The it is habitual (Hutchinson and Gatesy, 2001), rather

The origin of obligate bipedalism is one
of the most important questions of palaeoanthropology. What barriers are
preventing scholars from coming to a definitive conclusion on this issue?

Bipedal walking involves an inverted pendulum gait which balances
gravitational potential with kinetic energy (Schmitt, 2003). Bipedalism has a
spectrum of advantages including having forelimbs available for other tasks,
raising overall height to facilitate vision and minimising the upward surface
area facing the sun. Many of these maximise survival advantage (Geer, 2011).
However, disadvantages also exist, with greater pressure on the musculoskeletal
system and a more complicated circulatory system because of gravity, as well as
reductions in agility (Lovejoy, 1988). Obligate bipedalism is where an organism
is specialised for bipedal locomotion so that it is habitual (Hutchinson and
Gatesy, 2001), rather than a choice or occasional behaviour. Birds and humans
are considered obligate bipeds and the ancestry of birds is much more
straightforward than humans, with a clear phylogeny from archosaurs through
dinosaurs, theropods to modern birds, although the remainder of this account
will discuss bipedal hominins and the difficulty in concluding direct lineage.

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Several main theories
exist for the origins of bipedalism and cover a variety of options including
straightened limbs to facilitate locomotion from stiff legged quadruped to
stiff legged biped (Schmitt, 2003), and an evolutionary transition through
arboreal swinging or knuckle walking ancestors. Whilst one view is that larger
brains discovered efficiency inducing tools and required limbs to manipulate
these, the fossilised footsteps in Tanzania dating from Pliocene times (3.6-3.8
million years ago) (White, 1980) demonstrate bipedalism which must have
outstripped brain enlargement (Leakey and Hay, 1979) as it is only the genus Homo that are large brained. Whilst the
footsteps can be matched to bipeds an identification of the track maker is not possible
(Hutchinson and Gatesy, 2001) so could be early humans or a common ancestor.

The knuckle walking hypothesis
suggests that bipeds evolved from an ancestor that knuckle walked on the ground
but used forelimb based suspensory or climbing behaviours above ground
(Makedonska and Strait, 2001).

 

The main support for this theory is
that humans and chimpanzees are sister taxa suggesting a common ancestor
(Schmitt, 2003) although other evidence is less supportive and not all primates
share anatomical adaptations that allow knuckle walking (Kivell and Schmitt,
2009). The arboreal climber or suspensory hypothesis indicates that bipeds
evolved from ancestors that rarely travelled at ground level so had no need of
terrestrial locomotor adaptations (Makedonska and Strait, 2001). An effective
gait for such organisms would be a lower level shuffling one, which is
supported by anatomical data (Schmitt, 2003) and bone arrangement (Lovejoy,
1988). Experimental evidence appears to support this theory as the mechanics are
more similar than for other theories (Schmitt, 2003).

The idea that bipedalism evolved to
help with tool usage is not supported by early fossils of australopithecines,
which show hallmarks of bipedalism but no stone tools or larger brains
(Lovejoy, 1988). Instead, Lovejoy suggests that bipedalism arose to confer
reproductive advantage whereby the male could carry more food to enable his
monogamous mate to look after the children. As well as using forelimbs for
tools they could also provide a performance advantage whilst striking and
fighting which could add to the idea of sexual selection (Carrier, 2011). This
concept would be supported by the pelvic changes observed in early hominid
fossils (Grabowski et al., 2011). Other writers also indicate that tool usage
came after bipedalism as Homo only evolved some 2 million years after the
Tanzanian footprints (Hutchinson and Gatesy, 2001).

Hence the evolution of current
bipedalism is largely a specialised adaptation to surroundings and requirements
(Schmitt, 2003) but bipedalism overall is much earlier. For instance, endurance
running is an evolutionary adaptation to surroundings that aids survival and
maximises competitive hunting and gathering potential from a wider area
(Bramble and Lieberman, 2004). The very erect form of bipedalism is that modern
Homo appear to have developed in the
more open semi-arid environment that the earliest hominids wouldn’t have needed
to navigate. This is supported by the change in craniodental morphology and the
suggestion that earlier hominins had smaller brains and made less use of
technology, which restricted their survival in a changing environment
(Dunsworth, 2010). 

 

The main issue is that it is not
possible to know for sure based on fossil evidence from millions of years ago,
or indeed judge whether behavioural changes predated fossil evidence. Fossil
evidence can be patchy and subject to debate about what it relates to. There
are limited fossils of hominids and those that exist are often fragmentary (Schmitt,
2003) so their precise organisation is difficult to ascertain without
assumption. Many records involve a small component of a skeleton, such as a
mandible or temporal bone fragment or partial cranium (Dunsworth, 2010).
Without complete skeletons the link between cranial capacity and bipedal
behaviour cannot be made. If the need to develop bipedally was due to a change
in climate and terrestrial surroundings (fewer trees) rather than for more
sociological reasons such as suggested by Lovejoy (1988), this kind of
information cannot be gleaned from fossil records which show structure not
function.

Different bones may develop in a
varying pattern from modern skeletons so discrepancies from estimates from long
bones compared to flat bones could be genuine or a facet of the estimation
method (Dunsworth, 2010). Another issue is that even if the organism did move
bipedally this may not be the fully upright method of modern humans, but the
more shuffling method of chimpanzees (Hutchinson and Gatesy, 2001). Hence
correlation between observations may exist but this need not indicate
causation, especially as tool usage need not require actual bipedalism, given
that primate can manipulate these with their hindlimbs.

In conclusion, whilst bipedalism has
existed for millennia the reason for its origin cannot be definitively known
due to researchers not being there at the time and having to rely on fossils
which may provide some anatomical record but miss out the reason for that
anatomical change. Likewise, the absence of a complete fossil record means that
conclusions can only ever be based on assumptions and theories and without any
last common ancestor it is not possible to know whether bipedal hominids came
from existing quadruped ancestors or some other unknown origin.