A slime mold’s remembrance of things past

The paper “Encoding memory in tube diameter hierarchy of living flow network” by Kramar and Alim (1) may be an example of the unbridgeable gulf between physics and biology; I am not sure, but it may be. Kramar and Alim study how the giant unicellular slime mold Physarum polycephalum develops an associative memory by adjusting wall stiffness and removal of the tubing network to encode the location of a nutrient source. The paper is primarily a “how” and “what” physics-based model of how the mold remembers where nutrients are, and somewhat skirts the “why” question, as physicists are apt to do. Let me explain. In physics, the “why” question is forbidden; the “how” and “what” questions are perfectly OK. For physicists, things are what they are; they don’t exist for a reason. Things are different in biology: While the “how” and “what” questions are the primary ones as in physics, the “why” question is tolerted by evolutionary biologists. The tolerance comes from an understanding of evolution and how it has shaped biology. Billions of years ago, amazingly, the first self-reproducing ur-cell appeared, with the remarkable ability to reproduce itself based on an internal set of instructions, presumably some form of double-stranded DNA, or some other stable polymer, and able to store a long chain of information. Importantly, this chain of instructions can be mutated, and progeny can be produced with different instructions, and hence different phenotypes, from the parent. Darwin’s natural selection and subtle mathematics dictated that this ur-cell would then evolve into the millions of different species we see … [↵][1]1Email: austin{at}princeton.edu. [1]: #xref-corresp-1-1