Why would a single celled organism evolve to be multi-celled?

Why would a single celled organism evolve to be multi-celled?

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I read a story this week on Richard Lenski who has been 'evolving' E. coli for more than 50,000 generations now. One comment I read was from someone who doesn't accept Evolution who pointed out that we haven't seen a single celled organism 'evolve' into a multi-celled organism. Another person responded and said that a bacteria is not going to evolve into something that isn't a bacteria.

So, if Evolution created single celled organisms and then multi-celled organisms how might that change have happened? And is it possible to recreate that set of driving forces to make a bacteria something other than a bacteria?

To that end, what advantage does being multi-cellular have over being unicellular (if that's even a word)?

How did multicellularity evolved?

It is an ongoing field of research - Some insights about the origin of multicellularity

This is a big ongoing field of research. To start with an example, there was relatively recently (2012) an important article by Ratcliff et al. that shows that yeast can quickly evolve multicellularity under selection on the speed they sink to lower water layers. This article is one among many others and is far from being able to explain everything we would like to understand about the evolution of multicellularity. Typically, I think that this yeast species had a multicellular ancestor and we might think that this species would already have fixed alleles (=variants of genes that is fixed meaning that the whole population is carrying this variant today) in the population predisposing this species to easily (re-)evolve multicellularity. Also, they may have kept some standing additive genetic variance in their genome from their past and they would therefore very quickly respond to selection as they don't need de novo mutations. (Sorry if this last sentence was slightly technical).

One of the first traits that we usually refer to when talking about the evolution of multicellularity is the presence of sticky proteins allowing individual cells to paste to each other.

Some insights about the evolution from simple multicellular to more complex multicellular

Then, we could talk about more complex multicellular and argue how do these simple multicellular evolve into some more complex organisms. A common argument is that multicellular can have specialized cells are very could at doing what they're doing as they are specialized. Also, some level of complexity is thought to have raised due to the fact that multicellular organisms tend to have smaller population size than unicellular (see Lynch and Conery, 2003). It is important not to confuse evolution of complexity with the evolution of multicellularity although these two notions are somehow related.

What do you mean by multicellularity?

The evolution of multicellularity can be discussed in the context where sister cells form an organism together or when unrelated cells (among the same species or even cells from different species) come together to form an organism. Also, the multicellularity can be discussed at a different level depending on how we want to define multicellularity. Is a stack of cells reproducing individually, working for their own benefit a multicellular? Do we need a division of labor? Do we need a division between germline (reproductive caste) and soma line (non-reproductive case)?

How many times did multicellularity evolve independently?

Some people consider that there are multicellular bacteria (biofilms) but we will avoid discussions that are based on limit-case definitions. Let's talk about eukaryotes. Most Eukaryotes are unicellular and multicellularity evolved many times independently in eukaryotes. To my knowledge, complex multicellularity however evolved only (only?) 6 times independently in eukaryotes.

  • Metazoa (animals)
  • Ascomyceta (fungi)
  • Basidiomyceta (fungi)
  • Viridiplantae (green plants)
  • Florideophyceae (red algae)
  • Laminariales (brown algae)

Model organisms and interesting cases to study multicellularity

There are a bunch of specific clades that are particularly interested in studying multicellularity because they present transition states. For example Volvox is a chlorophyte genus and the species in this clade present different stages of multicellularity; Some species are exclusively multicellular, some form small groups, some create big colonies, some have some division of labor and some even have separation between the germline and the soma (Some castes don't reproduce). (ref1, ref2, ref3, ref4, ref5, ref6). Yeasts are also a good model organism for studying the evolution of multicellularity.

For one thing, larger organisms are much more energy efficient. This is what is known as Kleiber's Law where the caloric requirement scales as the 3/4 power to the body mass.

Another thing is that when all the cells cooperate to form a multicellular organism, each given individual is more likely to reproduce and less likely to die due to environmental variation because cooperation creates stability.

There are several theories about how this came about,but those are the elements of why. Collaboration and efficiency improve the chances of survival, which is to say that selection will favor multicellular organisms however they came to be.

