What was the "Cambrian explosion"?

The "Cambrian explosion" or "Cambrian radiation" was an event approximately 541 million years ago where the appearance of complex, multicellular organisms showed up in the fossil record. It was during this period when most of the major animal phyla (the taxonomic group that is below a kingdom, but above class) appeared. This also marked the time period when the variety of life found began to resemble life found on Earth today.

Textbooks from the United States traditionally used a system of six kingdoms (Animalia, Plantae, Fungi, Protista, Archaea/Archaebacteria, and Bacteria/Eubacteria). Essentially dividing organisms into plants, animals, fungi, and others. Phyla can be thought of as groupings of animals based on general body plan. For example, despite their obvious differences, spiders and barnacles both belong to the phylum Arthropoda.

Prior to this rapid period of development, taking place over a period of 13 to 25 million years (which is a relatively short period in the fossil record), and diversification of complex life, most multicellular life consisted of individual cells organized into colonies, rather than organisms with specialized cellular structures. The first fossils discovered from the Cambrian era were trilobites (an invertebrate marine animal with an exoskeleton). Fossils of earlier multicellular organisms have been found, but were fossilized remains of colonies of microorganisms known as stromatolites.