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Bipedal behavior and the emergence of erect posture in man.

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Bipedal Behavior and the Emergence of
Erect Posture in Man
B. A. SIGMON
Department of Anthropology, University of Toronto, Toronto, Canada
ABSTRACT
Bipedal behavior is present to some degree in most of the higher
primates. Field studies of living pongids have shown that an upright stance and
various types and degrees of bipedal progression are not uncommon and are thus
presumably advantageous. Bipedal behavior similar to that in living pongids
could also have been, and probably was, present in pre-hominids.
As the environment of the pre-hominids changed, the increased use of bipedalism was of survival value. That is, it was a behavioral response to environmental
circumstances and not just a random event. Once bipedal behavior became
habitual, it formed part of a pattern of natural selection which shaped a physical
type adapted for erect, bipedal posture.
BIPEDALISM IN NON-HUMAN
PRIMATES
Almost all of the higher primates can
assume an upright bipedal stance, although in some this locomotor behavior
is more common than in others. This is
well seen in pongids i n the wild. About
one tenth of the locomotion of the gibbon
is bipedal walking along the upper surfaces of branches of trees (Carpenter,
’64). Gorillas occasionally take a few
bipedal steps-r
stand upright on their
lower limbs as a display or threat (Schaller, ’63). Bipedal progression in orangutans in the wild has not been observed,
although it has been noted in captive
animals (Napier, ’64).
Most of the observations of bipedality
in pongids in the wild (and also in experimental conditions) are of chimpanzees
because they have been more frequently
studied than almost any other non-human
primate except perhaps the macaques.
Because the field data are more abundant
for chimpanzees, and because this animal is one of the closest relatives of man,
the following discussion of bipedality in
pongids will be centered around this particular African pongid.
Chimpanzees make use of bipedal locomotion more frequently in some environments than in others (Nissen, ’31; KortIandt, ’62; Goodall, ’65; Reynolds and
Reynolds, ’65). Differences in their beAM. J. PHYS.ANTHXOP.,
34: 55-60.
havior can be illustrated by the following
field studies.
Goodall’s study of chimpanzees in the
wild involved a group that inhabit a deciduous woodland environment (Goodall,
’65). Goodall observed bipedalism in these
chimpanzees in the following circumstances: (1) looking over tall grass, (2)
searching for a comrade, (3) watching
a n unusual part of the surroundings (for
example, a human observer), (4) when
carrying objects in the hands, (5) in
threat displays, and (6) in greeting and
courtship (standing upright and swaying
from foot to foot). I n a later monograph
(van Lawick-Goodall, ’68) she described
a n additional circumstance during which
bipedalism was used by the chimpanzees-when
moving over ground which
was wet after a hard rain. Under nonfield conditions, Kohler (‘59) also observed
that chimpanzees will walk bipedally
when the ground is damp o r cold.
Two other ethologists, Reynolds and
Reynolds (‘65) studied chimpanzees that
live in a rain forest, which is a habitat
that is quite different from that of the
animals in Goodall’s study. The only bipedal behavior observed in these rain
forest chimpanzees was bipedal walking
which occurred “rarely” on the ground
and “infrequently” along the branches
of trees (with the forelimbs holding onto
other branches for support).
55
56
B. A. SIGMON
Differences in the amount of bipedal
progression observed in these two groups
of chimpanzees would appear to be due
to differences in the type of environment
inhabited. Two additional studies shed
further light on this observation.
Nissen (’31) studied chimpanzees occupying an area similar to that of the
rain forest chimpanzees observed by
Reynolds and Reynolds (‘65). The two
studies agree with regard to the amount
of bipedal locomotion observed. Nissen
states:
. . I never saw a chimpanzee
in the bush walking upright, although
on several occasions I saw them standing
on their two hind limbs.” (’31:37). Normally, the chimpanzees studied by Nissen
progressed quadrupedally at a leisurely
pace; in addition, they were agile climbers
and used various forms of running and
leaping.
A fourth field study is that of Kortlandt
(’62) who observed chimpanzees living
near a pawpaw-banana plantation in a
habitat similar to that of the chimpanzees
in Goodall’s study. Kortlandt noted that
the chimpanzees that he observed were
mostly terrestrial, preferring progression
on the ground to swinging and climbing
in trees. Arboreal movements are undertaken hesitantly and cautiously. In the
plantation area he noted that bipedal
walking and running were used for 10
to 15% of the distance covered by the
animals, and he lists the following occasions during which an upright pose or
gait was assumed: (1) to free the hands
for eating or carrying food, (2) to view
the surroundings at better advantage,
(3) in jumping across brooks, and (4) in
threats.
