this is an applied math question. I could have posted this under a biological stackexchange, but the idea of emergent behavior or emergent properties of a system seems more appropriate to an applied math audience.
The question is why bacterial or uni-cellular chemotaxis represents an "emergent" behavior? I have been reading the original Keller Segel (1971) papers on chemotaxis--the movement of these organisms in response to nutrient gradients in the organism's environment. But I was not sure why this is considered an "emergent" behavior? Emergent compared to what? Is chemotaxis an emergent behavior compared to brownian motion or some sort of straight-line path (f = ma)?
I know that there is no really agreed upon definition of emergent behavior, but any suggestions would be appreciated.