You’ve heard of stem cell research and its promise of a medical revolution given the regenerative abilities of stem cells. But as it turns out, identifying what a stem cell is experimentally is not at all straightforward. Stem cells have two main abilities: cell renewal (division and reproduction) and cell differentiation (development into more specialized cells). The main problem is, there is no way to experimentally test whether one particular cell can both self-renew and differentiate to make more developed kinds of cells. Much like Heisenberg’s uncertainty principle, according to which we can’t measure a particle’s velocity and position at the same time, we can’t measure both properties that constitute a stem cell. Claims that any single cell is a stem cell are therefore inevitably uncertain, argues Melinda Bonnie Fagan.

The term “stem cell,” like so many others in biology, was introduced by Ernst Haeckel. In Haeckel’s speculative vision, an individual organism’s development (ontogeny) mirrors Darwinian evolution of diverse species (phylogeny), both processes emanating from a single “Stammzelle.” So the term originally referred to the root of the Tree of Life, for all species and for each living organism. The meaning of “stem cell” today is somewhat different. For one thing, the evolutionary aspect is has been jettisoned. Development is no longer thought to mirror evolution, and the two are now studied (for the most part) separately. Also, since Haeckel’s day, laboratory research on organisms, cells, and genetic material has generated enormous amounts of knowledge about development, especially in “model organisms” like fruit flies and inbred mice.