human evolution model answer information Flashcards
Describe physical traits common to all primates
Larger brains for their body size, forward-facing eyes, reduced snout, reduced number of teeth, digits are prehensile, sensitive fingers and toes with dermal ridges, nails instead of claws, and hands and feet are pentadactyl
Describe physical traits typical of brachiators, knuckle walkers and bipeds
Brachicators typically have a short lumbar spine, hands with long curved fingers and reduced thumbs (well suited for grasping) long forelimbs, and flexible, freely rotating wrists and shoulder joints. As such, they have the ability to support their full body weight with their arms and have an acute sense of balance. These adaptations allow them to move swiftly through the canopy without needing to travel on the ground.
Knuckle walkers are quadrupedal and have enlarged bones around the knuckle joints to enhance stability and weight-bearing. While knuckle-walking, the wrists are held in a stable, locked position to allow weight to travel down through the joints, while the outer fingers are held clear of the ground allowing them to carry objects as it walks. Knuckle walking spreads the bodyweight across all four limbs.
While knuckle-walking and brachiation are generally seen in arboreal primates, Humans are the only bipedal members of the primate family. Bipedal locomotion involves walking upright on two legs.
Describe the typical features of the primate skull (C,B,P,Z,N,S,L,F,P)
Cranium (part of the skull that encloses the brain and protects it from injury),
brow ridge (bony ridges above and around the eyes that protect the eyes from the powerful forces developed during chewing and biting)
prominent canine teeth (large pointed teeth used for holding and killing prey, as weapons or for aggressive display)
zygomatic arch, (through which important muscles for chewing pass through. These muscles are attached to the mandible and the skull)
nuchal crest (extension of the skull to which neck muscles attach, these muscles position, move and stabilise the head and cervical vertebrae)
sagittal crest (a ridge of bone running lengthwise across the midline of the skull to which strong jaw muscles attach),
large premolars and molars (large flat teeth that shear and crush food during chewing)
foramen magnum (large hole at the base of the skull through which the spinal cord and arteries enter the skull)
diastema (space between teeth for large canine teeth to fit into so the other teeth can come together)
prognathism (when the lower portion of the face markedly projects forward)
Discuss how and why the location of the foramen magnum varies according to the mode of locomotion
In chimpanzees, the foramen magnum is positioned toward the rear of the skull and angled backwards, as they are quadrupedal and have a relatively horizontal spine. In humans, this is central and angled downwards due to bipedalism, which results in the skull being positioned on tip of the spine. There is also less demand for nuchal muscles to hold the head upright and therefore a reduced nuchal crest.
Describe the differences in the cranium and mandibles of chimpanzees and humans and link these differences to their diets (sagittal crest, canines, jaw, zygomatic arch and brow ridge, foramen magnum)
Chimpanzees have a high degree of prognathism, a robust, rectangular mandible and a narrow, U shaped dental arcade. It has no chin but has a simian shelf. Chimpanzees are omnivores but the majority of their diet consists of fibrous plant material, requiring powerful bite forces to consume. Therefore, chimpanzees have a wide zygomatic arch for these muscles to pass through and a pronounced sagittal crest for them to attach to, as well as a pronounced brow ridge to help disperse the tension forces created.
Humans have little to no prognathism, a less robust mandible, a wider, parabolic dental arcade and smaller teeth. While also omnivores, our diet consists of greater consumption of cooked foods and meat, which required less chewing. As such, modern humans have lost the powerful jaw muscles, prominent brow ridge, sagittal crest and wide zygomatic arch seen in chimpanzees. These features would have been selected against due to the waste of energy and nutrients required to maintain them.
Discuss trends in canine teeth
Chimpanzees have larger canine teeth and therefore larger diastema due to the use of canines to display dominance and aggression, particularly in males. This is reduced in humans as there is less sexual dimorphism present, and less overt displays of aggression are favoured - language and vocalisation are more common.
What do the major traits associated with bipedalism relate to?
Bipedalism, which according to the fossil record developed 7-6 mya, is the main trait that defines the hominin lineage and is the driving force behind our subsequent evolution. The major traits relating to bipedalism involve the balancing of the body as a whole, keeping the downward transmission of forces as close to the midline of the body as possible, and keeping the legs stable to support the weight of the body.
Explain the skeletal differences between chimpanzees and humans specifically related to bipedalism (R,F,FM&NC,S,P&F)
Ribcage - Chimpanzees have a cone-shaped ribcage, to contain a relatively large gut due to a low-grade vegetable diet and to allow for a wide range of shoulder motion during brachiation. Humans have a barrel-shaped ribcage, allowing the arms to swing from side to side which assists balance while walking bipedally as well as the suspension of internal organs.
Feet - chimpanzees have longer toes and an opposable hallux, which allows grasping while tree climbing, and no longitudinal arch as they walk flat-footed. Their heel is less robust compared to humans. Humans have shorter toes and a larger, more robust hallux which is aligned with the rest to provide efficient toe push-off propulsion. They also have well developed longitudinal arches to aid toe propulsion and a large, robust heel bone to absorb the forces encountered during heel strike and provide stability when standing.