Disclaimer: Not my field of research, and not a field where I know the litterature well. See it as a complement to the other answers.

A distinct advantage of multicellularity is specialized functions of different cells. This can allow for higher efficiency of e.g. metabolic processes, and also that redundant functions can be removed from some cell lines, since they can be handled by other cells. Therefore, the constituent parts can become simpler, while the resulting organism becomes more complex at the same time. Mathematical modelling of cellular systems have shown how this type of division of labour can evolve from unicellular lines (Ispolatov et al. 2011), through the steps of aggregation and differentiation from preexisting functions.

An interesting intermediate step that can provide some clues to how multicellularity can evolve, is in cyanobacteria, where some unicellular species can show partial specialization e.g. when part of cellular biofilms. A phylogenetic study of cyanobacteria has also shown that they have reversed from multicellularity to unicellularity at least five times, and most extant cyanobacteria seem to descend from multicellular ancestors (Schirrmeister et al. 2011). This means that the evolution of multicellularity is not a one-way process, but seems to be a much more complex process.

I STRONGLY encourage to read work from the lab of Nicole King - she studies Choanoflagellates, which are the "out-group" for animals - they are, in some sense, the most animal-like single-celled organism that exists.

Chaonos are also amazing because they go through a single to multicellular transition in there own life cycle, so they provide an amazing opportunity to understand when it is more beneficial to be single-celled vs. multi-celled. Currently, one of the working hypotheses of the group is that one of the main drivers of the push towards multicellularity may have just been simple fluid dynamics: the flows around a spherical multicellular "rosette" of chaonos bring more food to them.

If you are interested in the evolutionary transition to multicellularity you must read work from the King Group.

If single cells are capable of surviving on their own then why did multicellularity evolve?

This situation can be compared with the evolution of family and society, in a way; during the time of crisis, the survival chances increase when someone stays in a group.

Similar conditions would have resulted in the evolution of multicellularity. The difference between being truly multicellular and just being a group of cells is that in multicellularity, the individual cells cannot survive in the absence of the other. Moreover, different cells in a multicellular organism perform different kinds of functions. However, it is certainly likely that grouping without a strong dependence would have constituted the early stages in the evolution of multicellularity.

One of the complex kinds of microbial colonies is biofilm. In a biofilm different "regions" of the colony have different kinds of functional roles; the "outer" cells take up nutrients for the colony from the surroundings whereas the inner cells reproduce and keep the colony thriving. Bacteria have also evolved a way of signalling (or "talking") to other bacteria (of the same species) by a mechanism known as quorum sensing, which in a way changes the behaviour of the bacteria when then stay in a group.

Dictyostelium or slime mold (or affectionately called dicty :) ) is an example of early evolution of multicellularity in eukaryotes. When there is plenty of food, dicty stays as unicellular amoeba. However, when there is a shortage of food, the dicty amoebae start grouping up and give rise to a multicellular "slug". The dicty slug roams around and when it encounters right conditions (such as humidity), it differentiates to give rise to a "fruiting body", which more or less looks like a fungal spore. In a fruiting body some cells form the spores (which will produce new dicties) whereas some cells form the stalk (which supports the spores). Apparently, the choice of what part will a cell become, is random and at this stage the individual dicty amoebae are no more selfish. The aggregation of dicty amoebae is co-ordinated by a signalling molecule called cAMP and this works in a way similar to quorum sensing.

Taken from Wikipedia

Volvox is another example of an early stage of multicellular evolution.

To sum up, as you said single cells can very well survive on their own. However, in some situations being multicellular would have given the organism some survival advantages. You should understand that this is just one of the survival strategies and not all organisms needed to adopt this. In fact, there are many more unicellular species in the planet compared to the multicellular ones.

I would reiterate Remi's suggestion that you should have a look at this site called Understanding Evolution, hosted by UC Berkeley.

You can also look at this post on our site about a recurrent doubt faced by many students and non-experts in the area of evolution: "Why do some bad traits evolve, and good ones don't?"

For the same reason that sociality evolved so many times among animals. There are lots of advantages in having similar fellows. And multicellular organisms are, after all, just a colony, sometimes a society, of individual cells.

