Single-celled organisms can sense and react. Much of what they do counts as behavior only in a very broad sense, but they can control how they move and what chemicals they make, in response to what they detect going on around them. In order for any organism to do this, one part of it must be receptive, able to see or smell or hear, and another part must be active, able to make something useful happen.
The organism must also establish a connection of some sort, an arc, between these two parts.#3582•
One of the best-studied systems of this kind is seen in the familiar E. coli bacteria, which live in vast numbers inside and around us. E. coli has a sense of taste, or smell; it can detect welcome and unwelcome chemicals around it, and it can react by moving toward concentrations of some chemicals and away from others.#3583•
An E. coli bacterium has two main motions: it can run or tumble. When it runs, it moves in a straight line, and when it tumbles, as you might expect, it randomly changes direction. A cell continually switches between these two activities, but if it detects an increasing concentration of food, its tumbling is reduced.#3587•
Organisms so small have a difficult time determining the direction light is coming from, let alone focusing an image, but a range of single-celled eukaryotes, and perhaps a few remarkable bacteria, do have the beginnings of seeing#3591•
But we're about to look at the transition from single-celled life to many-celled life. Once that transition is under way, the signaling and sensing that connected one organism to another become the basis of new interactions which take place within the new forms of life now emerging. Sensing and signaling between organisms gives rise to sensing and signaling within an organism.
A cell's means for sensing the external environment become a means to sense what other cells within the same organism are up to, and what they might be saying.
A cell's "environment" is largely made up of other cells, and the viability of the new, larger organism will depend on coordination between these parts.#3586•