Foramen magnum and nuchal crest - in chimpanzees, this is towards the rear of the skull and angled downwards, to position the head forward of the skull while walking quadrupedally. This requires strong neck muscles and therefore the presence of a nuchal crest for them to attach to. In humans, the foramen magnum is central and downwards opening, as the skull of balanced on top of the spine. As such, less neck support is needed and a nuchal crest is not required. These differences reflect the locomotion type of each species.
Spine - chimpanzees have a c-shaped spine, in order to support the abdomen and help absorb the forces generated by quadrupedal locomotion. Humans have an s-shaped spine created by a forward bend in the lumbar region and a backwards bend in the thoracic region of the vertebral column, which brings the body’s centre of gravity directly over the centre of the pelvis. This allows for the maintaining of balance and flexibility to absorb forces critical for bipedal locomotion.
Pelvis and femur - chimpanzees have a narrower, flatter and more elongated pelvis, which provides points for attaching muscles and ligaments. They have a smaller femoral head as their body weight is spread over all four limbs when walking quadrupedally and a shorter femur, which gives them a shorter stride. Their valgus angle is close to 90 degrees, meaning the knee is positioned directly below the hip, giving stability while walking quadrupedally. Humans have a bowl-shaped, shorter and broader pelvis, providing support for the abdomen while walking upright, positions body weight over the feet and gives a smooth gait. The femoral head is larger in order to give increased ability to tolerate weight transfer and better stability while bipedal, and the length of the femur is longer to provide a longer stride length and therefore more efficient bipedal locomotion. The valgus angle is less than 90 degrees, which places the feet directly below the centre of gravity.
Discuss the evolution of bipedalism in terms of selection pressures related to the environment and adaptive advantages
During the mid-late Miocene and the early Pliocene, the climate of Africa became cooler and drier, resulting in the predominantly forested environment giving way to grassy savannahs and open woodland. These environmental changes would have affected the selection pressures acting on ancestral humans. Originally, the ancestors would have relied on the forest for food, shelter from the sun and to hide from predators, but as trees became more sparse, early hominids who could move most efficiently between patches of vegetation while carrying food back to a “home location” would have been selected for. Natural selection alone would not have created bipedalism, but instead would have acted to favour it and the adaptive advantages it brought once the behaviour had arisen. Bipedalism’s most important advantage is freeing the arms, allowing those who could carry tools to obtain food, carry food back to a dependant family, carry offspring while moving or wield weapons to have an increased chance of survival, as would their mates and offspring, increasing the chances of their genetics passing on to the next generation. Bipedalism also offers an advantage in terms of thermoregulation, as less surface area is exposed to solar radiation, absorbing up to 60% less heat, meaning they could have ranged further without requiring an increased water intake, as well as energy efficiency - while bipedal locomotion is no faster than quadrupedal, it used only a quarter of the energy used for knuckle-walking and as such allowed for longer and more efficient foraging journeys.
Discuss how bipedalism led to changes in the manipulative ability of the human hand
Once hands were no longer needed for brachiation or knuckle-walking they could specialise and adapt for manipulation of objects instead.
Brachicating primates have long, curved fingers, but humans have relatively short, straight ones as excessive length inhibit the ability to form precision and some power grips. This allows the tips of human fingers to meet the thumb and palm for finer manipulative control, and for more mobile joints. Humans also have a relatively larger and fully opposable thumb, more muscular and mobile than in other primates. The human wrist is more mobile than knuckle-walking primates, as it no longer has to be rigid to transfer weight. Instead, it can rotate and flex to give a wider range of manipulative movements vital for tool making.
Discuss the endocranial features of the human and chimpanzee brain linked to speech, comprehension and intelligence
The two factors that indicate intelligence is the brain size to body mass ratio and the complexity of the brain, both of which are higher in humans. Human brains have a higher surface area due to being much more wrinkled and folded and has a greater number of connections between many of its parts. It also has a large frontal lobe, giving the ability to have abstract and logical thought, imagination, complex language and artistic skills. The large cerebellum in humans allows for coordination of fine motor skills, for example, manual dexterity and balance. The other key difference between human and chimpanzee brains are the two areas associated with speech - the Broca’s area, which is involved in producing the sounds associated with speech, and the Wernicke’s area, which is responsible for language comprehension. These areas are indicated by endocranial bulges.
Describe some disadvantages of bipedalism, particularly injury to the spine and the obstetrical dilemma
The human spine has evolved to be a vertical, weight-bearing column, which puts it under unprecedented stress. This leads to humans being at an increased risk of back injuries and pain, and as these forces travel down the spine and transfer to the knees, ankles and feet, they also have increased susceptibility to injury.
The obstetrical dilemma is two opposing trends - 1. As humans became increasingly bipedal, their pelvis narrowed and the birth canal decreased in diameter. 2. As humans became more intelligent, their cranium increased in diameter, making it harder to fit through the birth canal. Compared to other primates, this results in human birth being especially long, painful and risky.
