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ANTHR101 Study Guide - Midterm Guide: Prognathism, Bipedalism, Brain Size

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Francois Larose
Study Guide

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Trends in Human Evolution:
Adaptation to bipedal locomotion
o Started that we walked on all fours and most likely knuckle walkers o
Many anatomical changes
Gradual increase in body size
Gradual reduction in sexual dimorphism
Increased brain size to body size ratio
Reduced facial prognathism
o Muzzle type face; animals tend to have a face that appears pulled forward
o Modern-day humans have flatter faces
Reduced supraorbital torus (tori = plural)
o The bony ridges around the eyes are prominent in apes and chimps but have
flattened out in modern day humans
Incisors and canines becoming smaller compared to the premolars and molars
Non-honing canine complex
o The fact that apes have large canines and when the jaw closes, they need
a gap to close the mouth and as the canines and surrounding teeth rub
and grind together they create a sharpness to the canines. But humans
have lost this ability and now have the non-honing canine complex.
Increased dependence on tools for survival
o Changes significantly our evolution e.g. complexity of hands because we use our
hands a lot more
Origins of Bipedalism:
o Climate change from the rainforest to the open Savanna (tropical grasslands)
o For some ancestors the ability to walk upright was beneficial
o Reasons why they may have evolved:
Carrying - weapons, tools, water, and food
bringing food to mothers and infants, allowing females to
conserve energy to reproduce & care for infants
Spot predators or pray
Travelling between food trees
Less likely because we’re losing the ability to climb to feed from
trees not sensible
Feeding from bushes

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Anatomy of Bipedalism
(Remember, changes were gradual)
o Pelvis:
o Chimpanzee: long and flat because they stand on all fours as knuckle walkers and
there is little weight that’s going to be on the pelvic bones
o Humans: when we stand upright, all our body weight is being placed on our
lower body, therefore, we have a bowl shaped pelvis
o Base of the Skull:
o Looking at the an anatomical feature, the foramen magnum (meaning big hole) at
the bottom of the skull where the spine connects to the skull tells where the
animal’s head was in relation to the body
For those whose foramen magnum is on the back of the skull,
would walk on all fours
For those whose foramen magnum is on the bottom middle of
the skull, would walk bipedally
o Femur/ knees:
o Chimps: Femur goes straight down, therefore legs are slightly apart
Knees cannot lock upright.
o Humans: have legs that angle inward toward each other and fall under the body
(‘knock-kneed’ posture) helps legs move forward and helps maintain centre of
gravity while standing, walking, and running
We can fully extend our knee, so they can lock in an upright position.
o Feet:
o Bipedal = Robust heel which can withstand substantial force
o Arch in foot to help absorb forces

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Earliest Hominins (3 possible candidates) The Transition from Hominoids to Hominins
o Sahelanthropus tchadensis (7 6 mya)
o Orrorin tugenesis (6 mya)
o Ardipthecus ramidus (5.8 4.4 mya)
o Not classified as a biped (because of semantics)
o No clear connections because of missing links between the three fossils
General Characteristics:
o Small brains (1/3 of modern human brains)
o Mostly apelike characteristics
o Possibly bipedal (?)
Can’t say for certain until more of the fossils of an individual are
o Lived in forested environments
Recall that all origin models were developed with assumption that
species evolved to walk upright because of the geographical shift to the open
However, if they are the first bipeds, they lived in forests we
know because during the excavation we found plants and animals with the
fossils that clearly lived in forest environments
Could be that they only stood upright, not moved bipedally which is why
it’s controversial to call them bipedal (walk upright for the majority of the
time) because they may have never used their ability to walk upright as their
way of locomotion
When we study bonobos, their body size doesn’t allow them to stand on tiny
branches, they stand near the center and spend a lot of their time standing and
reaching the food on trees with their arms. Could be what these species did.
o Ardipithecus
Same as humans: some sexual dimorphism, pelvis, foramen magnum
Different from humans: opposable big toe, long arms and hands
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