The Advent of Bipedalism

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The Advent of Bipedalism
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• The development of upright walking in early
  hominids is still an area of some debate.
• The most commonly held view is that due to
  changes in the African environment open savannah
  was replacing forested areas.
• Our arboreal ancestors were increasingly forced
  to cross wide open areas of savannah to get from
  one patch of trees to another
• perhaps raising themselves and moving on their
  hind legs to obtain a wider field of view of the
  area and the potentials dangers it held.
• The trees would not only have offered a food
  source but would have been considerably safer
  than the open grassland, where large predatory
  cats roamed in search of prey.

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The Bipedal Adaptation

• Among living primates, only hominids have
  habitual, obligate, and efficient bipedalism.
• There are numerous modifications of the basic
  primate quadrupedal body-plan.
• The most dramatic modifications are in the
  pelvis.
• In quadrupeds, the two ossa coxae are elongated
  and oriented approximately parallel to the spine
  (i.e. dorsally).
• In bipeds, the ossa coxae are shorter and
  broader, and oriented more towards the side (i.e.
  laterally).
• The more basin-like shape of the bipedal pelvis
  alters the function of the gluteus maximus
  muscle, increasing its ability to extend the
  thigh, an important movement in running and
  climbing.

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Other Modifications for Bipedalism Include

• repositioning of the foramen magnum,
• increased spinal curvature,
• lengthening of the hind limb,
• inward angling of the femur,
• development of the longitudinal arch in the foot,
• realignment of the big toe.

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Repositioning of the Foramen Magnum
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increased spinal curvature
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lengthening of the hind limb
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• All the major structural modifications for
  bipedalism are present in the early hominid
  skeletons from Africa.
• Some new South African hominid fossils that
  include the ankle and the big toe indicate the
  heel and form of the longitudinal arch were
  adapted for bipedalism, while the big toe appears
  to be adapted for grasping.
• This (plus interpretations of fossil foot remains
  from Olduvai Gorge and Hadar) suggests to some
  that early hominids, though bipedal, were not
  necessarily obligate bipeds.

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Theories for Bipedal Origins

• The origins of bipedalism have remained an
  important, but controversial, discussion within
  the anthropological world since the discoveries
  of the first hominin fossils.
• To understand the significance of bipedal
  locomotion it is essential to appreciate the
  timeframe within which it is thought to have
  evolved.
• At least in a rudimentary form bipedalism may
  have been present with some of the earliest known
  hominin species.

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• By at least 3.7 mya an upright two-footed
  locomotion is confirmed by the Laetoli footprints
• a pair of fossilised tracks left by an
  australopithecine (A. afarensis) in Tanzania
• Lucy skeleton AL 288-1 shows significant
  bipedal adaptations (from Hadar, Ethiopia,
  between 2.9 and 3.5 mya).
• Other A. afarensis finds dating between 3.0 and
  3.8 mya have played an important part in
  assessing bipedal adaptations
• a proximal femur fragment (from Maka, Ethiopia)
• a right knee joint AL 129-1, 13 individuals
  the AL 333 family
• a near complete skull AL 444-2 (all from Hadar,
  Ethiopia)

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• But bipedalism was very different then to how it
  is now.
• Indeed the early hominins shared many features
  with our ape cousins, and modern completely
  upright habitual bipedal locomotion is unlikely
  to have developed until around 1.5 mya at the
  earliest with Homo ergaster / Homo erectus (see
  later).
• This form of movement is characterised by a
  striding gait.
• Many ideas have been proposed for causes of
  bipedalism first arising, each with a different
  emphasis and varying degrees of plausibility.

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• These ideas can broadly be divided into five
  categories
• The impetus was to free the hands and increase
  manipulative capabilities.
• A change in posture would have led to greater
  thermoregulatory efficiency.
• A change in posture would have widened the
  hominins feeding potential.
• Bipedalism would reduce predation pressures.
• Bipedalism was a more energetically efficient
  form of locomotion.

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Manipulation

• Carrying offspring.
• The suggestion is that by freeing the hands the
  hominins, which represent some of the larger
  primate species, would have better been able to
  carry their large and dependent offspring.
• But this seems unnecessary when we observe other
  primates.
• Indeed there are problems carrying young
  bipedally, especially when combined with a loss
  of body hair to which they can cling.

