A brainless city planner
In a 2010 experiment that sounds like science fiction, researchers from Japan and the UK demonstrated a deep form of biological computation. They took a single-celled, brainless slime mold, Physarum polycephalum, and challenged it to design a transport network. The organism didn't just create a functional network; it recreated a design strikingly similar to the Tokyo area rail system, one of the most complex transport networks in the world. The research, led by Atsushi Tero of Hokkaido University, showed how simple biological rules can produce optimized and complex structures.
The experimental setup was elegantly simple. Scientists placed the slime mold in a petri dish where oat flakes—a favorite food—were arranged in the same geographic pattern as 36 cities and major hubs around Tokyo. The organism, a bright yellow amoeboid creature, began to explore its environment from a central point representing Tokyo itself. At first, P. polycephalum spread out indiscriminately, creating a dense web of interconnected protoplasmic tubes. Over a period of about 26 hours, a process of refinement began. The slime mold reinforced the tubes that were most efficient for transporting nutrients between the oat flakes, while redundant or inefficient pathways were pruned away and disappeared. The final structure looked very similar to the actual Tokyo rail map, which took human engineers decades to plan and build.
Elegance in simplicity
The slime mold's success comes from a decentralized process of optimization. It has no central control system and operates on a simple feedback loop. Protoplasm flows through the network of tubes, and tubes with higher flow rates are reinforced and grow thicker. Tubes with less flow shrink and eventually vanish. This mechanism naturally finds a balance between efficiency, cost and resilience against faults. The resulting network was similar to the human-designed one but in some cases even suggested more robust connections.
This amoeba-like organism is technically a single cell, yet it can grow to be several square meters in size and contains millions of nuclei. Its entire body functions as a sensor and processor, constantly adapting its form to its surroundings. To make the experiment more realistic, researchers used light—which P. polycephalum naturally avoids—to represent geographical obstacles like mountains and lakes that would constrain a real-world rail network. The slime mold successfully navigated these "no-go" zones. The principles discovered from this experiment have inspired mathematical models for designing other types of networks, from computer and mobile communication systems to logistics and infrastructure planning.