If humans had gills like fish then the gills would need to be big enough to filter enough oxygen out of the water to keep us alive.
These big gills for humans would have to cover the entire back and possible more of the body to be effective.
Unless the gills on humans were large enough then we still would not get enough oxygen from the water and would still need to breathe air.
The oxygen levels in the water are much lower than in air, and gills are not a very efficient means of extracting it.
Gills on humans couldn't provide the body with oxygen fast enough to support the high metabolic rate that we and other mammals have.
If humans had wings we would not be able to fly unless we also had a smaller body and also the right metabolism which is the body's ability to use fuel from food we eat to make energy.
Human bodies are too large to be able to fly and stay in the air with just wings.
As an organism grows, its weight increases at a faster rate than its strength.
Thus, an average adult male human would need a wingspan of at least 6.7 meters to fly.
If humans had wings, they would be where wings are on every creature with wings—roughly, where our arms are now.
And mammals get four appendages.
We can have wings or we can have arms/forelegs, but you don't get both.
It's not possible for humans to have wings as humans lack the genes needed to grow wings.
If humans did however possess the Genes to grow wings then humans could grow wings then.
All animals have genes that decide and control the development of organs.
Because of these genes, it is impossible to find humans born with wings.
Human limbs will develop according to the instructions encoded in such genes.
It is because of these genes, a spider will form 8 legs but no wings.
To evolve useful wings, us humans would also need to become smaller, evolve honeycomb bones and lose most of the muscle mass in our legs and nearly all our teeth just to be light enough.
If our wings evolved from arms, we would become much clumsier and lose the benefit of our hands.
Even with artificial wings we cannot fly.
We cannot create enough lift to overcome the force of gravity (or our weight).
It's not only wings that allow birds to fly.
The birds light frame and hollow bones make it easier to counteract gravity.
Air sacs inside their bodies make birds lighter, which enables smoother motion through air.
And while you might grow taller thank your siblings, hox genes make sure you only grow two arms and two legs – and not eight legs like a spider.
In fact, a spider's own hox genes are what give it eight legs.
So one main reason humans can't grow wings is because our genes only let us grow arms and legs.