How Did Four Real Legs Evolve from Four Lobe-Fins?

When biologists first studied the skeletons of an ancient lobe-finned fish, Eusthenopteron, and an ancient four-legged, amphibian-like creature, Ichtyostega, there were no fossils to show the in-between steps of evolution.  What pressures of natural selection could have led from one to the other?
    Well, first of all, the tetrapod (four-legged animal) was a land animal, while the fish lived in water.  That involved quite a lot of evolution.  Animals that live in water don’t have to support their own weight, because the water does that for them.  Without the buoyancy of water, a land animal needs strong enough bones and muscles to resist being crushed by gravity and to move its weight around.  Land animals have to be able to get their oxygen from air, not water.  They have to keep body fluids from evaporating into the air.  And they must have a way of protecting reproduction and the development of offspring so they don’t dry out.
    In the 1950’s, Alfred Sherwood Romer, a paleontologist at Harvard, suggested a way that four-legged land animals might have evolved from lobe-finned fish:  Perhaps a species of lobe-finned fish lived in waters that became shallower and shallower, because of climate change or because of geological change.  Eventually, the water subdivided into small pools, and the pools held less and less to eat.  In search of food, some of the lobe-finned fish dragged themselves across narrow land bridges to new water pools.  
    As they gulped air instead of water, the fish best able to absorb oxygen through the linings of their mouths as well as their gills, would have been best able to survive these short land crossings.  Their mouth and gill tissues would have to remain very wet in order to take oxygen from the air without drying out.  The bones supporting their limbs and torsos had to be thick and strong, with big enough extensions for the attachment of enlarged muscles.   
    Gradually, such animals could have developed actual lungs from the wet surfaces of the esophagus at the backs of their mouths.  Their lobe-fins could have altered into legs and feet.  They could have stayed on land, returning to water to mate and lay their eggs.  So might the amphibians gradually have evolved.
    But as logical as all this seemed, it turned out that four-legged land animals evolved a different way.  Once the right fossils turned up, just as with Ardi, the earliest hominid (see my Oct. 12 and 14 posts), the true story emerged.  Tune in next time.