How Single-Cell Organisms Evolve Into Multicellular Ones
    By Michael White | February 20th 2009 03:15 PM | 55 comments | Print | E-mail | Track Comments
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    Show Me The Science Month Day 18

    The transition from one-celled microbes to multicellularity was a huge step in the evolution of life on this planet, but as daunting as this evolutionary step seems, it didn't happen just once. Today's plants, fungi, animals, and various types of algae are all descendants of separate transitions to multicellular life.

    All of these transitions from a single-cell lifestyle to multicellularity occurred in the very distant past, so how can we learn anything about them? It turns out that it is not hard to find living, modern examples that closely parallel the momentous evolutionary transitions that led to animals, plants, and fungi. Right now on earth there are primitive multicellular organisms that, in many ways, resemble the first multicellular creatures that existed a billion years ago. Researchers are using these organisms to understand what kinds of genetic changes are needed to turn a single-celled organism into a multicellular one.

    A group at the University of Arizona has published a study of of one group the these amazing organisms, the volvocine green algae. What's amazing about this group of algae is that you can find a range of multicellular sophistication in closely relate algae species. There are species that form simple sets of four identical cells stuck together, other that form balls of 32-64 not quite identical cells with some specialized functions, up to full-blown multicellular organisms with 50,000 highly specialized cells, including reproductive germ cells. The evolution of multicellularity is not an irrecoverable event from an unimaginably distant past; it is something we can observe, manipulate, and test in the lab today.

    With the availability of so many different types of green algae at varying levels of multicellular sophistication, the U. of Arizona researchers were able to put a timeline on the evolution of specific features of multicellular algae. They did this by calibrating DNA differences between species with absolutely dated fossils: DNA provides a relative time scale, since the more DNA differences there are between species, the longer it's been since their lineages diverged; and this relative time scale can be matched up against dated fossils that show when new major types of multicellular algae began to appear.

    Here is part of the time line the researchers came up with:

    1) ~223 million years ago, a species of single-celled green algae began forming aggregates of cells stuck together by a glue of secreted proteins and sugars (and we can see species which do this today).

    2) Also ~200 million years ago, the rate of cell division began to be controlled genetically. Unlike single-celled organisms, which reproduce whenever the surrounding environment is right, the new multicellular algae began controlling exactly how many daughter cells they produce. This is a critical step towards establishing a multi-cellular body-plan with genetically controlled dimensions.

    3) Roughly 10 million years later, the cells of some multicellular algae species began to orient their whip-like flagella in the same direction, so that all of the flagella would work together to control the swimming direction of the organism.

    4) By ~100 million years ago, some of the algae species had established separate reproductive germ cells, and ever since then, various volvocine algae species have developed more cells with highly specialized functions.

    One feature of this time scale is that the major innovations occur sporadically. The researchers suggested that these major events coincided with the inventions of new ways for resolving conflicts among individual cells in the organism: in other words, formerly independent cells had to learn how to be civilized. Single-celled microbes function very well as individuals. Some of that individuality has to be given up for the greater good when cells hitch their evolutionary fates together as one multicellular organism. A key example of conflict resolution is the evolution of genetic limits on cell division: to have a coherent, multicellular body plan, individual cells can't just divide with abandon, the way bacteria do. When cells escape these genetic controls on division in humans, you get cancer.

    The evolution of multicellular organisms is a major evolutionary step. In our history (the history of animals), how that step happened is lost somewhere in deep history. Nevertheless, the evolution of multicellularity has happened over and over again, and in the case of the volvocine algae, we can study this key evolutionary step in the lab.

    Join me tomorrow, here at Adaptive Complexity, for day 19 of 30 Days of Evolution Blogging Evolution as a science is alive and well. Each day I will blog about a paper related to evolution published in 2009.

    Are you a blogger and want to join in? Here's how.

    Front Page image of Volvox aureus by Dr. Ralf Wagner, courtesy the Wikimedia Commons, published under the GNU Free Documentation License.


