basal

Short definition from Wiktionary

Adjective

1. 1
   Basic, elementary; relating to, or forming, the base, or point of origin.
2. 2
   Associated with the base of an organism or structure.
3. 3
   Of a minimal level that is necessary for maintaining the health or life of an organism.
4. 4
   In a phylogenetic tree, being a group, or member of a group, which diverged earlier. The earliest clade to branch in a larger clade.

Noun

1. 1
   base, bottom, minimum
2. 2
   Any basal structure or part