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
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