    Fossil Huntress
    Great article, Michael! I do have an inexplicable fondness for algae and here they are being so useful again.
    But they apparently make terrible, frakking coffee.
    You lost me on that one. Algae coffee? I am a coffee snob, so I need to hear more.
    Coffee snob yes, but you are apparently not a geek.  That was a fantastic Battlestar Galctica reference I dropped there with the use of "frak" to give it all away.  In a recent episode, Admiral Adama complains about the crappy quality of the "coffee" they have, which is produced by processing algae (apparently the source of almost all their food).
    You've got me there - I haven't watched Battlestar Galactica since elementary school, back when the original series was out.
    Farscape remains the sci-fi show I can watch over and over, because the tone is much lighter, and Star Trek will make me stop and watch every time I come across it on TV, but I am going to get bold and say the new BSG is the best science fiction series ever made.     If, in the midst of raising kids, working and writing awesome stuff here you happen to have 80 hours of time to catch up, it's worth it.
    Firefly could have been in the running (same folks did the FX for at least the BSG miniseries), but Joss Wheedon and network executives can't play nice with each other.  The movie, Serenity, is terrible, though.  Goes against everything that was great about the show.
    Stephanie Pulford
    Agreed on Serenity.  They excised all of the westernness and compelling interpersonal relationships, and instead we got Wednesday Addams' boyfriend and a blow up doll as comedic relief.  I guess Summer Glau got some good practice for her TV Terminator role, though. 
    At its most basic, Serenity sucked like an Oreck infomercial because it was about, well,  everything. Firefly was awesome becaue it was generally bout nothing important unless you were in the crew, like most of our lives.

    FOr calibration puposes, it took me 3 minute  to write this after 12 hours of work at Mardi Gras.
    The biology of algae is fascinating. They have such diverse lifestyles and live in so many environmental niches. I'm sure ecologists have been interested in algae for a long time, but now molecular and cell biologists are realizing how ,uch insight they can provide into some basic molecular biology questions.
    VOLVOX sounds like the wheels of a higher-dimensional analogue of the wheels on a Swedish car!

    I must take some pond-water to work to look at under our microscopes.  Are they active at this time of year, or does one have to wait until the weather warms up?
    Robert H. Olley / Quondam Physics Department / University of Reading / England
    I have no clue, and Wikipedia, my ultimate source of information on anything I'm too lazy to look up elsewhere, has nothing.  There are over 100 different species, but I'm not sure where they're found.
    Ask Ursula Goodenough you lazy bastard.
    The Swedes are everywhere now! :-)
    Bente Lilja Bye is the author of Lilja - A bouquet of stories about the Earth
    Hi- I'm an artist looking at science for answers about 'why?'. so do excuse my uneducated musings.
    this article made me think that perhaps mankind is struggling right now to find a way to resolve its conflicts and succeed in living together as a complex multicellular organism? maybe in the evolutionary scale of time this could be compared to your description of the conditions of single cell algae coalescing into a multicellular organism 100 million years ago? that of course is if we succeed in not destroying the world before that.

    I sympathize with my artist friend here, as I am a mechanic and not a scientist. In my uneducated mind, it would seem that algae grouping together for survival purposes (this is why I am assuming they would do this, natural selection) is more akin to a pack of wolves, not the formation of a new multi-celled organism. Assorted pack animals each have specialized roles inside their overall pack. Algae, being single celled organisms, would have to join together in order to work together. Unless these groups of algae do merge and form a new organism, I fail to see this as examples of one species turning into another one. It could also be that I am way out of my league as this is not my area of knowledge. Possibly a more in-depth explanation on how this provides evidence to evolution is needed for uneducated minds like myself.

    What we see with today's living algae is not an example of one species evolving into another (although we do see that in the algae fossil record). The idea is this: for each major step that had to take place in the evolution of multicellular organisms, we can see a living example in at least one of today's algae species - there are single-celled algae species, there are species that form unordered clumps of cells, and there are algae species that come together into a multicellular body plan. In each case, researchers are finding that only minor genetics changes are needed to get from one stage to the next.
    Evolutionary intermediates that lived a billion years ago are unobservable except as fossils, but we can find organisms very similar to those intermediates alive today.

