Evolution of Multicellularity

EVOLUTION OF MULTICELLULARITY

Multicellular organisms consist of more than one type of cell. The cells are dependent on one another, which is not always the case with unicellular organisms. We talked about algae, which are considered multicellular. Things like slime molds and social amoebae are intermediaries between being a unicellular organism and a multicellular organism. There is such a thing as pluricellular organisms”, which are colonial organisms rather than truly multicellular.

Multicellularity did not happen in a single independent time in evolutionary history but instead evolved at least twenty-five separate times. It has occurred in eukaryotes and some prokaryotes, such as myxobacteria, cyanobacteria, and actinomycetes. It has only stuck throughout evolution in six instances, including brown, red, and green algae, fungi, animals, and land plants. The first sign of this phenomenon in evolution was 3 to 3.5 billion years ago among organisms similar to cyanobacteria.

A necessity for multicellularity to become possible is that the organism must be able to reproduce itself in order to make an entire organism. This means being able to have differentiation into sperm and egg cells, also called germ cells”, for the purposes of reproduction. In addition, there should be the ability to differentiate into different types of nonreproductive cells. There are more than 100 different types of cells in animals and about 10 to 20 different types of cells in fungi and plants. Finally, cells should have the ability to adhere to one another if multicellularity is to be possible.

Some organisms have been found to have lost their multicellularity as part of their evolutionary process. Certain fungi have reverted to become unicellular after a period of time being multicellular. This has also happened in some algae. It is believed that these organisms simply reverted to their previous unicellular state. Other organisms have reduced their multicellularity due to losses in the number of different types of cells. This has occurred in certain protozoa.

Multicellular organisms carry the risk of developing cancer, particularly in those that live long. Cancer can happen in plants and animals. When this happens, some believe this represents a loss of the organism s multicellularity because it involves a loss of differentiation of the cell.

So, how did multicellularity develop? Some believe that there were cells that aggregated into a grex, which is a slug-like mass, and that this functioned as a multicellular organism. Slime molds behave this way. Others believe that nuclear division occurred at one point without formation of a separating cell membrane in order to form a coenocyte that functioned as a multinucleated cell. This would have led to a functional, multicellular organism. Still others

believe that cells divided but did not separate from one another. This is what happens in embryos.

The early multicellular organisms did not leave a good fossil record because they weren t bony nor did they have hard body parts. Demosponges, however, did leave behind fossils and were early multicellular organisms. There are other early multicellular organisms that did yield a fossil. For this and other reasons, it has been hard to find ancestors of most multicellular organisms. What we do know comes mainly from studying the development of embryos.

There are theories as to how multicellularity evolved. One theory is the symbiosis theory. This suggests that it started with cooperation or symbiosis of different single-celled organisms that later became dependent on each other. This theory would involve an exchange in DNA eventually in order to create a new species. Clown fish and sea anemones are extremely codependent on one another, for example. The problem with this theory is that it isn t known how the DNA would be exchanged. The only time this is known to have occurred is when chloroplasts and mitochondria were involved in endosymbiosis.

Another theory is called the syncytial theory or cellularization theory, which involves a cell with multiple nuclei. Support of this theory is seen with slime molds and ciliates. This, however, doesn t completely explain multicellularity. This is because, in organisms that have more than one nucleus, there is usually one large macronucleus and smaller micronuclei used for sexual reproduction.

A third theory is the colonial theory is that organisms of the same species developed symbiosis among themselves. This is seen in some land-evolved organisms but not in aquatically-evolved organisms. This phenomenon is seen in certain protists, such as amoeba. These are called colonial protists. The combined groups of organisms will function as one organism but this probably indicates pluricellularity rather than multicellularity.

There are advantages to multicellularity. It allows organisms to be bigger than could be explained by diffusion alone. This allows for a certain competitive advantage to these types of organisms. Multicellular organisms also live longer than unicellular organisms because it allows some cells to die with others surviving. Despite these apparent advantages, unicellular organisms are more successful in the environment than animals but they are not more successful than plants.