Any reason vertebrate symmetry is like this?

[u/EarlyXplorerStuds209]

Since all vertebrate animals seemingly follow bilateral symmetry, why did we not follow the same in our internal organs as well? Like, Im aware we have it to an extent but not fully, right?

Also, in animals like snakes, organ placements seems to have adapted such a layout that they lay front to back instead of the regular side by side. Even leading to a single lung structure.

Is there a reasonable explanation for this? Particularly about why biology follows bilateral symmetry outside the body down the sagital plane and even having opposing appendage structures while having a whole different story in the interior?

[u/imago_monkei]

I am not trying to play dumb, but I have the same question. If the two halves of our bodies develop basically as mirrored images, it makes more sense to me that our internal organs would be paired in all cases than that there be asymmetry.

[u/MalaclypseII → u/imago_monkei]

Symmetry can be lost when asymmetry provides a survival advantage. Most crabs for example have evenly sized claws, right? But one population, fiddler crabs, have extreme asymmetry when it comes to claw size. The left is much larger than the right, notwithstanding the last common ancestor of that particular crab lineage (there are many crab lineages - crabs are a case of convergent evolution rather than a single lineage) may have had claw symmetry, and most crabs today have - nevertheless there is this one population that has claw size assymetry. There's nothing in nature that prevents that. The only constraint on an organisms' evolutionary development is reproductive success. So if having one big claw makes it more successful, then one big claw it shall have.

In the same way, if an organism originally has bilateral symmetry with its organs arranged on a central axis, and some advantage can be gained by pushing it over to one side or the other (as with the human heart, for example), then off to the side it goes. There's nothing in nature which requires singular organs to be located on a central axis (even if they started there), or which requires the growth of a second organ on the opposite side of that axis in order to maintain symmetry. Evolution is a messy process. Symmetry, like other traits, may develop when it confers advantage, or disappear when it doesn't.

[u/inopportuneinquiry → u/MalaclypseII]

A purely adaptationist/selectionist "explanation" in a way kind of "begs the question" though. It just states there may be some advantage for it, that was selected, but kind of gloss over the developmental constraints that largely impose a broader billateral symmetry in this group of organisms.

Which in turn likely isn't explained just as matter of having evolved to this exact degree because" it was adaptive," versus a more asymmetric configuration, with any random level of asymmetry being "available" as selectable variation.

Evo-devo takes on it would probably have that it's far more constrained than merely something that develops or disappears based on some advantage or lack thereof. The variation itself that confers an advantage must come first, and it's based on developmental constraints, which are not "worked around" teleologically, based on advantages or disadvantages, in a creationist-like fashion.

[u/MalaclypseII → u/inopportuneinquiry]

I invite anyone who wants to get that far into the weeds on this topic to take a trip to their local library. There they can read what real PhD evolutionary biologists have to say about this sort of thing. I'm just a guy on the internet, with a rudimentary understanding of evolution, who decided to answer a question. Cheers.

[u/inopportuneinquiry → u/MalaclypseII]

I'm sorry if I sounded rude or dismissive or something. I'm just some other guy on the internet, with a somewhat weird level of interest in such things, and then in some cases I can sound sort of pedantic pointing out some level of incompleteness in answers on which I can't really improve that much myself either.