Land Animals Appear
Plants created the terrestrial biomes that animals would inhabit once vegetation had settled in. The oldest animals to adapt on land were centipedes and spindly spiderlike arachnids the size of a flea.
The early biome on solid ground supported a bevy of plants that fed arthropods, who in turn became spider snacks and centipede feed. Perhaps the most surprising thing about early arthropods was that many of them could easily be placed within groups living today. The early critters got a lot right.
Insects Emerge
The earliest insects evolved on land some 500 MYA, as plants began to proliferate, and habitats stabilized.
Terrestrial hexapods descended from aquatic crustacean ancestors. Insects and crustaceans are distant cousins: both belong to the clade pancrustacea. (A clade is a biological group (taxa) which includes all descendants of a common ancestor.) The compound eyes of insects originated in water.
Insect success owed to serosa: the membrane that waterproofs insect embryos inside their eggs. This innovation greatly increased tolerance to a diversity of environmental conditions.
Emergent insects were at first flightless consumers of decayed plant matter. In evolutionary terms, plants succored the rise of insects.
Insects rapidly diversified, gaining exquisite individualization of functions from biomechanical enhancements: legs, antennae, and mouth parts, as well as other appendages. Wings sprouted during the Devonian.
Once winged insects originated, they diversified very, very quickly: so quickly that their diversification appears, from a geological perspective, to have been instantaneous. ~ American paleontologist Jonathan Payne
Arthropods evolved in codependence with the increasingly diversified vegetation. Interdependency between insects and plants developed during the Late Devonian.
As plants gained height ~400 MYA, insects developed wings to carry them aloft. This advance insured the prosperity of the little 6-leggers. Then they became not so little. Abundant oxygen, the success signature of the proliferating plant world, afforded 4-wing dragonflies with a 60-cm wingspan. The exoskeleton-based architecture of insects is eminently scalable in size: just add oxygen.
Chilling climatic oscillations during the last half of the Carboniferous Period provoked the innovation of metamorphosis: an adaptive practice of patience. This afforded variability in life cycle, to optimize environmental exposure.
Oxygen at 1/3rd of the atmospheric mix was not to last. Volcanic activity at end of the Permian, 250 MYA, abruptly halved atmospheric oxygen to 15%. Insects shrank in response.
Insects are by far the large living group of arthropods: the most prosperous animal throughout history. Their success owes to inherent adaptability, notably simple and pliable biomechanics, and their close relationship with plants, a reliable food source and homestead.
Early Herbivores
Insects were the first animal herbivores. Plants were more than a diet – they were a lifestyle: something to live on as well as eat. This required being able to hang on; no small feat in the face of stormy wind and rain. Caterpillars evolved sucker-like false legs with rows of small hooks. Crickets and beetles have broad pads on their feet thickly covered in fine hairs. Oily secretions enable the hairs to behave as suckers on smooth leaves.
Desiccation was another challenge. Some insects developed waterproof cuticles to retain moisture. Caterpillars drink the drops of water that plants exude at night. Then there are the sap suckers, such as aphids, which crowd together to reduce evaporation, as well as offering the relative safety of numbers.
The route to insect herbivory was from digesting decaying plant matter to consuming plants while they were still alive. Snacking on spores and pounding down pollen were an interim step.
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.
