When biologists speak of "simple" animals, they typically mean organisms with few cell types, basic body plans and limited nervous systems. The comb jelly — also known by its scientific phylum name, ctenophora — has historically qualified on most of these counts. Small, gelatinous and without the brain structures found in most of the animal kingdom, it was treated as a living fossil: an evolutionary throwback occupying a lower rung of the animal hierarchy.
New research is dismantling that characterisation. A team of marine biologists from institutions including the University of Cambridge and several collaborating European institutes has completed the most detailed analysis of comb jelly neural architecture ever undertaken, and the results suggest a degree of complexity that has no parallel among animals of similar body composition.
A Different Kind of Nervous System
What makes the finding particularly striking is not simply that the comb jelly's nervous system is more elaborate than expected, but that it appears to have evolved on an entirely independent trajectory from the nervous systems of other animals. Where most neural complexity is built from neurons using glutamate or acetylcholine as neurotransmitters — the chemical messengers that carry signals between nerve cells — the comb jelly uses an entirely different set of molecules to achieve similar functional outcomes.
This is significant because it implies that neural complexity, far from being a single invention in the history of life, may have arisen at least twice. If confirmed, this would represent one of the most remarkable examples of convergent evolution ever documented: two entirely separate lineages of life independently arriving at the same solution to the problem of coordinating behaviour in a complex body.
"What the ctenophore nervous system tells us is that there is more than one way to build a brain. Nature found a second path to complexity that we knew nothing about until very recently, and understanding that path may teach us things about our own nervous systems we could not have guessed."
What Was Found
Using a combination of electron microscopy and advanced computational imaging, the team reconstructed a near-complete wiring diagram of the neural tissue in a juvenile comb jelly specimen. They identified more than 150 distinct neural cell types — a number that approaches the diversity found in some simple vertebrate brains. Each cell type appeared to perform a distinct functional role, with some specialised for processing sensory information and others devoted to coordinating the rhythmic beating of the creature's comb rows, the hair-like structures that propel it through the water.
Particularly surprising was the identification of what the researchers describe as a "distributed processing" architecture. Rather than centralising neural computation in a single ganglion or brain structure, the comb jelly appears to distribute processing tasks across multiple semi-autonomous neural clusters arranged around its body. This arrangement has some parallels with the neural architecture of octopuses, where a significant portion of cognitive function is performed by neural ganglia in the arms rather than the central brain.
Implications for Evolutionary Biology
The findings have immediate implications for ongoing debates about the earliest origins of animal nervous systems. One school of thought holds that all animal nervous systems descend from a single ancestral neural network that predates the divergence of the major animal lineages. The comb jelly's distinctly different molecular toolkit challenges this view, suggesting either that the common ancestor lacked neurons altogether, or that the ctenophore lineage lost its ancestral nervous system and evolved an entirely new one — a hypothesis that, if true, would itself be biologically extraordinary.
For British marine biologists, the findings also raise fresh questions about species found in home waters. Several comb jelly species are regularly observed in the North Sea and around the coasts of Scotland and Wales, and their population dynamics have in recent years been linked to broader changes in marine food web structure. Understanding the sophistication of their neural function may inform how researchers model and predict their behaviour in response to environmental change.
The research team plans to extend their mapping work to adult specimens and to other ctenophore species, with the aim of establishing whether the complexity they have observed is consistent across the phylum or represents an outlier in a particular species.
