Why is the human body is symmetrical in exterior, but inside the stomach and heart is on left side? what advantages does it give to us?
In: 6507
Symmetry is important for locomotion, stereovision and so on. The body parts that are relevant for those features are all symmetrical. The organs inside the chest cavity and stomach don’t require full or any symmetry. A long symmetric intestinal tract is practically impossible and organs of which we only need a single one can’t all accumulate right in the center of the body, so, over time, they shifted to one of the sides where there was space for them and that’s where we have them now.
Not every feature gives us an advantage. Some of them just don’t give us a big enough disadvantage to hurt our chances of survival.
On the outside, it makes sense that our eyes, ears, and limbs would be symmetrical because it allows us to move and sense the world around us better.
On the inside, it doesn’t really matter where our organs are located as long as they can do their jobs.
Your heart is actually located in the middle of your chest and tilted slightly to the left. The left side of your heart is bigger and accounts for about 2/3rds of the hearts mass. That coupled with the tilt makes it easy to think it’s positioned to the left, but it is not. It’s positioned the way it is so that the sternum and ribcage protect it. The stomach just so happens to be on the left side, there is not a specific evolutionary reason behind it.
Evolutional traits don’t need to be advantageous, they just need to not be disadvantageous. The answer to a lot of these types of questions is simply: “it works well enough that way.”
Not sure about the heart/stomach in particular.
If you take two groups of any animal, and put them on two completely equal rooms for a very long time, they will look different eventually. Most of the time, evolution is random and does not yield any advantage. It could very well be that at one point, everyone with a right-side stomach died in a unrelated catastrophe, and now we only have left-side stomach individuals.
Genetic drift, which was mentioned previously. We have a very narrow view of human evolution because we’ve been around as sapiens for X amount of time, but we evolved from ancestors that survived because of some of those traits. Now some of those things aren’t as relevant, but because they don’t hurt our survival (and few things do with modern medicine) they happen to still be present in the majority of people.
One of the symptoms of not appreciating our entire evolutionary journey is not realizing we aren’t fully evolved and that there’s no end goal of evolution- it’s not heading in a specific direction, but more like all directions all at once because of variation. The directions that just survive long enough to reproduce have ‘suitable’ genes, which is how things become more prevalent without being ‘necessary for survival’.
They fit, so they sit.
Sometimes that’s all that matters to evolution.
(See the recurrent laryngeal nerve, which during the assembly of your body will loop down from your neck to your heart and back up to your neck)
The stomach and heart don’t have to be on opposite sides. Also since they aren’t similar in shape, they can’t be symmetrical
Unlike the kidneys and lungs,
On the inside, it has the advantage of space efficiency.
If everything needed to be symmetrical inside too, then all of our single organs would need to be on the centerline. Except the spine is taking up space there. The only way to fit everything would be to take a less efficient shape, like having longer bodies.
Realistically, all of our organs just need to fit in there and stay connected. Aside from that, they’re practically all misshapen water balloons that we need to keep from sloshing around. Symmetry doesn’t offer many advantages on the inside, so long as it doesn’t throw off balance.
On the outside symmetry offers plenty of advantages. For our face, symmetrical eyes and ears make for great sensory depth and direction.
With ears on both sides of our head, a sound wave that comes from straight in front arrives at both ears at the same time, while if it arrives from the side, it hits one ear sooner than the other and the wave pattern is out of sync by the width of our head. Without going into the exceptions, (which includes a strong theory on why dogs tilt their heads when hearing weird noises), this is what lets us determine the direction a sound is coming from.
With two eyes, we get the same effect, except using light instead of sound to triangulate and perceive distance.
With only one eye, we’d have a lot more trouble estimating how far away something is. We’re pretty much only have size to go off of. Two eyes also gives us decent peripheral vision in both directions. Our peripheral vision is terrible as seeing color, but is much more light sensitive than the center of our vision. Light sensitivity useful for detecting motion, which is great for both hunting and protecting yourself in low light. You can exploit this knowledge even in the modern world. If you’re trying to find your way around a dark room, try to use your peripheral vision more than your center vision. Avert your eyes from the things you’re trying to see, and while you won’t see in much clarity or detail, you’ll have better night vision in your peripherals.
Two symmetrical legs is important for balance and efficient walking. Uneven legs cause uneven stresses that would cause early joint failure and evolutionary failure. Two symmetrical arms is important for balance and torso mobility. Primates large arms are great for swinging. Not only are they just stronger, but they give them more mass that they can manipulate for rotating and changing their moment of inertia. Kind of like how a figure skater can increase their spinning speed by tucking their arms in.
Our biggest asset aside from our brains is our ability to throw objects. If you’ve watching a child throw something, they throw with their arm. It doesn’t go far, and often enough it knocks them on their ass. Watch even a beer league baseball player though, and they throw with their whole body. For a right handed throw, the left arm is constantly counterbalancing the upper body while the legs are propelling the body forward and the torso is rotating with the throw. We can push so much power into a thrown object, and that’s because we have the brains to do the “calculating” and coordinate all of those muscles at the same time, and it’s all enabled by symmetry.
Lastly, symmetry is attractive to others. It’s proven. Likely for the evolutionary reason of showing that a potential mate is healthy and able-bodied. This is a reinforcing trait, because it makes symmetrical featured people more successful at mating, and creates more symmetrical offspring for the next generation.
For all of these points, internal symmetry is rather unimportant. Lungs are a large cavity, which can affect balance which is likely a strong reason for their symmetry, but pretty much everything else has a similar enough density that it doesn’t significantly affect balance or health.
External symmetry allowed our aquatic ancestors to become streamlined for speed [1672, 1871, 2766], and it has been retained ever since because it is just as useful for walking as it was for swimming [517]. In contrast, the only locomotory restriction on our internal organs has been that their weight be distributed evenly relative to the midline.
If we trace the history of our anatomy back before the fish stage of evolution, we find that the inside of the body used to be as symmetric as the outside [512, 1462].
[source](https://www.cambridge.org/core/books/abs/quirks-of-human-anatomy/symmetry-and-asymmetry/6B91C73DA77BC52744BD18F486A651E1)
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