5 digits primitive, others derived from ancestral form

fish & tetrapods (article missing)

Pentadactyly (from Greek pente- = "five" plus δακτυλος = "finger") is the condition of having five digits on each limb. It is believed that all living tetrapods are descended from an ancestor with a pentadactyl limb, although many species have now lost or transformed some or all of their digits by the process of evolution

"Major branching events in vertebrate evolution occurred long ago. Sharks branched off the human lineage at least 430 million years ago.

The ray-finned fishes, technically known as Actinopterygians, branched off 420 million years ago.
"Teleosts, the largest subgroup of Actinopterygians, include the vast majority of fishes today," Coates said.

Acanthostega looked like "a large, grotesque salamander," Coates said. It had legs and digits, with rudimentary ankles and wrists, but also internal gills and a large tail fin. Acanthostega, along with new work by Coates and Clack on other previously discovered early tetrapods, contradicted long-held paleontological beliefs that early tetrapods all had five digits on each limb. But, Acanthostega had eight, while other creatures of the same period had seven and six digits."

[DD: Ancanthostega derived 8 from 5 primary digits for better swimming (just as the ray finned fish did), it probably lacked interdigital skin webbing (unlike todays' frogs and salamanders, think sturgeon) so more digits were selected, with multiple extra digits, some becoming mere t
hin needle-like bones in teleosts. Tiktaalik had 5 digits, and was ancestral to tetrapods.]

Interesting note:

Didactyly (from Greek di-="two" plus δακτυλος = "finger") or bidactyly is the condition of having two digits on each limb, as in the Hypertragulidae and Two-toed Sloth, Choloepus didactylus. In humans this name is used for an abnormality in which the middle digits are missing, leaving only the thumb and fifth finger, or big and little toes.