bipedalism

humans are…

bipedal (walking on two legs)

humans have unique locomotion compared to other primitive species because…

other primates (e.g monkeys, gorillas, chimps) are quadropedal, and humans have evolved to be bipedal.

for extinct human ancestors bipedalism must have been…

an advantage in the environment

what is the first evidence of bipedalism?

the Laetoli footprints that are approximately 3 million years oil made by Australopithecus, the footprints fossilised in the volcanic ash. there was only two feet in the prints. the feet showed adaptations for bipedalism (this doesn’t mean Australopithecus was the first to be bipedal)

what is bipedalism?

for humans it is to be able to stand on two legs and walk bipedal with strding gait.

for bipedalism…

the skeleton must evolve, the adaptation of bipedalism helped out human ancestors to survive, and hence has acted as a selective pressure during natural selection.

what are the selection pressures and evolution for bipedalism?

evolution by natural selection means the environment must have favoured bipedalism for human ancestors. hominids that evolved into human ancestors originally lived in woodland environments. but there was environmental changes in Africa 5-6 million years ago. the temperatures fell and rainfall decreased, therefore the forrest areas diminished and open grasslands increased, slow evolutionary changes towards bipedalism would have been favoured

what did the hominins that originally lived in a woodland environment look like?

they were ape like, arms and hands suited for arboreal lifestyle, may have used some bipedal movement to get through forrest cleanings, like modern orangutangs

if there is an open grassland….

bipedalism and other traits would have been advantageous.

why is bipedalism advantageous?

increase energy effiecny, increase range of vision to detect prey and predators, increased height and size to detect predators, the hands are free for carrying food and using tools etc, have a higher reach to pick fruit and collect honey, a decrease of exposure to sunlight, and a greater surface area for breezes and therefore better temperature regulation.

why was bipedalism a big selective advantage?

because food supply was limited as Forrests were replaced by open grasslands, it causes selection pressure which made bipedalism a big selective advantage. (e.g higher reach to pick fruit and collect honey)

extinct bipedal human ancestors show progression to what adaptations?

skeleton and muscles of the human species adapted for erect posture to enable bipedal locomotion

what are the adaptations for erect posture and bipedalism?

the position of the foramen magnum, S shaped spine(curvature of spinal column), the jaw (reduced prganthism), pelvis, carrying angle, knee, foot , centre of gravity, stance and muscle tone, striding gait.

what is the foramen magnum?

it is the hole in the base of the skull where the spinal cord joins the brain.

what is the evolutionary trend with the foramen magnum?

it gradually become more forward to become more central

where is the foramen magnum n quardrupeds?

towards to back of the skull

where is the foramen magnum in bipeds?

it sits at the centre of the base of the skull

because the foramen magnum sits on the centre of the skull in bipeds this allows…

the skull to balance on top of the vertebral canal

because of the non central position of the foramen magnum in quadrupeds…

an ape like gorilla needs large neck muscles to hold the neck in poistion

why is there no large neck requirements in bipeds?

because the weight of the skull is borne by vertebral canal.

what is the evolutionary trend of the curvature of the spine

the curvature of the spine was changed to allow for an upright posture

what is the curvature of the spinal column in apes/ gorillas?

Cshaped vertebral canal

what is the curvature of the spine in humans and why?

S shaped curve in humans, as this improved body balance in an upright position, this enables the head to balance on top of the neck

how is the S shaped curve achieved in humans?

the double curvature is achieved in the lower lumbar region (lumbar vertebrae support the lower back) being wedge shaped from front to back, thus forming a forward jutting spine. this cervical curve in the neck brings the vertebral column directly infer the centre of gravity of the skull

what is prognathism?

the protruding jaw (seen in Apes)

what is the structure of the human jaw?

the human jaw is much flatter to face (reduce proganthism)

what is the evolutionary trend of prognathism?

during evolution from an ape like ancestor, the size and protrusion of the human jaw has gradually reduced

why has the human skull showing a reduced amount of prognathism?

as it allows the skill to balance better on top of the spine, because the weight in the front of the foramen magnum is approximately equal to the weight behind it. therfore balance is achieved with minimal effort. (previously the front go the skull being heavier than the back, therefore less muscle is needed to raise the head)

describe the pelvis in humans

the pelvis in humans is broader, flatter, shorter and bowl shaped compared to apes. (this pelvis can be seen in bipedal human ancestors trending more bowl shaped over time),

what is the evolutionary trend of the pelvis

more bowl shaped over time

what does the broad, flat bowl shaped pelivis allow in humans?

it supports abdominal organs when standing up, supports developing foetus, suppourts stability for bipedal locomotion, have broad hip attachments for large buttocks so legs can move and keeps the upper body erect, the female pelvis is slightly broader than the male to supoourt a developing baby and for birth

what is the carrying angle?

it refers to the positioning of bones and muscles in the legs to carry body weight

what is the carrying angle in humans?

the shape and orientation of the pelvis results in the hip joint being directly under the trunk and head allowing the weight of the body to be transferred from the pelvis to the legs. the large head of the femur (or thigh bone)fits well formed hip socket (acetabulum) of the pelvis. therfore the hips are broad, hip sockets are wide apart, but the femurs and to converge towards the knees at angle to vertical

the arrangement of the femurs…

forms an angle to the vertical, termed the carrying angle, which ensured weight distribution remains close to the central axis of the body when walking. in humans the weight tends to fall through the outside of the femur, where as the reverse is true for apes

the carrying angle allows for…

stability in an upright posture, when walking it enables the body to be rotates about the lower leg and foot and each footstep follows a more or less straight line, enables the human to have a striding gait (upright and stable) instead of swaying side as gorillas and chimps when they are on two legs

in humans the hip joint…

is in line with the abdomen, chest and head, allowing for an upright position.

