Five hundred million years ago, Earth was in the midst of the late Cambrian Period, marking the early Paleozoic Era. This time followed the severe “Snowball Earth” glaciations of earlier eras, characterized by a world recovering from extreme cold and transitioning into a prolonged warm phase. The planet was a dynamic system of shifting landmasses and rising oceans, creating the conditions for a dramatic biological event. The surface, atmosphere, and biosphere were fundamentally restructured, setting the stage for the complexity of life that would follow.
Global Geography: The Fragmented Supercontinents
The lithosphere 500 million years ago was defined by the ongoing breakup of the older supercontinent Pannotia, resulting in several major landmasses separated by vast oceans. The largest of these continental blocks was Gondwana, which contained the future continents of South America, Africa, India, Antarctica, and Australia. This immense landmass was situated primarily in the Southern Hemisphere, stretching from the South Pole toward the equator, but not centered directly on the pole, which limited glaciation.
Other significant continental fragments floated in relative isolation, mostly clustered around the equator. Laurentia, the ancient core of North America, was a separate entity, as was Baltica, which would eventually form Northern Europe. Siberia was another distinct craton, positioned away from the others. The rifting and dispersal of these continents led to the formation of new, extensive ocean basins, such as the Iapetus Ocean. The configuration of these scattered landmasses, with their shallow, submerged edges, became a defining feature of the global geography.
Climate and Environmental Conditions
The climate of the late Cambrian Period was generally warm and mild, a consequence of the high levels of atmospheric carbon dioxide. Global temperatures were significantly elevated compared to the present day. The abundance of this greenhouse gas contributed to the planet’s warm state, despite the oxygen levels still being lower than modern concentrations.
A defining feature of this environment was the near-total absence of permanent polar ice caps. This lack of glaciation meant that vast amounts of water remained in the oceans, leading to high global sea levels. The rising water flooded the low-lying edges of the continents, creating expansive, warm, shallow marine environments. These shallow seas, which covered much of the continental margins, became the primary setting for the diversification of early animal life. The high \(\text{CO}_2\) concentration maintained a humid and generally ice-free world.
The Rise of Complex Life: The Cambrian Explosion
The immense, shallow seas of the Cambrian Period were the stage for the most rapid diversification of life in Earth’s history, an event known as the Cambrian Explosion. Nearly all modern animal phyla, the fundamental body plans of animal life, first appeared in the fossil record during this relatively short interval. This biological innovation was characterized by the evolution of hard parts, like shells and exoskeletons, which offered protection and structural support.
Among the most successful and widespread organisms were the arthropods, exemplified by the iconic trilobites. These segmented creatures, with their hard, jointed exoskeletons, were dominant in the shallow marine ecosystems and left an abundant fossil record. Complex predatory-prey dynamics also emerged, driven by the evolution of mobile predators like Anomalocaris, a large, shrimplike hunter that trapped prey with specialized appendages.
The experimentation with body plans extended to the earliest ancestors of vertebrates, the chordates, with primitive forms like Pikaia appearing in the fossil record. The remarkable preservation of soft-bodied creatures, such as those found in the Burgess Shale fauna, reveals the bizarre complexity of life, including organisms like Hallucigenia. While the oceans teemed with this rapidly evolving and complex life, the continents remained starkly barren. Terrestrial environments were essentially sterile, consisting of rocky landscapes and microbial mats, with complex plant or animal life yet to make the transition out of the water.