What two pieces of evidence led to the discovery of how an ice age can happen?

Two key pieces of evidence that led to the discovery of how an ice age can happen are the study of ice cores, which provide a record of past climate conditions, and the identification of Milankovitch cycles, which are variations in Earth's orbit and axial tilt that can influence climate over long periods of time. These pieces of evidence helped scientists better understand the mechanisms that can trigger ice ages.