Final answer:
Constantin Brancusi's early career was most significantly influenced by Cubism, as it directly impacted his sculptural work, in particular his abstraction and geometric reconfiguration of forms.
Step-by-step explanation:
The art predecessor most important to Constantin Brancusi's early career was Cubism. Brancusi's approach to sculpture was influenced by Cubist themes that involved the abstraction and reinterpretation of forms in three-dimensional space.
This is evident in Brancusi's work, Bird in Space, which is a simplification of form to express the essence of the subject. The development of Cubism was greatly influenced by Pablo Picasso and Georges Braque, who incorporated primitivist elements into their work which they, in turn, had assimilated from African tribal art.