Biology NCEA Level 3 Human Evolution

Humans are primates

Primates and Humans share many major morphological features

Genomes (DNA) are very similar (95%)

Primates that are most closely genetically related to humans

Chimpanzees (bonobos in particular)

Cranium

The part of the skull that encloses the brain and protects it from injury

Brow Ridge

Well-develop bony ridges above and around the eyes. They protect the eyes from powerful forces developed during chewing and biting

Canines

Large Pointed teeth used for holding and killing prey, as weapons or for aggressive display

Zygomatic Arch

Important muscles used for chewing pass through this arch in the ‘cheek-bone’. The muscles are attached to the mandible and skull.

Nuchal Crest

Extension of the skull to which the neck muscles attach. These muscles position, move and stabilise the head and cervical vertebrae.

Sagittal crest

A ridge of bone running lengthwise along the midline of the top of the skull. Strong jaw muscles attached to it.

Molar and Premolar

Large flat teeth that shear and crush food during chewing.

Foramen Magnum

A large hole at the base of the skull through which the spinal cord and spinal arteries enter the skull

Diastema

Space between the teeth for large canine teeth to fit into so that other teeth can come together

Prognathism

When the lower portion of the face markedly projects forward.

Difference of a Chimpanzee and Human Skull (Sagittal Crest)

Present in Chimpanzee but not in Human

The substantial jaw muscles needed to move the large, robust mandible are attached here. These muscles are reduced in human so no crest is necessary. This is most likely due to the differences in our diet.

Difference of a Chimpanzee and Human Skull (Relative size of the canines)

Present in humans and chimpanzee

Larger, with an associated larger diastema in chimpanzee, while reduced in humans

Canines are used by chimpanzee to display dominance and to display aggression particularly in males, whereas humans use less overt displays during aggressive interaction. There is also less sexual dimorphism in humans.

Difference of a Chimpanzee and Human Skull (Shape of jaw)

Present in humans and chimpanzee

U-shaped, no chin but with a simian shelf in chimpanzees, while V-shaped with protruding chin and no simian shelf in humans.

The reduction in size and specialisation of human teeth associated with rounded, less robust jaw, as diet shifted from low grade tough vegetation toward cooked (softened) food and meat. Less musculature needed so bone structure could be reduced (become more gracile)

Difference of a Chimpanzee and Human Skull (Zygomatic arch and Brow ridge)

Present in both humans and chimpanzee

Thick, wide bone structures - very prominent in chimpanzees, while reduced, finer zygomatic arch and minimal brow ridge in humans

Chimpanzees have a tougher diet requiring much larger chewing forces to be generated by much larger muscles. Their skulls require significant reinforcing to prevent tension fractures. The human diet removed the need for biologically expensive buttressing of skull as chewing muscles reduced.

Difference of a Chimpanzee and Human Skull (Foramen Magnum)

Present in both humans and chimpanzees

Toward the rear of the skull and angled backwards in chimpanzees, while central and angled downwards in humans

With bipedalism (in humans) the skull is balanced on top of the spine, with less demand for nuchal muscles to hold it upright. In chimpanzees, the head extend forward of a relatively horizontal spine, and so the foramen magnum is placed towards the rear of the skull and facing backwards so the spinal cord enters the spine horizontally

Arboreal

Tree-living

Quadrupedalism

Move in all four limbs when on the ground

Knuckle walking

A form of quadrupedal walking in which the forelimbs hold the fingers in a partially flexed posture that allows body weight to press down on the ground through the knuckles.

Brachiation

Move by swinging through the branches.

Bipedalism

Move in two limbs when on the ground using legs for walking

Developed between 7 - 6 million years ago and is thought to be the driving force behind our subsequent evolution (walking upright led to major changes in our anatomy and behavior)

Advantages of Brachiating

Allows them to move through the forest canopy without the need to travel on the ground. Also allows them to hang from slender vines and branches that they could not walk or balance on.a

Advantages of Knuckle-Walking

Allows animals to grasp and carry objects as it walks. It also spreads body weight across all four limbs.

Extant Relative

Still in existence - not extinct

Characteristic of a modern humans that came from a brachiating ancestor

A freely rotating shoulder joint

An acute sense of balance

Fingers well suited for grasping

Ability to support our body-weight by our arms.

Obligate Bipedalism

Walking only on two legs, with a reduced ability to walk on four

It is also the main trait that defines the hominin lineage and separates humans from the rest of the apes

Opposability of Hallux (big toe) in Humans

Absent, Hallux aligned with other toes

Provides efficient toe-initiated push-off for propulsion

Opposability of Hallux (big toe) in Chimpanzees

Present

Divergent hallux allows grasping during tree-climbing

Longitudinal arches in foot (Humans)

Present

It can push of ground with toes. Provide stability when standing.

Longitudinal arches in foot (Chimpanzees)

Absent

Walks flat-footed, however this is not the dominant method of locomotion.

