Humans have the highest prenatal growth rate of all extant primates, but how this exceptional rate came about has been a mystery up to now. Leslea Hlusko, a scientist at the Centro Nacional de Investigación sobre la Evolución Humana (CENIEH), has participated in a study led by Tesla Monson, a paleoanthropologist at Western Washington University (WWU) in the United States), looking at teeth, prenatal growth rates, and the evolution of pregnancy. This research has uncovered a key piece of this jigsaw in an unexpected place: the relative sizes of fossilized molars.