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?
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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.
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