Thus the nature of the habitat occupied by the four groups of chimpanzees
reported in the above studies appears to
have been a major factor in determining
the amount of bipedal behavior that was
observed. A semi-open environment, as
in Goodall’s and Kortlandt’s studies, appears to be a stimulus for increased use
of bipedal locomotion, while a dense rain
forest environment, as in Nissen’s and
Reynolds and Reynolds’ study, does little
to encourage a two-legged gait. This is
an interesting point worthy of emphasis
and will be discussed later in relation
to the origin of erect bipedalism in man.
‘I.
BIPEDAL BEHAVIOR AS
A PREADAPTATION
Some authors have attempted to explain the origin of erect bipedality in
terms of one or another behavioral characteristic of non-human primates. For
example, Hewes (‘61, ’64) discussed the
habit of food-carrying in non-human primates as a critical preadaptation for erect
bipedal posture. His experimental work
on stumptail macaques revealed that
monkeys are capable of assuming an upright gait when carrying food in their
hands. Hewes concluded that food-carrying in early hominids led to the f‘reeing of the upper limbs from locomotor
activity and to the increased importance
and use of only the lower limbs for progression. Wescott (’67) has suggested the
importance of the role of threat display
and “agonistic exhibitionism” as preadaptations for erect bipedal posture.
Instead of selecting one or another
behavioral trait, I suggest that the total
range of bipedal behavior that has been
observed in higher primates be taken into
consideration. In extrapolating then from
non-human higher primates to pre-hominids, it is probably reasonable to assume
that the latter were making use of bipedalism in circumstances similar to those
seen in living pongids: in courtship,
greeting, threat display, jumping across
barriers, while eating or carrying food,
in viewing their surroundings at better
advantage in order to look for predators,
companions, or for other activities that
we have neither observed nor thought of.
It is almost certainly incorrect to single
out one reason for upright locomotion
over another as being the most important,
or the one that was a major preadaptation for erect bipedalism in man. Bipedalism in non-human higher primates is
used in a number of different circumstances, but the key factor is that it is
used whenever it improves the chances
of survival of the organism. The fact to
stress then is that bipedal locomotion is
used, disregarding for the moment what
the circumstances may be, and that it is
a behavioral response on the part of an
animal attempting to adapt to its environmental circumstances.
BIPEDAL BEHAVIOR AND ERECT POSTURE
THE ROLE OF THE ENVIRONMENT
AND OF BEHAVIOR
Le Gros Clark (’55), Washburn and
Howell (’60), Robinson (’63), and others
have discussed the importance of changes
in the environment of the pre-hominids
as an impetus toward the adoption of
erect bipedalism. It is generally agreed
that the emergence of bipedal striding
occurred in savannah areas sparsely
dotted with forests where the pre-hominids would have been moving over the
grassy plains between clumps of trees.
Differences in environmental conditions have a n important effect on locomotion in non-human primates. This has
been illustrated by differences in locomotor habits of chimpanzees living i n
open deciduous woodland (Kortlandt, ’62;
Goodall, ’65) as compared with chimpanzees living in rain forests (Nissen,
’31; Reynolds and Reynolds, ’65). The
former use bipedal behavior much more
frequently and in more varied circumstances than do the latter. Perhaps an
analogy can be made between chimpanzees inhabiting a deciduous woodland
region and pre-hominids also in a n environment with decreasing tree coverage.
The latter environment could have provided the stimulus for the development
of a new locomotor habit, or the increased
use and refinement of one already being
used, namely bipedal behavior.
The emergence of erect bipedalism can
be viewed in terms of three stages of development: a preadaptive, a behavioral,
and a physical stage although the latter
two are probably highly interrelated.
During the preadaptive stage the prehominids would probably have been
making use of bipedal locomotion in a
number of circumstances (as has been
illustrated
to occur in chimpanzee
groups). During this period of development, however, the use of upright locomotion would not have been critical to
the survival of the species.
Now let us assume that there is a
change in the environment that is such
as to make erect bipedalism of high adaptive value. This leads us into the second
stage which is directly associated with
the behavioral response of the organism.
Before going into detail on the second
57
stage, let us consider the role of behavior
as a sclcctive force in evolution (see also
Ewer, ’60, Hardy, ’65). This capacity to
respond to changes in the environment
is a characteristic of the behavior of a n
animal, and can be a very significant
factor for the survival of the animal.