    For example, early in the evolution of multicellularity, one would expect that there were species that consisted of clusters of cells with some specialization, some sort of very primitive body plan. Well, we don't have to speculate about how that might have worked - we can observe a species of algae that has just such a primitive body plan, examine the types of genes involved, etc. It's almost like looking back 1 billion years in time to get a detailed glimpse of the first multicellular organisms.

    does anyone know the answer to my does a single cell organisms body or any living body of any creature know to evolve

    Gerhard Adam
    You're making too big of a jump.  Consider that the intermediate step is colonial organisms (like corals and jellyfish).  This will make the entire process a bit easier to grasp.
    Mundus vult decipi
    This article reminds me of a question I once asked in a biology lesson at school: How do the different organs in a body know how to grow 'in synch' together? How do they know when to stop together? I remember because the teacher looked at me as if I'd blasphemed. He wanted to make me look like an idiot but I'm perfectly capable of doing so myself consciously without assistance. I just thought to myself,"you don't know!!" Later a couple of friends agreed it was a good question and that the teacher was a moron trying to hide it. Why can't teachers just say," we don't know, in a few years you can research it."
    Why can't teachers just say," we don't know, in a few years you can research it."
    The more appropriate answer for a teacher to give is "I don't know, let's see if we can look it up." Whether a teacher knows the answer to a question isn't a good indicator of the current state of scientific research, it's just an indicator of that teacher's knowledge. But your question was certainly a good one, and these days a teacher could pull out a developmental biology textbook and give a reasonable answer.
    Hey Mike -

    Thanks for the great post.

    Question for you:
    WHY did cells first make moves to work together/ share functions?

    Thought you might be able to shed some light.



    Thanks .

    wondering what are your tjhoughts on;

    How to link the chromatin emerge
    with multicellualrity?

    Another interesting organism in this respect is Dictyostelium, which has also singlecellular and multicellular phases in it's life cycle. Multicellularity arises here by intercellular communication using cAMP. Dicty's life cycle is amply studied and it's complete genome published.
    As for the origin of multicellularity in evolution, I'm always disappointed why people fail to see the extreme toxicity of oxygen. When this started to accumulate in the atmosphere and hydrosphere, single cells, although already somewhat protected by their symbiosis with mitochondria, were further pushed to clump together, to minimise total surface and exposure to toxic oxygen. This also minimised there surface to take up food. The solution to this dilemma was cellular differentiation. Inside lots of surface for food intake, outside a closed layer of tough, maybe dead, cells.
    This differentiation was made possible by extended genome and guided by intercellular communication, which served as a messenger where to differentiate and when to stop growing.

    Hmmm....quite a bit of belief, but no science. 100% of observations using the scientific method prove that unicellular life always remains unicellular. Unicellular organisms are never observed to evolve into multicellular organisms.

    Gerhard Adam
    No one could be that clueless.  Did you really say that?
    Mundus vult decipi
    Gerhard, you can't be serious if you have a clue about using the scientific method. Single cell bacteria are never observed to evolved into multicellular organisms. Generation after generation, single cell organisms reproduce single cell organisms, they never evolve to become multicellular organisms. Why in the world would you think otherwise? Have you not studied the differences between unicellular and multicellular organisms?

    Gerhard Adam
    I gave you the chance with your last post.  Now I'm going to tell you that you're simply ignorant.

    Single cell bacteria are never observed to evolved into multicellular organisms.
    Of course not, since NO organism is ever observed to "evolve".  That's what populations do, not individual organisms.
    Mundus vult decipi
    You can't be serious. Are you claiming that populations of single cell organisms evolve to become multicellular organisms? That is a fairy tale, friend. All descendants of single cell organisms, are single cell, never multicellular. It matters not if it is a population of bacteria or a single bacteria. I suggest that you apply the scientific method, observe generations of bacteria reproduction, and verify it for yourself.

    "Each organism starts its life as a unicellular amoeba, but they aggregate to form a multicellular fruiting body when starved." That does nothing to change the fact that the next generation of amoeba is unicellular. They cooperate when starved, but they do not become multicellular. The genetics of unicellular creatures are apparently fixed to the degree that unicellular creatures never evolved to become multicellular. It is a fairy tale.

