Final answer:
Sahelanthropus tchadensis might not be classified as a hominin due to its small brain similar to that of a chimpanzee, large brow ridge, and the location of the foramen magnum which suggests it was not bipedal.
Step-by-step explanation:
Sahelanthropus tchadensis might not be a hominin for several reasons. Firstly, while it lived approximately 7 million years ago, around the time when the lineage leading to modern humans and chimpanzees bifurcated, its characteristics are a mix of primitive and evolved features. The skull exhibits a cranial capacity of only 350 cubic centimeters, akin to that of a chimpanzee, and a large brow ridge.
Additionally, the placement of the foramen magnum (the opening at the base of the skull) in Sahelanthropus tchadensis suggests it may not have been bipedal, which is a key characteristic in defining a species as a hominin. These features contrast with criteria used to classify species under the genus Homo, such as increased brain size, the ability to make tools, and bipedal locomotion. Therefore, despite living at the critical time of divergence, the physical features of tchadensis cast doubt on its status as a hominin.