Amphibians evolved from the lobe-finned fish. Most of this change happened in shallow water. The bottom fins (those little sharp parts that help a fish navigate) were in pairs AND were supported by internal bones.
Environmentally, as the amphibians took charge, conditions were unique. Parts of what is now North America, Europe and Asia were near the equator. They were hot, wet, humid with and were hot, wet and humid with oxygen levels up to 31% (compared to 21% now.) The heat and extra oxygen had much to do with rapid growth, leading to enormous and dense swamps filled with mosses, ferns and giant trees. With six-foot long poisonous centipedes and dragonflies the size of a seagulls, the swamps sound quite unpleasant. However, where this dense vegetation once was, in North America and elsewhere, much later it decayed and became coal.
For amphibians, conditions could not have been more ideal.
The lobe-finned fish lived on plants or smaller fish in that very shallow water. Some was on land. Lobe finned fish to follow it, slowly but surely those four fins strengthened, eventually to be called legs. That transition might have taken 30 or 40 million years. An animal living in water has buoyancy for support; as the lobe-finned fish spent more time out of water, those muscles developed.
The fish had a sort of bony arch through which water flowed. The gills in fish have the capacity to pull oxygen out of the water then merge oxygen with nutrients to provide the fish energy. As more time on land strengthened that strong skeletal structure, those gills became pretty inefficient oxygen providers. That had to change. Eventually, that fish-structure shut down and was lost. As adults, the amphibian has primitive lungs and slimy sort of skin that actually pulls in oxygen through the skin. The gills are gone.
Aha! Your arms and legs have developed.
Water was high; amphibians fit in everywhere. Fossils have been found all over the globe. This event happened at a variety of sites at about the same time. Amphibian fossils have been found on every continent; so clearly they prospered.
Well, those ideal started to change as land masses inched toward each other and climate began to cool. Between 375 to 360 million years ago two extinctions occurred, almost ending this long story. Each lasted for a short (by evolution timing) period of about a quarter million years. But — the party ends. Some think the rapid growth of vegetation increased oxygen levels too much. The temperature dropped sharply. An extinction was hard on water-dwellers; perhaps 75% of all water species became extinct.
When the party ended, though, the amphibians were well underway. They needed water to lay their eggs; but they could also escape to land. The timing of the extinction was fortunate for amphibians. One good thing: those deadly and vicious placoderms ruling the seas became extinct. Most species of insects and plants joined the amphibians in survival.
Building a skeleton to hold up the amphibian on land was a key part of the transition. Surely the process of fins pushing around in shallow water and spending a more and more time in shallower water played a key role in the transition from fins to legs. Artifacts also show this was the start of internal fertilization. The article points that the process of the male holding on to penetrate the female strengthened the legs of both genders. Internal fertilization becomes a key part of the evolutionary process.
Catch that? Internal fertilization started? Just checking.
Amphibians, clearly in the evolutionary line that leads to us, vary in reproductive techniques. Some require penetration, some not. Frogs lay eggs but the male and females are in contact at the time of fertilization.
Amphibian reproductive process is a precursor to the next evolving specie, reptiles. Amphibians leave an egg that produces a fish-like animal which then grows to be an amphibian. For example, tadpoles leave the egg as a fish but eventually transition to frogs. As the newborn gets older, though, the DNA directs a slow transition to adulthood. In this process, the adult amphibian moves from breathing through gills to breathing with primitive lungs and through the skin. The amphibian still needs water nearby to be available but has learned to live for substantial periods on land.
Amphibian fossils suggest the transition from fish to amphibians was complete by about 340 million years ago. Those full-blown amphibians quickly became the big guy on Earth, the dominant specie. If the continents had just stayed separated with a lot of land covered with shallow water, the amphibians would still rule. But the earth underneath them foiled their dominance.
As the earth cooled, those water sites amphibians needed were getting further and further apart. As birth sites diminished, what was a female with eggs to lay going to do? Next, I will summarize the steps upon which science seems to agree.