Archaeologists have long used changes in artifact form-in evolutionary terms, changes from one character state to another-to measure the passage of time (Lyman and O’Brien 2006). If evolved character states are ordered correctly, a historical sequence of forms is created, although independent evidence is needed to root the sequence-that is, to determine which end of the sequence is older. Such evidence could come, for example, from chronological dating (e.g., stratigraphy) or historical sources. Over the past several years, archaeologists have grown to appreciate that the methods biologists have developed to reconstruct the evolutionary, or phylogenetic, relationships of species can help them create not only historical sequences of artifact forms-what came before or after what-but sequences based on heritable continuity-what produced what.