Bipedalism and characteristics of humans vs apes

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

1. stone tool technology → allowed hominins to be better able to kill prey, cut up carcasses, process meat (pound to soften it)

2. 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