There is a common misconception that the earth went through three distinct phases in history: it formed, then there were dinosaurs, then we showed up. If you think about it though, this is a bit of an absurd assumption to make. There were actually several thousand million years of evolution leading up to the dinosaurs, each of which with unique forms of life that are in some cases so fantastic that it is a wonder that fewer people know about them. Diverse life truly came into existence around 540 MYA, and continued evolving until the Permian-Triasic extinction event, the largest extinction event in the history of the Earth. With inhabitants ranging from the massive insects of the Carboniferous, to the strange, most primitive forms of life found in the Cambrian.
Although life had existed for many millions of years prior, the Cambrian explosion set off the Cambrian period, and the evolution of life as we know it. Explanations for its cause range from an increase in calcium concentration of the oceans to the evolution of eyes triggering an “arms race” between predators and prey. The Cambrian was dominated by small invertebrate creatures known today as trilobites, but there was a fantastic amount of diversity present. Organisms such as the king of the invertebrate predators of the time, Anomalocaris, which grew to as much as six feet long, swam the seas, and preyed on smaller creatures such as the only known animal to have five eyes, Opabinia. The first vertebrates began to evolve during this time, generally in the form of minute fish-like creatures. The Cambro-Ordovician extinction event brought an end to these creatures, and gave rise to the Ordovician forms of life. The Ordovician period served as a template for life in the successive periods, and was on a much higher scale in terms of diversity and complexity than the Cambrian. The trilobites were replaced by primitive clams, vertebrates finally matured and the first real fish appeared on the scene, and Orthocones, creatures related to the modern nautilus, but that look more like squid, dominated the food chain. As you may have guessed, the end of the period was marked by the Ordovician-Silurian extinction events.
Image from Wikimedia Commons
The Silurian sees the colonization of land by early plants, the evolution of movable jaws in fish, and the evolution of leeches. The top predators at time were the Eurypterids, probably the coolest animals ever. They were basically giant sea scorpions, some of which grew to be meters in length. Extinction events led to the Devonian period. The Devonian finally marks a significant change in evolution, in that invertebrate life was starting to give ground to vertebrates in the contest for dominance. Coral reefs came into their own, and land was slowly starting to be green with plant life. Dunkleosteus, the greatest of the armored fish, sat at the top of the food chain, and helped give the period the name “Age of Fish”.
Carboniferous Plant Life
Image from Wikimedia Commons
Next comes the carboniferous period, significant because it is the first time animal life becomes firmly seated on land. Trees evolved from simple plants, and the first amphibians began to come to land. The ancestors of the dinosaurs came to be in this period, and this time is the last time invertebrates would play a part in the upper food chain. Invertebrates may have been making short trips to land since the Cambrian period, but the Carboniferous is the fist time they actually live there. Faster at colonizing than vertebrates, insect life dominated the land. The largest spiders grew to 20 inches long, dragonflies had wingspans of three feet, scorpions grew to 28 inches, and the largest land invertebrate ever, the centipede like Arthropleura, grew to be 8 and a half feet long. Primitive sharks dominated the seas, and the trilobites faded away for the last time. The end of this period marked the beginning of the last animals before the dinosaurs. Insects still ruled the skies during the Permian, but vertebrates finally made the full transition to land, and came to be the dominant life forms for all the future time. The Permian saw the rise of the Synapsids, which were reptiles that resembled mammals in key respects, and would later give rise to them. Diapsids would give rise to the dinosuars, and the most of them survived the Permian-Triassic extinctio event, the largest ever found.
Before the dinosaurs, there was a diverse array of life stretching for 200 million years, during which we see the rise and fall of the invertebrates, and the earliest predecessors to modern forms of life.