Teach-in
'99 : Evolution of Bipedal Gait
by Chris
Kirtley, The Hong Kong
Polytechnic University
One perennial question about bipedal gait
is "Why did it evolve?". As this wonderful Rubes cartoon nicely illustrates,
there must have been a stage when some humans walked upright and some still
used their knuckles!
Or was the transition more sudden? The fossil
footprints (see Teach-in #14 on Toe
Clearance ) found at Langebaan
Lagoon in Africa would seem to suggest that we have been walking the
way we do now for a long time. So what could have been the driving force
behind this change in locomotion style? It must have been a strong stimulus,
in view of not only the speed of transition, but also the additional control
and balance difficulties imposed by bipedalism. There must have been a
big "pay-off' in evolutionary terms.
There have been several suggestions as to what
this selective pressure was:
-
ability to use the upper-limbs
-
for using tools
-
for carrying babies
-
improved vision
-
as a feeding
adaptation
-
cooling of the brain, which was overheating as
it enlarged
At any rate, anthropologists seem to be agreed
that the transition took place at around the time of great climatic changes
in the eastern Rift valley area of Africa, about 7 million years ago (mya).
Dense rain-forest was being replaced by savannah, so our ancestors were
faced by the problem of how to move from tree to tree across open grassland,
pursued, no doubt, by lions and tigers. Darwin thought that bipedalism,
technology, and increased brain size came together as a 'package' However,
stone tools do not appear in the archaeological record until about 2.5
mya, well after the proposed origin of hominids at 7 mya. This seems to
refute the notion that we walk upright to use our hands.
There must therefore have been an evolutionary
pressure to develop a fast, efficient and stable locomotion pattern with
which to do this... but why bipedal gait? The question is not trivial,
since it presumably guided the development of the locomotor system and
very probably influences the compensation to gait disorders seen in patients.
For example, is the underlying goal of gait...
Inman & Saunders thought that energy efficiency
was the guiding principle, when they formulated their famous Determinants
of Gait. They thought that minimising the vertical and side-to-side
oscillations of the total body centre of mass was the guiding principle
of gait. However, recently a lot of these "determinants" have been challenged.
So what are we left with - what is the heuristic goal of the nervous system
when it is controlling walking?
References
Saunders, Inman and Eberhart, J. Bone & Jt.
Surg., 35-A(3): 543-558, 1953.
S.A. Gard, D.S. Childress (1997) Gait
& Posture, 5 (3): 233-238, Investigation of vertical motion of
the human body during normal walking.
S.A. Gard, D.S. Childress (1997) Gait
& Posture, 5 (2): 161, The effect of pelvic list on the vertical
displacement of the trunk during normal walking
Questions
-
What do you think was the reason
for the evolution of bipedal gait, and why?
-
What is the underlying goal which
guides the nervous system in controlling gait?
-
how could we test your hypothesis?
-
How could the nature of this goal
affect the compensation to walking disorders?
-
Can you use this knowledge to devise
better treament strategies?
Email your answers to [n/a] 
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