The skull has anatomical features that potentially indicate this primate had an erect spine, and therefore spent some of its time walking on two legs only.
But other researchers have since argued that this alone is not enough evidence to class Sahelanthropus as a hominin biped - a primate directly ancestral to humans - rather than a related, but not directly ancestral hominid.
Around the same time and at the same location where the skull was found, in Toros-Menalla in Chad, a partial left femur was also recovered.
"Based on our analyses, the partial femur lacks any feature consistent with regular bouts of terrestrial bipedal travel," Macchiarelli and team write in their paper.
Another paper still awaiting peer review from one of the authors of the original Sahelanthropus studies disputes this, claiming the femur has a hard top ridge that supports an upright stance.
They suspect Sahelanthropus may be an ancestral relative with no remaining living descendants - a primate lineage that went extinct.