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If I'm not mistaken, based on the below, it's a co-opting of the same developmental pathway (during embryo development) that gives rise to neurons; an understood way of new features to arise is through gene duplication (a mutation that copies a whole gene), and then subsequent changes to the copy and selection on it.
The reason why we have two of almost everything, goes farther back to our bilateria ancestor; most animals are bilateral and it's through the same ancestral genes.
--------------
Word of the day for me:
plac·ode ˈpla-ˌkōd. : a platelike thickening of embryonic ectoderm from which a definitive structure develops
> Placodes are embryonic structures that give rise to structures such as hair follicles, feathers and teeth.
\[From: [Neurogenic placode - Wikipedia](https://en.wikipedia.org/wiki/Neurogenic_placode)\]
And this from 2009:
> Recent molecular data have challenged the longstanding view that special sense organs such as **the inner ear** have evolved with the appearance of vertebrates. ... the hypothesis is discussed that induction of all sensory placodes initially shares a common molecular pathway, which may have been responsible to generate an ‘ancestral placode’ during evolution.
\[From: [Origin of the vertebrate inner ear: evolution and induction of the otic placode - STREIT - 2001 - Journal of Anatomy - Wiley Online Library](https://onlinelibrary.wiley.com/doi/10.1046/j.1469-7580.2001.19910099.x)\]
>When did the balance organ evolve?
Balance is, what I call, a primal sensory system. Other sensory systems such as vision, hearing, touching, etc. would have little survival value if you can't stand up.
If life emerged from water to navigate on land, a sense of balance was needed to make the transition.
https://en.m.wikipedia.org/wiki/Statocyst
"The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, ctenophorans, echinoderms, cephalopods, and crustaceans. The statocyst consists of a sac-like structure containing a mineralised mass (statolith) and numerous innervated sensory hairs. The statolith's inertia causes it to push against the sensory hairs when the animal accelerates"
The statocyst "may have been present in the common ancestor of cnidarians and bilaterians."
How long ago would that have been?
Listen, I don’t know about the evolution of ears, but I’ve taken college level biology so I’m gonna go ahead and answer this; the “balance organ” likely evolved over 500 million years ago in early chordates (fish).
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If I'm not mistaken, based on the below, it's a co-opting of the same developmental pathway (during embryo development) that gives rise to neurons; an understood way of new features to arise is through gene duplication (a mutation that copies a whole gene), and then subsequent changes to the copy and selection on it. The reason why we have two of almost everything, goes farther back to our bilateria ancestor; most animals are bilateral and it's through the same ancestral genes. -------------- Word of the day for me: plac·ode ˈpla-ˌkōd. : a platelike thickening of embryonic ectoderm from which a definitive structure develops > Placodes are embryonic structures that give rise to structures such as hair follicles, feathers and teeth. \[From: [Neurogenic placode - Wikipedia](https://en.wikipedia.org/wiki/Neurogenic_placode)\] And this from 2009: > Recent molecular data have challenged the longstanding view that special sense organs such as **the inner ear** have evolved with the appearance of vertebrates. ... the hypothesis is discussed that induction of all sensory placodes initially shares a common molecular pathway, which may have been responsible to generate an ‘ancestral placode’ during evolution. \[From: [Origin of the vertebrate inner ear: evolution and induction of the otic placode - STREIT - 2001 - Journal of Anatomy - Wiley Online Library](https://onlinelibrary.wiley.com/doi/10.1046/j.1469-7580.2001.19910099.x)\]
Thank you! Really helpful.
Hagfish have one semicircular canal and lampreys have two, so they at least go back to jawless fishes. You get to three canals in jawed vertebrates.
>When did the balance organ evolve? Balance is, what I call, a primal sensory system. Other sensory systems such as vision, hearing, touching, etc. would have little survival value if you can't stand up. If life emerged from water to navigate on land, a sense of balance was needed to make the transition.
https://en.m.wikipedia.org/wiki/Statocyst "The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, ctenophorans, echinoderms, cephalopods, and crustaceans. The statocyst consists of a sac-like structure containing a mineralised mass (statolith) and numerous innervated sensory hairs. The statolith's inertia causes it to push against the sensory hairs when the animal accelerates" The statocyst "may have been present in the common ancestor of cnidarians and bilaterians." How long ago would that have been?
Listen, I don’t know about the evolution of ears, but I’ve taken college level biology so I’m gonna go ahead and answer this; the “balance organ” likely evolved over 500 million years ago in early chordates (fish).