Okay but all animals at one point emerged from the sea, meaning they breathed underwater to respirate. We know lungs for breathing air developed afterward. So how did marine creatures adapt to such a hostile environment?
Ok good question.
You may have seen fish at the surface gulping air
when the water is low oxygen.
Gills have to be wet to absorb oxygen; works in air or
water. Gills are kind of impractical for land life
though many species live well for many hours,
even months out of water as long as gills are wet.
Gills or lungs are not necessarily even needed.
There's a group of lungless salamanders that get
all their oxygen through their skins. Fish that
respire through skin There's turtles...
But you get the idea.
At times and places there can be large areas of aquatic
environment where normal fish can't survive the seasonal
Iow oxygen.
Fresh water, not marine. The transitional forms were all fresh water.
Hostile environment? No. They'd not venture into a e hostile environment,rather, aadvantageous one. Behold the mudskipper.
Early land- venturing fish would be slow and clumsy.
Behold the walking catfish.
BUT! No hyenas or cats or coyotes to eat them. The fish were the Monsters from the lagoon, cone to prey on the helpress!
AlAlmostny design for a land venturing fish would work,
with no dangers or competition. At first.
Backing up a bit to oxygen starved fish...
Advantage, air gulping. And the more throat area,
the more and quicker they can get air. Helpful
if showing yourself at the surface invites predation.
But how to fit a big area of throat tissue into the body?
A pouch off from the throat works great...and theres the
start of lungs.
Lungs were ready made before the
fish ventured out of water. So, no,
we dont "know lungs debeloped after ".
Didn't happen, don't make sense.
There are living fish today at these different steps.
Make sense?