how did animals first develop air breathing lungs?

Question

how did animals first develop air breathing lungs?
When the first creatures poped out of the ocean,
How did they start to breath?

Answer 1

Well ... they didn't just "pop" out of the ocean. :-) The process took millions of years.

Contrary to a lot of people's understanding, lungs did not evolve from gills, but rather from the air sacs that fish have, and which have evolved into gas bladders that bony fish use for buoyancy. Many fish today actually have both lungs and gills ... most noticeably in the lungfish (see first source), which is a relative of the lobefinned fishes, and which is able to gulp and breathe air.

The modern lobe-finned fishes have many other characteristics that indicate that they are related to the ancestors of amphibians. Amphibians to this day still have what's called 'bimodal breathing', which means that they have gills and swim and breathe in water in early part of their life, but develop lungs and become airbreathers as adults.

As another example of a fish showing a transition to land, check out the modern mudskipper (see second source) ... a fish that wiggles out of the water and can survive for several hours by gulping air and keeping its gills moist in a mud burrow.

The transition from amphibian (which lays soft eggs in water, and becomes an air-breather in adulthood) to reptiles (which have hard-shelled eggs that it lays on land) was yet another transition that took millions of years.

Answer 2

As body temperature increases, the metabolic rate of ectothermic animals also increase. To fuel the increased metabolism, more oxygen is needed. An ectothermic land animal can maintain a body temperature higher than the surrounding air by basking or coming into contact with warm objects like rocks and road pavements (ever seen a snake pressed tightly against the road surface to get warm?). However, fish are unable to do so because water is a huge heat sink. It takes a lot of energy to raise the body temperature of a fish above that of the surrounding water. Therefore most fish simply maintain a body temperature that is the same as the water they live in. Tuna fish is a well known fish that can raise its CORE temperature to swim faster, since muscles are more efficient when they are warm. Nevertheless, it minimizes the amount of energy needed by maintaining a body surface temperature that is much cooler than its core temperature (BTW, some seals do the same to conserve energy). Since it takes a lot of energy to raise the temperature of water just a single degree C, most bodies of water are cooler than the surrounding air. Therefore, a lizard can often maintain a body temperature as much as 90-95 degrees F, while fish in the nearest body of water swim in temperature that is only about 70 deg. F or even lower. Therefore the often higher body temperature of the land animals are responsible for their greater need for oxygen. If you take a lizard, a frog and a fish, and they all have the same body temperature, their metabolic rates, and their need for oxygen, should be similar.