Robustness of heel bones in Humans

Present - Larger and stronger

Forces of heel strike are absorbed by heel-bone and balance while standing

Robustness of heel bones in chimpanzees

Present but reduced

Dense bone and point of attachment for ligaments and muscles

Orientation of foramen magnum in humans

Present - S-shaped (two curves)

To position the centre of gravity directly above the feet to absorb shock and provide flexibility

Orientation of foramen magnum in chimpanzees

Present - C-shaped

Weight of abdomen well supported on C-shaped spine (arched) while on all four limbs

Shape of rib cage in humans

Present - Barrel

Allows the arms to be swung from side to side during bipedal locomotion

Shape of rib cage in chimpanzees

Present - Cone

Contains the relatively large gut (low-grade vegetable diet) and allows wide range of arm motion for brachiation.

Shape of pelvis in humans

Present - Shorter, broader, bowl-shaped

Supports the gut when upright (with weight over the feet) and allows for a smooth gait.

Shape of pelvis in chimpanzees

Present - narrower, flatter, more elongated

Has points of attachment for muscles and ligaments but is shaped such that balancing on one leg is difficult - results in swaying motion when walking bipedally

Angle of femur in humans

Present - angled inward at knee (knock-kneed) valgus < 90 degrees

Places feet directly below the centre of gravity as knee joints are closer to the midline than the hips

Angel of femur in chimpanzees

Present - Parallel femur, Valgus ~ 90 degrees

Knee directly below hip giving stability when walking quadrupedally but poor stability when walking bipedally

Size of femoral head

Present - Larger

Increased stability to tolerate weight transfer and better stability when bipedal.

Size of femoral head in chimpanzees

Present - smaller

Weight spread across all four limbs when walking quadrupedally or not at all when brachiating

Length of femur in humans

Present - longer

A longer lever so more efficient bipedal locomotion (longer stride)

Length of femur in chimpanzees

Present - shorter

A shorter stride, even when upright/bipedal.

Advantages of Bipedalism (Thermoregulation)

Less surface area exposed to overhead radiation of sunlight - around 60% less solar energy nad upright so more surface area exposed to cooling effect of wind/air (increased convective cooling)

Advantages of Bipedalism (Freeing the hands)

Those who could carry tools to obtain food, carry food back to dependent family or wield weapons in self defence/ attack would have had a selective advantage

Advantages of Bipedalism (Predator avoidance)

Being upright allow hominids to obtain spot predators above the tall grasses of the savannah

Advantages of Bipedalism (Energy sufficient foraging)

Bipedal gait is more ergonomically efficient than knuckle walking and so they were able to forage for longer and further leading to increase in reliable food sources.

Difference between fingers of brachiating primates and humans

Long fingers are an advantage in brachiating grips for primates

Shorter fingers allow the tips to meet the thumb and palm for finer manipulative control in humans. Short (and straight) fingers suggest adaptation for life on the ground rather than in the trees.

Difference between the curvature of fingers in brachiating primates and humans

Curvature is related in brachiating as curved fingers give a stronger grip on branched.

Straighter digits of human hands allows more mobile joints for greater manipulative action and precision grips.

Humans have a relatively longer and more dextrous thumb than other primates

Large muscles around the human thumb give it a wide range of range of motion and strength. The thumb can perform more precise, independent movements with a high degree of precision

Human wrist is more mobile than knuckle-walking primates

Wrist of knuckle-walkers must be rigid to transfer weight

In humans, the wrist can rotate and flex giving a wider range of manipulative movements, vital for tool making etc.

Discuss how the evolution of bipedalism in human ancestors drove the subsequent evolution of our hands.

Once hands were no longer needed for brachiation or for knuckle-walking they could specialise and adapt for manipulation of objects instead. Those ancestors that could grip objects for weapons or tools, carry food back to dependent family, etc would have had an advantage. As bipedalism increasingly freed the hands for alternative activities, those with more developed hand/wrist muscles that gave even greater flexibility and motor control would have had an increasing advantage, setting up a positive feedback loop between bipedalism and manual dexterity. In positive feedback, a small change in one factor is amplified into a greater positive change in another factor which in turn reinforces the original change.

Broca’s area

A section of the brain that coordinates the muscular movements required to produce speech.

Wernicke’s area

A section of the brain that receives sensory inputs and is the region of the brain responsible for language comprehension.

How to determine an animal’s intelligence

Brain size to body mass ratio and complexity of the brain

Benefits of large frontal lobe

Greater abstract thought involved in such things as imagination, language, art and religion

Benefits of large cerebellum

Coordination of fine motor skills, e.g manual dexterity and balance.

Endocranial bulges

Organism most likely had the ability to communicate with language (i.e. makes sounds/language and understands them).

Obstetrical dilemma

Two Opposing trends

As humans became increasingly bipedal their pelvis narrowed and birth canal decreased in diameter

As humans became more intelligent their cranium increased in diameter so less likely to fit easily through the birth canal.

Human birth and infant development

Human babies are born immature, since any further cranial development (needing an increase in diameter of the head) would prevent safe delivery. Therefore they must complete development after birth and are completely helpless/are dependent on mother for longer than other primates.

The human birth process is slower since the cervix must dilate further and the baby must manoeuvre through an extremely tight birth canal, turning twice (both slow and risky).

Binocular

Forward-facing eyes allow for this type of vision

Hominins

Taxonomic name for human and ancestral species

Hominids

Group that contains the modern great apes, their extinct ancestors and modern humans

Apes

Family name for primates that do not have tails