Evolution Takes a Detour in Comb Jellies


New research into comb jelly genetics suggests that all of the animal kingdom may not have evolved from a single ancestor, as evolutionary theory states now. Comb jellies may represent a new branch of the tree of life, a branch that diverged from the rest of the animal kingdom in the early days of  animal life. If this is true, then evolution did take a detour in comb jellies.

According to a University of Florida study published Wednesday in the journal Nature, the translucent comb jelly, a ctenophore in scientific terminology, has a type of nervous system found nowhere else in the animal kingdom. This fact suggests that the jellies evolved separately from the rest of the animal kingdom. The ancestor of all animals has not been identified but is thought by some evolutionary biologists to be a sea sponge.

Neurobiologist Leonard Moroz, the study’s author, called this discovery a paradox because the animals have a complex nervous system built on a unique “chemical language.”

Moroz and his team mapped the genes of 10 comb jelly species. They found that the jellies do not use neurotransmitters like serotonin and dopamine found in other animals. Instead, they use bundles of unique neural cells. Comb jellies lack many of the genes other animals use to guide nervous system development. Evolutionary biologists had been operating on the assumption that there is only one way to make a nervous system.

All animals came from a single ancestor, but it is not clear what branches broke off first. This explains current thinking that a sea sponge was the ancestor of all modern animals. Early animals evolved over time to become more complex. Once the first systems of neurons develop, the networks grew in size and complexity to produce the types of brains found in modern animals.

The cells of the earliest animals could transmit signals directly to other cells. That communication ability built animal brains. About 550 million years ago, comb jellies went a different route, leading to parallel evolution. One line of evolution, call it a detour, produced modern comb jellies and the other route produced all other modern animals.

That theory is supported by previous research at the National Institutes of Health (NIH) in Bethesda, Maryland. In December, NIH researchers came to a similar conclusion about early evolution, suggesting that a comb jelly rather than a sea sponge might be the oldest member of the animal kingdom. Moroz and his colleagues suggest that comb jellies might need to be placed on a new branch of the tree of life.

Ctenophores range from a few centimeters to 1.5 meters in length. Aside from this unique nervous system, ctenophores are noteworthy for being the largest animals that move using cilia, or “combs.” Some ctenophores also have the ability to regenerate lost body parts including their brains.

Studying that regenerative ability might lead to new ways to study brain diseases like Alzheimer’s and Parkinson’s. If comb jellies can regrow brains, there might be hope of finding a way to use that mechanism as a treatment for human neurological disorders like Alzheimer’s and Parkinson’s. Current treatments can only slow the progression of those diseases.

Evolution, at least our understanding of it, appears to have taken a detour in the development of comb jellies with their novel type of nervous system. Research on comb jellies does show that the animal kingdom took a dramatic side road on the path to modern animals.

By Chester Davis


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