The eucaryote that became the modern organelle mitochondria, combined with procaryotes. This probably occurred as a result of the mitochondria being absorbed by the parent cell but not destroyed as it usefully created energy rich ATP molecules using Oxygen and Water through respiration.

These were the first Eukaryotic cells, they went on to become Eukaryotic, multicellular life.

Various benefits to Eucaryotes becoming multicellular include:

The volume to surface area of a cell gives cells a natural size of a few micrometers. Larger single cells find it increasingly difficult to absorb enough nutrients or oxygen for the volume of there cytoplasm.

Amoebas can be larger due to being so irregular in shape, this ensures that so nowhere inside the cell is it too far from the cells surface. Various more spherical centimetre scale single celled life also exists in nutrient rich parts of the deep ocean, such as the Valonia Ventricosa.

Another advantage is that structures can form between cells, outside of the cell walls, that can still be protected inside the creatures body. Such a connective network in animals called the extracellular matrix.

Note that creatures like the sea sponge are multicellular, but do not have distinct areas of the body such as organs in the same way as animals do.

"The genes I discuss in my article were not present in the common ancestor of all life on Earth. They do not exist in bacteria, for example. They do not even exist (as far as scientists know) in sponges. Only after the ancestors of cnidarians and bilaterians diverged from sponges did they emerge." (A planet of viruses, Carl Zimmer) This is the quote I could find, as I remember, relating to "bodybuilding" in many creatures, but not the sea sponge.

Multicellular organism

All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- and partially multicellular, like slime molds and social amoebae such as the genus Dictyostelium. [2] [3]

Multicellular organisms arise in various ways, for example by cell division or by aggregation of many single cells. [4] [3] Colonial organisms are the result of many identical individuals joining together to form a colony. However, it can often be hard to separate colonial protists from true multicellular organisms, because the two concepts are not distinct colonial protists have been dubbed "pluricellular" rather than "multicellular". [5] [6]

Flipping the Script

The authors of the Nature paper reject the colonial approach to multicellularity. Instead, they propose something completely different: “As an alternative, we posit that the ancestral metazoan cell type had the capacity to exist in and transition between multiple cell states in a manner similar to modern transdifferentiating and stem cells.”8 In other words, these researchers are proposing that the first multicellular organisms were not formed from balls of cells. Instead, they were formed from cells similar to stem cells that had the ability to differentiate into different functions. This is a radical idea in the evolutionary dogma. It overturns the entire evolutionary paradigm.

It gets worse. Along with doing yet another rewrite on the evolutionary dogma, this study also increases the difficulty of getting a multicellular organism by orders of magnitude. In order to understand why this is, it is necessary to understand some things about stem cells first.

Good response to typical evolutionary stretching of observations and definitions! There’s quite a difference between a trait being truly “obligate” (a necessary part of the creature’s survival) and something which is habitual — the algal cells would probably survive as individuals as well as in colonies. A similar study and claim was made for Chlorella vulgaris years ago. Apparently even the evolutionists didn’t find that very convincing and have been looking for other algae to provide a better case.

Similarly, a study of yeast (again in special laboratory conditions) claimed to have observed the origin of specialization in yeast. In truly multi-cellular organisms, cells can be very different and play special roles in different functions of the organism, and generally cannot survive long as individuals. In the yeast, the “specialization” amounted to no more than some cells dying before the others, allowing other cells to break away from the colony and form new colonies of their own. It wasn’t clear if this was something truly new, or even if the cells had died from internal activity or the stresses of their location in the colony. I haven’t heard any further reports or developments from that study, either.

Evolutionists used to ask, If you believe things can change from one generation to the next, what’s stopping them from evolving into something very different? However, I think they are beginning to realize, or knew it all along, that change isn’t a simple thing that can grow like the number of grains of sand in a sand dune, or the drops of water in an ocean. There are many different kinds of changes, and the ones we observe in these studies are not the kinds of changes that need to be able to happen if all life evolved from microbes.

