IN 1697, John Dryden, translating the Roman poet Virgil, wrote that “From one root the rising stem bestows a wood of leaves, and vi’let-purple boughs.” More than 150 years later, the German naturalist Ernst Haeckel used the green stem of a plant that rises into unfolding complexity as an analogy for the diversification of life from single-celled ancestors, which he called Stammzellen: stem cells. 

Haeckel believed that the development of a foetus followed a sequence corresponding to life’s historical evolution — from a single cell through a series of progressively more complex forms — often summarised in the phrase “ontogeny recapitulates phylogeny.” The single cell at the root of this sequence, and thus analogous to the single-celled organisms, was the fertilised egg. This Stammzell gave rise through repeated divisions to the approximately two hundred different cell types that make up a human body: from muscle cells to red blood cells, neurons to the liver. This definition of stem cell indicated that potential. Others later extended the definition again, to cover any cell capable of differentiating into different types. 

A stem cell is a bundle of potential: scientists call it “pluripotent.” The genome inside all human cells is the same, or very close to it.