Final answer:
The third oldest hominid is Sahelanthropus tchadensis, which lived around 7 MYA and displayed both primitive and evolved traits. Its small cranial capacity is similar to that of a chimpanzee, and its skeletal structure suggests it was not bipedal.
Step-by-step explanation:
The third oldest hominid is believed to be Sahelanthropus tchadensis, a fossil ape that lived approximately 7 million years ago (MYA). The discovery of a complete cranium of this species in Chad by a team led by French paleoanthropologist Michel Brunet shed light on its significant features. Sahelanthropus tchadensis exhibits a combination of both primitive and evolved characteristics, suggesting a close tie to the lineages leading to modern humans and modern chimpanzees.
The notable features of this ancient hominid include a cranial capacity of about 350 cubic centimeters (cc), akin to that of a chimpanzee. Despite this small cranial capacity, the specimen discovered possesses a large brow ridge, indicative of it being an early form of hominid. Notably, the placement of the foramen magnum in Sahelanthropus suggests that this hominid did not walk bipedally, as its head was likely not held directly over its spine.
While Sahelanthropus tchadensis represents a crucial link in our evolutionary past, its exact relation to modern humans remains subject to ongoing research and debate among scientists.