Base, bottom, minimum

Any basal structure or part

Basic, elementary; relating to, or forming, the base, or point of origin.

"A basal reader is a kind of book that is used to teach reading."

Associated with the base of an organism or structure.

"In the spring, basal leaves emerge from a stout taproot."

Of a minimal level that is necessary for maintaining the health or life of an organism.

"a basal diet"

In a phylogenetic tree, being a group, or member of a group, which diverged earlier. The earliest clade to branch in a larger clade.

"A magnolia is a basal angiosperm."