How Life Made the Leap From Single Cells to Multicellular Animals

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James O'Brien for Quanta magazine

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For billions of years, single-celled creatures had the planet to themselves, floating through the oceans in solitary bliss. Some microorganisms attempted multicellular arrangements, forming small sheets or filaments of cells. But these ventures hit dead ends. The single cell ruled the earth.

* Original story reprinted with permission from Quanta Magazine, an editorially independent division of whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.*Then, more than 3 billion years after the appearance of microbes, life got more complicated. Cells organized themselves into new three-dimensional structures. They began to divide up the labor of life, so that some tissues were in charge of moving around, while others managed eating and digesting. They developed new ways for cells to communicate and share resources. These complex multicellular creatures were the first animals, and they were a major success. Soon afterward, roughly 540 million years ago, animal life erupted, diversifying into a kaleidoscope of forms in what’s known as the Cambrian explosion. Prototypes for every animal body plan rapidly emerged, from sea snails to starfish, from insects to crustaceans. Every animal that has lived since then has been a variation on one of the themes that emerged during this time.

How did life make this spectacular leap from unicellular simplicity to multicellular complexity? Nicole King has been fascinated by this question since she began her career in biology. Fossils don’t offer a clear answer: Molecular data indicate that the “Urmetazoan,” the ancestor of all animals, first emerged somewhere between 600 and 800 million years ago, but the first unambiguous fossils of animal bodies don’t show up until 580 million years ago. So King turned to choanoflagellates, microscopic aquatic creatures whose body type and genes place them right next to the base of the animal family tree. “Choanoflagellates are to my mind clearly the organism to look at if you’re looking at animal origins,” King said. In these organisms, which can live either as single cells or as multicellular colonies, she has found much of the molecular toolkit necessary to launch animal life. And to her surprise, she found that bacteria may have played a crucial role in ushering in this new era.

Nicole King, a biologist at the University of California, Berkeley, studies the origins of animals, one of the big mysteries in the history of life.

In a lengthy paper that will be published in a special volume of Cold Spring Harbor Perspectives in Biology in September, King lays out the case for the influence of bacteria on the development of animal life. For starters, bacteria fed our ancient ancestors, and this likely required those proto-animals to develop systems to recognize the best bacterial prey, and to capture and engulf them. All of these mechanisms were repurposed to suit the multicellular lives of the first animals. King’s review joins a broad wave of research that puts bacteria at the center of the story of animal life. “We were obliged to interact intimately with bacteria 600 million years ago,” said King, now an evolutionary biologist at the University of California, Berkeley, and an investigator with the Howard Hughes Medical Institute. “They were here first, they’re abundant, they’re dominant. In retrospect we should’ve expected this.”

Although we tend to take the rise of animals for granted, it is reasonable to ask why they ever emerged at all, given the billions of years of success of unicellular organisms. “For the last 3.5 billion years, bacteria have been around and abundant,” said Michael Hadfield, a professor of biology at the University of Hawaii, Manoa. “Animals never showed up until 700 or 800 million years ago.”

The technical demands of multicellularity are significant. Cells that commit to living together need a whole new set of tools. They have to come up with ways of sticking together, communicating, and sharing oxygen and food. They also need a master developmental program, a way to direct specific cells to take on specialized jobs in different parts of the body.

Nonetheless, during the course of evolution, the transition to multicellularity happened separately as many as 20 different times in lineages from algae to plants to fungi. But animals were the first to develop complex bodies, emerging as the most dramatic example of early multicellular success.

To understand why this might have happened the way it did, King began studying choanoflagellates, the closest living relative to animals, nearly 15 years ago as a postdoc at the University of Wisconsin, Madison. Choanoflagellates are not the most charismatic of creatures, consisting of an oval blob equipped with a single taillike flagellum that propels the organism through the water and also allows it to eat. The tail, thrashing back and forth, drives a current across a rigid, collarlike fringe of thin strands of cell membrane. Bacteria get caught up in the current and stick to the collar, and the choano engulfs them.