Ewer (’60:163) says: “It would appear to
be glaringly obvious that what a n animal
does or tries to do, can determine what
characters are of survival value, i.e.,
can decide the direction of natural selection. . . .” She further states that
(‘60: 164):
“The adaptability of behaviour to varying
environmental conditions gives it a factor of
safety allowing a n immediate behavioural response to be made at once to a changed situation, without the necessity of waiting for
appropriate changes in the genetical structure of the population. Thus behavior will
tend to be always one jump ahead of structure, and so to play a decisive role in the
evolutionary process.”
Thus after the animal responds behaviorally to environmental changes, this
behavior can then become a part of the
total pattern of natural selection, making
it different from what it would have been
otherwise. That is, following the new behavioral response, the morphological/
physiological structure of the organism
can be modified by the modified selection
pattern resulting from the changed behavior. Those physical changes that assist the behavioral adaptation will be
selected for. It is in this sense that Ewer
(’60) and Hardy (‘65) speak of behavior
as a creative and a selective force in
evolution. Perhaps the concept can be
usefully applied to the process of the
evolution of erect bipedal posture in the
hominids.
Let us return now to the second stage
of development of erect bipedalism: the
behavior a1 response . Bipedal behavior
was probably present in the pre-hominids
as a preadaptation. With the change in
environment, the pre-hominids presumably responded behaviorally to increased
selection pressure favoring bipedalism.
The increased use of upright posture by
pre-hominids was probably not merely a
random event but improved their adaptation, hence their chances of survival.
The capacity of the pre-hominids to make
58
B. A. SIGMON
a behavioral response was probably a
major factor contributing to their survival. Once the bipedal behavior became
habitual, it influenced the natural selection pattern in a manner that favored
the development andlor refinement of a
physical type that further improved the
behavioral character, bipedalism.
The third stage involved the physical
adaptations that were necessary in
changing a pronograde animal to an
habitual biped. The most significant
changes can especially be seen in the
foot, pelvis, vertebral column, skull, and
the corresponding muscular systems of
these areas. The literature on this subject is vast, and I will attempt to summarize only certain of the more salient
points. It should perhaps be noted that
the ape grade of organization is frequently regarded as representing the earlier
pre-hominid
condition.
Consequently,
the postulated pre-hominid morphology
referred to in the following discussion is
based on our knowledge of the ape grade
of organization.
As the pre-hominids began using an
upright, bipedal gait more and more frequently, and as this means of locomotion
began providing them with survival advantages, any morphological changes that
would have increased the efficiency of
bipedal posture would of course have had
strong selective value.
The sequence of changes probably began in the lower half of the body-in
the
foot, the lower limb, and the pelvis. The
pre-hominid foot was adapted for tree
climbing, being flexible and having powerful grasping muscles, an abducted great
toe, and no arch. The lower limb was
short and the pelvis long and narrow. As
bipedalism began to be used more frequently, those pre-hominids deviating
from the above pattern would have had
a selective advantage. A more compact
foot with an adducted great toe and an
arch would absorb the shock produced in
walking or running in an upright posture,
while also providing a more efficient
structure for a bipedal animal. Correspondingly, an increase in the length of
the lower limb would have provided additional leverage needed by the muscles
used in bipedal striding.
Changes in the pelvis would also have
been necessary in the acquisition of erect
posture. The long, narrow pre-hominid
pelvis became shorter and wider. The
decrease in height is especially notable
in the ilium and ischium. Most of the
increase in width occurred in the ilium
which simultaneously changed in shape
from a blade-like structure to one with a
sigmoid curvature.
While the pelvic-thigh muscles of the
hominid and ape grades of organization
do not differ with regard to the type of
action that can be produced, the effect
of that action is significantly different.
Of particular importance are the gluteus
maximus and the hamstring muscles
which in both upright bipeds and apes
are important extensors of the thigh
(Sigmon and Robinson, '67; Robinson,
'68, Sigmon, '69). It was formerly thought
that the gluteus maximus in the apes was
primarily an abductor of the thigh and
that the change from abduction in pongids to extension in hominids enabled
the latter to become upright bipeds
(Washburn, 'SO). This hypothesis has
been shown to be incorrect; instead it is
the effect of the action that differs, not
the action itself (Robinson, '68, '70). In
the great apes the ischium is relatively
longer and the femur relatively shorter
than it is in upright bipeds. These two
skeletal elements are the major attachments for the hamstrings, and also for
gluteus maximus in apes. Thus, with a
long moment arm (ischium) and short
lever arm (femur or lower limb), these
muscles produce power of action, The
upright bipeds have the reverse proportions-a
relatively shorter ischium and
longer femur, so that the hamstrings
produce speed of action and a wider range
of movement. Gluteus maximus in upright
bipeds is more of a speed-of-action muscle compared to that of apes.
