Vertebrates Arrive on Land
Arthropods were existence-proof that terrestrial living had a bright future. They and the plants that fed them were a smorgasbord awaiting diners with backbones.
The first vertebrates crawled onto land 395 MYA. Then an extinction event wiped out half of them; probably a dramatic fall in atmospheric oxygen was partly responsible. Within 10 million years, recovery was underway. By 345 MYA, the land teemed with a variety of vertebrate creatures, some up to 2 meters in length.
Ancient lungfish lived in freshwater streams and lakes. They evolved the ability to breath air as an adaptation to the stagnant Devonian swamps, with waters bereft of oxygen.
By the Late Devonian, forest covered much of the land. In doing so, plants elevated atmospheric oxygen to over 30%; levels high enough to abet lungfish in their terrestrial transformation. Stabilized by their tails, these lobe-finned fish used their robust, bony fins to venture onto a land lush with plant food.
By 375 MYA, amphibious fish had evolved to tetrapods: 4-legged amphibians. The transition from aquatic to terrestrial was made independently by several lineages.
The fate of the lobe-finned fish family from which terrestrial vertebrates evolved was not so fortuitous. They were all wiped out in an extinction pulse 377 MYA. In an example of convergent evolution, other fish have since evolved the ability to walk on land. Mudskippers use their pectoral fins to amble through the mud of tidal pools. These amphibious fish are social and territorial.
From fin to leg was a relatively modest shift of skeletal structure. Most of the features needed for human hips were already present in our aquatic ancestors. Nonetheless, tetrapod backbones gained considerable complexity, to accommodate supporting the body out of water as well as facilitating locomotion.
Then, naturally, amphibians evolved to eat each other. Amphibians were the top predators during the Carboniferous and even into the early Permian, but they later faced competition from their descendants: reptiles.
Many amphibian lineages were wiped out during the Permian–Triassic extinction, though metoposaurs survived into the Jurassic. Metoposaurs had a passing resemblance to crocodiles, though the two are no relation. Both these river dwellers had their eyes far front of their triangular head. Metoposaurs had a finned tail that let them slither through water, resembling the crocodile tail.