No one is yet certain how multicellular life evolved. There are three prominent theories about how it happened, with one a strong favorite among scientists.
Throughout the world, many organisms live in symbiosis. Symbiosis is an association of two different species that may (though it does not always) equally benefit each member. Lichens, for example, consist of a fungus that provides support and shelter and an alga or cyanobacteria that provides food through photosynthesis. Another example is zooxanthella, which lives inside certain corals, jellyfish, and sea anemones, and provides them with up to 90% of their food through photosynthesis.
In eukaryotic cells, cells with a nucleus inside a membrane, the mitochondria serve as power plants, producing a chemical compound that stores energy for each cell. In plant cells, the chloroplasts are the engines that conduct photosynthesis. Both the mitochondrion and the chloroplast probably began as endosymbionts, formerly alien species living in symbiosis within the cell, which now are part of it.
However, the mitochondrion and the chloroplast carry their own DNA, separate from the main DNA of the surrounding cell. If the union of symbionts is how multicellular life began, the symbionts should share DNA, but they do not.
Syncytal means having more than one nucleus. Some of the slime molds and ciliates are syncytal. The syncytal theory posits that single cells developed membranes between their separate nuclei, and thus became multicellular.
Though syncytal organisms do have what appears to be more than one nucleus, each nucleus seems to perform a different role. Not only would each nucleus need to form its own cell, it would also have to take over functions performed by the other nuclei in its former cell. Biologists have found no organisms that do this.
Many unicellular organisms do not separate following reproduction. Certain bacteria are examples. Some form chains or even rafts of cells. Most scientists believe that multicellular life evolved from these colonies of cells.
Volvox is one example. A green alga, it forms hollow spherical colonies of up to 10,000 cells. The cells are connected by strands of cytoplasm that allow them to stay in formation. Each cell has two flagella, whiplike extensions that they use to move. Most of the cells of a Volvox colony are vegetative, asexual, like most of the cells of plants and animals. About eight cells in a colony reproduce asexually, and 15 to 25 can reproduce sexually.
In asexual reproduction, Volvox produces daughter colonies inside itself. When they are mature, the parent colony disintegrates, and the daughter colonies turn inside out. They are already forming their own daughter colonies within themselves. To reproduce sexually, male colonies send sperm out into the water to find the eggs of female Volvox colonies. Actually though, some species of Volvox are hermaphroditic, and fertilize themselves.
Since such species as Volvox exist, forming an intermediate step between colonies of like cells and organisms of different cells with the same DNA, scientists favor the colonial theory of how multicellular life evolved.
Multicellular organisms are not merely cells that group together. They must share the same genetic material. They must also divide functions among different groups of cells.
Symbionts have cells with different functions, but do not share the same genetic material. Syncytal organisms have multiple nuclei, but each nucleus does not appear to be capable of running its own cell. Colonial organisms appear to be the path that evolution took, because they have common DNA, and cells with different functions.