how is weight transmitted in bipedal species?

the weight of the body is transmitted down the outside of the femur to the knee.

describe the structure of the knee

two part hinge joint of the knee, with one hinge on either side of the ligaments in the middle of the joint. this is angled for weight transmission, the shaping of the outer joint allows the outer aspect to be larger and stronger than the inner one.

when weight is transmitted down the outside of each leg the centre of gravity tends to fall…

in a line just in front of the knees, as force pushes to bend the knee backwards, resisted by knee ligaments. this natural resistance requires no energy to hold the body in standing position

why does it require no energy to hold the body in a standing position?

as there is natural resistance, due to force pushing to bend the knee backwards but it is resisted by the knee ligaments

from the knee joint most of the weight is transmitted through to the…

tibia (the larger and stronger other bone in leg, the fibula) to the ankle

where is the weight transmitted from the ankle?

the body weight is transmitted from tibia through to the talus (ankle bone) to the other tarsal bones, then to the metatasals and phalanges via the arches in the foot.

the human…. is one of the most distinctive adaptation for bipedal locomotion

foot

why is the human foot one of the most distinctive adaotatuibs for bipedal locomotion?

because of the highly specialised locomotion organ, it has lost its grasping ability, or prehensility, e.g our big toe is now non opposable and is quite large and is aligned alongside the other toes (therfore has lost prehensility and opposability)

the human foot is described as “platform foot” why?

the bones in the foot between the toes and ankle (the metatarsals) are shaped in such a way that they form two archs a longitudinal arch running from front to back and the transverse arch running from side to side.

what are the two arches in the human foot?

longitudinal arch and transverse arch.

what is the longitudinal arch?

arch running from the from to back

what is the transverse arch?

arch running from side to side

which of the arches in the human foot are uniquely human?

the transverse arch, as these two arches allows striding gait as foot “rolls” with movement and allows for perfect bipedal locomotion

describe the centre of gravity of humans

humans have a lower centre of gravity, this makes balancing easier as humans have longer legs than arms (and an increased length in stride when walking)

what is the differences in the centre of gravity in humans and in apes?

centre of gravity is at ½ height of humans compared to 1/3 of chimps. the centre of gravity for us is at our pelvis and is at the chest for chimps

the lower centre of gravity in humans allows for…

stability when moving bipedal and standing erect.

what is an essential element for maintaining an upright stance?

muscle tone

what is muscle tone?

it is the constant and partial contraction of skeletal muscles . e.g to keep head erected so it doent slump on chest. the muscles in the neck are partially contracted, that is they have tone

when is the muscle tone of the neck evident?

if someone falls asleep and their neck droops to their chest

muscles are required for…

balance in upright stance need greater muscle tone. in humans these muscles that require extra muscle tone are those that bring about movement of the spine, hip, knee and ankle and abdominal muscles. also the sense organs and the nervous system work together to maintain the tine in these muscles and equilibrium in the body.

what is strding gait?

is walking in such a way that the hip and knee are fully straightened. hominins are the only animals to perfect this locomotion (even when walking on their hind legs other apes have their knees bent and bodies forward at the hips

what is a swaying gait?

it is the locomotion apes have when they are walking on their hind legs

what is the process after to foot hits the ground?

when the foot hits the ground, weight is transmitted form the heel along the outside of the foot as far as the ball, crosses the ball of the foot (via the transverse arch) and is finally borne. in the final moment of striding, the whole weight of the body is propelled by the big toe. this is why hominids have lost the opposability of the big tie (therfore it is no longer pensile)

the human foot has evolved into a weight bearing appendage rather than…

a grasping one

what happens during the body while walking?

the trunk rotates about the pelvis. the forward swaying of arms compensated for this natural rotation around the body.

what does the swaying of the arms do?

it tends to keep the shoulders at right angles to the direction of travel and therefore decreases the amount of energy expended. if our arms did not move like this the energy would be wasted in reversing the rotation of the body after each stride.

although the human pelvis is broad and hip sockets are wide apart…

the femurs converge towards the knees

what is the effect if the arrangement of the femurs converging towards the knees?

the arrangement of the femurs ensures that the weight distribution remains close to the central axis of body during walking. also for stability during walking as body can be rotated about the lower leg and foot, thus allows each footstep to be in a straight line.

what is the pelvis and carrying angle of a chimp

there is a lack of a wide pelvis and carrying ankle therefore when walking on two legs they must sway side to side therfire body weight is over each leg in turn

describe weight baring when walking

ball of foot, to outer foot, to just before big toe, then big toe

describe the weight bearing when standing still

ball of foot and on the metatarsal phalangeal joints

what are endocasts?

they are casts that are made of the inside of the skull, they provide the shape of the brain surface, may occur naturally or by scientific casts made of inside of the skull, may reveal an increase in the convolutions in the relative size of the frontal lobe trend can be seen as hominin species become more recent

describe the prognathism and brow ridge trend in non human apes and early homonins

they have a forward jutting jaw and also have a distinct brow ridge just above eye sockets. there is an evolutionary trend of a flatter face, it shifts the weight to a more central position, allowing the skull to balance during an upright stance and as the size of the frontal lobe increases the brain occupies a larger volume and therefore extending the cranium forward and therefore there is a decrease to brow ridge

how does the face change due to dietary changes?

dietary changes leads to smaller jaw and development of chin and prominent nose and therfore a flatter face (cooking food made eating easier)

what are the advantages of bipedalism?

it is more energy efficient, it leaves the hands free to reach foods and carry items, the upright stance therfore a greater height and therefore you can see further, less of the body is exposed to the sunlight, there is an increased exposure to breezes therfore increased cooling mechanisms and better thermoregulation