Back to evolution.
Ellie, it's important that you don't think of evolution as being goal-directed. It's just a process, like gravitational attraction or thermal equilibrium.
With regard to the lungs, let's look first at what lungs are for (this is a super-condensed explanation, btw, so no one harp on me for oversimplification). They put oxygen molecules close enough to the surfaces of capillaries where blood cells can bond and transport them to various cell membranes where they are needed.
Jump back a few hundred million years to the Devonian period, the age of the fishes. Fishes had developed gills which accomplished what our lungs do today: put waterborne molecular oxygen near cells that captured it and distributed it throughout the body. Over a period of time, some fish developed gaseous, slimy sacs attached to their gills. The actual origin of these sacs is debated; could it have been a miniscule elastic tissue that mutated in a normally cartilaginous structure? The point is that over time - millions of years - these sacs allowed lungfish to store oxygen within the sac.
Locomotion and oxygen acquisition go hand-in-hand. As we know, many sharks must keep moving to breathe, and the contraction/relaxation of locomotion muscles is believed to have contributed to the development of diaphragms and other lung-squeezing systems.
So you have the raw ingredients for lungs: sacs and periodic compression. All it took was a bit of selection pressure from Nature over the course of a few million years (and a few billion offspring) for the development of an easily recognizable lung.
What kind of selection pressure could do this? Consider that the Devonian was a time when fish were very plentiful, especially in fresh water lakes, rivers and streams. Also, the Earth's crust was under some pressures of its own - continents were drifting and crashing and changing the landscape. Severe weather swings, caused by volcanic ash choking the sky among other phenomena, created times of drought. So, fishies that were more efficient at processing oxygen in a dry environment were selected - their offspring had an advantage over the gill-users.
Many neutral alleles are accumulated in the genetic code of animals. The formation of primitive lung sacs didn't have any discernable advantage to fish in water, but when the water dried up, those that could bury themselves in the mud and continue to absorb oxygen were the survivors.
Hope that helps.
