ARCL 140 - Early Hominins

Hominin

Under the superfamily, Hominoidea

Sahelanthroppus tchadensis (7-6 mya)

Ape-Like Traits

• small cranial capacity

  • (320-380cc)

• ‘U’ shaped dental arcade

• large external occipital protuberance

Hominin Traits

• forward foramen magnum

• reduced canine honing complex

  • canine, diastema, sectoral premolar is smaller than apes but larger than humans

Orrorin tugenensis (6-5.7 mya)

Hominin Traits

• long femoral neck

• reduced canine honing complex

Derived Traits

• small molars

• thick enamel

Ardipithecus ramidus (4.42-4.15 mya)

Ape-Like Traits

• small cranial capacity (300-350cc)

• small body size (120cm, 42 kg)

• large molars

• opposable big toe

• curved fingers

Hominin Traits

• forward facing foremen magnum

• slight prognathism

• short and broad ilium

• vulgus knee

• reduced canine honing complex

Ancestral Traits of Early Hominins

• Y5 molars

• arboreal and suspensory locomotion

Derived Traits of Early Hominins

• bipedalism

• reduction in canine honing complex

Bipedalism

the ability to walk upright on 2 legs

Anatomy of Bidpedal locomotion

• suspensory upper body

• rotating shoulder

• flexible wrists

• round ribcage

• extending elbow

• short lower back

Derived Traits of Bipedal Locomotion

• ‘bowl-shaped’ pelvic complex

Osteology of Bipedal Locomotion

• foremen magnum underneath the skull

  • allows for spine to be directly under skull

  • distributes weight evenly

• S-curve spine

  • helps balance body over pelvic complex

• lumbar spine with short superior-inferiorly and transversely broad vertebral bodies for increased flexibility

• transversely broad sacrum

• anterior-posteriorly curved ilium

• os coxa is superior-inferiorly short

• narrow birth canal

• valgus knee

  • angled towards the middle

• long femoral neck

Superior-inferiorly

from up to down

Transversely

from left to right

Anterior-posteriorly

from front to back

Habitual Bipedalism

bipedal locomotion often used by hominins that retain ancestral anatomical structures

Obligate Bipedalism

major structural changes in the body occured to promote energy-efficient bipedalism

Advantages of Bipedalism

• hands are freed from locomotion

• upright stance allows for a wider view of surroundings

• reduces thermal load on body

• makes standing more efficient

Aridity Hypothesis

• expansion of savannah forced early hominins from an arboreal lifestyle to a terrestrial one

• first proposed by Darwin

• long term aridification of an environment diversified evolution in early hominins

Forest Hypothesis

• bipedalism arose not on land but in the trees

• stems from upright posture

  • orthograde

Turnover Pulse Hypothesis

• rapidly changing climates encouraged a selection for a generalized body plan

  • stabilizing selection

• animals with generalized diets fared better than those with specialized diets

• specialist eaters faced extinction at greater rates than generalist eaters

• periods with extreme climate change was associated with faunal turnover

Variability Selection Hypothesis

• High amount of climatic variability is linked to behavioural and morphological changes

• suggests that hominin evolution does not respond to habitat specific changes or to specific aridity/moisture trends

• long term environmental unpredictability over time influenced adaptations helping hominins to survive

• the constant varying degrees of natural selection allowed certain groups to develop genetic combinations that increased their ability to survive

• generalized body plans and adaptive behaviours allowed individuals to survive better over longer periods of time

Faunal Turnover

the rate at which species go extinct and are replaced with new species

Aridification

becoming increasingly dry