    Gerhard Adam
    The fact that you think genetics is "fixed" is the fairy tale.  Please just go away.  You aren't interesting in science nor a discussion.
    Mundus vult decipi
    Gerhard, we can observe that species are basically fixed with very little ability to change in any meaningful way. That is why the natural response to environmental change in nearly all species (excepting just a few such as people, cockroaches, rats, and scorpions) is to become extinct. Life doesn't evolve, it dies. That is why when we observe nature, we see a tree of life that is working in reverse. We don't see a trunk sprouting an ever increasing menagerie of limbs and leaves. We see the limbs and leaves dying, because they don't evolve to change with their environment. That is also what we find in the fossil record. Myriads of creature that did not evolve and responded to environmental changes by simply dying. It is observed science.

    Gerhard Adam
    Do you not pay attention to anything?  Even the most cursory examination demonstrates that antibiotic resistance in bacteria is a direct result of natural selection.  Are you truly so naive?
    Mundus vult decipi
    John Hasenkam
    <i>_  Single cell bacteria are never observed to evolved into multicellular organisms.</i

    You might want to think about biofilms ... 

    Thinking About Bacillus subtilis as a Multicellular Organism 
    Implicit in our title is a tribute to James A. Shapiro who nearly twenty years ago proposed multicellularity as a general bacterial trait [1]. 

    The finding that cell-cell communication mechanisms, a.k.a. “quorum sensing”, were present in virtually all species had an enormous influence in the change in thinking [6]. 

    OK, think of bacteria colonies as multicellular organisms. It doesn't change the fact that bacteria are single cells that reproduce single cells, always. A generation of single cells produces a new generation of single cells, even if there is some form of communication or cooperation between the cells. Evolutionary theory claims that multicellular organisms evolve from single cell organisms. It just doesn't happen.

    Gerhard Adam
    Don't waste my time with your tired, childish musings.  It is pretty clear that you understand neither unicellular or multicellular life.  Nor do you understand evolution or natural selection.  I'm not in a mood to engage in uninformed dialogue with someone that merely wants to make assertions that they have no evidence for.  Take your anti-evolutionary stance someplace else where you can pat each other on the back for your ridiculous ideas.
    Mundus vult decipi
    I am sorry that you are offended, Gerhard. I have only described observed phenomena, courtesy of the scientific method. I fully understand that it is not in accord with your beliefs and I respect every man's right to his beliefs.

    Gerhard Adam
    You are delusional.  It's unfortunate that you invoke the "scientific method" when you've employed nothing of the sort.  You don't even bother to get basic biology right.  Stop wasting my time.

    Let me be clear about one thing though.  Don't patronize me with your ignorant claims regarding beliefs and belief systems.  You know nothing about the topics and your continued intentional stance of remaining ignorant is simply insulting.

    While you may think that you're being smug and getting me [or others] all "riled up", the reality is that it is simply frustrating when one deals with an individual that chooses to intentionally ignore the data and then purports to quote "science". 

    I'm sure you'll respond with something even more inane, but I won't respond after this, since you've clearly demonstrated an inability to actually acquire information.  You are quite obviously impervious to logic, reason, and scientific evidence.  You should be proud.  Carry on.
    Mundus vult decipi
    Gerhard, your argument is not with me, your argument is with science. Apply the scientific method:


    We would observe that there are single cell bacteria and multicellular organisms of many classifications.

    Our hypothesis is that these single cell bacteria develop mutations over time which result in unicellular bacteria evolving into multicellular organisms.

    Observation of bacteria colonies over many generation will result in finding new multicellular organisms descended from original colonies of single cell bacteria.

    Conduct the observations and document the results.

    The null hypothesis is validated with each new generation of single cell bacteria. Descendants are never multicellular.

    You obviously have a belief about the evolution of multicellular organisms from single cell organisms that isn't based on science. You are not alone. You have a lot of very smart and wrong company.

    Gerhard Adam
    Please stop ... your ignorance is giving me a migraine.  Keep posting if you like.  Each time you do you demonstrate again and again that you simply don't understand.
    Mundus vult decipi
    I understand the difference between science and belief. Science proves that single cell organisms always replicate into single cell organisms. They don't evolve to become multicellular, despite beliefs to the contrary.

