to exist. But of course, we need a number of these organisms, right, so they can exchange cells with each other. You said internal cells, both at this point, internal, external, its all the same, I'm guessing. Internal cells would not provide any more advantage over external. The key is the exchange itself.
But even if such exchanges occur, they are of discrete cells. Where is the genetic mixing? And if no genetic mixing, where's the advantage to the exchange? More than that, this is still a somatic change. It has not affected the genetic structure of any of the individual cells making up the organism. So the organism, as a whole, is not capable of reproducing this advantage.
As you said, somehow, all of this has become centralized....
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posted
All exchanges are of discrete cells, including in humans. One sperm from the male and one egg from the female, with only the genetic material of half a cell each, in fact. That in no way stops evolution from occurring at a macro level, because those conglomerates of cells will all be mostly identical, having split from a common ancestor cell.
To really blow your mind, realize that humans, even for a lot of the time in the womb, cannot survive without certain symbiotic bacteria. We like to think of those bacteria as separate organisms because they can sort of move about independently, but so can our white blood cells, after all. Are they part of a human, or are they not ?
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posted
well yes, of course all exchanges are of discrete cells. Yet those discrete cells contain the genetic material for more than just themselves, but for the whole organism. That's the point. Morphogenesis, I think, is the specialization of cells in the organism. How can it's initial formation (the centralization, as KoM put it) begin through natural selection. That's part of my question.
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posted
So do the discrete cells of a conglomerate organism, usually, since they're typically all near-ancestors of one parent cell. And we observe borderline single-cell/multi-cell organisms like that all the time, including with specialized cells (typically organized along chemical gradients).
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posted
Wow... can I just say that this is the most awesome thread I've ever seen?
I'm at work, so I can't say much right now, but I do have to interject regarding KoM's theory of multicellularity. If I'm understanding you correctly (and I freely admit that I may not be ), you suggest that multicellularity may have occured when several different types of cells came together to form a colony, and due to the fitness benefits of their mutualism eventually develop interdependence.
However, I'm pretty sure that multicellularity evolved the other way around: that is, one type of cell developed the potential to generate more than one distinct phenotype from a single genotype. Thus, your most primitive multicellular organism would actually just be a big blob of cells that are identical in just about every way, and which, if separated, survive perfectly well as independent entities. The trick for evolving specialization, then, comes from turning *off* certain functions, and drastically ramping up others. This way, you don't need to evolve new functions (at least at first), because everything necessary for each cell's survival and propagation is already there: metabolism, reproduction, etc. To get a specialized "digestive" cell, for example, you upregulate genes that produce digestive and biosynthetic enzymes, while downregulating "nonessential" genes- those associated with, for example, reproduction and movement.
From an evolutionary standpoint, this is much simpler to achieve than trying to evolve entirely new strategies for functions that, ultimately, every unicellular organism has to be capable of anyway. Turning off genes can be as simple as a mutation in a necessary transcription factor or promoter site (or, as we see in most developmental genes, cell-specific selective gene methylation). Generating entirely new cellular machinery requires the whole arduous process of gene duplication, gradual functional deviation, and so on and so forth.
What does all this have to do with the evolution of reproduction? I'll try to get to that once I get home from work. Posts: 1321 | Registered: Sep 1999
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Wow, we have detailed information on how things evolved that far back? I'm impressed! How do we know this - observation of current 'borderline' species, maybe?
Apart from that, the details of how you get to multicellular aren't so important as all that; I just wanted to have a solid starting point for speculating on the sex part.
quote:So, if I envision this properly, you're saying that the 'organism' your describing (decentralized reproduction, some specialization for mutual advantage) exchanges DNA packaged in whole cells, which then is gradually stripped away to being nearly pure genetic material. The advantage to this is that two viable strategies become possible, which lead to the eventual formation of gender.
No, you are mixing up some steps of what I was thinking, here. The stripping away can happen at any time, either before or after the strategies that lead to gender. But at this stage, there's no real advantage; this is just stuff that's going to happen because these are very primitive, sloppy organisms that don't maintain a really sharp boundary the way modern ones do. They can't help exchanging cells, they don't have the defenses to stop it. The strategies for reproduction are separate from that.
Now, how does the specialisation happen? Well, basically, I think we are home free. All that needs to happen is that internal and external cells exchange some DNA, which bacteria do all the time anyway, and only express their genes differently according to which part of the chemical gradient they're on. (Or you can do it in Tarrsk's opposite order, where you begin with just one species; that skips the DNA-exchange step.) Then, with that done, why should the external cells bother to carry around all that useless responding-to-mating-time DNA? It won't matter if it gets damaged, since the internal cells are doing all the production of new cells anyway. So now we have the specialisation into skin cells and somatic cells, as it were. And the advantage is just the same as any other specialisation and division of labour : All the work gets done, and gets done better because the cells don't have to do everything.
Posts: 10645 | Registered: Jul 2004
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quote:Originally posted by King of Men: [QB] Wow, we have detailed information on how things evolved that far back? I'm impressed! How do we know this - observation of current 'borderline' species, maybe?
IIRC, it's based on the face that what we generally think of as multicellular organisms (as opposed to colonial unicellular organisms) are universally composed of many cell types with the same genotype. In the case of mitochondria, we can infer that they used to be an entirely separate organism that was eventually incorporated into their eukaryotic hosts, because mitochondria exhibit many of the characteristics of their bacterial ancestors and, although much of their genetic material has long since migrated to the host nucleus, plenty of mitochondrial genes remain located in the mitochondria. This plainly isn't the case for, say, muscle cells versus leukocytes.
In addition, as I mentioned, the mechanisms through which cells differentiate are based on the activation and deactivation of specific genetic pathways. These genes are intact in all cells in a given organism, and are simply expressed in a differential fashion. This suggests that the progenitor of the organism was, at some point, a cell capable of all of these basic functions.
Posts: 1321 | Registered: Sep 1999
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Wait a minute. So the fact that a cell contains the genes for a number of functions means that it was capable of all of them at some point? Now, I suppose I can see that a cell, for example, can become a nerve cell, muscle cell, blood cell, etc. And that only certain genes are expressed so that each cell handles only certain functions. But I still don't see how these cells naturally began to work together so that some cells could become muscle, other bones, other nerves, all according to the genome, and they harmoniously act as a single organism (to a certain extent), to become a rat, a fish, or a human. How does each cell know what to become and where it should be? How did the fully centralized organism's genome come to be, describing a large multicellular, multi-cell type organism? I'm trying to wrap my head around this.
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) Presumably, at first, the organisms do both, so near to 'mating time' (and it is clearly good if groups of organisms synchronise this, so the ones that do it in response to, say, seasons or tides have an advantage over the ones that just breed at random) the outer portions are filled with both 'eggs' and free-floating inner cells. But there are clear advantages to specialisation. Those who produce only 'sperm', if I can call them that, are using a lot less energy than those who produce both 'sperm' and 'eggs'. (Incidentally, this would be the situation where females reproduce sexually in the presence of males, but asexually otherwise.) ![[Wink]](wink.gif)
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