How do brains evolve to use new sense organs?

I recently watched a documentary about the evolution of eyes. Beginning with simple photoreceptive cells in jellyfish-like creatures, the organ eventually developed into compound and camera-style eyes, according to the documentary. I found it really interesting, but it got me wondering: How exactly does a creature’s brain (or general nervous system) derive advantage from the new sensory organ?

For example, if a jellyfish suddenly develops simple eyes, its nervous system receives a signal from the photoreceptive cells. How would the nervous system, not adapted to receiving this information, be able to make use of the new input? What if the initial response to the new signal actually caused the creature to behave detrimentally – wouldn’t that cause the presence of the organ to be selected against?

tl;dr: How would improvements in eye “hardware” be selected for if the “software” is not yet adapted to make effective use of it?

Gradually. . . .

Let me rephrase a little. In order for the brain to begin evolving to make use of the organ, the organ has to exist. But the organ does not present a survival advantage until the brain can use the organ. So, the organ (or the improvement) would have to exist in the organism, being largely useless, until a change in the nervous system makes beneficial use of the organ. Thus, until the brain catches up to the organ, the organ is not beneficial and so not selected for. Why, then, would the organ persist long enough for the brain to evolve?

Brains - neural networks or what - adapt incredibly quickly to novel changes in input. They are pre-wired for it. [They are already evolved to use ‘new’ sense organs.]

[The most startling section here is on echolocation in the blind.]

Well, a creature as complex as a jellyfish is not going to have a new sense organ pop up out of nowhere. Eyes probable started as a photosensitive chemical spot in a one celled organism, and probably even more primitively than that, with just a tendency to be attracted to the side that was lit by chemical reactions. There was a really interesting PBS video on slime molds, where these essentially one cell organisms grow and move towards food sources in a very intentional manner with no nervous system at all.

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This still assumes piecemeal or sudden additions of stuff, and is exactly what evolution itself tells us never happens. For example, the phrase…

already betrays a fatal misunderstanding of evolution. Nothing “suddenly develops”. Phil already touched on this too. To add to that … sensory organs and nervous system would all be developing together. One doesn’t “pop into existence” and then have to somehow prepare for a dramatic entrance of a hitherto entirely new sense. After all, any brain is useless in the absence of any input stimuli whatsoever. It needs to have something to work with. And it will develop to use whatever rudimentary somethings there are (or more likely a single rudimentary something) in the best possible way. … “move toward necessary stuff” … “move away from harmful stuff”. Any other very simple sensory stimuli in its most rudimentary form will also be part and parcel of the nervous system that hosts it or makes use of it. You wouldn’t be able to tease them apart just as our “vision processing center” now is not something separate from our brain. Richard Dawkins does a good job describing the process specific to vision development in his good book: “Climbing Mt. Improbable.”

Welcome to the forum, by the way!


Hi, thanks for your answer! The idea of the eye “suddenly developing” was actually derived from the documentary; it proposed that animal eyes may have originated from accidental genetic transfer from a dinoflagellate-like creature into a jellyfish-like creature’s reproductive cells. I should have made that more clear.

However, your criticism is still valid, as this would have only been the gene for rhodopsin, and likely doesn’t represent the development of any eye-like organs. Thanks!


It is worth mentioning that there are single celled organisms that have an eye:

As to multicellular organisms, the central nervous system would have preceded the eye. Therefore, the first eyes would have been cells that stimulated a touch receptor after absorbing light. Individuals who were able to take advantage of this stimulus would have had more offspring. For example, a small worm that sensed light as touch may move around until it gets into the shade at which time the sensation stops. This could help the worm escape detection.

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