  A cardinal feature common to embryonic development and tissue reorganization,
as well as to wound healing and cancer cell invasion, is collective cellular
migration. During collective migratory events the phenomena of cell jamming and
unjamming are increasingly recognized, and underlying mechanical, genomic,
transcriptional, and signaling events are increasingly coming to light. In this
brief perspective I propose a synthesis that brings together for the first time
two key concepts. On the one hand, it has been suggested that the unjammed
phase of the cellular collective evolved under a selective pressure favoring
fluid-like migratory dynamics as would be required so as to accommodate
episodes of tissue evolution, development, plasticity, and repair. Being
dynamic, such an unjammed phase is expected to be energetically expensive
compared with the jammed phase, which evolved under a selective pressure
favoring a solid-like homeostatic regime that, by comparison, is non-migratory
but energetically economical and mechanically stable. On the other hand, well
before the discovery of cell jamming Kauffman proposed the general biological
principle that living systems exist in a solid regime near the edge of chaos,
and that natural selection achieves and sustains such a poised state. Here I
propose that, in certain systems at least, this poised state as predicted in
the abstract by Kaufmann is realized in the particular by the jammed regime
just at the brink of unjamming.
