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Author | Topic: Cells into Organs: could it evolve? | |||||||||||||||||||||||||||||||||
LDSdude Inactive Member |
My question to evolutionists is as follows;
I have read much about evolution and understand the cellular theory, but I can't seem to find an answer to this question; if single celled organisms evolved into multi cellular organisms, that is cellular chains, when and how did each and every cell give up the, "every cell for himself" way of life and start relying on certain cells to do certain jobs for the good of the whole? An example of this in humans is organs and blood. Only the digestive organs provide nourishment to the rest of the body, and only the blood cells can deliver oxygen to every other cell in the body. Did the simple multi-cellular creatures pass out sign up lists for each cell to do something that benefits and helps the rest of the organism to survive? I think that if this question goes unanswered, it could really become one of TTOE's weaknesses. ---- And before anyone here gets started, let me also point out that without a digestive system, every cell in the human body would die. Without blood cells to deliver oxygen, every human cell would die. It's ALMOST an irreducibly complex system of organs. So HOW and WHEN did, according to evolution, cells abandon the "every cell for himself" doctrine and start the "all for one, and one for all" doctrine? (if you get my point)
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AdminSylas Inactive Member |
Thread moved here from the Proposed New Topics forum.
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Sylas Member (Idle past 5289 days) Posts: 766 From: Newcastle, Australia Joined: |
There are several kinds of single celled organism that will congregate together into a colony under some circumstances. You can see a similar thing with schools of fish. Sometimes there is an advantage being in a group. There are also cases in which single cells will, under stress, combine to form a colony and then differentiate to fit different roles within the colony.
A dramatic example is Dictyostelium discoideum. This is a "slime mold", and lives as a single celled organism, feeding on bacteria in forest litter. But when the food runs out, they form colonies, of about 10 to 50 thousand individuals. These colonies are effectively a multi-celluar organism... a "fruiting body", a bit like a slug. As part of this process, the individual cells differentiate into one of two types. This slug is better able to move up to the top of the litter in which it lives, and disperse spores which will then be new individual cells. There is a Dictyostelium-Homepage, which includes videos.
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Wounded King Member Posts: 4149 From: Cincinnati, Ohio, USA Joined: |
I think that if this question goes unanswered, it could really become one of TTOE's weaknesses. ---- And before anyone here gets started, let me also point out that without a digestive system, every cell in the human body would die. Without blood cells to deliver oxygen, every human cell would die. It's ALMOST an irreducibly complex system of organs. It is important to appreciate that these systems are not required for multicellular life, just our own particular brand of it. There are many multicellular species which are capable of absorbing nutrients simply by diffusion from the environment. As soon as any colonial form has developed a trade off with only a few cells reproducing the other cells are freed up to develop other specialised functions. Look at sponges and you will see many specialised cell types not organised into true 'organs'. TTFN, WK This message has been edited by Wounded King, 02-15-2005 05:02 AM
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Brad McFall Member (Idle past 5061 days) Posts: 3428 From: Ithaca,NY, USA Joined: |
That's a liget inquiry. I have seen a narrative in a fairly old botany textbook that replies to your question but I have not seen anything in the recent lit. On my own, I was able to reduce your "almost" to a topobiologist's "cell collectivity"
GIVEN a 70s advance in embryology about neural crest cells MOVEMENTS and the following: quote: The author is Ishizuya-Oka I think in chapter 4 "Cellular and Biochemical Changes"I dont have the booktitle, the material is in a book on Amphibian Metamorphosis. If you really need the reference I can sneak into a Cornell Library and find out. I get somewhat crossed up between the length of junk vs non junk DNA and the different sizes of DNA found in different cell death studies vs protein expression. The most important piece of information is that given the diploid state as the fulcrum of any morphospace the amounts of subdiploid DNA match the tertraploid content under cell death conditons whereas when not under cell death there is little to no DNA below the diploid level. I had to do some creative reading of Mendel to accomplish immunity from the lack of responseotherwise but it is possible.
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crashfrog Member (Idle past 1496 days) Posts: 19762 From: Silver Spring, MD Joined: |
if single celled organisms evolved into multi cellular organisms, that is cellular chains, when and how did each and every cell give up the, "every cell for himself" way of life and start relying on certain cells to do certain jobs for the good of the whole? It's called "kin selection", and its the same principle that explains why only one member of a beehive reproduces. An organ is a colony of cells, and because the cells are clones of each other, there's no difference (for instance) between each of 100 cells reproducing once, and one of those cells reproducing 100 times. Or more. So there's a survival/reproductive benefit if a colony of cells can ensure more offspring by specializing, then by all working to reproduce themselves.
So HOW and WHEN did, according to evolution, cells abandon the "every cell for himself" doctrine and start the "all for one, and one for all" doctrine? (if you get my point) I don't know when, but I know how - because there was survival benefit in doing so.
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jar Member (Idle past 423 days) Posts: 34026 From: Texas!! Joined: |
If you could see an example of something like that happening today would that help? If someone could point to an example where a single cell organism then splits and the different cells begin to specialize so that they are entirely different would it ease your understanding?
