Origins of Multicellularity: What Slime Molds Tell Us About the First Animals

One of the greatest “jumps” in the history of life was the transition from single-celled organisms to complex, multicellular animals. For billions of years, life on Earth was a solitary affair—individual cells competing for resources. Then, roughly 600 million years ago, cells began to cooperate, eventually forming the tissues, organs, and brains we see today.

Slime molds are a living “time capsule” of this transition. They provide researchers with a unique model for understanding how individual cells first learned to “glom together” and function as a single, coordinated entity.

The Social Amoeba Strategy

While Physarum polycephalum often exists as a single, giant coenocyte, many other slime molds (like Dictyostelium) begin their lives as independent, microscopic amoebae.

1. Independence: As long as there is plenty of food, these cells live solitary lives.
2. The Crisis: When food becomes scarce, the cells release a chemical “SOS” signal (cyclic AMP).
3. The Glomming: Thousands of independent cells begin to crawl toward the signal, merging into a single, mobile “slug” (pseudoplasmodium).

This “voluntary multicellularity” is the bridge between the single-cell world and the animal world.

Communication Without a Nervous System

The mystery for evolutionary biologists is how these thousands of cells decide what to do once they merge. In a human body, a nervous system tells cells how to differentiate. In a slime mold swarm:

• Chemical Democracy: Information is shared through the fluid cytoplasm. A discovery by one cell (like the location of food) is transmitted to the rest of the mass via rhythmic chemical pulses.
• Role Assignment: Even without a brain, the merged mass decides which cells will form the “stalk” and which will become the “spores.” The “stalk” cells eventually die to allow the “spores” to be carried away by the wind—one of the earliest examples of altruism in the biological world.

The Missing Link: Coenocytes vs. Multicellularity

Slime molds like Physarum take a slightly different approach to complexity. Instead of thousands of cells joining together, they allow their single nucleus to divide millions of times within one shared body.

• The Coenocyte Advantage: This “Super-Cell” architecture allows for massive scale without the need for complex cell-to-cell junctions found in animals.
• Evolutionary Parallel: Both the “swarming” strategy and the “coenocyte” strategy show that life has multiple ways to achieve large-scale coordination.

Why It Matters: Reframing Intelligence

By studying slime molds, we realize that “intelligence” didn’t start with the first brain. It started when the first single cells learned to process information as a group. Slime molds help us realize that we are essentially just “civilized swarms” of cells that have perfected the communication techniques first invented by the blob hundreds of millions of years ago.

---

Want to learn more about the biology of the super-cell? Read our guide on The Physics of the Coenocyte.

---

Origin and E-E-A-T

• Source: SciShow: “Slime Mold: A Brainless Blob that Seems Smart.”
• Key Organism: Dictyostelium discoideum (Social Amoeba) and Physarum.
• Evolutionary Concept: Transition from unicellularity to multicellularity.