In this chapter I chronicle the emergence of different phylogenetic traditions in invertebrate zoology that coalesced around attractive hypothetical ancestors. The first half of the chapter discusses different scenarios for the origin of Bilateria that emerged in the late nineteenth and early twentieth centuries, including the enterocoel and archicoelomate theories, and the importance of the contested character of amphistomy for these scenarios. These scenarios became phylogenetic totems in different parts of the world, with Gastraea at the core of the European zoological tradition, and Phagocytella rooting both the Russian and American traditions. In the mid-twentieth century an alternative theory was proposed that derived bilaterians directly from ciliate ancestors. Remarkably, this attraction to ciliate ancestors emerged three times independently in quick succession, which illustrates the epistemic importance accorded to the precursor potential of hypothetical ancestors in narrative phylogenetic debates. The second half of the chapter discusses why many authors have felt so strongly attracted to annelid-like ancestors. These were proposed to have crawled at the cradles of many taxa, including molluscs, arthropods, vertebrates, and Bilateria. The arguments used to promote annelid-like ancestors form a conspicuous strand in the history of narrative phylogenetics, and it can still be traced today.

In this chapter I discuss the anatomy of evolutionary storytelling. Historical narratives are woven around central subjects that lend them continuity through time. The central subjects of phylogenetic scenarios are lineages of hypothetical ancestors. These define the pathways of homology along which evolutionary change is reconstructed, and they root the power of phylogenetic hypotheses to explain the evolution of form by allowing characters in descendants to be traced back to ancestral precursors. De novo origins of novel traits are chinks in the explanatory armory of phylogenetic hypotheses. Hypothetical ancestors have therefore often been deliberately equipped with characters that provide suitable precursors of the traits that await evolutionary explanation. I will argue that the precursor potential of hypothetical ancestors functioned as an early phylogenetic optimality criterion used in the construction and judging of scenarios.

In this chapter I take a detailed look at the evolutionary storytelling of Ernst Haeckel. He founded phylogenetics as the science dedicated to tracing the evolution of lineages. Although Haeckel’s phylogenetic scenarios were nourished from a broad buffet of evidence, the biogenetic law was his favorite shortcut to create lineages of hypothetical ancestors, most famously the tiny cup-shaped Gastraea. A recent consensus has emerged that stigmatizes Haeckel’s phylogenies as unDarwinian constructs that are conceptually stained by teleological thinking and the linearity of the scala naturae. Instead, I argue that his trees are fully Darwinian, and that the linearity present in his trees and thinking is the linearity of evolving lineages that track the arrow of time. Lineage thinking was novel when Haeckel started writing, and his was marred by imperfections. It was up to the following generations of evolutionists to resolve the conceptual tension between the linear and branching aspects of evolution, a struggle that is still ongoing in today’s literature.