Early Homo Genus

Origins of Bipedalism

• Bipedalism is the defining trait of hominins; it predates brain size increases.

• Multiple models proposed for its evolution:

1. Tool Use Model

• Suggests larger brains and tool use led to bipedalism.

• Freed hands for carrying tools, food, and weapons.

• Critique: Fossil record shows bipedalism arose before tool use (~5-7 mya vs. 2.3 mya).

2. Predator Avoidance Model

• Bipedalism enabled better predator spotting on savanna.

• Critique: Standing to scan isn’t the same as habitual bipedalism; it may even increase visibility to predators.

3. Temperature Regulation Model

• Upright posture minimizes sun exposure and maximizes wind exposure.

• Critique: Other savanna animals aren’t bipedal; most avoid midday sun instead.

4. Food Acquisition Model

• Hands free to carry food; energy-efficient movement.

• Critique: Primates typically feed at source; bipedalism not often used for transporting food.

5. Bipedal Feeding Hypothesis

• Bipedalism for feeding efficiency, especially while standing in trees.

• Supported by early bipedal morphology: long arms, curved fingers, divergent big toe.

• Contrasts earlier savanna-based assumptions.

Environments of Early Bipeds

• Sahelanthropus (7–6 mya): Woodland and grassland mix.

• Orrorin (6–5.7 mya): Dry evergreen forest.

• Ardipithecus (5.8–4.4 mya): Forest-dwelling fauna evidence.

Genus Homo: General Traits

• Likely evolved from gracile Australopithecines, possibly A. afarensis.

• Key characteristics:

  • Larger brain size than apes/Australopithecines.

  • Smaller face and teeth.

  • Increased cultural reliance (tools, fire).

  • Emerged ~2–2.5 mya.

Notable Hominins and Brain Size

Homo habilis ("Handy Man")

• Lived ~2.33 to 1.7 mya, found in East Africa.

• First hominin definitively associated with stone tools.

• Skull more rounded; less prognathic face.

• Tooth structure: Less bulbous molars, large front teeth.

• Post-cranial traits similar to Australopithecines but with improved grip precision.

• Tool industry: Associated with Oldowan tools (choppers, scrapers, pounders).

• Brain specialization in frontal lobe linked to reasoning, social behavior, and early speech.

Cultural Behavior of Homo habilis

• Scavenger-gatherer lifestyle:

  • Processed animal bones, scavenged meat quickly, fled predators.

  • Possibly group coordination and division of labor.

• Potential use of camp sites or home bases for food processing.

• Likely shared both meat and vegetation among group members.

• Behavior marks shift from non-human primate patterns.

Killer Ape Theory (Raymond Dart)

• Suggests violence and weapon use drove bipedalism.

• Based on skull injuries and use of animal bones.

• Criticism: Early hominins were often prey, not predators—simpler explanations more likely.

Homo rudolfensis

• Appeared ~1.9 mya in East Africa.

• Proposed due to variation in H. habilis fossils.

• Compared to H. habilis:

  • Slightly larger brain and body.

  • More primitive teeth (larger/wider molars).