HUMAN EVOLUTION

HUMAN EVOLUTION

1. HOMININS

   • Humans and the great apes belong to the hominid family within the primate order of animals
   • Distinguished by: comparatively large brain relative to body size; well developed binocular vision; forward facing, enclosed eye sockets in the skull; colour vision; grasping hands with mobile fingers and sensitive finger pads, reduced sense of smell
   • Bipedalism developed at least 4.2 mya - driving force in our subsequent evolution

2. ANATOMY

   • SKULL
     • Foramum Magnum: hole on the underside of the skull through which the spinal cord passes. In all bipedal species, the foramum magnum is centrally located so that the skull balances on top of the spine. This requires little muscular energy to support the skull, so neck muscles are reduced in size and the nuchal crest (where the musucles are attached to the skull) is correspondingly small
       • In apes, which are quadrupedal, the foramen magnum is located at the rear of the underside of the skull. This requires large neck muscles to prevent the head from sagging forward, so the nuchal crest is correspondingly large
   • SPINE AND RIBCAGE
     • The spine is S shaped to act as a shock absorber and to keep the body weight above the hip joints for upright stance and movement. Cervical vertebrae have short neural spines as little muscle attachment is required to hold the head upright
       • The spine in apes is C shape so that the slight curvature counterbalances the downard force of organs and chest in the more horizontal stance
     • The ribcage is flattened from front to back so chest centre of gravity is as little far forward of the spine as possible, minimising tendency to fall forward
       • In apes, the ribcage is flattened from side to side
   • FEMUR AND KNEE
     • The femur is long and angled in from the hip (valgus angle) producing a knock-kneed stance and gait. This angling in brings the knees almost directly under the centre of the pelvis, so that the body weight is centred through the middle of the pelvis when walking. The lower end of the femur has buttresses of bone on the knee joint to prevent sideways tilting of the lower leg when walking. Can fully extend our legs
       • In apes, the femur is shorter and there are no buttresses at the knee joint end; the femur hangs vertically from the hip (no valgus angle) which enables a good swinging motion (brachiating) when moving through trees. Cannot fully extend their legs
   • FOOT
     • The big toe points forward in a line with the other toes, providing forward thrust to the stride when walking (at the end of each stride the foot pivots over the big toe) and the foot is arched, which acts as a shock absorber when walking and running (especially helpful over long distances)
       • In apes, the big toe spreads outwards from the foot and is opposable, allowing gripping of branches when moving though trees as well as grasping of food and other objects, and the toes curve (facilitates walking along branches) and there is no arch
   • HANDS
     • Thumb is relatively longer
     • Opposable tips most highly developed
     • Muscle flexes the joint nearest the tip of the thumb - unique to humans
   • TEETH
     • No difference in size of canines between sexes
       • In chimps, males have much larger canines than females; large male chimpanzee canines used in threat displays; killing and dismembering hunted monkeys; fighting or defence of the troop
   • BRAIN
     • Outer layer, or cortex of the cerebrum is folded, increasing surface area
     • Cerebellum, concerned with balance and muscular co-ordination is also greatly enlarged
     • Speech
       • Broca's Area
         • Involved in language processing, speech production and comprehension
       • Wernicke's Area
         • Involved in the understanding and comprehension of spoken language
       • Greatly improves the ability to co-ordinate activities, such as hunting of large prey and defence against predators. Greatly increases the chance of group success. The more successful the hunting, the better the diet, the healthier the members, the greater the survival ability and to produce and raise offspring. Evolutionary fitness increases, so any advances in the brain that improve language will be strongly selected for

3. ADVANTAGES OF BIPEDALISM

   • Upright Posture - Head Higher Up
     • Predators can be spotted earlier
     • Taller body may seem larger to a predator
     • More effective for brandishing sticks and throwing stones at predators

4. DISADVANTAGES OF BIPEDALISM

   • Intervertebral Disks
     • Of cartilage may become damaged (slipped disk); lower back problems
   • Arch of the Foot
     • Not always well developed - pain when walking
   • Posture
     • More difficult for blood to be returned to the heart from the legs