Animals began as ocean life with a shared gene kit
Kingdom Animalia gathers multicellular eukaryotes that eat organic matter, breathe oxygen, move with muscle cells, reproduce sexually, and pass through a blastula stage. Fossils appear by the Ediacaran; most modern phyla surface in the Cambrian explosion around five hundred thirty-nine million years ago. One ancestral toolkit of thousands of gene groups still unites living animals.
Five major clades frame the kingdom: sponges, comb jellies, placozoans, cnidarians, and bilaterians. Most species are bilaterians with mirrored sides and a head end, splitting into protostomes—including arthropods and molluscs—and deuterostomes such as echinoderms and chordates. Aristotle sorted animals by blooded versus bloodless; Linnaeus built a hierarchy in seventeen fifty-eight; Lamarck expanded phyla by eighteen oh nine; Haeckel in eighteen seventy-four separated multicellular Metazoa from unicellular Protozoa. Latin animalis meant having breath or soul; everyday speech often shrinks animal to nonhumans.
Life started marine. All living phyla except Micrognathozoa and Onychophora still include ocean species. Arthropods crawled ashore with early plants between about five hundred ten and four hundred seventy-one million years ago; vertebrates followed later via forms like Tiktaalik. Animals now fill vents, deserts, air, and other organisms' interiors, though few tolerate constant heat above fifty degrees Celsius. Size extremes run from microscopic invertebrates to blue whales up to about one hundred ninety tonnes and thirty-three point six metres—the largest animals ever—while African bush elephants top living land mammals near twelve tonnes.
Humans lean hard on other animals for meat, eggs, dairy, leather, wool, pets, and labour. Dogs were the first domesticate, later joined by horses, pigeons, and birds of prey in hunting and war. Ecologically, most animals sit downstream of photosynthetic producers. Counting described species by phylum shows arthropods dominating totals, yet predictive estimates dwarf formal lists. The kingdom's unity is developmental and genomic; its diversity is almost every lifestyle Earth allows.
Source: Animal
