Re: Why are frogs born in water, and as adults breath air like humans?

Frogs aren't the only creatures that have that sort of lifestyle. Lots of newts and salamanders have similar lives, and even some pretty distantly related things like dragonflies pass their childhood in water and then live their lives out of it. Basically, it's a strategy that works for some creatures. For frogs, it probably arose due to their evolutionary history as amphibians.

Frogs are one of the groups of amphibians, who evolutionarily lie between the aquatic fishes and terrestrial reptiles. Amphibians originally succeeded because they could get onto land and take advantage of the resources there; those that adapted permanently to life outside of water became reptiles. Although amphibians are tied to the water, they can live outside of it. Generally, though, they can't go too far from the water unless conditions are very humid, or they run the risk of drying out. And although they do have (inefficient) lungs, they mostly breathe through their skin, so they don't drown in water. There are, of course, exceptions: some toads actually live in the desert, and there are frog species whose eggs develop outside of the water. Still, the vast majority follow the rules above.

In short, frogs live their lives in the way they do because they are evolutionary intermediates between the fish and reptiles, and have some of the characteristics of each.