Basal

Adjective

 * 1) Relating to, or forming, the base, or point of origin.
 * 2)  In a phylogenetic tree, being a group, or member of a group, which diverged earlier.

Noun

 * 1)  Any basal structure or part

Thesaurus
ab ovo, aboriginal, basic, basilar, beginning, bottom, bottommost, central, constituent, constitutive, crucial, elemental, elementary, embryonic, essential, foundational, fundamental, generative, genetic, germinal, gut, in embryo, in ovo, lowermost, lowest, material, nethermost, of the essence, original, pregnant, primal, primary, primeval, primitive, primordial, pristine, protogenic, radical, rudimental, rudimentary, seminal, substantial, substantive, underlying, undermost