Concise Summary of Earliest Hominins

Earliest Hominins Fossils
  • Fossils: Preserved remains or traces of past organisms
  • Types include: bone, teeth, trace fossils, wood/leaves, subfossils
  • Preservation quality is highly variable.
Continental Drift and Climate Change
  • Earth’s history divided by major life shifts due to environmental change.
  • Cooler, variable climate seen over evolutionary history.
  • Major climate shifts linked to continental drift after 200 Ma.
  • Global temperature cooling during the Cenozoic (65 Ma) especially post mid-Miocene (15 Ma).
Fossils and Dating Methods
  • Understanding evolutionary history requires a time frame.
  • Two dating methods:
    1. Absolute dating: Provides age in years.
    2. Relative dating: Gives age concerning other fossils; establishes a sequence.
  • Radiometric dating: Uses unstable isotopes to estimate age (e.g., potassium-argon, carbon dating).
  • Biostratigraphy: Dates rocks by comparing fossil sequences.
Closest Living Relative
  • Genetic analysis shows Pan (chimpanzees) as our closest relative.
  • Molecular clock suggests separation occurred ~ 6-8 Ma.
What is a Hominin?
  • Members of the ‘human clade’ post-split from chimpanzees.
  • Shared derived traits:
    1. Dental changes (smaller canines).
    2. Larger brain-to-body ratio.
    3. Slower maturation/development.
    4. Complex culture, incl. language.
    5. Habitual bipedalism.
Bipedalism and Anatomical Changes
  • Bipedalism is a defining feature seen in the fossil record.
  • Human pelvis: Adaptations for bipedalism vs. large-brained baby births.
  • Hip abductor muscles and posture adaptations support efficient walking.
  • Bipedal adaptation allows for less oxygen consumption compared to quadrupedalism.
The First Hominins
  • Late Miocene (~9 Ma): decline in fossil ape diversity characterized by drying/cooling.
  • Earliest hominins:
    1. Sahelanthropus tchadensis (6-7 Ma): Near-complete cranium indicating bipedalism.
    2. Orrorin tugenensis (6 Ma): Bipedal traits with primitive characteristics.
    3. Ardipithecus kadabba (5.2-5.8 Ma): Mixture of primitive and derived dental traits.
    4. Ardipithecus ramidus (4.4 Ma): Significant skeleton find showing adaptations for bipedalism.
Summary of Earliest Hominins
  • The earliest hominins are controversial regarding classification and bipedalism evidence.
  • Key derived features: bipedalism, thicker enamel, reduced canines.
  • Early bipedal environments likely wooded rather than open savannah.
  • Relationships among early hominins remain unclear; more fossils needed for insight.