Aslan is not a Tame Lion
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Quetzal Member (Idle past 5901 days) Posts: 3228 Joined: |
Jar's probably going to use my favorite example of this. I'll let him get first crack and then fill in the gaps.
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jar Member (Idle past 423 days) Posts: 34026 From: Texas!! Joined: |
Go for it. The fewer respondents the easier it is to keep the baby ducks herded for us Admins. I fully relinquish my place in line.
Aslan is not a Tame Lion
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Quetzal Member (Idle past 5901 days) Posts: 3228 Joined: |
Hokey dokey. I'll wait to see if LDS responds to your question. Otherwise not much point (hint: it's an algae).
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LDSdude Inactive Member |
quote:______________________________________________________________________ Depends. Remember that I'm talking about uses to the multi-celled organism that are not merely beneficial, but are NEEDED for survival. If all you're talking about is an example that shows where the cells are still working for themselves, then your example would not help answer the question. Whatever the example is, it has to have a situation where certain cells will only perform a few of the tasks they would need to survive, and would rely on other cells to accomplish the rest of the tasks it needs to survive. So it IS kind of like irreducible complexity.As I mentioned earlier, that is how human beings bodies operate. Certain organs perform only certain tasks. Like the heart. It uses blood to pump oxygen to the rest of the body. But without the rest of the body (like if you just set a lone heart lying on the pavement), it would definately die. I guess the key word in all of this is SYSTEM. An irreducebly complex system of cells or organs working for the whole instead of themselves. I bet you have the idea by now, but if not, compare it to this: Countries trading. One country might have lots of water but little farmland to grow food on. Another country might have lots of land but little drinking water. If the two countries trade water for food, both survive. But if not, both die. They are DEPENDANT on one another for survival. I still can't see how this could have arisen in cells, Crashfrog. fixed quote tags, square brackets are used not parentheses - The Queen This message has been edited by AdminAsgara, 02-16-2005 15:53 AM
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LDSdude Inactive Member |
Sylas, I reviewed your example(thanks by the way), and I think you're comparing two different things. The reason is becuase the cells in Dictyostelium discoideum are working together, not for eachother. This is just what I warned about at the end of my very first post in this topic. Each and every cell in this "slug" is not dependant on the other cells to live. They come together to work together, but they are not required to do so for survival.
Your example also branches off to a different subject that still proves my point. It is naturally programmed into these cells to form together in order to reproduce most effectively. If this 'evolved', tell me how! Did once along time ago some of these dying cells happen to form into this little slug? If they did, how did their offspring figure it out also later on? Then THEIR offspring would have to figure it out, and then THEIR offspring would, and so on, and so on until it somehow became part of their instinct. How? Wait, wait. I'm already digressing form the main subject. We would have to open another topic to answer these other crucial questions.
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Sylas Member (Idle past 5289 days) Posts: 766 From: Newcastle, Australia Joined: |
Depends. Remember that I'm talking about uses to the multi-celled organism that are not merely beneficial, but are NEEDED for survival. Stop right there. Why this extra constraint? What you should be looking for is beneficial; not "needed for survival". Needed for survival is not a useful or sensible extra constraint. Environments are very complex. They don't have a sharp boundary between do this and you survive, do that and die. Something is "beneficial" if it makes survival (or propagation) more common. You will still get cases in which an organism adopting a colony behaviour will die, and other cases in which individuals that go it alone survive. But in some lineages and some environments, single cells have adapted for a behaviour in which they form colonies and differentiate roles. Some organisms do this in reaction to stress, but in times of plenty they live as single independent cells. This conveys some benefits to the organisms, which means the trait propagates. The fact that they can also live as individuals gives a clear evolutionary pathway. And there is also scope for development in which organisms become committed to the colony lifestyle. Sometimes this works out, sometimes it doesn't. The ones for whom it worked out are the ones who propagate more relatives to themselves. End result, we have living creatures today showing a range of living strategies, with no sharp boundaries. The ones living today are the ones for whom circumstances and strategies worked out for their ancestors.
I bet you have the idea by now, but if not, compare it to this: Countries trading. One country might have lots of water but little farmland to grow food on. Another country might have lots of land but little drinking water. If the two countries trade water for food, both survive. But if not, both die. They are DEPENDANT on one another for survival. I still can't see how this could have arisen in cells, Crashfrog. I'm not crashfrog; but your analogy fails. Nearly all nations do have resources, in differing degrees. They are not absolutely dependent on each other, but going it alone means they can only support smaller populations, or accept a lower standard of living. So they adopt strategies of co-operation that have a payoff. They also adopt strategies of conflict; and this has a side effect of driving to the wall those competitors that opt for less effective strategies. I don't doubt that you can't see how co-operative behaviours and differentiation can arise from single cells. But it seems to me that you aren't thinking this through, or looking at the examples available to show how co-operation can be "chosen" by living cells or by nations, who are also able to live independently. Cheers -- Sylas
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Sylas Member (Idle past 5289 days) Posts: 766 From: Newcastle, Australia Joined: |
Our last two posts passed each other in the pipeline. I was writing number 13 while you were writing number 12. Actually, my previous Message 13 already answers your objections in Message 12, but let me spell it out.
