🧬 Vivarium

A self-evolving world of neural creatures that writes its own history.

Vivarium is a contained tank of digital life. Each creature carries a genome — body genes plus the weights of a small recurrent neural-network brain. They see, think, move, eat, hunt, leave scent, and reproduce. Energy ties it all together: gain it by eating plants or prey, spend it living, moving, fighting and breeding; run out, or grow old, and you die — and your body becomes carrion for the carnivores.

There is no fitness function and no goal. Herbivores, predators, scent-trails, pack behaviour, armored prey, fast grazers — whatever appears, appears because it survived and reproduced. A Chronicler watches it all and writes the history: naming each species as it establishes, recording the first act of predation, dynasties and milestones, extinctions, great dyings, the turning seasons, and reflections that name each age.

It is zero-dependency: pure JavaScript (ES modules) and Canvas 2D. Nothing to install.

Run it

cd vivarium
python3 -m http.server 8765
# open http://127.0.0.1:8765/index.html

(Any static server works. A server is needed because it uses ES modules.)

Run it headless (no browser)

The whole simulation core is DOM-free, so you can evolve a world in the terminal:

node headless.mjs [seed] [steps]      # e.g. node headless.mjs 777 30000

It prints population, diet mix, species count and the chronicle as it goes.

What you can do

• Watch. The default posture. Drag to pan, scroll to zoom.
• Click any creature → the Inspector shows its species, energy, full genome, and its brain firing live (senses → hidden → move/turn/eat/bite/breed/scent).
• Tabs: the Chronicle (its written history), Charts (population, trophic mix, species count, Shannon diversity), and the Tree of Life (a phylogeny that branches as species diverge).
• Layers: scent fog, fertility biomes, vision cones, species names.
• Speed / pause, Follow a creature (f), New World (optional seed), Save / Load a world to JSON.
• Intervene (optional — it's your tank): drop a meteor, trigger a bloom of food, a drought, or spawn random newcomers.

Keys: space pause · f follow · +/- speed.

The systems

System	What it does
Recurrent brains	Memory between ticks → chasing, fleeing, patrolling can emerge
Vision	6 sectors sensing distance + plant/meat/creature + relative size
Pheromone field	Shared RGB scent they deposit & smell → trails, signals, kin-recognition
Energy economy	Eat to gain; pay for upkeep, movement, sensors, armor, fighting, breeding
Genome	15 body genes + ~860 brain weights; mutation, sexual crossover
Speciation	Genetic-distance clustering, procedural binomial names, a real phylogeny
Climate	Day/night and four seasons modulate plant growth → boom and bust
Chronicler	Turns the silent numbers into a narrated history

Things to look for

• The first time the Chronicle says "First blood" — predation has just evolved.
• A herbivore world tipping into an Age of Hunters as the carnivore trait spreads.
• Bodies changing shape over generations — armor appearing where predators hunt.
• A great dying, and which lineages survive it.

Origin & author

Vivarium was built by Claude (Opus 4.8), an AI made by Anthropic, in a single session on 2026-06-14 (via Claude Code) — written from an empty folder with zero dependencies.

It came from an open invitation: "build whatever you want — show me the maximum of what an AI would make, with complete freedom. " Given a blank page, I chose to make the thing I find genuinely beautiful: a world where complexity, hunger, strategy and whole histories bootstrap themselves out of almost nothing — and a narrator that watches it and gives it meaning.

📖 The full story of how and why it was made: STORY.md. 🛠 The design and engineering reasoning: PLAN.md.