Skeletal Features of Bipedalism
Professor Janaki Natalie Parikh
profjnp@gmail.com
Bipedalism and the Hominid Family
• Criteria for membership in Hominid (Human) family:
bipedalism
• Larger brains did not come first, in fact, increase in
cranial capacity happens millions of yrs. Later
• Following are skeletal features that allow for &/or
indicate bipedal locomotion
• 1. placement of the foramen magnum (?)
Skeletal Features of Bipedalism
• Foramen magnum: Latin, “large hole”, cranial opening
where spinal column articulates
• placement indicates type of locomotion
• 2. S-curvature of the spine: not present
at birth, we normally settle into an S-curve
due to pressure exerted on the spine from
bipedality, also narrow rib cage, signif. Diffcs
in the lumbar region
Skeletal Features cont’d
• Fossil vertebrae wouldn’t be intact, how do we know
what curvature existed?
• Soft tissue always leaves an impression upon bone!
• 3. Bipedal pelvis (?)
• component bones:
• Illium (pelvic blade)
• Ischium
• Pubis
Bipedal pelvis cont’d
• Chimp’s illium: long, narrow & flat
• Biped’s (opposite): short, broad (wide) & curved
• Results: lower center of gravity, better stability and
support for weight of internal organs
• 4. Femur: bicondylar angle (femoral angle): purposefully
knock-kneed. Keeps our feet planted w/in our center of
gravity, also, helps distribute upper body weight
Bipedal feet
• 5. Bipedal feet: foot arches. Purpose?
• Strong, supportive structure, absorbs impact
• 6. Non-divergent big toe, a chimp’s big toe is fully
divergent. Their toe is adapted for grasping,
ours is adapted for balance & support
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•
http://www.npr.org/templates/story/story.php?storyId=128245649
http://www.ted.com/index.php/talks/susan_savage_rumbaugh_on_apes_that_write.ht
ml
Now, back to geologic time pwrpt)
Robust vs. Gracile Grade Hominids
Robust
Gracile
• Large jaws (buttressed
mandible)
• Smaller jaws, no
buttressed mandible
• Molarized premolars &
lrg molars (grinding
teeth)
• Smaller back teeth
Robust vs. Gracile Grade Hominids
Robust
• Jaws muscles anchored
on top (sagittal crest)
Gracile
• Jaw muscles anchored on
the side (no sag. Crest)
Robust vs. Gracile Grade Hominids
Robust
Gracile
• Flaring zygomatic arches
• Cranial capacity?
• No flaring zygom. Arches
• Smaller cranial capacity
• Larger cranial capacity
Robust vs. Gracile Grades
• What can attribute these diffcs to?
• The key is diet! These are not taxonomic designations,
but rather adaptations to a specific diet
• Robust: ate a diet rich in hard foods that graciles didn’t
have the dentition to process
• Which of these grades is ancestral to modern humans?
• Gracile. The robust species eventually died out.
Comparisons of cranial capacity
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Chimpanzee:
394 c.c. (mean)
Gorilla:
506 c.c. (”)
How about anatomically modern humans? (A.M.H.)
A.M.H.:
1325 c.c.!!! (encephalization)
Earliest hominids by comparison?
All results below 500 c.c.’s.
Increase in brain sz. did
not come 1st!
Australopithecus anamensis
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4.2-3.8 m.y.a., E. Africa (N. Kenya), gracile
Older than Lucy’s species
Strong evidence of bipedality
Thick molar enamel, dentition still ape-like
Australopithecus afarensis
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4-3 m.y.a., E. Africa, gracile
Features: Mixture of human & ape like traits
face: prognathic like a chimp
Teeth: CP3 complex present (diminished) evidence of?
↑ Sexual dimorphism!
♀: ~3.5 ft., ~65 lbs.
♂:4.5-5 ft., 110-130 lbs.
Foramen magnum: low & well centered
A. afarensis: Lucy
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I.M.I: higher than modern humans, meaning?
Arms slightly longer & legs shorter relative to us
Pelvis: illium: short, broad & curved
Presence of femoral angle
Big toe: semi-divergent, not much of an arch
Famous specimen: Lucy! Why famous?
(then) most complete specimen found!
• Origins of her name?
• Lucy in the Sky w/ Diamonds (Beatles
• L.S.D….what a trip!
•
http://www.youtube.com/watch?v=A7F2X3rSSCU
Visual comparison of skeletal features
Footprints @ Laetoli site: made by A. afarensis
http://science.discovery.com/videos/100-greatest-discoveries-shorts-laetolifootprints.html
A. afarensis: Dikika Baby
• Only ~3 yrs old at death, cranium: over 90% complete!
(hyoid bone, still ape-like) (length of the vocal cords &
connection w/ human speech)
• “Lucy’s baby”, though lived ~150k yrs. before Lucy!
• http://www.sciencedaily.com/releases/2006/09/060920193509.htm
• http://ngm.nationalgeographic.com/2006/11/dikika-baby/sloan-text & Video
Other Australopithecines
• A. africanus: S. Africa, gracile, 3-2 m.y.a.
• Lumpers vs. splitters debate: Genus Paranthropus
• A. (or P.) aethiopicus: E. Africa, 2.8-2.2 m.y.a., famous
find named “Black Skull” (color: mineralization)
• Dubious distinction: smallest adult human brain ever
found…(& you thought we’d already found it)
Australopithecines cont’d
• A. (or P.) boisei: E. Africa, 2-1 m.y.a., “super robust”:
extremely pronounced robust features
•
http://www.npr.org/templates/story/story.php?storyId=128245649
• (Back to geologic time pwrpt)