It is widely accepted that animals evolved from a unicellular ancestor with an apical cilium surrounded by a microvillar collar that structurally resembled present-day sponge choanocytes and choanoflagellates. Here we test this traditional view of the origin of the animal kingdom by obtaining the transcriptomes of three primary sponge cell types choanocytes, mesenchymal pluripotent archeocytes and epithelial pinacocytes, to compare with choanoflagellates and other unicellular holozoans.