Each and every cell in this "slug" is not dependant on the other cells to live. They come together to work together, but they are not required to do so for survival. As far as I can see, they are pretty much required to do so for survival. We are not only talking about one species here; there are a range of species with this kind of behaviour, and I would expect them to differ in the degree of commitment they have to the colonial phase of their life cycle. Others may well be less committed. This behaviour persists because the individuals don't propagate all that well as individuals. They are individuals for some of their life cycle, but some slime moulds (I think) are now pretty much committed to going through the colonial phase for the next generation. As for how it evolved; this is pretty obvious. We don't have a record of all the exact ancestors, but we have a clear case in which individual cells sometimes come together for mutual benefit. There are plenty of other examples, in which individuals come together in much more loosely defined amalgamations, so there is no major gap or hurdle to overcome. Species evolve by accumulation of change. There is no question of "figuring out" anything. Living creatures just do their thing. Some survive, and some don't. This is due mostly the vagaries of fortune; a predator happens by, food runs out, a tree falls in the forest and sets up a suitable environment. But there is also a contribution of how well adapted living things are to take advantage of opportunity or to surmount obstacles. Two things drive evolution. First, living creatures are not exact replicas of their parents. Second, there are many more organisms born than manage to survive to the next generation. The sets up a kind of competition, in which change that works out persists, and change that does not is removed from the gene pool. This is not "working things out". This is just doing what comes naturally, with what comes naturally changing over time. The existence of many degrees of colonial organization plainly establishes that there are credible evolutionary pathways for cumulative change taking you from single independent cells to obligate multi-cellularity. Cheers -- Sylas This message has been edited by Sylas, 02-16-2005 17:44 AM
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Quetzal Member (Idle past 5901 days) Posts: 3228 Joined: |
I think I understand your questions, especially in light of your response to Sylas. I've got a few examples, mostly for your information. None of them resemble a "heart lying on a sidewalk", and not all of them are "irreducibly complex". However, each of them is suggestive in one respect or another.
The first example I'd like to give you is Volvox. This is an autotrophic protist - an algae. It's very common in the East and Northeastern US (I don't know whether they have them out in Utah, however). Each cell has two flagellum, and the usual protist accoutrements (chloroplast, eye spot, etc). It is motile, and its eyespot is a simple light sensitive pigment that allows the protist to use its flagella to move toward light. During the spring, these independent organisms (quite capable of independent existence) form colonies in ponds of between 500-1000+ cells, some of which are large enough to be seen with the naked eye. The cells link together with cytoplasmic threads, and then secrete a mucus-like jelly that serves as a colony boundary. So far nothing special, right? What happens next is truly amazing (to me). The colony develops a distinct "front" and "rear" end as a colony. The eyespots at one "end" of the colony grow substantially larger than those at the "rear" or "interior" - although all cells retain the basic parts. The flagella of these "eye" cells stop beating. The cells at the "rear" of the colony increase the frequency of their flagella. Somehow, and no one knows for sure exactly how this is accomplished, the entire colony now functions as a single organism. The "front", with its enlarged eyespots draws the colony toward light, and the flagella at the "rear" push the colony forward. IOW, we have a very primitive version of the cellular differentiation we see in "higher" metazoans. Note, in addition, that during its colonial phase, a very small fraction of the cells become gonidia - reproductive cells very similar to metazoan germline cells. The rest remain same-old motile Volvox. My second example is the Portuguese man-of-war (Physalia physalis). Although commonly called a "jellyfish", it isn't a cnidarian. It's a colony composed of four different types of organism: a pneumatophore (single individual which serves as a floatation device and supports the rest of the colony); numerous dactylozooids which detect and capture food and convey their prey (the tentacles) to the digestive gastrozooids (basically living mouths); finally reproduction is carried out by gonozooids. The colony is irreducible - none of the organisms can live without the others. Each member of the colony performs a unique function that aids the whole organism. This is a bit different than the Volvox set up. In Physalia, these are evolved from different, formerly free-living organisms. In Volvox we're seeing cooperation from a single type of organism that is differentiating "roles" within the colony. My final example is another odd-ball. Mixotricha paradoxa is quite possibly the poster-child of endosymbiosis. It is a composite organism that outwardly resembles a protist. It lives inside the gut of a termite (Mastotermes spp.), and helps the termite digest cellulose. However, M. paradoxa is actually four organisms. It has a nucleated cell, but in place of mitochondria, it contains spherical bacteria. Instead of cilia and flagella, it contains two different types of spirochete for motility. Although I don't know whether each of the subordinate organisms can live without the others, it's certain the Mixotricha can't live without its symbionts - each of which perform a different function for the whole. To sum up: Volvox shows primitive differentiation at the cellular level in a single organism. Physalia and Mixotricha show irreducible symbiosis between different organisms functioning as a single organism. These examples are suggestive, rather than absolute, answers to your question.
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