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 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 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.

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. 

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.

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.

Bipeds have their arms free to use!

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

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.

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.

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 

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.

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.

Ancestral traits:

Primitive teeth 

Small Brain

Apelike back of skull

Derived traits:

Less prognathism (protruding jaw)

Central foramen magnum

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"

Ancestral traits:

Chimp-like teeth 

Post-cranial climbing adaptations

Derived traits:

Postcranial characters indicate bipedality

Small teeth with thick tooth enamel

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" - southern ape

small-bodied, small brained bipeds

Protruding jaw

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

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.

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: 

bipedal

small canines

large molars 

large lower jaw

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:

bipedalism

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