After every major extinction, there has been a blossoming of diversity and abundance. The most prominent and abundant species are the most likely to disappear, and the most inconspicuous and humbled species are the ones most likely to flourish. After the “Cambrian-Ordovician” extinction, the strangest animals, with their structure on the inside and their flesh in the outside, became dominant. Ostracoderms, from their earliest record 0.505 bya, were similar to today’s lamprey or the hagfish. The jaw, teeth (0.420 bya), and what we think of when we think of as eating would take 65 million more years, and another major extinction, to evolve.
Meanwhile, life was beginning to occupy the final frontier, land. The first land plants colonized land 0.475 bya, giving them 25 million good yers before their consumers, arthropods evolving from crustaceans, followed them onto land (0.450 bya) The first land plants were similar to moss, as they had to keep all of their cells moist without the advantage of immersion in fresh or salt water*. Similarly, the first centipedes to crawl ashore had to avoid desiccation through frequent visits to water and develop a harder exoskeleton than ever before. 50 million years after segmented and multi-legged species first colonized land, they had specialized into more land-adapted forms with fewer legs, like today’s insects (0.4 bya). The form was humble, like a silverfish, but in 5 million years 0.395 bya they had evolved in to spiders and scorpions, and by 0.28 bya they were evolving into beetles, bugs (0.19 bya) and bees (0.1 bya).
The arthropod colonization of land was only the first. The first lobed finned fishes, similar to today’s Coelocanth (0.4 bya), like the modern Coelacanth, evolved 20 million years after developing the jaw. Only 25 million years later, the first salamanders (0.375 bya), who could spend part of their days or even years on land, as long as they had access to water for reproduction and breathing. After another major extinction (0.368 bya) another blossoming of life occurred, this time from the amphibians, forming the first lungs and jointed limbs by 0.365 bya. This was also the time of the first two-lobed brain (0.365 bya) in what would become reptiles, birds and mammals, the Tetrapod.
The first plants with seeds (0.36 bya) enable rapid plant propagation on land, but require much larger plants to provide their progeny with sufficient structure to succeed. After the formation of a modern walking gait, the first proto-mammals – Synapsids – formed our modern brain position in the skull (0.324 bya) while reptiles diverged to remain cold blooded and covered in scales (0.310 bya), The first eggs (0.3 bya), allowed reproduction entirely on land. In this period, the first proto-dragonflies gained the ability of flight (0.3 bya). With flight, insects diversified anew in their new 3D world. The earliest dinosaurs, the Archosaurs, grew from smaller reptiles by 0.256 bya, while still-hairless Synapsids skulked about their feet.
The fourth major extinction event, the Permian-Triassic (0.251 bya), was the worst ever known, but left enough of the insect, dinosaur, and Synapsid lines around to develop into what we know today of life. It was still the largest extinction event of the six, with 83% of all genera wiped out.
Now 15/16ths through the history of life.
* I can’t find when the first freshwater fish or worm evolved. It would be an interesting departure date to have.