The cranium of the oldest human ancestor would have differed significantly from modern human skulls in terms of size, shape, and certain anatomical features. Here is a general idea of what the cranium of our oldest ancestor might have looked like:

1. Smaller Overall Size: The cranium of our oldest ancestors would have been smaller compared to modern humans. The average brain volume of early hominins ranged from about 350 to 650 cubic centimeters (cc), while modern humans have an average brain volume of about 1350 cc.

2. Thicker Skull Bones: The cranial bones of our oldest ancestor would likely have been thicker than those of modern humans, providing greater protection for the brain. This feature is observed in some early hominin fossils, such as the renowned "Australopithecus afarensis" specimen known as "Lucy."

3. Pronounced Brow Ridges: The brow ridges above the eyes would have been more prominent and pronounced than in modern humans. These ridges helped to anchor powerful chewing muscles, as early humans consumed a diet that required significant chewing force.

4. Receding Forehead: The forehead would have been less rounded and more sloping compared to modern humans. A more pronounced brow ridge and a sloping forehead gave our ancestors a distinctive facial appearance.

5. Larger Jaw and Teeth: The jaws and teeth of our oldest ancestor would have been larger and more robust, reflecting their diet of tough plant materials and occasional meat. The dental arcade would have been wider, with larger molars for grinding vegetation.

6. Position of the Foramen Magnum: The foramen magnum, the opening at the base of the skull where the spinal cord connects to the brain, would have been positioned more towards the rear of the skull. This indicates a more horizontal posture compared to the more upright posture of modern humans.

7. Absence of a Chin: The cranium of our oldest ancestor would likely lack a well-defined chin. The modern human chin is a relatively recent evolutionary development associated with changes in jaw structure and muscle attachments.

8. Low Cranial Vault: The top of the skull, known as the cranial vault, would have been lower and more rounded compared to the high and rounded vault of modern humans.

It's important to note that the appearance and characteristics of the cranium can vary among different early hominin species, as our ancestors evolved and diversified over time. The exact features would depend on the specific species and evolutionary stage being considered.