Evidence Suggests that Ancient Marine Animals May Have Helped Shape the Evolution of Neurons

A recent study published in the journal Cell explores the evolution of neurons in animals, focusing on placozoans, millimeter-sized marine animals. Researchers at the Center for Genomic Regulation in Barcelona have discovered that specialized secretory cells found in placozoans could potentially be the precursors to neurons in more complex animals.

Placozoans are simple creatures that lack body parts or organs. They are approximately the size of a large grain of sand and feed on algae and microbes in shallow, warm seas. These animals are estimated to have first appeared on Earth around 800 million years ago and are one of the five main lineages of animals.

The researchers used various molecular techniques and computational models to map the different cell types in placozoans and understand how they evolved. They created “cell atlases” that allowed them to identify clusters or “modules” of genes associated with specific cell types. By comparing different species, they reconstructed how these cells evolved over time.

The study revealed that placozoans have nine main cell types connected by intermediate cell types that transition between them. These cells grow and divide to maintain the balance necessary for the animal to move and eat. Interestingly, the researchers discovered a distinct group of peptidergic cells in placozoans that share similarities with neurons. These similarities were not found in other early-branching animals such as sponges or comb jellies.

The researchers found that the peptidergic cells in placozoans differentiate similarly to the process of neurogenesis in more advanced animals. They also have gene modules required for the pre-synaptic scaffold of a neuron, which allows them to send out messages. However, they lack the components necessary for the receiving end of a neuronal message or for conducting electrical signals.

Additionally, the study showed that placozoan cell types communicate with each other using specific proteins called GPCRs and neuropeptides, similar to how neurons communicate using neuropeptides in various physiological processes.

The findings suggest that the evolution of neurons began around 800 million years ago with the emergence of secretory cells in placozoans. Over time, these cells gained new gene modules that allowed for the development of post-synaptic scaffolds, axons, dendrites, and ion channels, leading to the formation of true neurons.

This study sheds light on the early stages of neuron evolution and provides insights into the origins of complex nervous systems seen in more advanced animals today.

– [Cell Journal](https://cell.com)
– [Center for Genomic Regulation](https://www.crg.eu/)