show Abstracthide AbstractThe origin of animals, which occurred over 600 million years ago, left no evidence in the fossil record. To trace the earliest events in animal prehistory, we compare extant animals to their closest living relatives, the choanoflagellates, in order to reconstruct the gene content of their last common ancestor and how it evolved on the stem lineage leading to animals. In this project, we increase the accuracy of ancestral animal and choanoflagellate gene content reconstructions by sequencing the transcriptomes of 19 species of choanoflagellates selected for their phylogenetic diversity.