Biological Evolution

Hominins

living and fossili species belonging to human lineage

Hominds

Group that includes both humans AND great apes.

biological evolution

The changes in genetic information as it is passed from one generation to the next

Arboreal

living in trees

brachiation

primates swong from tree to tree using their arms

quadrupedalism

walking on 4 limbs

Bipedalism

wakling on 2 limbs

selection pressures for bipedalism

bipedalism is thought to evolved due to enviornmental changes africa around 4-5million years ago

climate became cooler and drier, near-continuous forests were replaced by more open, wooded savannas with widely spaced trees.

food resources spread out, more time was required to collect food, had to experiment with new food resources in wooded savannahs

near complete arboreal life, pre-hominins had to leave the tree to seek out resources.

advantage of bipedalism

1. more energy efficent locomotion ( bipedlaism gives energy effcient method favours low speed, long distance movement)

2. seeing over the grass ( upright allowed humans see better over tall grass. Spot predators or locare carcasses and food resources, upright stance made hominins look more menacing to predators)

3. freeing the hands (carry objects -tools, food, babies and use objects - weapons and tools, and make tools and weapons - freeing hands to make tools)

4. decrease the exposure of the body to direct sunlight (70%) (helps body cool in hot cliimate in adican savannah)

individuals that have efficient locomotion can use energy to access more resources, greater chance of survival and successful reproduction

Disadvantages of bipedalism

1. Narrow birth canal

2. slipped disc

3. flat feet ( feet may suffer strain as body rests on 2 limbs. Arches feet collapse resulting in flat footedness)

selection for nakedness

1. Thermoregulation

• Shorter, finer body hair increased heat loss via radiation.

• Well-developed skin glands enabled highly effective heat loss.

2. Parasite Control

• Reduced parasites (fleas, lice), especially around home bases where reinfection was likely.

3. Head Hair Retained

• Reflects and radiates heat, protecting the scalp from direct sun exposure.

Sagittal Crest

Change: From large to reduced

• Diet changed from tough, fibrous food to softer, cooked food.

• Less need for strong chewing muscles and heavy jaw movement.

• Smaller space needed for muscle attachment.

Zygomatic arch

Change: From large to reduced

• Diet changed from tough, fibrous food to softer, cooked food.

• Less need for strong chewing muscles and heavy jaw movement.

• Smaller space needed for muscle attachment.

Brow ridge

Change: Large to reduced brow ridges

Reason:

• Diet changed to softer, cooked food.

• Less strain on the face from chewing.

• Less need for reinforced brow ridges to reduce strain.

Prognathism (and Jaw)

Change: Large to reduced jaw and tooth size

Reason:

• Diet changed from tough, fibrous to softer, cooked and processed food.

• Less need for space for large teeth and strong jaw muscles.

teeth

Change:

• Large, specialized teeth —→ smaller, uniform teeth

• Prominent canines —→ reduced canines

• Diastema present —→ no diastema

• U-shaped dental arcade —→ V-shaped (parabolic) arcade

Reason:

• Diet shifted to softer, cooked, and processed food.

• Less need for grinding and tearing, leading to smaller teeth.

• Uniform teeth allowed more efficient chewing of varied foods.

Cranial capacity

Change: Small to increased brain size / cranial capacity

Reason:

• Higher energy diet and anatomical changes from bipedalism.

• Allowed more energy and space for the brain.

• Led to an increase in cranial capacity.

shape of skull

Change: Flat —→ more rounded braincase

Reason:

• Accommodates a larger cranial capacity.

• Reduced jaw and neck muscle attachment sites allowed the skull to expand into a rounded shape.

Nuchal muscle/Nuchal crest

Change: Large —→ reduced neck muscle attachment sites

• Shifted to bipedalism, so the head balanced on top of the spine.

• Reduced need for large neck muscles to support a forward-leaning skull.

• Smaller space needed for muscle attachment.

Foramen Magnum

Change: Back of the skull $\rightarrow$ center underneath

Reason:

• Hominins shifted to bipedalism.

• Helps balance the head over the spine.

• Less energy is required to hold the head upright.

Occipital condyles

Change: Back of the skull —→center underneath

• Hominins shifted to bipedalism.

• Helps balance the head over the spine.

• Less energy and neck muscle required to hold the head upright.

How does the rib cage differ between apes and humans, and how does it aid bipedalism?

Apes: Cone-shaped and deep from front to back.

• Humans: Barrel-shaped and flattened front-to-back.

• Why it helps: 1. Brings arms to the side so they can swing for balance. 2. Shifts upper body weight backward over the center of gravity. Balance upper body, energy efficient walking

What is the difference in spine shape between apes and humans?

• Apes: C-shaped curve.

• Humans: S-shaped curve.

• Why it helps: 1. Positions the majority of body weight directly above the pelvis. 2. Acts as a shock absorber to reduce physical stress during walking. Providing balance, and energy efficient walking.

Compare the pelvis shape of apes vs. humans.

• Apes: Long and narrow.

• Humans: Broad, short, and bowl-shaped.

• Why it helps: distributes weight of upper body to enable balance, for energy efficient walking

How do leg and arm proportions differ between apes and humans?

* In apes, legs are shorter than arms but in humans legs are longer than arms.

• This increases stride length, making walking more energy efficient.

How does the femur differ between apes and humans?

In apes, femur is shorter and broader but in humans femur is longer and straighter.

• This supports the body’s centre of gravity so that one foot can be lifted while the other maintains the balance for bipedal walking, contributing to energy efficient walking.

What is the difference in valgus angle between apes and humans?

There is no valgus angle in apes and their femur falls straight down. However in humans, there is increased valgus angle where the femur slants downwards and inward, bringing the knees under the body’s centre of gravity.

• It reduces side to side movement (swaying), improving balance and makes walking more energy efficient.

How do knee joints and buttressing differ between apes and humans?

In apes, they have smaller knee joints with less lateral buttressing at the end of the femur, whereas humans have larger knee joints with greater lateral buttressing at the end of the femur.

• This is because with increased valgus angle, the pressure over the knee became greater. Buttressing prevents the knee from collapsing sideways and stabilises the leg when one foot is lifted off for walking making walking more energy efficient.

How does the big toe differ between apes and humans?

In apes, the big toe is out to the side whereas in humans, the big toe is aligned with other toes (points forward).

• The big toe provides thrust (push off) against the ground to propel the body forward, making walking more energy efficient.

Compare the foot arch in apes vs. humans and its function.

Apes: Has no arch.

• Humans: Has an arch under the foot.

• Function: Acts like a spring under tension, absorbing shock and allows transfer of weight down the foot to the big toe. Energy efficicent

Compare the thumbs and fingers of apes vs. humans and what they allow.

Apes: Short thumbs and long, curved fingers.

• Humans: Long and fully opposable thumbs.

• Function: Allows both power grip and precision grip for making and using tools and fine manipulative movements.

Define power grip.

Definition: Stronghold where fingers and palm are partly bent and pressure is applied by the thumb.

precision grip.

The ability to perform fine manipulative movements using the long thumb and fingertips.