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Anthro 105 Chapter 10
Chapter 10
Undergraduate 1

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General Anatomical Changes for Bipedalism

Bipeds must balance the body's weight over two limbs. 


Quadrupeds must move its weight between two limbs. 


Bipeds' body weight fall naturally between their two feet. 

Vertebral column in Quadrupeds

Vertebral column attaches to the back of the skull.


C-shaped spine that makes thoracic region of spine slightly curve outward. C-shaped spine puts the center of gravity in front of the quadruped's feet, causing it to fall forward when on two legs.


Weight is equally distributed throughout quadruped's vertebrae.


Vertebral column in Bipeds


Vertebral column attaches to the bottom of the skull.


S-shaped spine from two curves: at the neck and the lower back. This is an addition to the quadruped's C-shaped spine that brings the center of gravity to the hips, resting OVER the biped's feet.


Weight increased as you go down a biped's vertebral column.


Anatomical Change: The Pelvis

Modified to keep the body's center of gravity over one foot while walking.


Composed of Ilium, Ischium, and Pubis, ordered largest to smallest. 


Ischium: the bone you sit on.


Ilium: the bone felt when hips are touched.


Pubis: the bony portion near the pubic region.




Pelvis in quadrupeds 


Pelvis in bipeds


Pelvis in quadrupeds: long and flat and situated on the back of the animal


Pelvis in bipeds: short and broad. bowl-shaped, allowing for support for abdominal organs that are pulled down by gravity. 

Anatomical Change: The Leg

Broad pelvis in bipeds places the femur to the side, allowing the foot to fall directly below center of gravity


Bipeds have angled femur.


Bipeds have longer legs compared to trunk, increasing stride length and walking efficiency.


Quadrupeds have straight femur, placing the foot far to the side of the center of gravity.


Quadrupeds have straight femur.

Anatomical Change: The Foot

Composed of tarsals (heel/ankle), metatarsals, and phalanges (toes).


Bipedal feet are stout and can accommodate the great weight place on them.


They also have arches that act as shock absorber that store and return energy during walking.

Anatomical Change: The Arm

Bipeds have their arms free to use!


Arms have shortened relative to trunk length. Thumb became opposable and the phalanges were shortened.

Energy Efficiency of Bipedalism

Much more efficient that quadrupedalism. 


Definitely more efficient than knucklewalking apes: e.g. oxygen consumption is greater in chimpanzees.


Greater ability to dissipate heat: less surface area is exposed to the sun.

Ecological Influences on Bipedalism

Environmental changes 5 to 8 MYA may have favored bipedalism: grasslands expanded and forests decreased.


Grasslands: wider scattering of foods (requires a more energy efficient mode of traveling). 


Gained a better view of one's surrounding: see over tall grass or scan for predators.



Dietary Influences on Bipedalism

Feeding advantages: easier to pluck ripe fruits or hunt efficiently, to walk atop tree limbs or shuffle between food patches.


Safety advantages: increase in patchiness of food requires the crossing of savannas = increase in body size and group size for protection 

Sexual Selection, Mating Strategies, and Bipedalism

Walking upright allows males to look impressive and get more mates.


Bipedalism raised energy efficiency and allowed males to walk and search for food to carry back to the females they were mating with.


Female's were fortified by extra nutrition received by males: intervals between births shortened.

Sahelanthropus tchadensis (7-6 MYA)

Fossilized skull found in northern Chad in 2001.


Nicknamed "Toumai."


Sahelanthropus tchadensis: "the Sahara hominid from Chad" 


5.2-7 million years old. Oldest member of the Hominids.

Sahelanthropus tchadensis characteristics

Ancestral traits:


Primitive teeth 


Small Brain


Apelike back of skull


Derived traits:


Less prognathism (protruding jaw)


Central foramen magnum

Orrorin tugenensis (6 MYA)

Discovered in Kenya in 2000 - "Millennium Man"


Consists of fragmentary cranial and postcranial remains; mainly thigh bone fragments.


Orrorin tugenensis - "hominid from the Tugen Hills"



Orrorin tugenesis characteristics

Ancestral traits:


Chimp-like teeth 


Post-cranial climbing adaptations


Derived traits:


Postcranial characters indicate bipedality


Small teeth with thick tooth enamel

Ardipithecus ramidus (4.4 MYA) and characteristics

Ardipithecus ramidus - ("ground-living root hominid")


Fairly complete fossil


Ancestral traits:


Small molars


Thin enamel


Derived traits:


Locomotion: grasping hands and feet; bipedal


Small canines

Australopithecus genus

"Australopithecus" - southern ape


small-bodied, small brained bipeds


Protruding jaw

Australopithecus anamensis (4.2-3.9 MYA)

Found in Lake Turkana, Kenya


Consists of: upper and lower jaw, teeth, tibia


Ancestral traits: 


U-shaped dental arcade


Receding chin


Climbing adaptations


Derived traits:


Large molars


Thick enamel


Small canines


Bipedal (based on shape of knee & ankle bones)


Ecology: mixed habitat - dry woodlands, riverine forests, open grassland

Australopithecus afarensis (3.9-2.9 MYA)

Also known as Lucy (from Beatles song).


Found in Ethiopia in 1974.


Very complete fossil human.


Clear-mosaic of human-like and ape-like features.


Modest brain size equal to a chimpanzee's. Ape-like skull, but bipedal: angled femur, tibia supports more weight, non-divergent toe.


Cranium and teeth intermediate between ape and human.


Cranial crests--bony ridges on the skull where muscles attached--indicate Lucy heavily chewed.


U-shaped dental arcade with large anterior teeth, parallel rows of cheek teeth, and a shallow palate: all ape-like traits.


Small canine teeth.


Large vegetal diet. 


Arms longer relative to leg length.


Sexually dimorphic; not monogamous, but polygynous.

Australopithecus africanus (3.5-<2.0 MYA)

Original finding, the Taung child, in South Africa, hence "southern ape-man"


Post-cranial remains similar to Lucy (A. afarensis)


Small-bodied and broad, short pelvis, as well as structural adaptations in spine, leg, and feet resemblant of modern bipeds.


High sexual dimorphism 


Ape-like developmental rate


Ancestral features: 


Small brain


Derived features: 




small canines


large molars 


large lower jaw

The Robust Australopithecines (or Paranthropines)

Evolutionarily dead end: extreme anatomical specialization.


Hard-object feeding: large bite forces maximized the size and placement of muscles of chewing (muscles of mastication).


Diet consisted of tough objects most hominids could not eat: nicknamed "nutcracker man;" omnivores.


This resulted in extreme postorbital constriction and flared zygomatic arches (cheek bones) to accommodate bigger muscles.


Ancestral features:


small brains


Derived features:




enormous molars and lower jaw 


sagittal crest


flaring zygomatic arches


Specialized chewing indicated different ways of living compared to other hominids --> given own genus, Paranthropus

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