Human evolutionary trends Flashcards
Hominid
Any member of the group consisting of all modern and extinct humans and great apes (including gorillas, chimpanzees and orangutans) and all their immediate ancestors
Naming organisms
Named according to the genus and species to which they belong (E.g homo sapiens belong to the genus homo and species sapiens)
Hominid evolutionary trends
Directional change in a character state, or set of character states, resulting in a significant change through time: Relative size of cerebral cortex, Mobility of the digits, Prognathism and dentition, Locomotion- adaptions to bipedalism and quadrupedalism
Evolution in the relative size of the cerebral cortex
An average increase in cerebrum, in particular the cerebral cortex (frontal lobe), Cerebral cortex is important for its role in vision, memory, reasoning and manipulative ability, A larger cerebral cortex facilitates more accurate visual and tactile perception along with better co-ordination, Both have increased enormously as primates have evolved, Number of cerebral convolutions has also increased dramatically as primates have evolved, Greater surface area of cortex has had far-reaching effects on development (tool making and use, behavioural responses, greater survival chance)
Evolution in the mobility of digits
Primates are pentadactyl, Digits are highly mobile due to arboreal (living in trees) ancestry and prehensile (capable of grasping), Evolutionary trend is toward increasing ability to move digits independently of one another, thumb is most evolved, Thumbs are independent and opposable (can be placed opposite the fingers of the same hand), There have been varying degrees of opposability throughout evolution (longer the relative length of the first digit, the more opposable it is)
Opposable thumb
Allow the digits to grasp and handle objects which enables a precision grip
Power grip
Any carrying or grasping task where the finger flex toward the palm, most forceful grip available to the human hand and allows gross motor skills
Precision grip
When an object is pinched between the thumbs and fingers
Evolution in prognathism
A projecting lower jaw or chin, Human jaw is smaller and less-prognathic than in the ape groups
Evolution in dentition
Dental arcade in humans- parabolic with all teeth very similar in height, Dental arcade in apes- have parallel sides, large canines and diastemas, Trend- towards smaller, more uniform teeth in hominins
Locomotion
Humans require a modified muscular and skeletal structure to stand upright and walk with a striding gait, These modifications are considered adaptions (characteristics that help an organism survive and reproduce in its natural environment)
Postural adaptations (6)
Position of foramen magnum and prognathism, Curvature of spine, Pelvis shape, Carrying angle of femur, Knee structure, Foot structure
Foramen magnum
Large hole at the base of the skull, In humans- directly under the skull, In quadrupeds- further towards the back of the skull, Having the foramen magnum directly under the skull enables the skull to balance on the top of the vertebral column, Non-humans require much stronger neck muscles to hold the head in position, Because the human jaw is smaller and less-prognathic, the skull can balance on the spine as the weight in front of the foramen magnum is approximately the same as the weight behind it
Spine curvature
Non-humans have their vertebrae arranged in a single (C-shaped) curve meaning the body tilts forward and its centre of gravity is at chest level, Humans have a double curvature (S-shaped spine), Lumbar vertebrae are wedge-shaped, providing a strong lumbar curve which brings the head directly over the spine meaning the centre of gravity runs straight through the head and spine
Pelvis shape of humans
Broader and shorter than that of other primates, Supports the abdominal organs during upright stance, Broad hip bones provide space for large buttock muscles to attach, Broad pelvis allows the femurs to attach further apart
Carrying angle
Angle away from the vertical, ensures that the weight distribution remains close to the central axis of the body when walking, allows the body to be rotated about the lower leg and foot, Enables the body to be rotated about the lower leg and foot, producing a striding gait (instead of swaying) when walking
Evolution in carrying angle
Hip joint is directly under the head and body trunk allowing the weight of the body to be transferred to the legs, Pelvis is wide with two acetabular, Head of the femur is large and fits into the acetabulum, Femur converges towards the knees and is not directly vertical, Weight is transmitted through the outside of the femur, Means each footstep can follow a straight line, Apes lack a wide pelvis and carrying angle meaning, when bipedal, they must sway from side to side so that the weight is over each leg in turn
Evolution of the knee
Weight of the body is transmitted down the outside of the femur to the knee, Consists of two ‘hinges’ with the ligaments between, Although the weight of the body is transmitted down the outside of the joint, the centre of gravity falls through a line just in front of the knee, Force tries to bend the knee backwards, but the knee ligaments naturally resist it meaning no energy is required to support the body in a standing position, Unlike apes’ knees that are always flexed, human knees are capable of full extension and they lock into place when the other leg is in swing phase, providing a stable supporting leg
Larger knee hinge
Accommodates the carrying angle as weight is distributed through the outside of the femur
Foot structure of humans
Humans feet have changed dramatically from a mobile, grasping structure to a rigid, supporting one, Human foot doesn’t have an opposable toe and is no longer prehensile, Human toes are parallel which provides a pedestal, Human heel bone is large and robust providing support for balance, Humans have both a longitudinal and transverse arch
How foot structure helps walking
Big toe provides strength and push off during bipedal locomotion, Arches facilitate a striding gait and provide from spring and shock absorption, In a striding gait, the weight is transmitted from the heel along the outside of the foot to the ball of the foot, at the final moment of the stride, the weight is transferred to the big toe, Toes are parallel as they are used for weight bearing rather than grasping, Arms swing to oppose the rotation of the trunk around the pelvis keeping shoulders perpendicular to direction of travel which reduces the need for energy to reverse trunk rotation
Laetoli
Name of a site in Tanzania where the footprints of three hominids were preserved in ash
Muscle tone
Continuous and passive part contraction of muscles helps to maintain posture
Adaptations for climbing trees (5)
Rotating shoulder joint, Short clavicle, high shoulder blades, Long arms/shorter arms to leg ration, Long curved fingers with long thumbs well adapted to grip branches/hang from branches
Adaptations for climbing trees
Rotating shoulder joint, Short clavicle, high shoulder blades, Long arms/shorter arms to leg ration, Long curved fingers with long thumbs well adapted to grip branches/hang from branches
Human brachiation
Due to the shape of the pectoral girdle, humans are the only living hominid that can throw objects accurately and at speed, Homo sapiens have a straight shoulder line whilst Australopithecines and chimpanzees have an upward curve to their shoulder line, Curved shoulder line supports climbing and suspension in trees
Speciation (5)
Variation- present in individuals, wide variety of genes present, Isolation- isolation of gene pool/barriers to gene flow, Struggle- individuals with suitable genes are more likely to survive and produce off spring, Selection- only suitable genes are passed on, Speciation- no longer able to reproduce with others/new species
