Physarum polycephalum as an Active Mechanical Medium

One of the current challenges in Biology is to understand how living systems self-organize to achieve emergent properties that improve fitness. In order to act coherently as a unit, each organism must display system-level dynamics that span it. As organisms evolve and become more sophisticated, they must be able to develop increasingly complex organism-wide behaviors that result from the local activities and interactions of subparts. This can lead to a hierarchy of structures that integrate system behavior. At the cellular scale, these can correspond to metabolic networks or cell signaling networks. In higher organisms, it involves the development of integrating systems that can ensure, for example, the distribution of nutrients, the coherent motion of the individual as a whole, or some level of centralized control. In simpler organisms, however, such coordination of subparts required to integrate system activity, referred to here as coherent dynamics (CD), must be achieved without requiring complex developmental structures. Furthermore, from an evolutionary perspective, various forms of CD were likely required before organisms had evolved integrating structures. Life must therefore take advantage of biophysical mechanisms that spontaneously lead to CD, either as a final solution to achieve organismlevel behavior in simple cases or as a step in developing the required integrating systems. Understanding these mechanisms could not only lead to a detailed description of certain biophysical processes in simple organisms, but it could also provide key insights into the underlying principles that lead to the emergence of structure in living systems, in general.