During the Cambrian Explosion, which occurred roughly between 541 and 530 million years ago at the start of the Cambrian Period, there was an unprecedented rapid emergence and diversification of complex life forms in Earth's oceans. This event marked the appearance of almost all major animal phyla that constitute modern animal life, with estimates ranging between 20 and 35 phyla emerging during this relatively short geological period

. Before the Cambrian Explosion, life was mostly simple, consisting of unicellular organisms or small multicellular colonies. The explosion introduced a dramatic increase in complexity and diversity, with animals developing new anatomical features such as legs, compound eyes, and protective shells. Arthropods, for example, evolved legs and armor-like chitinous shells, while other groups developed features that allowed for more active predation and complex ecological interactions

. Several factors are believed to have contributed to this rapid diversification:

• A modest rise in atmospheric and oceanic oxygen levels, which may have enabled higher metabolic rates and more complex body structures. Recent research suggests that only a small increase in oxygen was sufficient to drive these ecological changes

• Evolutionary innovations such as the development of eyes, which could have triggered new predator-prey dynamics and an evolutionary arms race, accelerating diversification

• The formation of the Earth's ozone layer, providing protection from harmful ultraviolet radiation and allowing more complex life to thrive in shallow waters

• Ecological factors including the emergence of predators, which created selective pressures leading to rapid adaptation and diversification

• Possible mass extinction events that opened ecological niches for new groups to evolve and diversify

Notable organisms that appeared during or shortly after the Cambrian Explosion include early arthropods like Anomalocaris , primitive chordates such as Pikaia , and other unique creatures like Wiwaxia and Spriggina that exhibit body plans distinct from modern animals

. In summary, the Cambrian Explosion was a pivotal evolutionary event characterized by the rapid appearance of most major animal groups, driven by a combination of environmental changes, biological innovations, and ecological interactions, fundamentally reshaping the complexity of life on Earth