Why cancer is still so prevalent in many species — and why Hydra breaks the rule 🧬 Cancer is often framed as a failure of biology, but in evolutionary terms it’s a by-product of multicellularity. Any organism built from billions of dividing cells faces a trade-off: rapid growth, tissue repair, and long lifespan all increase the chance that some cells accumulate mutations and escape control. Enter the Hydra effect. Freshwater hydra (Hydra vulgaris) show negligible senescence — they don’t measurably age, and spontaneous tumors are extremely rare. Why? Hydra are powered by constantly active adult stem cells that divide symmetrically and are continuously culled by apoptosis (programmed cell death). Instead of “protecting” old cells, hydra replace everything, all the time. There’s no long-term cellular memory for mutations to hide in. Most animals (including humans) evolved the opposite strategy: • Long-lived differentiated cells • Restricted stem cell pools • Strong tumor suppression ...