Early Human Evolution and Australopithecine Divergence

The Fossil Record and Early Hominins

  • Fossil record spans 7 millon years; most informative fossils from 5 millon years ago.

  • Key adaptations: Bipedalism; Savanna diets.

Divergence and Early Ancestry

Diverged from chimpanzees around 7.8 million years ago.

  • Ancestry in forests; still arboreal.

  • Ardipithecus ramidus (~4.4 million years: bipedal and arboreal traits; standing in trees to gather food; reaching with bipedal posture; grasping with free hand and opposable toe.

Mio-Pliocene Climate Shift

Starting around 12 million years ago: cooling trend; onset of glacial periods; increased aridity; expanding grasslands; tropics diminished by 5 million years ago.

Mio-Pliocene Environmental Shift

  • Climate change gradual; forest margins patchy; intersected by grasslands.

  • Potential selective influence on bipedalism and knuckle-walking.

  • Fewer resources; new strategies; bipedalism possibly first adapted to arboreal conditions.

  • Continued resource decline and increased competition likely led to migration out of forests; shift to terrestrial life.

Australopithecus: How to identify the first hominins to occupy the savanna?

  • Bipedal adaptations: maybe, but not a signifier of a savanna.

  • Diet: changes in dentition for harder-to-process foods; expansion on post-canine dentition (molars and premolars); reduction of incisors; thickening of enamel.

Au. afarensis

  • Time range: 3.9 million to 2.9 million years ago.

  • Morphology: complete biped with some arboreal tendencies; full suite of bipedal adaptations; long arms relative to legs; elongated fingers; muscles arranged on ischium for upward propulsion.

Au. afarensis Savanna Adaptations

  • Savanna-related traits: larger post-canine dentition (premolars and molars).

  • Skull/face: Post-orbital constriction; larger chewing muscles (temporalis) and zygomatics; larger masseter.

  • Brain size: similar to chimps ~450\,\text{cc}.

Australopithecine Divergence

  • Australopithecus lineage divergence reflects diet.

  • Robust australopithecines: hardy diet of grasses, tubers, roots, seeds; hard to chew and digest; abundant resources to collect.

  • Gracile australopithecines: possible omnivory; inclusion of new, easy-to-process, high-energy foods.

Robust Australopithecines

  • Dorsal view features: broad cheekbones (zygomatics); large backward-extending zygomatic arch; cranial capacity 410-530\,\text{ml}; postorbital constriction; sagittal crest; small incisors and canines; front view shows very large molars; lateral view also noted.

Au. aethiopicus

  • Ethiopia ~2.5 million years ago.

  • Heavy-duty chewing apparatus; sagittal crest; wide zygomatics; post-canine megadontia; scaled up version of Lucy.

Au. boisei

  • Tanzania ~2.2 million years ago.

  • Larger version of Au. aethiopicus.

Au. robustus

  • S. Africa ~2-1.2 million years ago.

  • Three species of robust Australopithecus; existed for more than 1 million years.

Gracile Australopithecus

  • Compared with Au. afarensis: smaller build; slightly smaller cheek teeth; larger cranial capacity 430-520\,\text{cc}.

  • Smaller body with larger brain; increased encephalization.

  • Why: brain growth facilitated by nutrient intake; new food source.

Meat Eating and Encephalization

  • Higher energy yield; easier to digest; reduction in intestinal tract; energy conservation in digestion.

  • Nutritious diet paired with reduced energy requirements → demand for sophisticated tool kit and cooperation.

  • Implications: development of cutting, scraping, and chopping.