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what is the defining characteristic of hominins
bipedalism
what does bipedalism mean
a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs
habitually walking on two legs
how is bipedalism significant in human evolution
it’s likely that it was the main driving force for subsequent hominin evolution
what is the most important advantage of bipedalism
it freed the hands to carry out a variety of functions therefore:
bipedalism freed the hands for purposes that would make a large brain particularly advantageous
what functions did freeing hands aid in
carrying foods
tools
weapons
babies
picking fruit and other foods
wielding weapons
manufacture of tools
what other advantages did bipedalism allow
greater height → look above grass to see predators, find food, locate shelter + appear more intimidating
energy efficient → walking long distances, less energy used walking on two legs compared to knuckle walking on four limbs
too keep cool in hot savannah → less body surface in direct sunlight + increased airflow around body
disadvantages of bipedalism
moving more slowly
less stable on two feet than on four
smaller pelvis → disadvantage for females and birthing young
why are quadrupeds more stable that bipedal
a body is balanced when the centre of gravity falls within the base of support
quadrupedal organisms have a wide base of support (four legs) whereas bipedal organisms have a narrower base of support (two legs)
therefore any changes in skeletal structure that maintains centre of gravity inside the body and thus balance will be selected for
what is the foramen magnum
the hole where the spinal cord exits the brain
where is the foramen magnum in humans
it’s near the centre of the skull → which allows the skull to balance on top of the spine
where is the foramen magnum in apes
it’s located at the rear of the skull → the skull projects forward and requires strong neck muscles to support it
how does the position of the foramen magnum relate to the nuchal crest size in humans and apes
in human the centred skull does not require strong muscles to hold in place
thus little energy is required to support the skull
therefore the nuchal crest is greatly reduced in humans compared to apes as there skull projects forward
what is nuchal crest
where the neck muscles are attached
what kind of shape spines do humans have
a S-shaped spine
why do humans have a S-shaped spine
to act as a shock absorber
keep the body weight above the hip joints for upright movement
what shape spines do apes have
C-shaped spines
why do apes have C-shaped spines
the slight curve counterbalances the downward force of organs and chest
ribcage characteristics of humans
flattened front to back
centre of gravity body is close to the spine
ribcage characteristics of apes
flattened side to side
making it more funnel-shaped to accomodate the large gut → herbivore
pelvis characteristics of humans
short and wide → bowl shaped
to reduce stress of upper body weight on hips
to support internal organs
strong bones support the muscles that move the legs when walking
wide pelvis allows babies with large skulls to be born
pelvis characteristics of apes
long and narrow → provide large surface area
needed for the attachment of the leg muscles
shape does not allow a passage suitable for the birthing of a large brained infant
femur characteristics of humans
long and angled/curved in
large valgus angle → cause knees to be almost directly under the centre of the pelvis so body weight is centred through middle of pelvis when walking
buttress knees → prevent sideways titling of the lower leg when walking + wider surface area so less pressure and can support weight over large area without dislocation
can fully extend legs
femur characteristics of apes
femur is curved out
small to no valgus angle → legs hang vertically from the hips which facilitates brachiation when apes are moving through trees
shorter and no buttress knee
cannot fully extend their legs
feet characteristics of humans
toes are straight
foot has an arch → act as shock absorber when walking/running + act as spring to walk efficiently and save energy
big toe points forward → pivot point
feet characteristics of apes
flat foot → grip and wrap around things
big toe goes sideways and is opposable → grip things eg. grasping trees
what were the two major developments from bipedalism freeing the forelimbs of early hominins
stone tool technology → allowed hominins to be better able to kill prey, cut up carcasses, process meat (pound to soften it)
controlled use of fine → enabled cooking
which is the enabling of cooking so vital in the evolution of early hominins
cooking makes food easier to ingest
physically break down and digest food so less energy is required to process the food + nutrients in the food can be accessed quicker
overall bipedalism affect on evolution of hominins
bipedalism resulted in hands being freed to handle objects
which eventually included making and using stone tools
hunting became more effective + more meat was added to diet
meat supplied more protein and fats to grow the brain, resulting in an increased brain size and development
cooking began and released more nutrients from food
addition of fish from cooking populations added further protein to the diet
brain increased in size and complexity and so did learning and communication
resulted in improved hunting and tool technology
which both increased amount of fat and protein in the diet and a further increase in brain development
positive feedback loop of bipedalism in hominin biological and cultural evolution
bipedalism → manipulative hands → improved diet → bran development → increased ability to think, learn, communicaton → improved diet → improved survival chances
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