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|  | Summary
The evolution of large multicellular organisms depended on the ability of eucaryotic cells to express their hereditary information in many different ways and to function cooperatively in a single collective. In animals one of the earliest developments was probably the formation of epithelial cell sheets, which separate the internal space of the body from the exterior. In addition to epithelial cells, primitive differentiated cell types would have included nerve cells, muscle cells, and connective tissue cells, all of which can be found in very simple present-day animals. The evolution of higher animals and plants (Figure 1-38) depended on production of an increasing number of specialized cell types and more sophisticated methods of coordination among them, reflecting an increasingly elaborate system of controls over gene expression in the individual component cells.
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