What intrigued King about choanoflagellates was their lifestyle flexibility. While many live as single cells, some can also form small multicellular colonies. In the species Salpingoeca rosetta, which lives in coastal estuaries, the cell prepares to divide but stops short of splitting apart, leaving two daughter cells connected by a thin filament. The process repeats, creating rosettes or spheres containing as many as 50 cells in the lab. If this all sounds familiar, there’s a reason for it — animal embryos develop from zygotes in much the same way, and spherical choanoflagellate colonies look uncannily like early-stage animal embryos.

When King began studying S. rosetta, she couldn’t get the cells to consistently form colonies in the lab. But in 2006, a student stumbled on a solution. In preparation for genome sequencing, he doused a culture with antibiotics, and it suddenly bloomed into copious rosettes. When bacteria that had been collected along with the original specimen were added back into a lab culture of single choanoflagellates, they too formed colonies. The likely explanation for this phenomenon is that the student’s antibiotic treatment inadvertently killed off one species of bacteria, allowing another that competes with it to rebound. The trigger for colony formation was a compound produced by a previously unknown species of Algoriphagus bacteria that S. rosetta eats.

S. rosetta seems to interpret the compound as an indication that conditions are favorable for group living. King hypothesizes that something similar could have happened more than 600 million years ago, when the last common ancestor of all animals started its fateful journey toward multicellularity. “My suspicion is that the progenitors of animals were able to become multicellular, but could switch back and forth based on environmental conditions,” King said. Later, multicellularity became fixed in the genes as a developmental program.

King’s persistence in studying this humble organism, which was overlooked by most contemporary biologists, has won her the admiration of many of her fellow scientists (as well as a prestigious MacArthur fellowship). “She strategically picked an organism to gain insight into early animal evolution and systematically studied it,” said Dianne Newman, a biologist at the California Institute of Technology in Pasadena, who studies how bacteria coevolve with their environment. King’s research offers a thrilling glimpse into the past, a rare window into what might have been going on during that mysterious period before the first fossilized animals appeared. The research is a “beautiful example” of how bacteria shape even the simplest forms of complex life, Newman said. “It reminds us that even at that level of animal development, you can expect triggers from the microbial world.” The bacteria system in S. rosetta can now be used to answer more specific questions, such as what the benefit of multicellularity might be — a question King and her collaborators at Berkeley are now working to answer.

The first bacteria may date back as far as 3.5 billion years. But animals, the first complex multicellular life form, took much longer to emerge.

Bacteria Are Models Of Efficiency

The bacterium Escherichia coli, one of the best-studied single-celled organisms around, is a master of industrial efficiency. This bacterium can be thought of as a factory with just one product: itself. It exists to make copies of itself, and its business plan is to make them at the lowest possible cost, with the greatest possible efficiency.

Efficiency, in the case of a bacterium, can be defined by the energy and resources it uses to maintain its plant and produce new cells, versus the time it expends on the task.

Dr. Tsvi Tlusty and research student Arbel Tadmor of the Physics of Complex Systems Department developed a mathematical model for evaluating the efficiency of these microscopic production plants. Their model, which recently appeared in the online journal PLoS Computational Biology, uses only five remarkably simple equations to check the efficiency of these complex factory systems.

The equations look at two components of the protein production process: ribosomes &ndash the machinery in which proteins are produced &ndash and RNA polymerase &ndash an enzyme that copies the genetic code for protein production onto strands of messenger RNA for further translation into proteins. RNA polymerase is thus a sort of work &lsquosupervisor&rsquo that keeps protein production running smoothly, checks the specs and sets the pace.

The first equation assesses the production rate of the ribosomes themselves the second the protein output of the ribosomes the third the production of RNA polymerase. The last two equations deal with how the cell assigns the available ribosomes and polymerases to the various tasks of creating other proteins, more ribosomes or more polymerases.

The theoretical model was tested in real bacteria. Do bacteria &lsquoweigh&rsquo the costs of constructing and maintaining their protein production machinery against the gains to be had from being able to produce more proteins in less time? What happens when a critical piece of equipment is in short supply, say a main ribosome protein? Tlusty and Tadmor found that their model was able to accurately predict how an E. coli would change its production strategy to maximize efficiency following disruptions in the work flow caused by experimental changes to genes with important cellular functions.