    Gerhard Adam
    I can only conclude that you're being intentionally ignorant despite the article and links that have provided counter-evidence.  As you should know, science doesn't "prove" anything that you've asserted.  In addition, your statement regarding evolution is either disingenuous or just stupid.  You've been told repeatedly that individual organisms don't evolve, but rather that populations do.

    Nevertheless you've insisted on continuing to advance your foolishness, therefore I'm putting you on notice that if you persist in simply parroting back such nonsense, I will simply begin delete your posts.  No more discussion.  You've elected to not inform yourself, so there's nothing further to be done.
    "We observed adaptation of multicellular traits, indicating a shift in selection from individual cells to multicellular individuals."
    So, as I said.  Any further attempts at insisting on your nonsensical position will simply result in deletion.
    Mundus vult decipi
    Gerhard, I greatly enjoyed the article. I printed it out, and read it carefully. You do realize that the scientists grew colonies of unicellular yeast from several unicellular yeast, right? "Multicellular" means single cell yeasts adhering to each other in a colony. "Multicellular" does not mean that single cell yeast become organisms that are multicellular.

    "Understanding the evolution of complex multicellular individuals from unicellular ancestors has been extremely challenging, largely because the first steps in this process occurred in the past, greater than 200 million years ago. As a result, transitional forms have been lost to extinction, and little is known about the physiology, ecology, and evolutionary processes of incipient multicellularity."

    In other words, they simply believe it happened. There a single cell organisms and multicellular organisms, then and now. There are no transitions. A snowflake cluster of single cell organisms is a cluster of single cell organisms. The organisms are not multicellular.

    John Hasenkam
    <i>_ The genetics of unicellular creatures are apparently fixed to the degree that unicellular creatures never evolved to become multicellular.<i/>

    This belies a fundamental misunderstanding of genetics. You can be excused for that because Mr. Dawkins played a major role in promulgating the misapprehension and it is still widespread. Nonetheless many others know that to think of the genome as a "blueprint" or "set of instructions" or "containing all the information necessary to build a cell" is fundamentally mistaken. You think of genes acting in very specific ways towards very specific ends so I imagine you have never encountered Susan Lindquist's ideas of hsp90 as an "evolutionary capacitor". 

    Absence of evidence is not evidence of absence. If it were there would be no religion. There is data to demonstrate that the preconditions for single to multi-cellular life are already present. It is no great leap to multi-cellularity. Here is a primary example. Apoptosis is programmed cell death, an internally or externally initiated series of events that causes the cell to die but most importantly to die in a very specific way. This process is absolutely fundamental to multi-cellular life forms in shaping organs, sculpting organs, and preventing viruses in particular replicating within cells. Failure of apoptosis is a leading problem in cancer.  

    I was confused some months ago when I read that even single celled creatures have death programs. What advantage does a single celled creature have in killing itself? None. But it has this group selection advantage: bacteria are under constant attack by bacteriophages, viruses that infect them. An infected bacteria in a colony, by committing suicide, is preventing the replication of the pathogen and its spread. Apoptosis even prevents the release of viral material because unlike necrosis where the cell contents are spilled out along with the viral particles whereas in apoptosis the cell membrane collapses inwards. It is a great trick. It also challenges the "selfish gene" and "no group selection" ideas that have been so fashionable of late. 

    That's one example. There are hundreds. I suggest before you start asserting that it is a fairy tale you make an effort to see just what evidence is in support of the argument. I know that is difficult because you'll have to read scientific papers and that is not easy, I tire of my struggles in that regard but persevere.But if you are serious about arguing your position you need to know the position of your enemy. If you can't demonstrate the effort and integrity to do the basic homework then piss off and stop wasting my time. 
    John, thank you for taking the time to write such an interesting and educational response. I am not at all reluctant to read articles on the topic. I would absolutely love to read about the observations of bacteria colonies and their evolution to multicellular organisms. I must say, I don't believe it happens based upon my understanding of observations made in the natural world since development of the microscope, but that doesn't mean I am not open to proof validated by the scientific method. I am certainly open to the hypothesis. I have to be, otherwise I am not open minded.

    "I don't believe it happens based upon my understanding of observations..."

