The story of the evolution of the lung is actually quite ancient and start in the middle of the silurian, around 420 million years ago.
At that time, Osteichthyan are just on the verge of branching out between the actinopterygians (ray-finned fishes) and the Sarcopterigian (lobe-finned fish).
This is also the time were fish are colonizing fresh waters for the first time. It's a new environment, and quite a rich one at that, so it is a pretty good move, but there is a problem with that...
You see, freshwaters mean shallower bodies of water and these can become quite hot and the ideal gaz law tells us that warmer temperature contain less dissolved oxygen than cooler one. So, the fish were often confronted to oxygenation problem.
One thing they did to compensate that, and contemporary fish are still doing it, is to go to the surface to "gulp" some air. Air above the surface was at a roughly constant level, one significantly higher than in the hot waters.
So the fishies swallowed big gulp of air. This air came into contact with the mucosa of the digestive track and started to diffuse into the blood vessels, after all, the mucosa had already evolved as an optimal area for the diffusion of small molecules from nutrients.
Clearly, that wasn't a very efficient way to do so, but it allowed the fish enrich their blood-stream in sweet, sweet oxygen and that gave them an advantage.
Over time, the digestive track started to invaginate, slowly, into a pocket. This allowed to keep more air at a time, and allowed more gaseous exchange to take place.
Not surprisingly, this invagination took place rather high in the digestive track, just by the heart, in order to provide this very important muscle in oxygen.
As you'd predict, this innovation was selected upon for many generations of fish that stayed in shallower water; mud ponds or swamp environments. Indeed, some probably started to wonder at the surface, maybe to escape predators, or to go look for some food they wouldn't have to share with other fish species, or to move away from a pond that started to dry up and into a safer one. Progressively, their primitive lungs grew and became more vascularized allowing them to stay for longer and longer periods of time.
And, the fish that did best at staying out of the water did better, they could find more food, or avoid predators better (remember the old joke, 'I don't have to outrun the lion, I just need to outrun YOU'; these fish didn't have to stay out of the water all their life, they just had to stay longer than the neighbor and only get back when the big bad predator was no longer hungry...).
Today, several species of catfish have adopted a similar behavior, wandering, sometime for miles at a time, in the night. Some other species, such as the Australian mudfish, are able to bury themselves in the mud and wait for several weeks for the return of the rains...
On the other hand, some among these fish actually moved away from these shallow waters. They did not loose their "proto-lung", however, instead, in these species, the organ took a different evolutionary root and started moving toward a more dorsal position.
There, it allowed the fish to equilibrate himself in the water without having to constantly swim. Not only was it great to save energy, it allowed the fish to become more stealthy, wonderful for ambushing preys.
This was so useful that this swim bladder soon become dominant among bony fishes (the condrichtyan, sharks and rays, and the agnathean don't have it, though, as the organ appeared after their split from the osteichthyan lineage).
After a while, however, one more evolutionary innovation took place in the swim bladder. Rather than going to the surface to gulp air, some fish started reversing the initial gas diffusion process. Rather than a breathing organ where the gas diffused from the organ to the blood stream, the same diffusion laws allowed the gas to diffuse from the blood stream toward the organ. This allowed the fish to adjust its swim bladder without going back to the surface (and also allowed the 'closing' of the swim bladder, hence isolating from pathogens). This is the beginning of the physoclistic fish but, even among them, the organ start, during the embryological stages, as an invagination of the digestive track.
Neil Degrasse Tyson wonders in his funny examples of
stupid design video about why the design of our respiratory track is such that we can choke on our food, but there is a very good reason for that and, as usual, the reason is evolutionary.
The story of the limbs is actually quite boring in comparison, I think.
As we have seen, we had fish using their primitive lung and starting to wander in the muddy banks of the swamp they live it.
It is also likely that some species did not actually live the
These fish had most of the features required to make a reasonably efficient land animal, they had ribs, they had a pectoral girdle and they had limbs connecting to this pectoral girdle. More importantly, they had little predators to worry about and only had to out-compete their cousins.
So, the pectoral girdle reinforced itself, and the hyelomedular bone decreased in size, allowing for a better mobility of the neck. More strikingly, the fins reinforced itself, gaining new bones allowing for a better support, and these bones progressively evolved into something strikingly similar to ours:
http://scienceblogs.com/pharyngula/upload/2006/04/tiktaalik_limb_lg.jpg
Edited by LashL:
Removed hotlinked image. Please see Rule 5.
Tiktaalik is probably a good example of what the first primitive tetrapod limbs looked like...
