Evolution - Bipedalism

Vocabulary

• Bipedal: Walks on two legs.

• Canine: Pointed tooth for tearing food.

• Cranial capacity: Internal volume of the cranium, indicating brain size.

• Cranium: The part of the skull enclosing the brain.

• Evolve: To develop gradually through natural selection.

• Foramen magnum: Hole in the skull where the spinal cord exits, its position indicates bipedalism.

• Hominid: Includes all great apes (e.g., humans, chimps, gorillas).

• Hominin: Subset of hominids leading to humans, characterized by bipedalism.

• Index: A ratio comparing measurements.

• Maxilla: Upper jawbone.

• Orbit: Eye socket.

• Palate: Roof of the mouth.

• Skull: Framework of bones enclosing the brain.

Key Features for Identifying Human vs. Chimpanzee Skulls

• Humans:

  • Larger cranial capacity (more room for the brain).

  • Flatter face and shorter jaw.

  • Centrally located foramen magnum (upright posture).

  • Teeth contained within the jaw.

• Chimpanzees:

  • Smaller cranial capacity.

  • Longer jaw with protruding face.

  • Foramen magnum towards the back (indicating walking on all fours).

  • Teeth may extend outside the jaw.

Foramen Magnum and Bipedalism

• The opisthion index is key for determining bipedalism:

  • Humans: High index (~34.05), foramen magnum near the center (upright walking).

  • Chimps: Low index (~8.89), foramen magnum at the back (knuckle-walking).

• Hominins characterized by bipedalism (high opisthion index):

  • H. habilis, H. erectus, H. heidelbergensis, H. sapiens neanderthalensis, and H. floresiensis.

• Early species like A. afarensis and A. africanus had lower indexes, showing early bipedal traits but less adaptation.

Cranial Capacity and Evolution

• Humans: 1,030 cm³ cranial capacity.

• Chimpanzees: 450 cm³.

• Largest capacity: Homo sapiens neanderthalensis (1,125 cm³).

• Cranial capacity reflects brain size and is linked to cognitive abilities like reasoning and tool use.

• Bipedalism evolved first, as early species like A. afarensis walked upright but had smaller brains (~405 cm³).

Comparison of Human and Chimpanzee Foreheads

• Humans:

  • Large, vertical forehead.

  • Space for a developed frontal lobe (reasoning, planning).

• Chimps:

  • Small, sloping forehead.

Evolutionary Sequence

1. Bipedalism evolved first (e.g., A. afarensis).

2. Larger brains developed later in species like H. erectus and H. sapiens.

The foramen magnum is a hole in the base of the skull where the spinal cord connects to the brain. Its position in the skull provides important clues about how an animal moves (its posture and mode of walking).

Here’s how the foramen magnum relates to bipedalism (walking on two legs) and knuckle-walking (walking on all fours using the knuckles):

1. Bipedalism (Humans and Other Bipedal Hominins)

• In bipedal species (like humans), the foramen magnum is positioned closer to the center of the skull.

• This alignment allows the head to be directly above the spine, which is necessary for maintaining balance when standing or walking upright.

• The spine connects more vertically to the skull, allowing for an efficient upright posture and walking on two legs.

2. Knuckle-Walking (Chimpanzees and Other Apes)

• In knuckle-walking species (like chimpanzees), the foramen magnum is positioned further back (toward the rear of the skull).

• This placement allows the spine to enter the skull at an angle, which supports a forward-leaning posture.

• It helps them balance their head while walking on all fours or using their knuckles to support their weight.

Why the Position Matters

• The foramen magnum's position indicates posture:

  • A central position (like in humans) suggests upright walking (bipedalism).

  • A rearward position (like in chimps) suggests knuckle-walking or walking on all fours.

Example:

• Humans: The foramen magnum is near the center of the skull, helping us walk upright (bipedalism).

• Chimpanzees: The foramen magnum is towards the back, helping them balance while walking on all fours (knuckle-walking).

In Short:

• If the foramen magnum is central, the species is likely bipedal.

• If the foramen magnum is rearward, the species is more likely to be knuckle-walking or quadrupedal.