What&rsquos the optimum? The model predicts that a bacterium, for instance, should have seven genes for ribosome production. It turns out that that&rsquos exactly the number an average E. coli cell has. Bacteria having five or nine get a much lower efficiency rating. Evolution, in other words, is a master efficiency expert for living factories, meeting any challenges that arise as production conditions change.

Dr. Tsvi Tlusty&rsquos research is supported by the Clore Center for Biological Physics.

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Early Unicellular Life

Now, there is some debate about the first origins of life on the planet, with estimates ranging from 3.77 billion years to 4.5 billion years ago&mdashless than fifty million years after our planet formed! While the timeline may not be exact, there is little debate that the first forms of life existed on hydrothermal vents deep in the oceans, as the earliest evidence of life comes from hydrothermal vent precipitates. These first forms of life were simple microorganisms, and may have appeared almost immediately after the formation of the oceans.

However, the first undisputed and direct evidence of life on Earth dates back to about 3.465 billion years ago&mdashfossilized microorganisms&mdashwhile earlier claims are typically dependent on the presence of substance involved in biochemical processes, though not remains of the organisms themselves. The early direct examples of life that have been found, however, already show some cellular complexity, including cell walls encasing the protein-generating DNA, so more rudimentary forms of life likely existed much earlier.

(Photo Credit : Nasky/Shutterstock)

Basically, beginning 3.5 billion years ago, single-celled organisms ruled&mdashdespite early multicellularity in cyanobacteria-like mats&mdashmost of which were prokaryotes, until the rise of eukaryotes (cells with a nucleus, organelles, and more complex functionality). Bacteria and Archaea are the first two domains of life that arose, followed by Eukarya. These simple organisms were able to maintain their individual metabolism and survive all on their own, requiring only one cell, rather than additional specialization. There are still many unicellular species on the planet, including bacteria, plankton and amoeba, as well as all protists (which are eukaryotes), and some fungi.

Multicellular Vs. Unicellular Organisms

As the name suggests, the main difference between multicellular and unicellular organisms is the number of cells that are present in them. This leads to the development of all other characteristics and properties of these living organisms. Read about the distinction between these two types in this BiologyWise article.

As the name suggests, the main difference between multicellular and unicellular organisms is the number of cells that are present in them. This leads to the development of all other characteristics and properties of these living organisms. Read about the distinction between these two types in this BiologyWise article.

Cells are the building blocks of all life forms. Every living thing has cells within its body. The composition, distribution, and the number of cells that are present in an organism determine whether it is multicellular or unicellular. Cells in the human body play a vital role in the sustenance of life.

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In simple terms the difference between unicellular and multicellular organisms arises from the number of cells that are present in them. As the name suggests, unicellular organisms contain one single cell, while multicellular organisms contain more than one cell within them. All their physical characteristics and traits can be traced to the difference in the number of cells they comprise.

Unicellular Organisms

Due to the presence of only one cell in them, these organisms are much smaller in size and are very simple in structure. Most of these organisms fall under the category of ‘prokaryotes’, or ‘prokaryotic entities’, because their composition and structure is not complex. The structure known as the cell nucleus is completely absent in these prokaryotes, and this leads to their inability to handle their surface area to volume ratios. Owing to this reason, their sizes are very small.

Most unicellular organisms are so small and microscopic in nature, that they are almost invisible to the naked human eyes. They do not have internal organs as well, and this means that the membranes which are the organic coats around the organs are also absent. Due to their highly simplistic life form, these can exist in areas that are perceived to be hazardous to human life and are highly acidic or radioactive in nature.

It is believed by many scientists that the human race is the result of long term evolution of many unicellular organisms that existed millions of years ago. The two sets of organisms exist in harmony with each other on our planet. Besides this, all these organisms have their own specific roles to play in nature’s ecosystem.

Examples: All forms of bacteria, amoeba, yeast, and paramecium.

Multicellular Organisms

On the other hand, these organisms are those forms of life that have more than one cell present in them. In fact, they have millions of cells present in them.

