4.7.2 Miocene Hominids

The first hominid fossils appear in the late Miocene, 10 to 5 MYA. Sometime between 7 MYA and 4 MYA, hominids moved out of the trees and began to adapt more fully to a ground-based living niche. Unfortunately, the fossil evidence from this time period is extremely sparse, but new finds continue to be discovered.

A complete cranium of Sahelanthropus tchadensis was found in 2002 by French paleoanthropologist Michel Brunet and his team in Chad in West Africa. Sahelanthropus is a fossil ape that lived approximately 7 MYA and is claimed by some researchers to be the last common ancestor of humans and chimpanzees. Genetic studies indicate that humans and chimpanzees diverged from one another sometime between 5 MYA and 7 MYA, so this species lived right at the time of the divergence. The cranial capacity is a mere 350 cubic centimeters (cc), which is equivalent to a chimpanzee; the modern human cranial capacity is approximately 1,400 cc. Sahelanthropus also has a very large brow ridge (the large bone above the eyes), and the location of the foramen magnum, the opening at the base of the skull where the spinal column enters the skull, suggests that its head was not held over its spine and thus it was not bipedal.

Orrorin tugenensis was found in Kenya in 2001 by geologist Martin Pickford of the Collège de France and paleontologist Brigitte Senut of France’s National Museum of Natural History. Orrorin tugenensis was dated to approximately 6 MYA. Orrorin was proposed to be a hominin due to anatomical traits that suggest bipedalism. For example, the femoral head (the big, rounded ball at the top of the leg bone that connects the leg to the hip) is much larger than in quadrupedal apes, suggesting the femur was being used to support the weight of the upper body. The muscles attached to the femur also suggest bipedal movement. Another feature that suggests that Orrorin is truly a hominin is the teeth, which exhibit thick dental enamel and small, square molars, much like modern humans.