Bipedalism: how do we know?
*Position of the foramen magnum
*Shape of the ribcage
*Pelvis
*Curvature of the spine
*Shape of distal femur
*Foot
Curvature of spine; ribcage
- Ribcage has changed shape so the torso can flex
and arms swing freely, aiding balance - Primate cone-shaped ribcages accommodate a large
gut below and allow a wider range of movement at
the shoulder for climbing - Curvature of spine, especially neck and S-shaped
lumbar region brings body’s centre of gravity closer to
midline and allows spine to flex
Orrorin tugenensis: bipedal?
Femur: has a few ape-like
features but many more derived features:
* Elongated femoral neck
* Anteriorly twisted head (vs
posterior twist in
australopithecines)
* Thicker cortex bone in the upper
part
* Well-developed gluteal
tuberosity (gluteus muscle
attachment area)
* Shallow superior notch
* Antero-posteriorly compressed
femoral neck
Ardipithecus ramidus: bipedalism?
Feet:
* Divergent big toe for good grasping (more apelike)
* Supportive mid-foot and heel to allow better
‘pushing off’ while walking bipedally
Pelvis:
* Upper part of pelvis is more human-like, lower
part more ape-like
Hands
* Long, ape-like fingers
* But palm is short and robust
* Wrist is flexible
* NOT adapted to knuckle-walking or brachiation
but some arboreality
Limb proportion:
* Relatively long arms
* More like old world monkeys, not great apes!
Mix of features: more bipedal than living apes,
but some arboreality?
* Bipedal on ground, quadrupedal in trees?
* Or was there more than one way of being
bipedal?!
Danuvius guggenmosi
- Extinct species of Great Ape
- Dated to 11.6mya, found in southern Germany
- Suggestions of both below-branch suspension, and bipedalism
- Extended lumbar region of spine and curvature
- Extended hip and knee
- Stabilised ankle joint
- Grasping big toe
- Femur: more vertically oriented neck and expanded joint surface
implies titled iliac blade - Flat patella
- Near vertical orientation of tibia relative to plane of ankle joint,
suggests valgus knee
Facultative bipedalism
species has the
ability to adopt bipedal locomotion at
need but usually uses another mode of
locomotion
Habitual bipedalism
species often uses
bipedal forms of locomotion but can use
other modes and retains adaptations for
them
Obligate bipedalism
species is so well-adapted to bipedal locomotion that other
forms of locomotion are significantly less
efficient and rarely if ever used
Postural feeding
- Bipedality in arboreal setting: * Chimpanzee and orang-utan eat foods
found in higher levels of canopy of trees - Bipedalism lets them balance on
branches and reach higher ones too
flimsy to bear primates’ weight - Habit transferred subsequently to
terrestrial settings - Arboreal traits seen in skeletons of early
ancestors: some anatomical adaptations
to arboreality retained - Assumes hominins were actually eating
these kinds of resources
*Dietary/palaeoenvironmenal information
so far supports (or at least, doesn’t
refute
Savannah-based theory/theories
Strong and weak alternatives:
* Strong version: savannah as modern
grasslands, i.e. very open landscapes
with few trees (traditional)
* Weaker version: more of a mosaic
environment (more recent
palaeoecological evidence supports
this!)
* Lack of trees meant they needed
their hands less for climbing around
in trees
* Once hands were freed up we could
use them for other things, i.e. mix of
selective pressures!
* Climate causes switch from forested
environments to open grasslands
* Less shelter from predators
* Different foraging techniques required
* Open grasslands meant moving further
between resource patches, particularly for
food
* Some debate over precise nature of
climatic/environmental changes
* Carbon isotope analysis shows changing
woodland % through time, difficult to spot a
clear trend around this time
* Fossil evidence associated with early
hominins suggests mixed environments
* IF bipedalism is an adaptation to savannah,
why retain adaptations to arboreality?
