Final answer:
The primary subfield of biological anthropology addressed in the fossil species research that lived about 4.4 million years ago in Africa is paleoanthropology. This subfield studies the evolution and behavior of hominids through the fossil record and is crucial in understanding human evolution.
Step-by-step explanation:
Primary Subfield of Biological Anthropology
The research regarding the discovery of a new fossil species that has adaptations both for climbing trees and walking on two legs corresponds to the subfield of paleoanthropology. This subfield specifically deals with the study of ancient humans and their primate relatives' fossil records. Paleoanthropologists aim to understand the evolution and behavior of hominids, the biological family that includes modern humans, Australopithecus, and other early human ancestors.
Biological anthropology is a diverse field with several subfields, but when it comes to the study of human and primate evolution, particularly through fossilized remains, paleoanthropology is the most relevant subfield. This branch of anthropology examines various species in the hominin lineage, their environmental adaptations, and the emergence and migration patterns that have led to the diversity of species observed in the fossil record. The research into the newly discovered species dating back 4.4 million years helps contribute to the broader understanding of our evolutionary history within the context of biological anthropology.