Life › The Cambrian explosion › Cambrian explosion
First vertebrates: jawless fish
c. 518 million years ago (Haikouichthys)· Organism or group· Animals
Strong evidenceGood evidence; dates or details still being refined.
The earliest known backbone animals: inch-long jawless fish from about 518 million years ago in southern China.
See it on the Life map →What happened
The oldest accepted vertebrates, Haikouichthys and Myllokunmingia, were described from the Chengjiang fossils in 1999. About 2–3 cm long, they had a stiffening rod along the back (notochord), zigzag muscle blocks, gill pouches, eyes and a cartilage skull, but no jaws and no bone. Jawless fish later grew bony armor and spread through Ordovician and Silurian seas. Their only living jawless relatives are lampreys and hagfish.
Why it matters
They stand at the base of the lineage that leads to every fish, amphibian, reptile, bird and mammal, including humans.
The evidence (fossils, genes, rocks)
Hundreds of Chengjiang specimens showing head, gill and muscle structures; Metaspriggina from Canada's Burgess Shale area; and comparisons with living lampreys and hagfish.
Still debated
Whether Haikouichthys and its kin are true vertebrates or close relatives just outside the group, and when vertebrates split from other chordates.
Connections (3)
▸Chengjiang fossils (China) preserve First vertebrates: jawless fishWell established
The connection
Chengjiang's fine mud and rapid burial preserved soft tissues that normally rot, including the cartilage, gill pouches and muscle blocks of the earliest fish. Without this rare preservation, the first jawless vertebrates, which had no bone or teeth, would probably be invisible in the fossil record.
When and how
Near Haikou and Chengjiang, Yunnan province, China, about 518 million years ago; Haikouichthys and Myllokunmingia were described by Shu Degan and colleagues in 1999.
The evidence
Over 500 specimens described in 2003 show consistent vertebrate features: eyes, gill arches, zigzag muscle blocks and simple backbone elements.
Debates and open questions
Whether these animals are full vertebrates or close relatives just outside the group, and whether some features are artifacts of decay.
▸Pikaia is a chordate cousin of First vertebrates: jawless fishStrong evidence
The connection
Pikaia and the first vertebrates share the chordate body plan: a notochord stiffening the back, blocks of muscle for swimming, and a nerve cord along the back. But Pikaia lacked the clear skull, eyes and gill structures of Haikouichthys, and it is younger, so it is a side branch, not an ancestor.
When and how
Burgess Shale, Canada (Pikaia, about 508 million years ago) and Chengjiang, China (first vertebrates, about 518 million years ago).
How sure are we?
Comparisons of the fossils with living lancelets and lampreys; family-tree analyses place Pikaia near the base of the chordates.
Debates and open questions
Whether Pikaia is closer to vertebrates, to lancelets, or sits outside both depends on how its strange head features are interpreted.
▸First vertebrates: jawless fish led to Jawed fishWell established
The connection
Over about 80 million years, jawless fish evolved bony armor, stronger skulls and, in some groups, paired fins. In one branch, the front pair of gill-supporting arches became a hinged jaw. That change, combined with paired fins, produced jawed fish by about 440 million years ago.
When and how
Ordovician and Silurian seas, roughly 485–440 million years ago; many key early jawed-fish fossils come from southern China.
The evidence
Fossils of armored jawless fish showing steps such as paired fins, and embryo studies in sharks and lampreys showing how jaws develop.
Debates and open questions
Whether jaws truly came from gill arches is debated, as is which jawless group is closest to jawed fish.