system
Given the following post, respond as if you're commenting on Reddit:
human
Why is there mirror-like symmetry on the outside (limbs, face, etc.) yet a lot of internal systems are asymmetric (location of the heart, digestive system, vein structure in opposite limbs, etc.) ?
gpt
First, why *are* we bilaterally symmetrical? Our left/right halves are mirrored, but there's a distinction between front/back and top/bottom due to how selection would favor differentiation between these. Consider an animal that moves with its belly facing the ground (a worm, a fish, a rhino, whatever).


The animal kingdom is fairly unique in that animals are mobile. As such, a general animal that is seeking food will naturally have a front end that encounters food first. As the first end that encounters stuff during movement, it makes sense for sensory structures to evolve at this front (cranial, head) end, giving a rise to a distinct front/back end. 


In a similar fashion, the top/bottom face different selection pressures. On land, the bottom is either in contact or closer to the ground, while the top is facing the open environment. Even in water, where gravity is less of an issue, top and bottom face different selection pressures. A fish viewed from above will be against a backdrop of the ocean floor; viewed from below will be against a backdrop of clearer water lit from above. This can explain why many fish are camouflaged with dark colors on its top side, and light colors on its belly.


So, while top/bottom and front/back have been differentiated, there is no reason why an animal would experience different things to its left or right. Sensory organs and limbs are thus symmetrical along this left/right plane, as asymmetry along this plane (the same direction as movement) would make locomotion difficult, or make animals less perceptive of one side of them, or some other maladaptive outcome.


Internally, movement and sensory organs don't have the same selection pressure and are free to evolve in non-symmetrical ways if it's advantageous. And once just one thing becomes asymmetrical, the others have to as well, to fill the remaining space efficiently. 


One big player in this is the digestive system, which is essentially one long continuous tube from mouth to anus. In some animals, like worms, this really is a straight, symmetrical tube. But other animals have different nutritional requirements, and one way to increase the efficiency of absorption is increasing the length (not necessarily width) of the digestive tract. To do so, you have to start bending the tube around the body cavity in a non-symmetrical way. This initial asymmetry could have paved the way for further internal developmental asymmetries that were selected for because they made the most use out of the remaining space.