The history of life on Earth is far older and more complex than the 165-million-year age of dinosaurs, which represents only a recent chapter in the planet’s story. The creatures that came before them set the stage for all future terrestrial life, spanning hundreds of millions of years. This vast evolutionary timeline began with the sudden appearance of complex life in the oceans and culminated in the dominance of creatures that were immediate evolutionary cousins to the first dinosaurs. The life forms that existed during this time were diverse, resilient, and adapted to environments ranging from shallow seas to humid swamps and dry, inland deserts.
Life’s First Major Expansion in the Oceans
The earliest part of this history saw a profound shift in the oceans, beginning approximately 541 million years ago with the Cambrian Explosion. This rapid burst of evolution saw the appearance of nearly all animal body plans seen today. Before this time, life consisted mostly of simple, soft-bodied organisms, but the Cambrian event introduced creatures with hard shells, jointed limbs, and complex tissues.
The seas quickly became populated with diverse forms of life, including the iconic trilobites, a group of arthropods that flourished for hundreds of millions of years. These segmented, shelled creatures were among the most successful organisms of the Paleozoic Era. Simultaneously, early chordates, the group that includes all vertebrates, began to appear, represented by primitive forms like Pikaia.
Following the Cambrian, marine ecosystems continued to diversify with the evolution of the first vertebrates—the jawless fish. These creatures, like ostracoderms, lacked true jaws but possessed bony armor plating for protection. Later, jawed fish, or gnathostomes, appeared near the end of the Silurian Period.
The development of jaws allowed for predation and crushing hard-shelled prey, leading to the rise of apex predators like the armored placoderms in the Devonian Period. The Devonian is sometimes called the “Age of Fishes” because of the variety and abundance of both jawless and jawed fish that dominated the global oceans.
The Emergence of Terrestrial Vertebrates
The shift from sea to dry land began with the lobe-finned fish, or Sarcopterygians. These fish possessed fleshy fins with internal bones structurally homologous to the limb bones of land animals. Certain lineages developed adaptations that allowed them to navigate shallow, oxygen-poor waters and briefly haul themselves onto land.
During the Devonian Period, this lineage led to the first four-limbed vertebrates, known as tetrapods, represented by forms like Acanthostega and Ichthyostega. Although these early tetrapods possessed fully formed limbs with digits, they were still largely aquatic, using their appendages to move through dense swamps. The earliest fully terrestrial vertebrates appeared later in the Carboniferous Period.
The Carboniferous saw the proliferation of humid coastal swamps and rainforests, providing the environment for the first terrestrial ecosystems to flourish. These extensive wetlands led to the rise of early amphibians, which became the dominant land vertebrates of the time. The period is sometimes called the “Age of Amphibians” due to the diversification of groups like the temnospondyls, which thrived in the warm, wet climate.
The dense plant life in these swamps provided food and shelter, leading to the first complex terrestrial food webs. This lush environment also supported the evolution of giant arthropods, including enormous dragonflies and millipedes, as plants and insects had already colonized the land ahead of the vertebrates.
The Reign of Mammal-Like Reptiles and the Great Extinction
As the continents converged to form Pangaea, the climate dried out, setting the stage for the rise of amniotes in the Permian Period. This group, which includes all reptiles, birds, and mammals, evolved the amniotic egg—a self-contained structure with a protective shell. The amniotic egg eliminated the need for water for reproduction, allowing these animals to conquer drier inland environments.
The dominant terrestrial animals of the early and middle Permian were the Synapsids, often called mammal-like reptiles, which are the lineage that eventually gave rise to mammals. Early Synapsids, such as the sail-backed Dimetrodon, were the top predators in their ecosystems. Later Synapsids, known as Therapsids, were more advanced and diversified into a range of herbivores and carnivores.
The Synapsids shared the land with early Sauropsids, the lineage that would eventually produce the reptiles, dinosaurs, and birds. Although both groups adapted to the arid conditions of the supercontinent, the Synapsids remained the more diverse group for much of the Permian. This Synapsid-dominated world was ended by the most severe extinction event in Earth’s history: the Permian-Triassic Extinction, or “The Great Dying.”
Around 252 million years ago, volcanic eruptions in Siberia released large amounts of carbon dioxide and sulfur dioxide into the atmosphere. This caused rapid environmental changes, including global temperature spikes, ocean acidification, and widespread oxygen depletion. The catastrophe wiped out approximately 81% of all marine species and 70% of all terrestrial vertebrate species, fundamentally resetting the course of life on Earth.
The Early Triassic Survivors
The early Triassic Period, immediately following the Great Dying, was a time of severe ecological recovery characterized by lower species diversity. The harsh, post-extinction environment was dominated by a handful of resilient “disaster taxa” that had managed to survive. The most notable survivor was Lystrosaurus, a sturdy, herbivorous Therapsid that looked something like a pig with tusks.
Lystrosaurus became the most common terrestrial vertebrate on the planet for a time, with its fossils accounting for up to 95% of all land vertebrates found in some early Triassic fossil beds. Its success was likely due to burrowing adaptations, which allowed it to shelter from the harsh climate, and a tolerance for low-oxygen air. Its widespread distribution across Pangaea also provided evidence for the theory of continental drift decades later.
While Lystrosaurus briefly reigned, the ecological niches left vacant by the extinction allowed other, smaller groups to begin diversifying. Among the most important emerging groups were the Archosaurs, a lineage of Sauropsids that includes crocodiles, pterosaurs, and the direct ancestors of the dinosaurs. These early Archosaurs were small and agile, such as the predatory Thrinaxodon, and possessed features that would characterize their descendants, including a more upright posture.
The first true dinosaurs appeared during the later stages of the Triassic Period, but they were initially small, bipedal creatures that were not yet the dominant force. The world immediately preceding the Age of Dinosaurs was one of recovery, dominated by the resilient Synapsid survivors like Lystrosaurus and the first steps of the Archosaurs, who would eventually rule the Mesozoic Era.