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Alternative views and further studies

Key physical features

This species was a facultative biped and stood upright on the ground but could move on all four limbs in trees. Features of the anatomy are extremely primitive.

Brain

  • about 300-350cc, similar in size to modern female chimpanzees and bonobos

Body size and shape

  • similar in size to modern chimpanzees. The most complete specimen, a female, stood about 120cm tall
  • males were only slightly larger than females
  • the body shape was more ape-like than humans, but differed from living African apes in a number of significant features

Limbs

  • mix of primitive and derived features suggest this species was able to walk upright on the ground yet efficiently climb trees
  • long powerful arms that were not used for weight-bearing or knuckle-walking as with quadrupedal apes
  • bones in the wrist (particularly the midcarpal joint) provided flexibility and the palm bones were short. These features suggest this species was not a knuckle-walker and that the palms could support the body weight when moving along branches
  • finger bones were long and curving, both features useful for grasping branches
  • upper and lower legs bones (femur and tibia) have features consistent with bipedalism
  • feet were relatively flat and lacked arches, indicating this species could probably not walk or run long distances
  • they had grasping abducted toe characteristic of gorillas and chimps
  • the foot was more rigid than chimpanzees with the bases of the four toe bones oriented to reinforce the forefoot when pushing off. Chimps have a highly flexible midfoot that improves their ability to grasp and climb but are less effective for propulsion when walking on ground

Pelvis

  • has a mix of features useful for both climbing and upright walking and suggests the species still spent significant time in the trees
  • shape of the upper blades (ilium) appear short and broad like Australopithecus afarensis, indicating that the gluteal muscles had been repositioned. This lowered the body’s centre of mass so to balance on one leg when walking
  • the lower pelvis is large and the angle of the ischial surface does not face upward as it does in humans and Australopithecus. These are primitive features that suggest this species had massive hindlimb muscles for climbing and did not walk like A. afarensis
  • the sciatic knotch is similar in size and shape to later hominins. This is a derived feature and is not found in chimpanzees