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The larger number of cells means that these organisms are much bigger in size and are very complex and intricate in their composition as well as structure. Human beings are the best example of multicellular organisms, and the large number of cells leads to the birth of many different organs for carrying out different functions. Most ‘eukaryotes’ or ‘eukaryotic entities’ are multicellular. The cell nuclei present in eukaryotes and the DNA of the organisms are separately placed, unlike the case of unicellular organisms. All these cells work in tandem with each other to keep the life form alive, and this leads to a variety of complex functions occurring simultaneously.

The organisms in both the categories differ greatly in their appearance, and even though multicellular organisms grow to large sizes, some of them are still microscopic in nature. These are also known as ‘myxozoa’.

Examples: Human beings, animals, plants, myxozoa, and all kinds of fungi.

Scientists discovered all the differences between multicellular and unicellular organisms and this laid the foundation for the rest of biology to develop. The advanced study of the structure of all animals and plants would not be possible without proper knowledge about the cell structure of these organisms, since the cells are the primary life forces and no organism can be alive without the presence of cells in them.

The desire to know the differences between organisms is an important event in human history, and medical science would not be where it is today without this discovery.

Related Posts

Unicellular organisms refer to living entities that have only one cell, and the cell is responsible for performing all the functions. Some examples are amoeba, paramecium, bacteria, and cyanobacteria.

The following article presents before us monocot vs. dicot differences by considering their various features. Read on to known more about dicotyledon and monocotyledon classifications.

Exocytosis is the reverse of endocytosis. This article gives you a brief explanation of these processes and also compares the two.

Scientists Explore Why Single Cells Band Together

Researchers are discovering how multicellular organisms evolved.The first evidence of multicellularity happened about 2 billion years ago.

Let's turn now to the very first cells that swam all alone in the primordial soup. Those cells figured out a way to get together, and that led to the explosion of complex plants and animals we see on Earth today. It's a process scientists would like to know more about.

And as NPR's Joe Palca tells us, a new study sheds light on that.

JOE PALCA: This is a story that starts a long time ago.

JOHN KOSCHWANEZ: It's believed life started around four billion years ago, and the first evidence of multicellularity is a little over two billion years ago.

PALCA: John Koschwanez is a researcher at Harvard. He's investigating how those first cells joined together.

KOSCHWANEZ: We can't take a time machine back two billion years ago to find out exactly what happened, but instead we can do a couple things in the lab.

PALCA: One of those things is to see if there any benefit for cells that are normally loners to gather their pals and form a clump. Koschwanez tried this with yeast. A single yeast cell has a problem. It can get energy from sugar, but it spills lot, so it doesn't make the most efficient use of the sugar around it. But if the yeast clump together into a group, instead of spilling the sugar on the floor, as it were, they're all spilling it on their neighbors.

KOSCHWANEZ: So in essence all the cells within the clump of cells are feeding each other.

PALCA: So Koschwanez tried growing loners and clumpers in conditions where sugar was scarce.

KOSCHWANEZ: And we compared the single cells against a group of cells.

PALCA: Sure enough, as Koschwanez reports in the journal Plos Biology, the clumpers beat the loners hands-down. So if in the course of evolutionary time a group of yeast cells did happen to clump together, they would probably stick around and thrive.

Rick Grosberg is a biologist at the University of California Davis. He says the new study shows you don't need some unique and strange event two billion years ago to encourage cells to form into groups.

RICK GROSBERG: The conditions that really promote group formation in organisms as simple as yeast are very simple conditions. There's nothing complicated or surprising or special about them. They must have been very general conditions.

PALCA: Now there is multicellularity, and then there is multicellularity. Nicole King is at the University of California, Berkeley.

NICOLE KING: There are simple forms of multicellularity, in which cells typically are living on their own but they can come together under certain environmental conditions.

PALCA: But King thinks more interesting is the kind where different cells take on different tasks within a single organism. In the case of humans, that would mean heart cells or brain cells or something specialized like that.

KING: That's the kind of multicellularity that most of us think about, and know and love.

PALCA: And how that happened is really complicated. In her work, King studies tiny single-celled organisms called choanoflagellates. These are organisms that have already begun to develop special bits, like a kind of tail for swimming around on a collar of tentacles to grab bacteria for dinner.

