A study in the journal Cell sheds new light on the evolution of neurons, focusing on the placozoans, a millimeter-sized marine animal. Researchers at the Centre for Genomic Regulation in Barcelona, Spain find evidence that specialized secretory cells found in these unique and ancient creatures may have given rise to neurons in more complex animals. The open-access Cell article was published on September 19, 2023 and is titled “Stepwise Emergence of the Neuronal Gene Expression Program in Early Animal Evolution.” Placozoans are tiny animals, around the size of a large grain of sand, which graze on algae and microbes living on the surface of rocks and other substrates found in shallow, warm seas. The blob-like and pancake-shaped creatures are so simple that they live without any body parts or organs. These animals, thought to have first appeared on Earth around 800 million years ago, are one of the five main lineages of animals alongside Ctenophora (comb jellies), Porifera (sponges), Cnidaria (corals, sea anemones and jellyfish) and Bilateria (all other animals). The sea creatures coordinate their behavior thanks to peptidergic cells, special types of cells that release small peptides which can direct the animal’s movement or feeding. Driven by the intrigue of the origin of these cells, the authors of the study employed an array of molecular techniques and computational models to understand how placozoan cell types evolved and piece together how our ancient ancestors might have looked and functioned.