Why is chemotaxis considered an emergent behavior? 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.

It seems that that the term _emergent_ might refer here to some behaviors or properties which appear _when a number of simple entities (agents) operate in an environment, forming more complex behaviors as a collective_ than was expected in view of the simplicity of the mechanisms ruling them at an individual level.

An example of this perspective in the context of chemotaxis is in the paper Chemotaxis as an Emergent Property of a Swarm by Rion G. Taylor and Roy D. Welch.