Term for "Ultimate Parent" or "Ultimate Ancestor"

I'd say the word your looking for is primogenitor, or possibly progenitor, which are seen as synonyms by Oxford Dictionary:

From https://en.oxforddictionaries.com/definition/primogenitor

primogenitor NOUN

An ancestor, especially the earliest ancestor of a people; a progenitor.

progenitor NOUN

1A person or thing from which a person, animal, or plant is descended or originates; an ancestor or parent. ‘his children were the progenitors of many of Scotland's noble families’

I like primogenitor better, because it is rooted in the root primo-, meaning "first."