“Presumably there are steps between the two with an intermediate state that mostly tended to make sense at each step. Anyway, there are tons of people looking for the evidence of fossils to back fill this theory.”
In my wildest imagination I can’t figure out how partial evolution of a leg might produce some kind of survival advantage whereby it would be kept by the selection process because it might be of use to the organism further down the road.
If you want to argue irreducible complexity, there are probably better examples.
"In my wildest imagination I can’t figure out how partial evolution of a leg might produce some kind of survival advantage whereby it would be kept by the selection process because it might be of use to the organism further down the road."
First, you began in this period to get plants invading the water margins, forming dense swamps and forests around the water's edge. And some of the fishes living in the shallows would have found it advantageous perhaps to lose the fin webbing from their fingers and develop separate digits. Fish have bony supports for the fin web. If you lose those and make the digits separate, then you can begin to grasp things and push aside the vegetation. You can also grasp the vegetation to hold your position in the water. There are quite a lot of modern fish that do this.This is from a Nova series on evolution, in this case, the creature that may come closest to providing the link between water-dwellers and land-crawlers that may be found, Acanthostega.
What we think happened is that these creatures, which were developing this mode of life in the shallows, developed legs with digits before they ever started really to walk on the land at all. So they would have got their legs first, then gradually perhaps moved into shallower and shallower water—more and more vegetation and less and less water—and eventually emerged onto the land. But it took a very long time.The show was fascinating, and the article is a very interesting read. Fins to fingers. Whoulda thunkit?