Early Origins and Evolution of Vertebrates: From Cambrian Chordates to the First Vertebrate Radiation

The emergence of vertebrates represents one of the most significant evolutionary transitions in animal history. The earliest definitive vertebrate fossils appear in the Lower Cambrian Period (approximately 525-520 million years ago), with Myllokunmingia fengjiaoa and Haikouichthys ercaicunensis from the Chengjiang biota of China being among the oldest known specimens. These early vertebrates evolved from chordate ancestors, likely similar to modern lancelets (amphioxus). The key transition occurred through the development of neural crest cells, a mineralized skeleton, and a more complex brain. The foundation for vertebrate origins was laid in the Pre-Cambrian with early chordates like Pikaia gracilens, which exhibited basic chordate characteristics. The early Cambrian period witnessed a rapid diversification of vertebrate forms, leading to the establishment of major vertebrate lineages, including both jawless (agnathan) and later jawed (gnathostome) groups.