Metazoa

Metazoa is a major clade within the eukaryotic supergroup Opisthokonta that includes all multicellular animals. In contrast with unicellular or colonial relatives, metazoans are characterized by multicellularity, differentiated tissues, and heterotrophic nutrition. Sponges (Porifera) are considered the most basal metazoan lineage, with all other animals sharing a common ancestor after diverging from that group.

Key traits of Metazoa include multicellular organization with specialized cell types and, in most groups, true tissues and organs. They typically secrete an extracellular matrix rich in collagen and other connective components. Most metazoans possess nervous and muscular systems, though some basal lineages lack true tissues. Development usually proceeds through embryonic stages that involve gastrulation. Reproduction is predominantly sexual, and many species exhibit distinct larval stages.

Origin and fossil history: Metazoans are believed to have originated from a choanoflagellate-like ancestor during the Neoproterozoic era. The oldest probable metazoan fossils date to roughly 600–700 million years ago, with rapid diversification during the Cambrian period beginning about 541 million years ago, leading to the broad range of animal phyla seen today.

Diversity and classification: Extant metazoans include the phyla Porifera, Placozoa, Cnidaria, and Ctenophora, as well as the extensive Bilateria, which comprises the majority of animal diversity. Within Bilateria are the major clades Protostomia and Deuterostomia, with further subdivisions such as Ecdysozoa and Lophotrochozoa. The metazoan lineage thus encompasses a wide range of forms, from simple, sessile organisms to highly complex vertebrates.