How they're made
Five ideas, one cell at a time
01 A row of cells and a rule
A creature lives on a single row of cells. Each cell holds one of a few colors, and one color means empty (black here). At the start, every cell is empty except one.
To make the next row, each cell looks at three cells in the row above: the one directly above it and that cell's two neighbors. The rule is a lookup table that says, for every possible trio of colors, what color the new cell becomes.
That table is the creature's genome. Here is the complete rule for the creature in the figures below. It uses four colors, and each little icon is one entry: three cells on top, the result below.
02 Apply it everywhere, row after row
Every cell updates at once, using the same rule. Time runs down the page: the top row is the single starting cell, and each row below is one step later.
Watch the outlined trio slide across the row. The highlighted entry in the table is the one being looked up, and its result fills the outlined cell.
03 Mirror-image rules make mirror-image bodies
The rules are symmetric: a trio and its mirror image, the same three colors in reverse order, always give the same result. Since the starting cell is symmetric too, the left half of a creature is always the mirror of its right half. That is where the bilateral, animal-like look comes from.
Symmetry also shrinks the genome. Four colors make 4 × 4 × 4 = 64 trios, but only 40 of them can be set independently.
On the right, the same creature with one entry changed for one trio . The body comes out lopsided.
04 Fitness is a long life that ends
A creature's lifetime is the number of rows it lasts before every cell is empty again. Most rules fail one of two ways: they die almost at once, or they never die, spreading forever or settling into a pattern that repeats forever.
The interesting ones live a long time and then stop. That finite lifetime is the creature's fitness, and immortal creatures score zero.
In general there is no way to know in advance whether a rule's pattern will ever die out. It is a version of the halting problem. So in practice, anything still alive after 1,000 steps, or more than 300 cells from where it started, counts as immortal.
05 Evolution: mutate, test, keep
Start from the null rule, whose creature lives exactly one step. Then repeat:
- Mutate. Pick one entry of the rule at random and change its result to a different random color (its mirror image changes with it).
- Grow. Run the mutant from a single cell and measure its lifetime.
- Select. Keep the mutant if it still dies and lives at least as long as the current creature. Otherwise, throw it away.
That's the whole algorithm. There is no designer and no crossover, just one random change at a time and a single number to judge it by.
Keeping ties matters. Most mutations that survive are neutral: they don't change the lifetime, but they let the genome drift until some later mutation finds a longer life. Without that drift, evolution gets stuck almost immediately.