The common evolutionary belief is that life originated in water, and that it developed to survive on land later on. Eventually, mammals evolved on land.
Cetaceans, which include dolphins, whales, and other marine mammals, then evolved from land-dwelling creatures, going back into the water.
Some believe this change has made it unlikely for cetaceans to revert back to life on land again. Notably:
- Their legs atrophied over time and now they rely on one pair of fins and a tail. These are not as suited as the two pairs of fins and relatively smaller, lighter tails that the first amphibious animals had.
- Some have twice as much hemoglobin as most mammals, so they don't have to come up for air as much. If they breathed air constantly, they would receive twice as much oxygen - which may cause fluid buildup in their lungs, pain, and eventual suffocation.
- Many of them are enormous. The square cube law means that they would have to evolve to support their own weight out of water, which is simply not feasible for larger species.
- Echolocation is suited for use underwater - some cetaceans cannot communicate on land.
- Baleen designed to filter small food is useless out of the water, so there is no feasible way for some species to eat out of the water.
Of course, if mammals completely unsuited to life underwater can evolve into whales and dolphins, it must be feasible that cetaceans can eventually evolve to walk on land once again. Just as these changes can be done, they can be undone.
What natural evolutionary pressures would cause these problems (and others) to vanish, allowing both whales and dolphins to become land mammals?
Assume there is no human intervention, and the world is normal. General-purpose answers work - you do not need to address specific concerns for every subspecies, just "whales" and "dolphins" in general. Assume the whales in question are species with baleen.