Placozoans and the Origins of the Nervous System

A Much-Magnified Placozoan Stained to Reveal Different Cell Types

Some fascinating new science concerning the primitive nervous systems of the nearly miscroscopic, pancake-shaped animals known as placozoans. Placozoans have no neueons, yet they make "coordinated responses to stimuli." Scientists have suspected for a long time that they use some kind of chemical signaling to achieve this, and it was confirmed a few years ago that they use small peptides – short chains of amino acids – for this signalling. 

NY Times:

Placozoan bodies are simple, only three cell layers thick. But that’s enough to glide around, absorb and digest food, and respond to their surrounding environment. Instead of being controlled by neurons, some of these behaviors are regulated by peptidergic cells, which release short chains of amino acids that activate surrounding cells.

Because the activity of peptidergic cells is reminiscent of more complex nervous systems — like the one in humans — Dr. Grau-Bové and his colleagues were intrigued by the possibility that these cells and their connections might represent the nervous system of an ancient animal ancestor.

The research team began by analyzing gene expression — which bits of DNA are converted into RNA used to make cell proteins — in more than 65,000 individual cells across four placozoan species. They discovered that placozoans have 14 types of peptidergic cells that are also important for building neurons in cnidarians and bilaterians. However, they also found that peptidergic cells were not true neurons given their lack of electrical activity and inability to receive messages.

The researchers then created a map showing potential interactions between peptidergic cells and other cells in placozoans. They identified a complex signaling network as well as specific pairs of neuropeptides and receptors. These cellular relationships support what scientists call the chemical brain hypothesis, the idea that early nervous systems evolved as networks of cells connected through chemical signals that would diffuse across an animal and bind to specific protein receptors.

They then compared what they had found to the nervous systems of more complex animals like cnidarians and simple bilaterians (we are bilaterians) and found a lot of similarities. Some scientists are now speculating that the ancestor of all animals with nervous sytems was a placozoan, or something like one.

Of course chemical signaling is a lot slower than electrical signalling, which is why placazoans have remained so small. Sponges, which use a similar system, do grow large but they have a sedentary, low-energy lifestyle that makes slow responses (usually by only part of the organism) a viable approach. To grow larger, animals had to evolve proper neurons and a faster signalling system.

The understanding of these primitive "nervous systems" fills in yet another "missing link" in the evolution of animals, showing how yet another supremely complex system could have evolved from very simple origins. As the original paper puts it, "peptidergic volume signaling may have pre-dated synaptic signaling in the evolution of nervous systems."