Key Concepts: Early Hominins

Evolutionary Themes

• Four main themes in human evolution:
  • Bipedalism
  • Distinctive dentition: reduced canines
  • Bigger brains
  • Culture
• Genus Homo emerges ca. $2.6\text{ Ma}$; this is our genus.

Reconstructing Human Evolution

• Evidence sources:
  • Fossil evidence
  • Molecular clock
  • Comparisons with living primates and humans
  • Paleoenvironmental reconstruction: climate data and other animal species

Epochs and Key Timeline

• Miocene: $23\text{–}5\text{ Ma}$ – Ape diversity
• Pliocene: $7\text{–}1.3\text{ Ma}$ – Early human ancestors: bipedalism and dentition
• Pliocene–Pleistocene transition: climatic instability around $2\text{ Ma}$

The Evolution of Bipedalism

• How to infer bipedalism in fossils:
  • 1) Pelvis is short and broad
  • 2) Knock-kneed alignment to maintain balance (valgus knee)
  • 3) Forward position of the foramen magnum
  • 4) Arms and legs change length over time

The First Bipeds: Morphology and Evidence

• Indicators include pelvis shape, knee alignment, skull base (foramen magnum), and limb proportions

Bipedal Advantages

• Long-distance travel energy efficiency
  • Example: $150\text{ lb}$ adult walking at $2\ \text{mph}$ for 1 hour ≈ $170\ \text{cal}$
  • Running: $150\ \text{lb}$ adult at $7\ \text{mph}$ for 1 hour ≈ $700\ \text{cal}$
• Ability to carry and manipulate objects
• Less sun exposure

Pliocene Climate Context

• $\text{Shift from wet forest to drier savanna/mosaic environments}$
• 2 Ma: climatic instability and mosaic habitats

Bipedal Disadvantages

• Birth more difficult as pelvis narrows
• Lower back stress
• Infant clinging more difficult

The First Bipeds: Sahelanthropus tchadensis

• Age: $6\text{–}7\text{ Ma}$
• Key features: sagittal crest (variable), small cranial capacity (~$360\text{ cc}$), relatively forward foramen magnum in some assessments
• Significance: possibly the earliest hominid; occurs in a mosaic environment; relevant to hominin–ape split debates

Sahelanthropus tchadensis: Foramen Magnum Question

• Some analyses suggest forward foramen magnum position; role in bipedality remains debated

Sahelanthropus tchadensis: Fossil Context

• Cranium from deposits ca. $6\text{–}7\text{ Ma}$ in Chad; fossil and virtual reconstructions discussed in early accounts

Sahelanthropus tchadensis: Summary

• Possibly earliest hominin (~6–7 Ma)
• Mosaic environments align with genetic split timing between hominins and apes

Ardipithecus ramidus (Ardi)

• Location: Middle Awash, Ethiopia (Aramis region)
• Age: >$4.4\text{ Ma}$; well dated
• Specimens: >100, including a partial skeleton (36 individuals)
• Size: ~$120\text{ cm}$ tall; ~$50\text{ kg}$; little sexual dimorphism
• Key features:
  • Reduced canine/premolar complex
  • Curved fingers
  • Facultative bipedalism: spent time in trees and walking upright
  • Opposable big toe

Ardipithecus ramidus: Ardi Skeleton

• Ardi: 4.4 Ma adult female; about 120 pounds; walked upright but lacked arch-like foot structure found in later hominins
• Long arms and hands; relatively short legs; suggestive of agile tree-climbing
• Found along the Awash River in Ethiopia

Geography and Fossil Context: Aramis Region

• Middle Awash region, Ethiopia
• Key sites: Aramis, ARA-VP-1 (Yonas Molar Site), ARA-VP-6/500 skeleton discovery
• >100 specimens; well dated to 4.4 Ma

Ardipithecus ramidus: Summary

• 4.4 Ma hominin with facultative bipedalism and tree life adaptations
• Reduced dentition and curved fingers indicate a mosaic of locomotor modes

Recap

• Indicators of bipedalism: pelvis, knee, foramen magnum, limb proportions
• Sahelanthropus tchadensis: 6–7 Ma; possibly bipedal
• Ardipithecus ramidus: 4.4 Ma; definitely bipedal; also spent time in trees

Key Questions

• Do earliest hominins derive from Miocene ape lineages tied to forested, fruit-rich environments?
• Are Sahelanthropus tchadensis and Ardipithecus ramidus truly hominins, or remnants of Miocene ape radiations?

Why bipedalism? Why reduced canines?

• Why bipedalism?
  • Late Miocene environmental change: mosaic environments (forests, woodlands, grasslands)
  • Energetics: knuckle-walking vs. bipedality; hands freed for manipulation
  • Overall: bipedalism as a versatile adaptation to mosaic environments
• Why reduced canines?
  • Large, sexually dimorphic canines in primates with intense male-male competition
  • Reduced canines may reflect reduced male-male competition or reduced social aggression
  • Possible roles for female choice and social structure adjustments

Mosaic Evolution Timeline

• Bipedalism and terrestriality: ~$6\text{ Ma}$
• Canine reduction: ~$6\text{ Ma}$
• Stone tools (material culture): ~$2.5\text{ Ma}$
• Modern bipedalism: ~$2\text{ Ma}$
• Brain expansion: ~$2\text{ Ma}$
• Further brain expansion: ~$300\text{ ka}$
• Fully modern human behavior: ~$50\text{ ka}$