by Janet Grischy

Created on: September 01, 2009

Evolution favors efficiency. Since living things compete for living space, food, and mates, efficient organisms that are better competitors are more likely to leave successful offspring. The division of labor leads to efficiency in living things as it does in society, and therefore, different cell types in multicellular organisms evolved.

Since multicellularity is so adaptive, plants, animals and fungi each developed it independently. Multicellular members of each kingdom competed more successfully by breeding offspring with special protective, digestive, or

neural cells.

Evolution of multicellular life

There are different theories about how multicellular life evolved. Symbiosis, sharing of resources as in lichens, may have led to fusion, according to one theory. The syncytal theory suggests that cells developed multiple nuclei, and then partitioned off each nucleus with membranes into separate cells

The colonial theory, though, is the one most biologists favor. They point to actual colonies of single celled organisms as proof of the concept. Eudoria live in colonies of up to 128 cells. They, and many other colonial species, are proof that unicellular organisms prospered by grouping together. These are still not multicellular organisms, because there is no differentiation of function among the colonists, no division of labor. Each cell behaves exactly as the one next to it does.

Volvox is an alga that lives in a hollow spherical colony in water. It has cells that are asexual, and also cells that divide and reproduce, just as animals have. Once a daughter colony has been created, it matures inside the hollow Volvox sphere, which then breaks open to (in effect) give birth to it.

To a biologist, a multicellular organism has cells with different functions, that all share the same genetic material. Therefore, a mere group of cells that are all the same is not multicellular.

Choanoflagellates

Choanoflagellates may have been the first step in the evolution of many-celled animals with different types of cells. Unicellular, they live in underwater colonies. They cling to rock and use their whip-like flagellum to draw in prey in a water vortex. Their colonies are not a single organism though, because there is no specialization, it is every cell for itself.

Sponges

Sponges are metazoans, multicellular animals. They reproduce sexually. If certain pieces of sponge are broken off though, each can recombine and live on. This is because a sponge has no organs, no

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