KING: All of them are single-celled, but the exciting thing is that some of them can form little multi-celled colonies as well.

PALCA: Inside the colonies, individual cells can start to take on these specialized functions, like swimming or digesting. King doesn't know precisely how that happens, but she does know one thing. If you compare the genomes of these tiny single-celled creatures with the genomes of some of the first true animals, they look remarkably similar.

KING: So we in fact think that a cell that looked like a choanoflagellate was probably the ancestor of animals.

PALCA: In other words, we all have some pretty humble origins.

Joe Palca, NPR News, Washington.

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Bacteria with bodies - multicellular prokaryotes

Bacterial cells are fundamentally different to the cells of multicellular animals such as humans. They are far smaller, with less internal organisation and no nucleus (they have DNA but it is not packaged safely within a membrane). Because of this bacteria are almost exclusively single-celled organisms, with their own autonomy and often mobility.

Of course many bacteria form large interlinked structures such as biofilms and colonies. These show impressive cellular organisation, but they cannot really be considered one single multicellular organism. In order to be considered a multicellular creature, and organism must fulfil certain criteria:

Are there some bacteria that can do all that? Not very many of them can, true, or there would be large multicellular bacterial 'animals' roaming the plains. But there are a number of photosynthetic bacteria are able to form truly multicellular structures, albeit rather small ones.

Those long chains are technically all one organism, a photosynthesising cyanobacteria. The outer cell wall surrounds the whole organism in one continual envelope, and fulfills the first requirement for multicellularity, keeping the cells together. The arrows point towards larger cells which fulfill the both the third and the fourth. These larger cells are very different from the ones surrounding them they have differentiated to form specialised cells whose only job is to take up inorganic nitrogen from the surroundings and 'fix' it into a usable organic form.

This is a very important development, as the enzyme required to fix nitrogen does not work in the presence of oxygen, which is vital for respiration. That's why most animals and plants can't fix nitrogen and instead rely on food sources, or surrounding soil bacteria for the organic form. Bacteria have different ways to respond to this problem. Some rely on outside food sources, others become totally anaerobic (not using any oxygen at all) and some, like the cyanobacteria, have differentiated to form special nitrogen-fixing cells.

(There is a third strategy, which is to become a nitrogen fixing bacteria by night, and an aerobically respiring bacteria by day, but this requires huge amounts of energy as it means that the cell has to do a complete enzyme turnover every twelve hours)

The differentiated cell is called a heterocyst. It has a thicker cell wall to stop oxygen diffusing into the cell, and all cellular processes that might produce oxygen have been removed. Once the cell has turned into a heterocyst it cannot change back again, and is completely dependant on the cells surrounding it for the products of respiration, which it cannot carry out by itself as the process requires oxygen. Likewise, the surrounding cells are dependant on the heterocyst for the provision of nitrogen.

The cells also communicate between themselves, using a feedback system of chemical messages to determine which of them will differentiate into a nitrogen cell and which ones will stay as normal respiring cells. They can also choose to differentiate into hormogonia, which are little lines of very tiny cells that act as invasive reproducing particles. Hormogonia have some pretty awesome properties, they can glide through slime, scuttle around with pili and even float on water due to internal gas vesicles. Unlike the nitrogen-producing cells though, hormogonia are not terminally differentiated, and can turn back into normal cells once they've reached a good destination to reproduce in.

Can this be considered 'true' multicellular behaviour? There are arguments either way, but as far as I'm concerned this is a multicellular bacteria. It's certainly the closest a bacteria can get to loosing it's single-celled autonomy and existing within a larger organism.

This post is based on an older post from my previous blog, Life of a Lab Rat.

Flores, E., & Herrero, A. (2009). Compartmentalized function through cell differentiation in filamentous cyanobacteria Nature Reviews Microbiology, 8 (1), 39-50 DOI: 10.1038/nrmicro2242

The views expressed are those of the author(s) and are not necessarily those of Scientific American.


A biochemist with a love of microbiology, the Lab Rat enjoys exploring, reading about and writing about bacteria. Having finally managed to tear herself away from university, she now works for a small company in Cambridge where she turns data into manageable words and awesome graphs.