If only humans were able to breathe under water, we could have ruined the oceans as well. Too bad 70% of the planet is water and not so habitable. (To give credit where it is due, humans still managed to screw the oceans to a large extent).
So, why can't we breathe under water? The obvious answer is that our lungs are not made for absorbing oxygen from water! So a better way to phrase the question would be -
How come humans did not evolve with lungs that can breathe on land and water?
Before we answer that, lets refresh some basics and clear some myths -
Its common knowledge that water is basically a combination of Hydrogen (2 parts) and Oxygen (1 part). This unfortunately leads to the misconception that fish somehow breathe this oxygen that forms the water molecules. However fish are not able to break the bonds in water molecules to meet their Oxygen needs. Instead they breathe Oxygen that is dissolved in water.
Many liquids including water can hold some dissolved oxygen (DO). It is either directly absorbed from atmosphere or is produced by the aquatic plants / algae. The DO is often used as an indicator of a water body's ability to support aquatic life. It is the same oxygen that we breathe from air but dissolved in water (Two molecules of Oxygen double bonded).
One key difference though is its availability. The amount of oxygen available in water is much lesser than what is available in air. For a given volume, air has approximately twenty times more oxygen than water.
|A graphic to demonstrate the levels of Oxygen in water vs air and the structural properties of water and breathable Oxygen. Its just for demonstration and the proportions are not to scale.|
So, to answer our question, we did not evolve to breathe under water because the amount of Oxygen in water is just not sufficient to meet our Oxygen demands and thus nature did not take that route.
This explanation however begs the question as to how aquatic life, sometimes bigger than humans, is able to meet its Oxygen demands under water. The answer lies in the fact that most aquatic animals are cold blooded whereas humans are warm blooded. Since, cold blooded animals do not regulate their body temperatures from within like we do, the Oxygen demand of those animals is much lesser compared to warm blooded animals and thus are able to survive in an environment with limited Oxygen supply.
It is interesting to note that aquatic warm blooded animals do breathe from air (by coming to the surface for each breath) because the Oxygen available in water would not be sufficient to meet their demands.