Travelling efficiency theory
- Connected to savannah theories: more open
landscapes means hominins had to start travelling
further between trees to access food sources - IF the shift to savannah happened: fewer stands of
tress left larger distances to travel between them,
should have favoured shift to bipedalism to move
quickly and efficiently between patches of
resources - Assumption: bipedality is good for walking longer
distances more efficiently - BUT tests on efficiency of chimpanzee
quadrupedalism suggest no real difference between
quadrupedal walking and bipedal walking (though
some debate…) - Longer stride is important but doesn’t appear until
later on - Habitual mobility relates to group size and speed of
resource use; larger groups use resources more
quickly, requiring more mobility to exploit widely
spaced resource patches
Provisioning/sharing theory
- Complements the other carrying models …
- Walking frees up hands to allow carrying of provisions
- Assumption: early hominins had monogamous
relationships; males would go out and find food and return
it to the home base where the mothers/offspring waited - Males carry food to females; females carry young and food
so stay at ‘base camp’ and move less - Assumes that human ancestors had monogamous
relationships and shared supporting young - Is this just a projection back into the past of what is assumed to be
the norm for our own society? - Reduced sexual dimorphism may suggest more co-operation
in raising young? - male canine teeth smaller, body size more similar between males
and females - BUT some archaic hominins e.g. A. afarensis had large sexual
dimorphism, suggesting not monogamous? - Humans certainly more k-selected compared to old-world
apes and monkeys: more emphasis on a smaller number of
offspring, but how far back does this date?
Carrying
- Bipedalism frees up the hands for carrying things: food, branches (for
weapons); stone, other raw materials - Related in part to provisioning/sharing: could be carrying children, food,
tools; stone; other raw resources - Pre-dates stone tools (as far as we know at the moment!)
Coping strategy when faced with widely spaced resources-
* Carrying tools to kill-sites allows access to otherwise unavailable
resources
* Carrying material away from kill-sites avoids confrontation with (other)
scavengers and carnivores
- Chimpanzees are seen to walk bipedally more often when carrying food
but no evidence for physiological postural change; only used during
specific activities - Might relate to value of resources? More important where resources are
more precious, moved resources further, easier when bipedal
Warning/threat (and other)
displays
- Bipedal posture gives advantage when striking with
forearms: during fights, many animals (lions; horses;
bears…) stand on two legs and use their upper limbs to
fight - Advantage in striking your opponent, especially
downwards (more force); also easier to aim - Standing on two legs makes an individual looks bigger
– more threatening to other males, more attractive to
females - Also displays testicles/penis or breasts: sexual
selection - Male to male aggression as competition for resources
(including females)
Thermoregulation
- By standing up, bipeds are presenting less
surface area of their bodies to the sun (and
hence less heat gain) than quadrupeds - Standing upright allows you to lose more body
heat and spend longer doing more exercise in
the savannah - Regulating temperature allowed more exertion;
running, long distance walking - Bipedalism and hair loss allows better
temperature regulation in hot environments - Problem: at midday, not too much difference? * Problem: heads already get quite hot, as
brains produce quite a lot of waste energy - Connected to savannah hypothesis: assumes
that species have already moved out into the
savannah (which is far from demonstrated!) - Models show difference is probably minimal
compared to hair
-covered early hominins:
needed less hair and more human sweat
glands and systems to be really useful
Wading/aquatic ape theory
- Strong version by Elaine Morgan suggests a phase of
hominin evolution during which hominins became aquatic - Pattern of fat deposition and hair growth among humans
unusual among apes? - May be later development related to thermoregulation among
hairless infants…? - No evidence for a fully aquatic stage of evolution: more
likely is the weaker variety: - Food sources located across/in/near water meant hominins
would have to wade, standing on two legs - Then moved to a drier habitat where it was no longer necessary
- Primates who are typically quadrupedal usually switch to bipedalism to get food in water
- Lived near water habitats; spent time in riverine habitats and trees in swampy ground; tooth microwear suggests
variety of foods, including those found near river habitats - Tooth microwear; glossy enamel similar to animals feeding on
water-based plant - Isotopic evidence of use of water-based plants
- Patterns of body hair/body fat similar to those of aquatic
creatures
