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bipedalism: Miocene hominoids & modern analogues

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  • Marc Verhaegen
    Acquisition of bipedalism: the Miocene hominoid record and modern analogues for bipedal protohominids Masato Nakatsukasa 2004 J Anat 204:385­402 doi
    Message 1 of 1 , Mar 10, 2013
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      Acquisition of bipedalism:
      the Miocene hominoid record and modern analogues for bipedal protohominids
      Masato Nakatsukasa 2004 J Anat 204:385­402
      doi 10.1111/j.0021-8782.2004.00290.x

      1) Proconsul (Early Miocene, Kenya) was a non-specialized arboreal
      quadruped with strong pollicial/hallucial assisted grasping capability.
      It lacked most of the suspensory specializations acquired in living
      hominoids.

      2) Nacholapithecus (Mid-Miocene, Kenya), although in part sharing with
      Proconsul the common primitive anatomical body design, was more
      specialized for orthograde climbing, "hoisting" & bridging:
      - glenoid fossae of scapula = probably cranially orientated,
      - forelimbs = proportionally large,
      - toes = very long,
      - tail loss suggests rel.slow movement (may already have occurred in
      Proconsul).

      Nacholapithecus-like positional behaviour might thus have been a basis for
      development of more suspensory specialized positional behaviour in later
      hominoids.
      After 13 Ma, there is a gap in the hominoid postcranial record in Africa
      until 6 Ma,
      so a scenario for later locomotor evolution prior to the divergence of
      Homo & Pan cannot be determined with certainty.
      The time gap also causes difficulties when we seek to determine polarities
      of morphological traits in very early hominids.
      Interpretation of the form­function postcranial relationships in incipient
      hominids will be difficult:
      it is predicted that they had incorporated bipedalism only moderately into
      their total positional repertoires.

      Japanese macaques, which are trained in traditional bipedal performance,
      may provide useful hints about bipedal adaptation in the proto-hominids.
      Kinematic analyses revealed that these macaques walked bipedally with a
      longer stride & lower stride frequency than used by ordinary macaques,
      owing to a more extended posture of the hindlimb joints.
      The body centre of gravity rises during the single-support phase of stance.
      Energetic studies of locomotion in these bipedal macaques revealed that
      energetic expenditure was 20­30 % higher in bipedalism than in
      quadrupedalism, regardless of walking velocity.

      _____

      Orthogrady (= vertical lumbar spine) can in principle be any combination of
      - (with feet) bipedal standing, walking & running on land or between
      mangroves etc., wading on 2 legs,
      - (with hands) climbing vertically, hanging (suspensory) or swinging
      (brachiating), arms above the head,
      - (with body) vertical treading or floating (airsacs) at the surface.

      IMO the transition from more OWM-like above-branch pronogrady to more
      ape-like below-branch orthogrady can most parsimoniously be explained by
      spending more time in swamp/gallery/mangrove/flooded forests: spending
      more & more time in the water (like present-day lowland gorillas still do
      parttime), grasping branches above the water, climbing vertically & later
      hanging from branches (& still later in gibbons arm-swinging). This can
      also explain the apes' larger & broader body, dorsal scapulas, cranially
      oriented glenoid fossae & tail loss.
      Nacholapith, Morotopith etc seem to be a step further on the way to
      becoming vertical aquarboreals than Proconsul.

      --marc
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