Cambrian explosion, the unparalleled emergence of organisms between 541 million and approximately 530 million years ago at the beginning of the Cambrian Period. The event was characterized by the appearance of many of the major phyla (between 20 and 35) that make up modern animal life. Many other phyla also evolved during this time, the great majority of which became extinct during the following 50 to 100 million years. Ironically, many of the most successful modern phyla (including the chordates, which encompass all vertebrates) are rare elements in Cambrian assemblages; phyla that include the arthropods and sponges contained the most numerically dominant taxa (taxonomic groups) during the Cambrian, and those were the taxa that became extinct.

The beginning of the Cambrian Period is marked by the evolution of hard body parts such as calcium carbonate shells. These body parts fossilize more easily than soft tissues, and thus the fossil record becomes much more complete after their appearance. Many lineages of animals independently evolved hard parts at about the same time. The reasons for this are still debated, but a leading theory is that the amount of oxygen in the atmosphere had finally reached levels that allowed large, complex animals to exist. Oxygen levels may also have facilitated the metabolic processes that produce collagen, a protein building block that is the basis for hard structures in the body.

Other major changes that occurred in the Early Cambrian (541 to 510 million years ago) include the development of animal species that burrowed into the sediments of the seafloor, rather than lying on top of it, and the evolution of the first carbonate reefs, which were built by spongelike animals called archaeocyathids.

By the Early Cambrian the bulk of the biosphere was confined to the margins of the world’s oceans; no life was found on land (except possibly cyanobacteria [formerly known as blue-green algae] in moist sediment), relatively few pelagic species (biota living in the open sea) existed, and no organisms inhabited the ocean depths. Life in the shallow regions of the seafloor, however, was already well diversified. This early aquatic ecosystem included the relatively large carnivore Anomalocaris, the deposit-feeding trilobites (early arthropods) and mollusks, the suspension-feeding sponges, various scavenging arthropods, and possibly even parasites such as the onychophoran Aysheaia. Thus, it seems likely that a well-developed aquatic ecosystem was already in operation in the ocean shallows by this time.

Following the Cambrian Period, the biosphere continued to expand relatively rapidly. In the Ordovician Period (485.4 million to 443.4 million years ago), the classic Paleozoic marine faunas—which included bryozoans, brachiopods, corals, nautiloids, and crinoids—developed. Many marine species died off near the end of the Ordovician because of environmental changes. The Silurian Period (443.4 million to 419.2 million years ago) marks a time when a rapid evolution of many suspension-feeders in the oceans occurred. As a result, pelagic predators such as nautiloids became abundant. Gnathostome fishes, the oldest craniates, became common near the end of Silurian times.