Confocal microscopy image of nuclei, coloured by depth, of Trichoplax sp. H2, one of the four species of placozoan for which the authors of the study created a cell atlas for. Credit: Sebastian R. Najle/Centro de Regulación Genómica
Key points:
- A study on an animal as small as a grain of sand has revealed information about human brain cell formation.
- The authors say the study demonstrates that the building blocks of the neuron were forming 800 million years ago in ancestral animals.
- The researchers believe the origins of neurons will become clear as scientists continue to sequence high-quality genomes from diverse species.
A study in the journal Cell sheds new light on the evolution of neurons, focusing on placozoans, a millimeter-sized marine animal. These animals, thought to have first appeared on Earth around 800 million years ago, are one of the five main lineages of animals. They are so simple they have no body parts of organs.
The sea creatures coordinate their behavior thanks to peptidergic cells, special types of cells that release small peptides that can direct the animal’s movement or feeding. Focusing on these cells, the researchers first made a map of all the different placozoan cell types, annotating their characteristics across four different species.
Surprisingly, the peptidergic cells shared many similarities to neurons—a cell type that didn’t appear until many millions of years later in more advanced animals. Cross-species analyses revealed these similarities are unique to placozoans and do not appear in other early-branching animals, such as sponges or comb jellies (ctenophores).
The authors say the study demonstrates that the building blocks of the neuron were forming 800 million years ago in ancestral animals grazing inconspicuously in the shallow seas of ancient Earth. From an evolutionary point of view, early neurons might have started as something like the peptidergic secretory cells of today’s placozoans. These cells communicated using neuropeptides, but eventually gained new gene modules which enabled cells to create post-synaptic scaffolds, form axons and dendrites and create ion channels that generate fast electrical signals – innovations which were critical for the dawn of the neuron around one hundred million years after the ancestors of placozoans first appeared on Earth.
However, the complete evolutionary story of nerve systems is still to be told. The first modern neuron is thought to have originated in the common ancestor of cnidarians and bilaterians around 650 million years ago. And yet, neuronal-like cells exist in ctenophores, although they have important structural differences and lack the expression of most genes found in modern neurons. The presence of some of these neuronal genes in the cells of placozoans and their absence in ctenophores raises fresh questions about the evolutionary trajectory of neurons.
The authors of the study believe that, as researchers around the world continue to sequence high-quality genomes from diverse species, the origins of neurons and the evolution of other cell types will become increasingly clear.