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• Using tools and weapons.
• Hominins were bipedal long before the origins of
  stone tool technology, but the use of wood and
  bone tool technology may have been important,
  especially if the early hominins were scavenging
  or hunting for meat resources.
• However no tools have been positively identified
  with the earliest hominins, although it may be
  argued that bone and wood are easily missed as
  part of the fossil record because they are
  naturally expected to be found.

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• Manipulating vegetal foods and carrying water.
• Vegetal foods must be dug up or cut from their
  roots, which requires use of the hands.
• Similarly water containers (perhaps gourd-like
  plants) must be transported.
• This would be especially important in light of
  deteriorating environmental conditions and
  central-places.
• The watering hole is a dangerous place, made more
  perilous if it was shrinking.
• Although water-carrying is not observed among
  non-human primates today, many are able to
  successfully forage for plant resources.

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• Feeding on grass-seeds.
• This requires free and dextrous hands to
  manipulate grasses and obtain food.
• Again the impetus seems lacking, given that the
  gelada baboon is a significant consumer of grass
  seeds, yet adopts a terrestrial quadrupedal
  locomotive strategy

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Thermoregulation

• Convection effects.
• Above around three feet from the ground the air
  is much cooler and circulates to a greater
  extent.
• The circulating air or breeze immediately has a
  direct cooling effect, but also serves to
  increase the sweating potential of an individual
  as air flow better removes moisture from the
  bodys surface.
• Combined with a loss of body hair and a set of
  physiological adaptations that separate humans
  from the non-human primates as the most efficient
  sweater the upright posture would certainly have
  led to a cooling effect on the body.

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• Radiation effects.
• By maintaining an upright posture it has been
  demonstrated that there is also a reduction in
  the surface area of the body exposed to thermal
  radiation.
• Direct radiation from the sun is restricted to an
  area of the shoulders and head (in part
  explaining the presence of head hair?), but
  critically the quantity of reflected solar
  radiation is reduced.
• These serve to reduce thermal stress among bipeds

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Feeding Potential

• Feeding from trees.
• If australopithecines had become terrestrial
  because of diminished forest and woodland regions
  it is argued that they may have been ground
  feeders.
• Therefore an ability to reach up and remove
  fruits from the trees would have been
  advantageous.
• Hence an upright posture was selected for and
  this led to the potential for bipedal locomotion.
• However, even the first bipedal hominins would
  have been good climbers (as are chimpanzees), so
  would they really need to feed from the ground
  where they were susceptible to predation?
• The basis for this theory is a study on chimps
  that showed that individuals feeding on two feet
  had a greater abundance of resources and required
  fewer moves than those that remained seated

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Predation Pressure

• Running from predators.
• It has been suggested that humans can run faster
  on two legs than on four.
• Thus bipedalism may have adapted as a mechanism
  for running between fruiting trees to avoid
  terrestrial predators.
• But running short distances is less energetically
  efficient on two legs than four, and the earliest
  hominins would not have possessed the same
  skeleton as anatomically modern humans for whom
  bipedalism is very specialised.

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• Long-distance vision to spot predators.
• There is certainly a benefit for spotting
  predators if the field of vision is maximised by
  an upright posture.
• But equally an upright gait exposes an animal
  more to its predators.
• And any adaptation must be viewed in the context
  of the cost-benefit relationship.
• Would the benefits of seeing a predator have
  outweighed the costs of being seen by a predator?
  
• We must also consider the fact that early
  australopithecines were largely occupying
  woodland habitats where it would have been
  impossible to see predators because of the trees

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Energetics

• Efficiency between fruiting trees.
• Bipedalism is a more energetically efficient
  terrestrial strategy than quadrupedal locomotion
  at slow speeds, and over long distances.
• The proposal is that as the east African
  grasslands expanded (due to environmental
  deterioration in the late Miocene)
  australopithecines may have been forced to search
  out high energy, poorly distributed fruiting
  trees over larger day ranges.
• Terrestrial locomotion would predominate and
  bipedalism may have been an energy-saving
  strategy.