Key features: notochord, 2) dorsal (hollow nerve cord), pharyngeal gill slits, and 3) post- anal tail. Pharyngeal gill slits: regresses and disappears in amphibians, reptiles, birds and mammals. For apes and humans, appears during embryonic stage. Humans are a part of phylum chordate- since we have spinal chord. Adults only have nerve cord, embryos have all four features. For embryo: tail disappears completely, notochord is replaced by backbone, gill slits attribute to lower jaw formation. Ecothermic: rely on environment for heat ex) frogs. Lose notochord and dorsal, hollow tubular nerve cord when adult. As adults, possess all four main chordate features. Vertebrates- have a backbone, made of bone or cartilage. Increased size and complexity of the brain and sensory structures. Earliest fossils found are 530 million years old. In adulthood: notochord (no vertebral column) partially replaced by cartilage. Reproduce like salmon (lay eggs in freshwater and spawn travel to saltwater in adulthood.