Conway's Game of Life, but in a solid block instead of on a flat sheet. Each cell counts its living neighbours and a birth and survival rule decides whether it lives.
The extra dimension changes things more than you might expect. On a flat grid a cell has eight neighbours. In a block it has twenty-six. That spreads the counts across a much wider range, so a rule tuned for the flat version either fills the whole volume solid within a few steps or dies out immediately. The rules that produce anything interesting in three dimensions are a completely different set, found by search rather than by adapting the famous one.
Alongside whether a cell is alive, its age is tracked, so the renderer can shade freshly born cells differently from settled ones.
The first frame seeds itself, either at random or from an upstream volume.