I was recently reading about artificial life and came across the statement, \"Conway’s Game of Life demonstrates enough complexity to be classified as a universal machine.\"
Paul Rendell implemented a Turing machine in Life. Gliders represent signals, and interactions between them are gates and logic that together can create larger components which implement the Turing machine.
Basically, any automatic machinery that can implement AND, OR, and NOT can be combined together in complex enough ways to be Turing-complete. It's not a useful way to compute, but it meets the criteria.