  About 45,000 years ago, symbolic and technological complexity of human
artefacts increased drastically. Computer simulations of Powell, Shennan and
Thomas (2009) explained it through an increase of the population density,
facilitating the spread of information about useful innovations. We simplify
this demographic model and make it more similar to standard physics models. For
this purpose, we assume that bands (extended families) of stone-age humans were
distributed randomly on a square lattice such that each lattice site is
randomly occupied with probability p and empty with probability 1-p.
Information spreads randomly from an occupied site to one of its occupied
neighbours. If we wait long enough, information spreads from one side of the
lattice to the opposite site if and only if p is larger than the percolation
threshold; this process was called "ant in the labyrinth" by deGennes 1976. We
modify it by giving the diffusing information a finite lifetime, which shifts
the threshold upwards.
