The building blocks of our brain cells may have begun to form back when our ancestors were still a blobby collection of animal cells oozing through the ocean's shallows.

Researchers from Spain and Germany have discovered characteristics of specialized secretory cells in simple animals called placozoans which could identify them as a prelude to neurons in other organisms.

Roughly the size of a grain of sand, these basic creatures have no organs, consisting of little more than a colony of different cell types. Much as they still do today, placozoans once hunted microbes and browsed algae in the warm shallows of seas 800 million years ago.

To achieve this feat, they rely on cells that help coordinate their bodies.

Centre for Genomic Regulation cell biologist Sebastián Najle and colleagues discovered these so-called peptidergic cells have a number of uncannily similar traits to the cells we owe our own smarts to.

"We were astounded by the parallels," exclaims Najle.

"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."

Diagram showing key features of neurons in different animal groups.
Features of early neurons (left) compared to those found in later species (right). (Najle et al., Cell, 2023)

A genetics analysis and microscopic screenings revealed these cells coordinated the tiny animal's behavior by releasing peptide signals much as neurons do.

The researchers identified 14 different types of peptide cells in the small marine animals. Comparing the tissues in four known placozoa species as well as with other early animals, the team found these cells even have some of the same genes as our neurons.

But they're missing other specialized components our neurons have to receive peptide signals or generate electrical signals. Instead they use receptor proteins (GPCR) to receive the chemical messages, which are common across many animal cell types and not specific to neurons.

Nucleus of Trichoplax placozoan with brightly colored fluorescent markers highlighting different molecules. (Najle et al., Cell, 2023)

The genes of the peptide-releasing cells were highly conserved in all the placozoans, but absent in other early animals like sponges and comb jellies (ctenophores).

This suggests the peptide-secreting cells may have evolved before other neuron-like cells.

"Ctenophores have neural nets, with key differences and similarities with our own," explains Xavier Grau-Bové.

"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."

The first known modern neuron arose around 650 million years ago in the common ancestor of jellyfish and the bilaterian group of animals that lead to us.

"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. Placozoans, ctenophores, sponges and other non-traditional model animals harbor secrets that we are only just beginning to unlock," concludes evolutionary biologist Arnau Sebé-Pedros.

This research was published in Cell.