Five hundred million years ago, Earth’s landscape differed vastly from today. This period, known as the Cambrian, was a time of significant geological shifts and biological innovation. It reveals a planet undergoing transformations in its landmasses, oceans, and the life within them.
The Global Landmasses
The configuration of Earth’s continents 500 million years ago was markedly unlike the familiar map of today. A dominant supercontinent, Gondwana, was the largest landmass, primarily situated in the Southern Hemisphere and extending towards the equator. This immense continent comprised land that would eventually become present-day South America, Africa, Australia, Antarctica, India, and parts of Asia.
North America, then known as Laurentia, was a separate continent located near the equator, positioned apart from Gondwana by the Iapetus Ocean. Other smaller continental fragments, such as Baltica (modern-day Scandinavia, Eastern Europe, and European Russia) and Siberia, were also present, drifting across the ancient seas. These landmasses were generally low-lying, with mountains forming along zones of continental collision.
Earth’s Climate and Atmosphere
The global climate during the Cambrian Period was considerably warmer than today, characterized by a general absence of polar ice caps. This warmer state was influenced by elevated atmospheric carbon dioxide levels, which were significantly higher than current concentrations, reaching up to 4,000 parts per million (ppm) at times, compared to about 420 ppm today. Such high carbon dioxide levels contributed to a pronounced greenhouse effect, leading to mild land temperatures even near the poles.
Atmospheric oxygen levels were also on the rise throughout the Cambrian, increasing from approximately 3% to 14% of the atmosphere, though still below the modern level of 21%. This gradual oxygenation of the atmosphere and shallow oceans played a role in supporting the developing life forms. The warm climate and lower oxygen levels in marine waters also meant that deep ocean basins experienced widespread anoxic, or oxygen-depleted, conditions.
Vast Shallow Oceans
Earth’s surface was largely covered by extensive marine environments during the Cambrian Period. Global sea levels were considerably higher than they are today, leading to the widespread inundation of continental landmasses. This created vast, shallow epicontinental seas that stretched across large portions of the continents, such as the Kaskaskia Sea covering much of North America.
These shallow seas were warm and nutrient-rich, providing ideal conditions for the proliferation of marine life. The retreat of the late Proterozoic glaciers contributed to this significant rise in sea level, expanding the available habitats for marine invertebrates.
The Cambrian Explosion of Life
The Cambrian Period is renowned for the “Cambrian Explosion,” a span of approximately 13 to 25 million years marked by a rapid diversification of complex animal life. During this time, nearly all major animal phyla, the fundamental blueprints for animal body plans, appeared in the fossil record. This included the emergence of early chordates, the group that encompasses all vertebrates.
A significant development during this evolutionary burst was the widespread evolution of hard parts, such as shells and exoskeletons. These mineralized structures, like those found in trilobites, greatly enhanced the preservation of organisms, providing a rich fossil record from this era. Trilobites, an extinct group of marine arthropods, were particularly abundant and diverse, quickly populating the ancient oceans.
They were among the first animals to develop complex eye structures. Predators also emerged during this period, including creatures like Anomalocaris. This large marine arthropod, reaching up to 60 centimeters or even over a meter in length, possessed large compound eyes and specialized appendages for capturing prey. The development of such predators likely exerted evolutionary pressure on other organisms, contributing to the rapid diversification and the emergence of defensive features like hard shells.