Variation within the family Hominidae

• Overlapping characteristics within the family Hominidae:

  • larger, more complex brain → increased cognitive ability
  • five cusps in molar teeth of the lower jaw
  • arms freely rotating at the shoulder
  • wide, shallow chest cavity
  • no external tail
  • appendix
  • diurnal
• Varying characteristics within the family Hominidae:

  • cerebral cortex size
  • mobility of the digits
  • locomotions - adaptations to bipedalism and quadrupedalism
  • prognathism and denition
• Changes in the relative size of the cerebral cortex:

  • primates large brain size is a consequence of natural selection in an arboreal environment
  • more accurate visual, tactile perception, better coordination
  • human brain size rage from 900 com to 2200 cm
  • convolutions contributed to a 50% increase in surface area
  • enables primates to move about, locate food and make tools
  • brain size of hominins determined by measuring the volume inside the cranium
• Mobility of the digits:

  • grasping digits essential for climbing by wrapping long fingers around trees
  • humans’ big toe not opposable at all when the human foot become weight-bearing
  • precision grip is a hallmark of being human
  • power grip - fingers apply force against the palm
• Locomotion:

  • bipedalism has acted as a selective pressure during natural selection
  • for humans to be able to walk bipedally with a striding the, the skeletons and muscles had to evolve
  • adaptation to bipedalism helped our human ancestors survive by enabling the freedom of the arms
• Foramen magnum:

  (the hole in the skulls where the brain joins the spinal cord)

  • gradually moved forward to become more central
  • allows skull to balance on top of the vertebral column
  • humans do not have large neck muscles as the weight of the skull is beared by the vertebral column'
• Curvature of the spinal column:

  • C-shaped curve of spine has evolved to an S-shaped curve
  • allows upright posture and improves body balance
  • head can also balance on top of the neck
  • double curvature achieved by the lumbar spine being wedge-shaped from front to back → forward-jutting curve
  • cervical curve in the neck brings the vertebral colum directly under the centre of gravity of the skull
• The jaw:

  • apes have prognathism (protruding jaw)
  • size and protrusion of human jaw has gradually been reduced
  • weight in front and behind of foramen magnum is equal allows skull to balance on top of the spine
• The pelvis:

  • broader, shorter from top to bottom and bowl-shaped in humans
  • bowl shape supports abdominal organs when standing, provides greater stability for bipedalism and supports developing foetus in pregnancy
  • broad hip bones provide space for attachment of the large buttock muscles which move the legs and keep the upper body erect
• The carrying angle:

  • shape and orientation of pelvis in humans results in the hip joint being directly under the trunk and head → allows body weight to be transferred from pelvis to legs
  • femurs converge towards the knees, forming the carrying angle which ensures weight distribution remains close to the body’s central axis when walking
  • allows for greater stability in upright posture
  • enables the body to be rotated about the lower leg and foot when walking with each footstep following a more-or-less straight line → striding gait
• The knee:

  • weight of body is transmitted down the outside of the femur to the knee
  • knee joint is a two-part hinge joint
  • outer hinge is larger and stronger
  • cannot be bent backwards due to ligaments making up the knee joint as body’s centre of gravity falls through in front of the knees
  • no energy required to support the body in a standing position
• The foot:

  • from knee joint, most of the body weight is transmitted through the tibia to the ankle
  • human foot has lost its grasping ability
  • metatarsals form a longitudinal and transverse arch which enables humans to perfect bipedalism
• Centre of gravity:

  • long legs increases the length of the stride and lowers the body’s centre of gravity
  • apes have only one-third of total height taken in leg length → centre of gravity at chest level
  • lower centre gravity contributes to stability in bipedalism and erect standing
• Muscle tone:

  (the partial contraction of skeletal muscles)

  • essential element for maintaining an upright stance
  • to keep head erect, muscles at the back of the neck are partially contracted
  • sustained muscle tone most evident in parts supporting the upright position (spine, hip, knee, ankle, abdominal muscles)
• Striding gait:

  (walking upright where the hip and knee are fully straightened)

  • only perfected by hominins
  • when the foot hits the ground, weight is transmitted from the heel, along the outside of the foot, crossing the ball of the foot and finally to the big toe
  • whole weight propelled by big toe → hominins lost the opposability of the big toe
  • trunk rotates about the pelvis when walking but the forward swinging of the arm compensates for this
  • swinging of the arms keeps the shoulders at right angles to the direction of travel and reduces energy output
• Advantages of bipedalism:

  • more energy-efficient
  • leaves hands free to use tools and to carry items
  • greater height = can see further
  • less of the body exposed to sunlight from upright stance
  • upright stance increases exposure to breezes, increasing cooling mechanisms
• Number and shape of teeth:

  • dental formula - number of each tooth type expressed in a quarter of jaw
  • natural selection led to decrease in the teeth number in primates
  • linked to reduction in size of face and jaw
  • Old World monkeys, apes and humans have 32 teeth (2:1:2:3) but different structure and arrangement
  • diastema in Old World monkeys and apes to fit their large canines
  • apes and humans evolved with four cusps in upper molars and five in the lower molars
  • evolved due to the predominantly fruit diet of the apes
  • dental arcade (shape of tooth row) evolved from U-shape to parabolic shape
• Prognathism and brow ridges:

  • brow ridge - bony ridge above the eye socket
  • non-human apes and early humans have both prognathism and a distinct brow ridge
  • with evolution, the tooth size gradually decreased → flattening of the face (development of chin and prominent nose)
  • flatter face shifts the weight to a more central position
  • the brain occupies a larger volume, extending the cranium forward and reducing the brow ridges