In their publication in Science, Professor Guojun Sheng (Kumamoto University, Japan), Professor Alfonso Martinez Arias (Universidad Pompeu Fabra, Spain) and Professor Ann Sutherland (University of Virginia Health System, USA) offer a phylogenetic and ontogenetic overview of the primitive streak and its role in mediating amniote (vertebrate animals that develop on land) gastrulation, and discuss the implications of embryonic stem cell-based models of early mammalian embryogenesis on the function of this structure.