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The evolution of life on this planet required organisms to sense chemicals suspended or dissolved in water for their survival. Some such chemicals were destructive and required avoidance, whereas others provided nourishment and sustenance. Single-celled organisms such as the bacterium Escherichia coli that inhabits our own digestive systems, developed multiple chemical receptors that determined whether they should approach or avoid a given situation. In the case of E. coli, for example, the direction of rotation of the flagella—whip-like appendages that propel them through their environment—is influenced by the type of chemical they encounter. Thus, chemicals important for sustenance produce a counterclockwise rotation of the flagella that facilitates a smooth and somewhat linear swimming path, whereas other, seemingly noxious chemicals induce a clockwise flagellar rotation ...

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