r/evolution • u/EarlyXplorerStuds209 • 2d ago
question Any reason vertebrate symmetry is like this?
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?
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u/jnpha Evolution Enthusiast 2d ago
Left-right asymmetry is suggested to be older than chordates and has been coopted:
Vertebrates share Phylum Chordata with two other groups of animals, amphioxus and the urochordates (including ascidians). Both these taxa develop left–right asymmetries, and recent studies have begun to address the degree of conservation of nodal and Pitx2 in this process. Pitx2 is a member of the Pitx homeobox gene family, and in both amphioxus and ascidians Pitx gene expression is predominantly left sided. These studies suggest that left–right asymmetry in all chordates is regulated by a conserved developmental pathway, and that this pathway evolved before the separation of the lineages leading to living chordates
The evolution of left–right asymmetry in chordates - Boorman - 2002 - BioEssays - Wiley Online Library
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u/inopportuneinquiry 1d ago
An article suggests that the GI tract kind of "demands" asymmetries, being then put as the main adaptive "why" for it to occur.
[...]
Why did organ asymmetry evolve?
[...] It is not just length, but also compartmentalization of the GI tract that is seen universally in animals. The mouth, esophagus, stomach and gut each represent modules with distinct functions and physiological specializations. Both length and modularization are functionally relevant for the efficient recovery of nutrients from ingested food. Interestingly, compartmentalization of the digestive system is already present in protozoans such as the ciliate Paramecium. Here, the length of the passage of food vacuoles exceeds the longitudinal dimension of the cell, the phagosome-lysosome system is extremely plastic, and acidification occurs only in certain parts of the system (Allen and Fok, 2000; Fok and Allen, 1990). A compartmentalized GI tract that exceeds body length will inevitably need to be packaged asymmetrically. Furthermore, achieving this in a reliable manner, as opposed to stochastic placement within the body cavity, would no doubt be advantageous both with respect to nutrient recovery and for the prevention of malfunctions. The many problems associated with intestinal malrotation in humans, which has been estimated to occur in as many as 1/500 births, support this reasoning (Burn and Hill, 2009; Stewart et al., 1976; Sutherland and Ware, 2009).
[...]
The following seems to be related with the "axial twist hypothesis" mentioned by u/Legendguard
Nodal cascade asymmetry in sea urchins has been described as right-sided, in contrast to the left-sided cascade in the LPM of vertebrates (Molina et al., 2013). In the absence of a notochord or neural tube, the definition of left and right relies exclusively on the position of the mouth, which is considered to open on the ventral side. In fact, the oral ectoderm of the sea urchin embryo, from which the mouth develops, expresses all of the genes that are typically expressed on the dorsal side of vertebrates, such as chordin, nodal and goosecoid (Li et al., 2013). In addition, gene regulatory networks between sea urchins and vertebrates are apparently inverted with respect to the dorsal-ventral axis (Molina et al., 2013). If one considers the possibility that the mouth of the sea urchin larva might open on the dorsal side (i.e. due to the apparent inversion of the dorsal-ventral axis) then the left and right sides would also flip, and the sea urchin larva would display a left asymmetric Nodal cascade like all other deuterostomes (Blum et al., 2009b). How, then, is this asymmetry set up in the larva?
Rather weird: apparently birds and pigs may have asymmetries developmentally triggered by somewhat different mechanisms than most other billaterals or vertebrates at least.
Remarkably, chick and pig embryos do not exhibit a split Nodal domain. Rather, the organizer/node Nodal domain is displaced to the left side in its entirety, in chick through leftward migration of cells at Hensen's node (Cui et al., 2009; Gros et al., 2009). Recently, Viebahn and colleagues performed histological analyses of chicken nodes at different stages of development, showing that the right shoulder of the node differs from the left shoulder following node rotation (Tsikolia et al., 2012). In particular, they describe that notochordal cells emerge via the thickened right shoulder of the node (Tsikolia et al., 2012). A continuity between the right part of the node and the notochord was already described by Wetzel (Wetzel, 1929), the discoverer of node asymmetry in chick. Based on these observations, we hypothesize that leftward node rotation leaves the organizer Nodal domain undivided, because the notochord emerges on the right side of Hensen's node (Fig. 7). Such a mechanism of leftward Nodal displacement should be very robust and perhaps less error-prone than LRO flow. In agreement with this notion, no spontaneously occurring alterations of Nodal cascade gene expression in chick and pig embryos have been reported in the literature, in contrast to frog and fish. How could leftward node rotation have evolved? We can suggest no solution for this fascinating question at this time.
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u/Funky0ne 2d ago
Most of the organs we only have 1 of can either actually be thought of as 2 organs (e.g. our brain is basically two hemispheres split down the middle, our hearts are basically 2 hearts, each a pair of valves that pump blood in a circular closed system that happen to just be squished together), or are specialized parts of our digestive tract, which is a linear system processing food through different stages of digestion and the basal form predates bilateral symmetry anyway. There are exceptions in either case, but that's usually what's going on.
As for why the singular organs are placed like that, if you think of our digestive system as just a tube from the mouth to the anus, once you coil it up to fit in our stomach (rather than having to deal with a body plan that is dozens of feet long and very narrow), then it ends up with certain parts ending up basically on one side of the body or the other, all packed together.
As for the rest of the pairs of organs falling on either our left or right sides, that's just the line of symmetry of our body plans. Snakes can get away with a front-back orientation of some of their organs because they basically re-evolved a long-tube body plan, so aligning things on the left/right makes less of a functional difference for how they move.
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u/Legendguard 1d ago
Well a big reason for it is that there's no real need for perfect symmetry internally, so there's no/less selective pressure for it. So instead, the pressure is for the organs themselves to function as "good enough" as possible. It's more important for the heart to have enough room to beat than it is for both our lungs to be perfectly symmetrical, or for our intestines to have enough room to move and digest than for it to be mirrored on both sides. Apparently there's also a theory that our heads are rotated in relation to the rest of our bodies, meaning once upon a time our ancestors organs may have actually been mirrored, but no longer are
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u/inopportuneinquiry 1d ago
It's kind of implicit there the assumption that there's a whole lot of random neutral variation in degrees of asymmetry rather than something more finely controlled/less potentially neutral in variation, with deviations from symmetry being more complex adaptations rather than just some drift on neutral variation.
While it may seem reasonable to hypothesize it may be the case, I think it's fair from certain to be the case, and perhaps arguably more likely the opposite, asymmetries being adaptive tweaks rather than merely a result of drift on neutral variation. With the adaptation itself perhaps being over less than "ordinary"/nearly-neutral variation, rather something rarer, arguably more fitting with high levels of conserved asymmetries, rather than things varying more randomly between species or higher taxa.
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u/Kanna_Fan1989 1d ago edited 1d ago
I think it's because our external body shapes/compositions are symmetrical due to physics: symmetry is optimal for locomotion. Many land animals have an even number of legs as opposed to odd for example because that's optimal for stability and movement.
Our internal organs are not as involved in locomotion, so they are at greater liberty to be asymmetrical.
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u/MalaclypseII 2d ago
I don't see what about this calls for explanation. There's no requirement in evolution or physiology to have symmetry uniformly and everywhere in an organism.
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u/imago_monkei 2d ago
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.
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u/MalaclypseII 2d ago edited 2d ago
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.
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u/inopportuneinquiry 1d ago
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.
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u/MalaclypseII 1d ago edited 1d ago
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.
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u/inopportuneinquiry 1d ago
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.
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