Can we breathe underwater

My history teacher said that we humans can retrain our bodies to breath underwater because we used be able to when we unborn babies. I thought we had gills as babies then transformed into lungs before we were born.

To sum it up, I do not believe him. Is it possible to breathe underwater if we practice and remember how to do it from when we were babies?

During a stage of early development we have structures related to the developing gill structures of other animal embryos, but they don't change into lungs, they just develop in another way. We don't actually breathe liquid - a fetus gets oxygen from its mother's blood - but our lungs are filled with amniotic fluid and a fetus's diaphragm will make rhythmic "breathing" motions in the womb.

So it's hypothetically possible that we could move liquid in and out of our lungs like breathing. But we would have to somehow suppress the spasm of the larynx that occurs when water enters the airway. This is what happens to many drowning people - the water does not enter the lungs to drown them, they suffocate with their throat closed. Presumably overcoming that reflex is somehow possible ... but ..

The lungs are designed to release and accept gases. Blood picks up oxygen in the gas state in the lungs. So any liquid we "breathed" would have to have the property of being able to pass oxygen gas dissolved in it to the blood without affecting the cells. Water might be problematic because of osmosis - it would tend to flow into the lung cells and swell them. And the amount of it we'd have to breathe to get enough oxygen, or dissolve enough carbon dioxide, might be a problem.

I remember reading somewhere about research trying to develop a liquid oxygen carrier that could be breathed. So maybe that is what inspired your teacher and he just forgot the details.

No, we can't breathe underwater. Maybe we could if we were a fish.