Origin of neurons revealed using tiny marine organisms

Researchers at the Center for Genomic Regulation in Barcelona have found evidence that specialized secretory cells found in ancient millimeter-sized marine animals gave rise to neurons in more complex animals. 

The scientists studied tiny multicellular organisms from a type of platypus (Placozoa) common in shallow waters in the tropics and subtropics. Animals without distinct organs, resembling blobs and feeding on algae and microbes, appeared on Earth about 800 million years ago. Plastids coordinate behavior thanks to peptidergic cells, special types of cells that secrete small peptides. These molecules control the animal's movement and feeding.

The researchers mapped all the cell types of the plastids, noting their characteristics in four different species. Each cell type has a specific role that is determined by a set of genes. The scientists created a map of the regulatory regions of DNA that each gene module controls, revealing a clear picture of what each cell does and how they work together. 

The main nine plastid cell types are linked by many "intermediate" cell types that change from one type to the next. The cells grow and divide to maintain the balance the animal needs to move and eat. The researchers also found fourteen types of peptidergic cells that were different from all others, showing no intermediate types or any signs of growth or division. 

The scientists showed that the peptidergic cells had much in common with neurons, cells of the nervous system that appeared many millions of years later in more evolved animals such as bilaterians. The signals that control the formation of such cells resemble neurogenesis. They also contain many of the gene modules needed to build the part of the neuron that can send a message. Finally, chemical signals resembling brain neuropeptides are used for interaction.

The researchers believe that peptidergic cells provided the basis for the evolution of the neuron. Early cells communicated using neuropeptides and, through mutations, acquired new gene modules that allowed them to create postsynaptic scaffolds, form axons and dendrites and ion channels that generate fast electrical signals.