A key feature of stable multicellularity is reproductive division of cellular labour. In other words, nearly all multicellular organisms have a dedicated group of cells that are uniquely capable of passing their genetic information on to the next generation. This lineage of cells is called the germ line. In all sexually reproducing animals, it is specified during development, and is required for reproduction. The molecular and developmental mechanisms that ensure the specification of the germ line during development have been elucidated in a small number of animals, but the nature of an ancestral animal mechanism to specify this cell type is unclear. Moreover, while animals display a variety of mechanisms that can specify germ cells, the evolutionary relationship between these mechanisms remains mysterious. Here we address both of these problems by examining and comparing germ line specification mechanisms across a range of different arthropods. We compare data from these studies to those from well-established model systems like the fruit fly Drosophila and the mouse Mus. We propose that co-option of genes and mechanisms ancestrally used for somatic development, may have played an important role in the evolution of germ line specification.