    Would you please cite one paper or study of these "observations" that you base your understanding on. I would like to evaluate your data for myself.


    Take a look at the link posted by Gerhard Adam. I am reposting for you, here. Researchers filtered single cell yeasts, Saccharomyces cerevisiae, through a solution to create descendants that adhere to one another and form clusters in the shape of snowflakes. Since the descendant yeast would form clusters, where previously that had not clustered, this change is referred to as "evolving multicellularity".

    My point is that evolution claims that single cell organisms evolved into multicellular organisms. Clustering single cells are simply clusters of single cells. They are not remotely multicellular as in evolving from single to multicellular organisms. Take a look at the pictures of the snowflakes. You do not see a single living organism composed of numerous cells. You see many single cell organisms clustered together.

    Researchers who conducted the experiment, point out that there are no transitional organism between single cell and multicellular organisms. They are assumed to have lived hundreds of millions of years ago.

    "Understanding the evolution of complex multicellular individuals from unicellular ancestors has been extremely challenging, largely because the first steps in this process occurred in the past, greater than 200 million years ago. As a result, transitional forms have been lost to extinction, and little is known about the physiology, ecology, and evolutionary processes of incipient multicellularity."

    Sorry Achilles, I can't agree with your understanding of the observations in the link you provided. "You do not see a single living organism composed of numerous cells." ????? This is exactly what I see, a single living organism composed of numerous cells. Oh well, I guess you'll have to consider me a part of "a lot of very smart and wrong company", and I'm not wrong to be glad to be part of it.

    Gerhard Adam
    I'm sure you're aware that achilles is playing a child's game here.  (S)He's trying to create an anti-evolution position by claiming that no single generation has spontaneously changed from being unicellular to being multicellular.

    Of course, a child would actually understand that such a thing has never been proposed by evolutionary or biological explanations, but it's a type of "gotcha" reasoning that makes him/her feel like they've outsmarted everyone else.

    In short, it's the same kind of stupidity that occurs when people claim that since no chimpanzee ever gave birth to a human, then evolution most be wrong.  As I said ... stupid is simply stupid, and I have no patience for people that choose to remain intentionally ignorant.
    Mundus vult decipi
    Gerhard, what is stupid about pointing out that there are no transitions between single cell and multicell organisms, either living, or in the fossil record? Further, since there are none, why do you believe they exist or ever have existed? What is scientific about that belief?

    Gerhard Adam
    Oh please.  You know that this is simply another variation of how we don't see fish become birds, or cows become whales.  It's the same transitional argument that is foolish on the face of it.

    You're simply arguing the same old tired creationist bullshit, so just go away.  Your perverted sense of what is science, is simply annoying.
    Mundus vult decipi
    Gerhard, the only thing I'm aware of is that Achilles' understanding is not supported by the facts presented in the link provided. Since he does not provide any scientific argument to support his understanding, I do not see any possibility in having a scientific discussion about it. Introducing the assumption that he's trying to create an anti-evolution position brings up the very thing that I do not want to discuss here on this forum. It has nothing to do with the subject of this article. I'm very happy not discussing it any further and I think you could be too. Just let it be.

    John Hasenkam
    Eat my shorts ... 

    These cells indeed represent a peak of social sophistication among procaryotes, for when food supplies are exhausted, the cells aggregate tightly together and form a multicellular fruiting body  

    In some ways Volvox is more like a multicellular organism than a simple colony. All of its flagella beat in synchrony as it spins through the water, and the colony is structurally and functionally polarized and can swim toward a distant source of light. The reproductive cells are usually confined to one end of the colony, where they divide to form new miniature colonies, which are initially sheltered inside the parent sphere. Thus, in a primitive way, Volvox displays the two essential features of all multicellular organisms: its cells become specialized, and they cooperate. By specialization and cooperation the cells combine to form a coordinated single organism with more capabilities than any of its component parts.

    Very interesting. The individual cells operate in the fashion of cells in a multicellular organism. Since they have such a successful survival strategy, there is no environmental pressure to develop into unibody multicellular organisms.

    John Hasenkam
    No worries achilles, I knew there was evidence but could not find the specific reference I had in mind. Nonetheless your demand for evidence was valid so today, luckily, I found something better.