Lecture 12 - Early Hominin Evolution

Question 1

what makes a hominin

modern human traits

Answer 1

• bipedalism
• increased encephalization
• reduced canines, dental changes
• sexual dimorph. reduction

Question 2

early hominins

Sahelanthropus tchadensis

Answer 2

7 Ma, Chad
first hominin based on cranial evidence
- reduced canines
- foramen magnum more forward than chimps

Question 3

early hominins

Orrorin tugenesis

Answer 3

6 Ma, Tugen Hills, Kenya
postcranial bipedality
- large femoral heads, long femoral necks, hip joins

no cranial evidence - only few isolated teeth

Question 4

early hominins

Ardipithecus kadabba

Answer 4

5.8-5.2 Ma, Ethiopia, Afar region
similar to A. ramidus, more primitive traits

Question 5

early hominins

A. ramidus

Answer 5

~4.4 Ma
* relatively small brain
* prognathism as in apes, pronounced snout
* more forwardly placed foramen magnum - consistent with bipedalism
* reduced canine size
* foot retains divergent big toe
* lacked other features for suspension (vertical climbing, knuckle-walking)
* bipedal on ground, walk on branches rather than swing

Question 6

early hominins

Au. anamensis

Answer 6

• ~4 Ma, Kenya/ Ethiopia
• evidence of bipedalism in tibia
• expanded molar teeth, larger than humans

derived traits
* reduced canines
* larger molars, thick enamel
* undisputed evidence of bipedalism

compared to later hominins, retains:
* more U-shaped dental arcade

• chimp range brain
• open woodland/gallery forest environment and open savanna environment
• first species starting to be found in C4 vegetation landscapes

Question 7

early hominins

Au. afarensis - general

Answer 7

~ 3.9 - 2.9 Ma
Kenya, Tanzania, Ethiopia
“Lucy”

Question 8

early hominins

Au. afarensis cranium

Answer 8

relatively small brain size relative to face size
sagittal crest
strong nuchal crest for neck muscle attachments
relatively prognathic

Question 9

early hominins

Au. afarensis dental

Answer 9

thick enamel on teeth suggest they may have eaten nuts, grains, or hard fruit pits, also probably still eating fruit
canine further reduced
lower 3rd molar bicuspid usually
lower jaw

Question 10

early hominins

Au. afarensis postcranial

Answer 10

more forward placement of foramen magnum
lumbar curvature
valgus knee
short toes, enlarged heel, arches, non-opposable big toe

Question 11

early hominins

Au. afarensis arboreal features

Answer 11

highly mobile shoulder joint and upwardly oriented shoulder blade
relatively long and curved hand bones
relatively long upper limbs compared to lower limbs

Question 12

why did later hominins (Au.) keep arboreal features?

Answer 12

argument 1: natural selection takes long time to work

argument 2: retention of adaptations means they will be used (not ignored)

humans still climb trees without adaptations, why wouldn’t Lucy?

Question 13

where does most of evidence for early hominins come from

Answer 13

East - Great Rift Valley
Southern Africa

Question 14

difference between eastern and southern African sites that preserve hominins

Answer 14

southern - accumulation of fossils in caves
eastern - open air sites

Question 15

general trend of C4 grasslands through past 8 Ma in africa? cause?

Answer 15

• more grasslands, C4 domininant
• fire, herbivory etc limit C3 vegetation
  reduced CO2 atmosphere - C4 plants do better
• variation of precipitation (Milankovitch cycles)