Comb Jellies Had Something Like a Brain 550 Million Years Ago

Imagine trying to understand the origin of the human brain by studying an animal that has no brain, does not share our distant relatives, and lives by pulsing through the ocean on eight combs of iridescent cilia. That is, more or less, what biologists who study ctenophores are trying to do.

Ctenophores - comb jellies - are gelatinous marine animals that diverged from the rest of the animal kingdom roughly 550 million years ago. Depending on which phylogenetic analysis you believe, they may be the most ancient animals still alive. And while they lack the centralized nervous system that connects sensory input to motor output in most animals we think of as complex, they have something called the aboral organ - a small, gravity-sensing structure at one end of the body that has long been a source of debate about what, exactly, the earliest nervous systems looked like.

17 cell types, 11 of them previously unknown

A team led by Pawel Burkhardt at the Michael Sars Centre, University of Bergen, teamed up with Maike Kittelmann at Oxford Brookes University to produce high-resolution three-dimensional reconstructions of the aboral organ using volume electron microscopy - a technique that images tissue at nanometer resolution across thousands of serial sections. The results, published March 4 in Science Advances, revealed 17 distinct cell types within the organ, including 11 that had never been documented before.

That diversity is striking. Most sensory organs in the animal kingdom are functionally specialized structures built around a small number of cell types. The aboral organ, it turns out, is a multimodal sensory system - capable of integrating information about gravity, pressure, and possibly light through a remarkable variety of cellular machinery.

"I was amazed almost immediately by the morphological diversity of the aboral organ cells," said Anna Ferraioli, a postdoctoral researcher at the Michael Sars Centre and the study's first author. "The AO has a striking complexity when compared to apical organs of cnidarian and bilaterian. It is so unique."

A hybrid communication system - synapses and chemical clouds

Beyond counting cell types, the imaging data revealed something more functionally important: the aboral organ is tightly integrated with the ctenophore's nervous system - a continuous network of fused neurons that differs from the more familiar discrete-cell nerve nets of other animals. This nerve net forms direct synaptic contacts with cells of the aboral organ, establishing a clear path for two-way communication between the sensory structure and the broader neural network.

Many aboral organ cells also contain abundant vesicles - small membrane-bound sacs that release chemical signals. This points to what biologists call volume transmission, where signals diffuse broadly through tissue rather than traveling along dedicated wires. The combination creates what the authors describe as a hybrid signaling system, one that uses both the targeted precision of synapses and the diffuse reach of chemical clouds.

"I think our work provides an important perspective on how much we can learn from studying morphology," Ferraioli said. "I would say that the AO is definitely not like our brain, but it could be defined as the organ that ctenophores use as a brain."

Evolution may have invented centralized nervous systems more than once

The researchers also looked at how developmental genes are expressed in ctenophores. Many genes that define body organization in other animals - bilaterian "patterning" genes - are present in ctenophores, but their expression patterns are considerably different. This suggests the aboral organ may not be directly homologous to brains in other animal lineages.

"Evolution seems to have invented centralized nervous systems more than once," Burkhardt said - a concept known as convergent evolution. If true, it would mean that the basic logic of having a dedicated sensory-processing center is not a one-time invention passed down from a common ancestor, but a solution that different lineages stumbled upon independently when the selective pressure demanded it.

Complementary support for this view comes from a separate study by Kei Jokura at the National Institute for Basic Biology, Japan, and Gaspar Jekely at Heidelberg University - a collaboration that also involved Burkhardt. That team reconstructed the complete neural wiring of the ctenophore's gravity-sensing system, combining high-speed imaging with three-dimensional reconstructions of over 1,000 cells. They showed how fused neurons coordinate ciliary beating to let the animal maintain its orientation in water - and found similarities to gravity-sensing circuits in very different marine organisms, consistent with independent evolution of comparable solutions.

What comes next

The aboral organ's 11 newly identified cell types remain molecularly uncharacterized. Ferraioli says the next step is to establish their molecular identities - what proteins they express, what signals they send - and to test more directly whether the organ modulates specific behaviors. Those experiments will likely determine whether the hybrid signaling system is a general feature of early nervous systems or something peculiar to ctenophores alone.

Either way, a 550-million-year-old gelatinous animal has just handed neuroscientists a more complicated story about where brains come from.