18. evolutionary trends in hominids Flashcards
what is a tribe?
a relatively new level of classification between subfamily and genus, hominin is used to distinguish humans and other ancestors from gorillas and apes
What are the categories for humans?
homindae (great apes)–> homininae (chimp, gorilla, human) –> hominini (human and extinct ancestors)
What are adaptations for bipedalism?
foramen magnum Curvature of the spine Jaw Pelvis Carrying angle knee Foot Centre of gravity
how does the foramen magnum allow for bipedalism?
-located centrally underneath the skull in humans but located towards the back of the skull in quadrupeds
how has the foramen magnum evolved?
- gradually moved forward until the skull is able to balance on top of the vertebral column
How is the weight distributed around the foramen magnum in paes vs humans?
humans
- the weight of the skull ir borne by the vertebral column, so large neck muscles arent required
apes
- skull weight is placed in front of the vertebral column so large neck muscles are requied
How does the curvature of the spine allow for bipedalism?
- humans have double curvature of the spine giving it a S-shape that allows for an upright stance
- lumbar curve has wedge shaped vertebrae to form a forward-joining curve, improves bosy balance and enables the head to balance on top of the neck
How does the curvature of the spine influence the centre of gravity?
the cervical curve brings the vertebral column directly under the centre of gravity of the skull
how does jaw shape differ in humans and apes?
apes have a protruding jaw
Humans facial profile is flatter
How has jaw size evolved in hominids?
size and protrusion has gradually reduced
allowing skull to balance on top of spine
Balance achieved with minimum muscular effort
shape of pelvis in humans
human pelvis is broader and shorter from top to bottom
broad hips provided space for attachment of gluteus muscles
hwo has the carrying angle evolved in hominids?
- 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 pelvis to legs
- head of femur is large and fits into the acetabulum of the pelvis, with the sockets wide apart and femurs converging towards the knees, forming carrying angle which ensures the weight distribution remains close to the central axis of the body when walking
- allows for greater stability as it enables body to be rotated about the lower leg and foot so footsteps follow straight line
- ->enables striding gait rather than swaying side to side
- -> weight transmission tends to fall through the outside of femur
how does the knee allow for bipedalism in hominids?
- weight of the body trandsmitted down the outside of the femur to the knee, a 2 part hinge joint surrounded by ligaments
- trasnmitted down outside of legs with COG close to knees - ligaments resist force trying to bend knees backward
- natural resistance produces a joint requiring no energy to support body in standing position
how do feet support bipedalims?
weight transmitted through tibia to ankle through talus to metatarsals and phalanges via arch of foots
-foot lost prehensility
metatarsals shaped to form longitudinal arch and transverse arch
- humans evolved weight bearing arch not grasping one
how has COG allowed for bipedalism?
humans have longer legs than arms, increasing the length of stride, lowering COG in the body
pelvis level COG contributes to stability when moving bipedally or standing erect
human adaptations for bipedalism?
- foramen magnum located centrally in base of cranium
- jaw bone small and non-protruding, enabling the skull to balance on vertebral column
- lumbar vertebrae is edge shaped to produce S curve bringing vertebral column under centre of skull
- pelvis is broad and shallow, supporting abdominal organs
- attachment of femur is wide apart to contribute to carrying angle
- large head of femur for carrying angle
- outer hinge is large and strong, taking weight of the body, knee able to be straightened
- legs longer than arms for lower COG
- large heel bone aligned with big tow form pedestal on which body is supported
- foot has longitudinal and transverse arches
What is muscle tone and how does it keep upright stance?
partial contraction of skeletal muscles
required for upright stance as it keeps head erect via partial contraction of neck muscles
What is the striding gait in humans
method of walking where the knee and hip are fully straightened
- weight transmitted from heel along outside of foot to the ball, crosses ball of foot via transverse arch and borne by big toe
- trunk rotates around pelvis
- swinging of arms keeps shoulders at right angles to direction of travel and reduced energy expanded
- femurs converge close to knees ensuring weight distribution remains central allowing for stability
what is the most developed part of the brain?
the cerebral cortex
what has enlarged the most in the brain?
the frontal lobes surface area, as it controls higher functions of thinking, reasoning, planning and processing
Why has the nose become more prominent in humans?
because the face has flattened but bones of nose stayed
how has the brow bone developed in humans?
cranium size increase
brow tends to be vertical and lacks prominent brow ridges as more of the skull is used in housing the brain
evolution of the human head:
- cranium size increased
- increased number of convultions
how has the jaw evolved in hominids?
-early hominins had ape like face, large teeth and evident diastema
now:
- dental arcade evolved to be more parabolic as canines take up less room
- rows of teeth parallel rather than curved
- trend towards smaller molars and decreased robustness of teeth due to diet change
Apes + early hominins have forward-jutting jaw and distinct brow ridge
now–> flat face, prominent forehead, chin and prominent nose
