Stem Cells

Stem cells are undifferentiated cells - they haven't committed to becoming a specific type yet. They're flexible, uncommitted, capable of becoming whatever the organism needs. And they can self-renew, making more stem cells while also differentiating into specialized cells.

Stem cells vary in flexibility. Some can become anything - every cell type, even a whole organism (early embryonic cells). Others are more limited - they can become several related types but not everything (adult stem cells in bone marrow can make various blood cells, but not liver cells or neurons).

You still have stem cells right now, primarily in your bone marrow, churning out about 200 billion red blood cells daily. When you get a cut, stem cells in your skin activate and differentiate to heal the wound.

But here's the constraint: you can't maintain unlimited stem cells. The more cells remain undifferentiated, the fewer are doing specialized work. An organism made entirely of stem cells would be infinitely flexible and completely useless. The trade-off is between flexibility (uncommitted capacity) and efficiency (specialized function).

Healthy organisms maintain a small percentage of stem cells - just enough flexibility to respond to emergencies and changing conditions, but not so much that they sacrifice function for potential.