Abstract
Studies of the evolution of axis formation have shown that, consistent with the developmental hourglass model, a great deal of variation occurs at the earliest stages of embryogenesis. This variation has come to be thought of as due to development systems drift, rather than adaptive change. By linking variation in the biology of reproduction in the parental generation with variation in early development, turning the hourglass into a twisted ribbon, I aim to show that early embryo variation is adaptive change shaped by natural selection in the parental generation.