bipedalism: Miocene hominoids & modern analogues
- Acquisition of bipedalism:
the Miocene hominoid record and modern analogues for bipedal protohominids
Masato Nakatsukasa 2004 J Anat 204:385402
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
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
Nacholapithecus-like positional behaviour might thus have been a basis for
development of more suspensory specialized positional behaviour in later
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.