Early Hominins Notes from 2 20 14 Earliest hominins What o Bipedalism Reorientation of spine limbs and head Swinging gate Knuckle walking phase Wrist morphology suggest knuckle walking retention What is the selective value of bipedalism Savannah hypothesis Stood upright to see over the savannah Problems o Grass is short o Bipedalism evolved before the forests were gone Efficient but slow locomotion Free hands to carry and throw Problems o Would need a group structure to make it effective Look more threatening when upright Sun hits you at an angle to keep you Predator protection Thermoregulation cooler Running backwards Niche Expansion Fruits in trees reaching up Long distance running Sexual selection Male provisioning Exposed genitalia Attract a mate To carry child No single selective advantage Slinky hypothesis As foramen magnum moved forward brain changes shape to add surface area and less on instinctual o Molecular clock suggests 5 7 mya Chimps and humans diverge Based on regular mutations of mitochondrial DNA When Where o Africa What to look for o Toumai Hope of Life Djurab Desert Chad Sahelanthropus tchadensis itsahellofananthropous 6 7 mya Desert woodland conditions Plesiomophies ancestral traits U shaped dental arch Wide inter orbital distance Small endocranial capacity 320 380 cc Post orbital constriction Sagittal crest Large nuchal crest Apomophies derived traits Smaller canines non honing No canine diastema no gap Thickened enamel Thick continuous supraorbital torus eyebrow bone Orthognathic face Bipedal Ape anscestor Sahelpithecus o Orrorin tugenensis 6 mya Tugen Hills Kenya Wooded environment Plesiomorphy Primitive canine Apomorphies Thick enamel Large head of femur Bipedal o Other Tugen Hills sites Lothagam 5 5 mya Tabarin 5 mya o Aridipithecus ramidus Molars with buccal flare serrate root pattern 4 4 5 8 mya variety of sites Middle Awash Ethiopia Wooded environment Plesiomorphies Thinner enamel toward crown Small molars Humerus with mixed features Small cranial capacity Apomorphies Canines less projecting No honing facet on mandibular P3 P3 relatively small Foramen magnum anteriorly placed Was Ardi bipedal Pelvis only slightly reoriented Hallux big toe adductable but not opposable Sticking out Explanded second metatarsal for propulsion FACULTATIVE BIPED not habitual Mosaic evolution Different features evolve at different times