Brain cell origins traced to creatures that lived 800 million years ago

A new study reveals that brain cell components were forming in shallow seas around 800 million years ago. Experts at the Centre for Genomic Regulation have traced brain cells back to placozoans, tiny marine animals, shedding light on the evolutionary journey of neurons.

Placozoans, roughly the size of a large grain of sand, feed on algae and microbes in warm, shallow waters. Along with ctenophores, sponges, cnidarians, and bilaterians, placozoans represent one of the five primary animal lineages, even though they lack organs or distinct body parts.

Peptidergic cells 

Central to the placozoans’ existence are peptidergic cells which release peptides directing their feeding and movements. Through extensive molecular and computational analyses, the researchers crafted “cell atlases” to understand the evolution and function of these cells. They were found to be precursors to modern neurons.

The findings indicated a network of “in-between” cell types connecting the main nine cell types in placozoans. 

The peptidergic cells, distinguishable from other cells, bore a surprising resemblance to neurons that emerged millions of years later in advanced organisms. This unique similarity to neurons was not observed in other early-branching species like sponges or ctenophores.

Evolutionary connections

Three notable parallels between peptidergic cells and neurons emerged from the study. First, placozoan cells differentiate in a manner akin to neurogenesis observed in cnidarians and bilaterians. 

Second, while these cells possess many components of a neuron’s messaging end, they lack the attributes of a true neuron, such as conducting electrical signals. 

Furthermore, using deep learning, it was found that these cells communicated through GPCRs (G-protein coupled receptors), which are initiated by neuropeptides, a feature of neuronal communication.

“We were astounded by the parallels,” said Dr. Sebastián R. Najle, co-first author of the study. “The placozoan peptidergic cells have many similarities to primitive neuronal cells, even if they aren’t quite there yet. It’s like looking at an evolutionary stepping stone.”

Critical features

Highlighting the evolutionary timeline, the research indicates that the neuron’s foundational components were taking shape in the ancient seas 800 million years ago. 

Placozoans’ peptidergic cells, with their neuropeptide communication, likely evolved to include features critical for neurons, such as ion channels and post-synaptic scaffolds, about a century after their ancestors first appeared.

The evolutionary narrative of the nervous system remains incomplete. While the first modern neuron is believed to have appeared around 650 million years ago, there are neuronal-like cells in ctenophores with distinct characteristics. Their existence raises significant questions about the evolutionary trajectory of neurons.

Many questions remain

“Placozoans lack neurons, but we’ve now found striking molecular similarities with our neural cells. Ctenophores have neural nets, with key differences and similarities with our own,” explained Dr. Xavier Grau-Bové, co-first author of the study.

“Did neurons evolve once and then diverge, or more than once, in parallel? Are they a mosaic, where each piece has a different origin? These are open questions that remain to be addressed.”

As genomic sequencing progresses, the evolutionary paths of neurons and other cell types are expected to become clearer. 

“Cells are the fundamental units of life, so understanding how they come into being or change over time is key to explain the evolutionary story of life,” said study co-author Professor Arnau Sebé-Pedro. 

“Placozoans, ctenophores, sponges and other non-traditional model animals harbor secrets that we are only just beginning to unlock.” 

The research is published in the journal Cell.

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