Fossil Record and Early Hominins

Defining Characteristics of Hominins

• Hominins are defined by several key traits that distinguish them from other primates:
  • Earliest Hominins:
  • Facultative Bipedalism: Ability to walk on two legs intermittently.
  • Dental Characteristics: Changes in tooth shape and size.
  • Reduction in Facial Prognathism: Less protrusion of the face compared to ancestors.
  • Later Developments:
  • Obligate Bipedalism: Fully adapted to walking on two legs.
  • Larger Brains in Relation to Body Size: Increased brain size relative to body.
  • Slow Development and Long Juvenile Period: Extended timeframe for growth.
  • Material and Symbolic Culture: Development of tools, culture, and language.

Bipedal Locomotion vs. Chimp Quadrupedalism

• Differences in anatomical features:
  • Base of Skull & Foramen Magnum:
  • Humans: Centered under the skull for upright posture.
  • Chimpanzees: Positioned towards the back for quadrupedal stance.
  • Vertebral Column:
  • Humans: 'S' shaped for bipedal support.
  • Chimpanzees: More linear.
  • Pelvis & Limb Structure:
  • Humans: Shorter arms, longer legs.
  • Chimpanzees: Longer arms, curved hand bones for grasping.

Mosaic Evolution

• Refers to the concept where different traits can evolve at varying rates:
  • Traits from both ancestral and derived forms can coexist in hominins.

Dental Adaptations in Early Hominins

• Mix of ancestral (ape-like) and derived traits:
  • Derived Features:
  • Smaller canines.
  • Smaller diastema.
  • V or parabolic shaped dental arcade.
  • Thicker enamel.
  • Decreased canine sexual dimorphism.
  • Non-honing complex (C/P3).

Ecological Context: Miocene Africa

• Period: 23-5 million years ago.
• Climate changes leading to:
  • Tropical forests; dominance of over 40 genera of apes.
  • Increasing variability in environment leading to diverse habitats.
• By mid-Miocene, the world became cooler and drier:
  • Adaptations in hominins due to shifting environments.

Significant Geological Features

• Africa's Great Rift Valley:
  • Intersection of multiple continental plates.
  • Importance in fossil studies: exposure of fossils, climate change impact, and the availability of datable layers.

Earliest Known Hominins (Basal Hominins)

• Timeline: 7-4.4 million years ago, possibly multiple species.
• Notable Species:
  • Sahelanthropus tchadensis:
  • Age: 7-6 million years.
  • Characteristics: Centered foramen magnum, flat face, small canines.
  • Orrorin tugenensis:
  • Age: 6 million years.
  • Features: Thick enamel, shape of femur indicates bipedalism.
  • Ardipithecus ramidus:
  • Age: 4-5 million years.
  • Mosaic traits indicating both bipedality and arboreal activity.

Notable Traits of Sahelanthropus tchadensis

• Primitive features include brain size (320-350cc), but with a flat face and smaller canines typical of later hominins.

Orrorin tugenensis

• Habitat: Partially wooded/open habitats.
• Hominin features include thicker enamel and a femur suggesting bipedalism.

Ardipithecus ramidus

• Characteristics:
  • Mix of ancestral and derived features, such as a divergent big toe and curved finger bones, but also had adaptations for bipedal locomotion.

Summary of Dental and Skeletal Features

• Ancestral features: larger canines, thinner enamel, and small brain size.
• Derived features: bipedal pelvis, stable foot structure, less sexual dimorphism.

Evolutionary Cladistics of Hominids

• Evolutionary Path: Relations amongst various hominins, highlighting key transitions such as canine reduction, brain expansion, and changes in life history traits.