early hominins

foramen magnum

the hole in the bottom of the skull through which the spinal core enters

bipedality

walking on two legs

lumbar lordosis

inward curvature at the base/lower spine

sacrum

triangular shaped bone in pelvis located at the base of the spine that continues the lumbar curve

center of gravity

the specific point where the body’s weight is balanced in all directions

center of gravity is affected by…

lordosis

quadrupedal

walking on four limbs

iliac blades

wings of the pelvis

valgus knee

knees angle inward and are brought more under the hips

convergent toe

big toe is aligned with/parallel to the other toes

divergent toe

big toe is separated from the other toes

use for divergent toe

grasping

hominid

humans and all great apes

hominin

group of bipedal hominids after the split of the last common ancestor of chimps, bonobos, and humans

how many of the living hominids are habitually bipedal?

one

honing premolars

premolar sharpens the upper canine

intermembral index

forelimb length/hindlimb length x 100

position of foramen magnum in habitual bipeds

• positioned directly under the skull

• head is balanced on vertebral column

  • makes a right angle

shape of ribcage in habitual bipeds

• barrel-shaped

• advantageous for maintaining balance

  • easier arm-swinging

curvature of the spine in habitual bipeds

lordosis curve brings the hips forward

vertebral column in habitual bipeds

gravity is parallel to the spinal cord

shape of pelvis in habitual bipeds

• shorter and wider iliac blades

• basin-shaped

  • supports abdominal organs

arm/leg length in habitual bipeds

• legs are longer than arms

• IMI < 100

angle of femur at knee joint in habitual bipeds

• slanted inward under pelvis

  • NOT a right angle

  • knee is brought more under hips

foot arch in habitual bipeds

• acts as a shock absorber/spring for the foot

• absorbs all of the body’s weight

convergent big toe in habitual bipeds

big toe is parallel to the rest of the toes

shape/length of toe bones in habitual bipeds

straighter, shorter, fatter

mate provisioning model

males would gather food and bring it back to females— hands need to be free to carry items

mate provisioning model flaws

• would mean that pair-bonding evolved earlier, but sexual dimorphism suggests otherwise

• in pair-bonded socialism, males & females are of similar size— here, males are larger

tool use model

hands needed to be free to make and use tools to hunt

tool use model flaws

no evidence of stone tools until long after bipedalism

thermoregulation model

reduces solar radiation

thermoregulation model flaws

many quadrupeds live in savannas

climbing mechanical model

retention of suspensory morphology: maybe arboreal habitat became patchy— perhaps arboreal ancestors needed to travel on the ground and walk bipedally due to their morphology

climbing mechanical model flaws

• evidence of knuckle-walking as solution for long arms by contemporary apes

• anatomy of living african apes and fossil hominins suggests humans evolved from a knuckle-walking ancestor

why did bipedality evolve?

we do not know— likely a combination of many factors

locomotion of early hominins

bipedal

brain size of early hominins

slightly increased cranial capacity/brain size

honing premolars of early hominins

trend towards smaller canines and non-honing premolars

what are the earliest known hominins?

• sahelanthropus tchadensis

• orrorin tugenensis

• ardipithecus kadabba

• ardipithecus ramidus