[removed]

Bilateral (humans) or radial (jellyfish) symmetry helps keep you balanced so you can move with purpose, and reduces the needed genetic complexity.

You don’t need totally separate developmental pathways for your left arm and your right arm, just a genetic copy-paste.

You’ll just swim in circles with one large fin and one short fin.

Both the vertebrates and the arthropods settled on bilateral symmetry long ago, so practically all land animals are symmetrical like that.

Radial symmetry is more popular among the jellyfish/anemone/sea urchin types in the ocean.

Asymmetric anarchy like bivalves and sponges still has a few very ancient holdouts.

It reduces instruction complexity and central neural system load. Your brain constantly makes calculation to errect musles in right way to keep body balanced.

Imagine if instead of left arm you would have 2m long tentacle which grows from stomack. In such situation center of mass during movement will be a mess. It would increase brain work immensivly just to keep you straight

Furthermore, tentacle and arm have completly different moveset and thus require different instruction set. That also increase load on brains as it cannot reuse same thing twice. It also would take huge extra memorry to remember all possible moves each limb can make.

One small part of the answer is evolution. Some asymmetries can be an evolutionary disadvantage. Take different leg lengths for example: the uneven load on each leg, knee, and foot increases the risk of stress fractures, recurring pain, and more. Not a great trait to pass on.

Also, it is more efficient for DNA. Simply code for half a body, then tell it to make a mirror image.

Consider that essentially anything asymmetric is imbalanced and less predictable and usually less efficient.

What makes carrying a backpack on your back easier than in an outstretched hand? The weight distribution is asymmetrical in the latter case.

If given the tasks of creating the most efficient thing that rolls on a flat surface, a sphere is unbeatable. Any asymmetry justs adds chaos.

The points about DNA and evolution are nice, but might be putting the horse before the cart. DNA adjusts to what works. Creatures aren't asymmetrical just because it's easier for DNA to code for half.

what reason is there for it TO be symmetrical? youd get weird balance and one side would do diff than the left and if imagine that put a strain on the brsin

Our bodies are not symmetrical on the inside. For example, the liver is on the right, the stomach and spleen are more towards the left.

Most animals come from or branched from bilateral organisms. Flatworms, chordates, fishes, ichthyostegans, amphibians, lizards, early shrew-like mammals, spider monkeys, gorillas, humans. The other main symmetry is radial, like starfishes. I'm not an expert, but I saw it on youtube hehe.

> Imagine if instead of left arm you would have 2m long tentacle which grows from stomach.

I'm ready for such an upgrade.

Just to nit-pick, there's no need for any "copy-paste". All¹ of our cells contain our entire genome.

¹Well, almost all. Red Blood Cells eject their nucleus, and thus don't retain the DNA when mature.

Personal belief: the force of gravity affects life on a cellular level. If life could develop in zero gravity, "symmetry" wouldn't matter.