Freshwater or marine origin of the vertebrates?

1. Paleontological data indicate that the earliest recognizable vertebrate remains, bone fragments of Upper Cambrian and Lower Ordovician heterostracan fishes, were deposited in a marine situation. 2. Since these earliest fossils are sporadic in occurrence, from atypical marine deposits and since they only represent the full grown adult stage, the possibility of a freshwater developmental stage or estuarine habitat cannot be excluded. 3. The hagfishes, supposedly the most primitive of living vertebrates, are exclusively marine and possess an osmoregulatory strategy (monovalent ion levels nearly identical with sea-water with little capability of regulation) that is consonant with a strictly marine evolutionary history. Possibly, but less parsimoniously, this strategy and habitat could be secondarily derived. 4. The hagfish has a glomerular kidney, renal sodium reabsorption and branchial pumps for the uptake of sodium and chloride which are indicative, but not unequivocally diagnostic, of a freshwater ancestry. 5. A scenario in which the earliest vertebrate was anadromous, breeding in fresh water and migrating to the sea, is consistent with the paleontological data and with the physiology and life history of living 'primitive' fishes. It also leads to more coherent explanations for the origin of bone and for the evolution of vertebrate special senses than do alternative marine scenarios.