More than 40,000 years ago, Ice Age humans were carving repeated patterns of dots, lines, and crosses into tools and small ivory figurines. A new computational study of more than 3,000 of these Paleolithic signs reveals that they were not random decorations but structured sequences with measurable complexity. Surprisingly, their information density rivals that of proto-cuneiform, the earliest known writing system that emerged around 3,000 B.C.E.