Your Nervous System May Have Evolved from Sponges

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Issue 6, Volume 112

By Andy Chen 

Sponges are rudimentary creatures that lack basic structures present in many other aquatic creatures such as lungs, gills, nerve cells, and muscles. Like the ones you wash dishes with, these aquatic animals absorb and release enormous amounts of water every day, gathering nutrients rather than soap. The mechanism through which sponges do so is not as simple as it sounds, and the sponge’s mastery of this mechanism without a brain shows that they are far from being basic organisms.

A recent study explains that sponges have a sophisticated cell communication system that assists them in absorbing food and defending against bacterial infections. The interactions between the cells resemble how nerve cells and the brain coordinate, hinting at a connection between a sponge’s system and real nervous systems present in higher-order animals, where communications between neurons occur through the transmission of chemical and electrical signals across junctions called synapses. Previous experiments have determined that sponges contain specific genes that code for synapse-supporting proteins.

Figuring out how sponges use these genes and which cells express them are by no means easy, especially considering the sponge’s odd structure. The insides of a sponge consist of many linked channels and digestive chambers, which contain a variety of specialized cells. The unique body plan in sponges is incomparable with other animals, allowing for their extraordinary ability to filter water.

To understand these processes, Detlev Arendt, an evolutionary biologist at the European Molecular Biology Laboratory (EMBL), and his team employed single-cell sequencing on several cells from Spongilla lacustris, a freshwater sponge. The team discovered 18 distinct types of cells in sponges, including some that resembled certain cells found in humans and animals. One type called secretory neuroid cells aids sponges in expanding and contracting, similar to muscle cells in animals. A few other cell types that contained active synaptic genes were scattered around the sponges’ digestive chambers. Specifically, one cell type has active genes that code for synapses that fire signals. The digestive cells next to those express active genes for receiving fired signals. This suggests that some type of cellular communication is responsible for regulating the sponges’ filter feeding. Moreover, the presence of a sender and receiver cell for signals is a hallmark of the nervous system. Hence, this study hints that the first type of nervous system in animals evolved to monitor feeding.

To confirm the relationship between the two cells, Arendt and his colleagues used X-ray imaging and electron microscopy, a process that transmits electrons to amplify an image of a specimen to generate a 3D view of the digestive chambers. They determined that the neuroid cells form arms that come into contact with choanocytes, cells with cilia, or hair-like extensions, that handle water movement and nutrient intake in the sponge. Based on the closeness and connection between the digestive cells and the neuroid cells, the team believes that chemical signals are sent to choanocytes, which may regulate the foreign particles and water that flow into the sponge. If sponges do coordinate their cells similar to the way neurons communicate, it may change the perception that sponges lack neuron-like behaviors. In fact, the relationship between the two cell types may represent an evolutionary precursor of the current nervous system, which used electrical signaling passed through small spaces of synapses rather than chemical messengers.

Other scientists argue that there is little evidence that the same synapses that allow neurons to communicate at rapid speeds are found in sponges that contain neuroid cells. Linda Holland, an evolutionary biologist at the University of California, San Diego, believes that it is unlikely that the nervous system evolved from the sponges’ cellular communication system. Previous theories propose that the nervous system may have developed from an earlier time and perhaps evolved many times already. But none of these claims have been proven, leaving the discussion open.

While the research regarding sponges and their relationship to true nervous systems have been preliminary, the identification of essential cell types will be invaluable to future research in the origins of the nervous system. The discoveries of neuroid cells in the digestive chambers have already become part of the argument that neurons originate from digestive cells. Additionally, the methods of research and findings of this study could be a platform for further inquiries about the relatedness between humans and sponges. Though there have been contrasting views on the implications of this study, the sponge seems to play a role in the evolution of the nervous system one way or another.