2. Evolutionary Trends in Hominids

What is a hominid?

A member of the Hominidae family - including humans and the great apes

What is a hominin?

A member of the Hominini tribe - which was used to separate humans and the great apes, as included in the hominin tribe are humans and extinct humans

What are some of the variations caused by DNA sequences in the Hominidae family?

• relatively hairless

• upper and lower limbs allow for bipedal locomotion

• walk with an upright posture

• development of speech

• having a striding gait

• great brain development

• change in teeth

• development of sexual characteristics

What are the adaptations to the foot to contribute to walking?

• Humans have a larger heel bone. Weight transmitted from the heel along the outside of the foot as far as the ball, crosses the ball and finally along the big toe

• Humans big toe is large and aligned with the other toes. It propels the weight of the body forwards (hence why no longer prehensile)

• Longitudinal and transverse arches in human feet allow bipedal locomotion (walking)

What are the adaptations to the leg stance to contribute to walking?

• Humans have long legs (lower centre of gravity) and a long stride

• Human femurs converge towards the knees keeping the weight distribution close towards the central axis of the body

• Humans to have a straddling gait instead of swaying when walking

What are the adaptions to the knee joint to contribute to walking?

• Body weight transmitted down the outside of the femur to the knee in humans

• Human centre of gravity in the body is in line with the front of the knees

• Little energy required to support the body in a standing position

• In modern humans, the knee joint is adapted to bear weight through the lateral condyles, which are larger than the medial condyles

• In chimpanzees, the knee joint is adapted to bear weight through the medial condyles

What are the adaptations to the curvature of the spine to contribute to upright posture?

• Humans have an S - shape curvature (contributes to an upright stance)

• Lumbar curve improves the body balance and allows the head to balance on the top of the neck

• Cervical curve brings the vertebral column directly under the centre of the gravity of the skull

What are the adaptations to the position of the foramen magnum and jaw that contribute to upright posture (humans vs apes)?

Humans:

• Skull balances on top of the vertebral column

• Flatter face (shorter jaw) helps to balance skull

• Foramen magnum located centrally, underneath the skull

Apes:

• Strong neck muscles to keep head in position

• Protruding jaw

• Foramen magnum towards the back of the skull

What are the adaptations to the pelvis that contribute to upright posture?

• Humans’ pelvis is wide and short

• Supports abdominal organs when standing and foetus in pregnant women

• Wider in females than males (helps with childbirth)

What are the adaptations to the stance that contribute to an upright posture?

• Sustained muscle tone required to keep the body upright

• Muscles bring about the movement of the spine, hips, knees, ankles, and abdominal muscles

How do adaptations to locomotion contribute to an upright posture?

• Trunk rotates at the pelvis

• Arms swing forwards to compensate body rotation (left arm forward when right leg goes forward saving energy)

• Broad hips and converging femurs provide stability and prevent side to side swaying as in apes

What had been an adaptation made to locomotion over evolution?

• Transition from quadrapedalism to bipedalism

• In the past, brachiation was a form of arboreal (tree - living) in which primates swung from tree to tree only using arms

What has happened to the relative brain size and why?

• The difference in brain size between apes and humans is associated with the cerebral cortex

• It is highly convoluted (increases SA)

Apes:

• Orangutans (275 - 500cm3)

• Chimpanzees (275 - 500cm3)

• Gorillas (340 - 750 cm3)

Early Hominids:

• Homo Sapiens (1300cm3)

• Homo Erectus (1100 cm3)

• Homo Habillus (600 cm3)

• Australopithecus Africanus (500 cm3)

• Australopithecus Afarensis (450cm3)

What is an endocast?

• Impressions of the inside of the skull made from rock or other material

• Occur naturally

• Can show cranial capacity, brain size, and convolutions

How has the skull evolved?

• Reduction in prognathism (forward sticking out jaw)

• Reduction in brow ridge (bony ridge located above eye sockets, more distinct than forehead)

• Face becomes flatter, nose becomes more prominent

• Cranium increases in size

How has dentition evolved?

• Human canine teeth are not elongated as in monkeys and apes

• The shape of the tooth row (dental arcade) has become wider

• Diastema between the canine and incisors has disappeared

• Human teeth are about 10% smaller now than 100,000 years ago

• Modern humans are gradually losing their wisdom teeth