The western European rediscovery of geometric perspective during the fourteenth century revolutionized the understanding of spatial relationships in general, and the structure of nature in particular. Renaissance artists-naturalists wrought this revolution, and the roots of the earth sciences can be traced to achievements such as Leonardo da Vinci's oldest known work of art, a drawing of the hills of Tuscany. A perspectival analysis of it reveals a distinct sequence of laterally continuous, horizontal strata which scaffold the Tuscan hills the way that bones give structure to the flayed human body in écorché images that Renaissance anatomists such as Vesalius as well as da Vinci produced. The drawing depicts original horizontality. superposition, and lateral continuity nearly two hundred years before Steno defined them in words in his Prodromus. Steno was educated in mathematics and anatomy, and his Prodromus is clearly an attempt to apply the geometric principles he had learned to further advance the Renaissance understanding of the three-dimensional continuity of nature.