Power would naturally be of more value
to an animal that spends much time
climbing, whereas speed and range of
movement would be of greater advantage
to a biped who has become more or less
independent of trees both for food and
for protection, and more dependent on
his abilities to cover a lot of ground in
the shortest possible time. In the upright
BIPEDAL BEHAVIOR AND ERECT POSTURE
biped, therefore, power of action has been
sacrificed for speed of action.
With the increased use of bipedalism
the vertebral column became vertical and
in the process acquired a n S-shaped curvature necessary for flexible movement
and also important in balance. A s the
orientation and curvature of the vertebral
column changed, the occipital condyles
on the skull shifted to a more anterior
location, and as a result the skull occupied a more “balanced” position. I n the
quadrupedal pre-hominid, the robust
skull extended forward of the vertebral
column and therefore had to be supported
by strong neck muscles that originated
high on the occiput. With the skull in a
more balanced position, the neck muscles decreased in size and their bony attachment (nuchal crest) shifted to a
lower position on the occiput and decreased in size as well.
The adoption of upright posture also
saw a shift in the center of gravity from
the lumbar to the sacral region.
Other changes were occurring in the
face, jaws and teeth. The pre-hominid
face was probably robust, and the jaws
and teeth were large and strong due
partly to their use in grasping and defense. As the forelimbs became freed with
the use of upright locomotion, they took
over the function formerly performed by
the jaws and teeth, and as a result the
latter were reduced in size. Subsequent
changes in diet later continued this trend
(Robinson, ’63, ’68).
These and other changes could, of
course, be discussed a t greater length.
This abbreviated discussion serves only
to highlight some of the major anatomical
changes involved in the acquisition of
erect bipedal posture. What should be
stressed is that behavior itself played a n
important role in the selection that
shaped the physical structures that were
adaptive for upright locomotion.
CONCLUSION
Three points are emphasized in this
paper. First, the data from field studies
on chimpanzees show that the animals
behave differently in different environmental situations. Those groups of chimpanzees living in a deciduous woodland
59
environment make use of bipedal behavior more frequently and in more vaned
circumstances than those groups inhabiting the rain forest. This fact indicates
that the animals are capable of adjusting
their behavioral responses to environmental differences.
Second, stressing that a single specific
behavioral trait is the major preadaptation for erect bipedalism in man overlooks
the fact that bipedal behavior i n nonhuman higher primates is used in a number of different circumstances and, more
importantly, is used whenever it improves
the chances of survival of the animal.
Third, as the environment of the prehominids changed, bipedal behavior must
have become of greater importance than
it had formerly been. The nature of behavior is such that it will often allow a n
immediate behavioral response that can
enable animals to adjust at once to
changes in their environment. The genetical structure of a population does not
change so rapidly. This capacity to make
a n immediate behavioral response must
have been a major factor enabling the
pre-hominids initially to adapt to, and
survive in, a changing environment. The
subsequent physical changes would have
evolved along the lines that were adaptive for the behavior, and thus bipedalism
as a behavioral character was probably
a very important force i n the evolution
of erect bipedal posture in the hominids.
ACKNOWLEDGMENTS
It should be specially noted that many
of the ideas in this paper have been developed through numerous conversations
with Dr. J. T. Robinson of the University
of Wisconsin. His participation i n discussicns and in his critical reading of
the manuscript are gratefully acknowledged.
LITERATURE CITED
Carpenter, C. R. 1964 A field study in Siam
of the behavior and social relations of the gibbon (1940). I n : Naturalistic Behavior of Nonh u m a n Primates. C. R. Carpenter, ed. Penn.
State Univ. Press, University Park, Penn., pp.
145-271.
Ewer, R F. 1960 Natural selection a n d neoteny. Acta Biotheoretica, 13: 161-184.
Goodall, J. 1965 Chimpanzees of the Gombe
Stream Reserve. In: Primate Behavior. I. Ue-
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B. A. SIGMON
Vore, ed. Holt, Rinehart and Winston, New
York, pp. 425-473.
Hardy, A. 1965 The Living Stream. Collins,
St. James Place, London.
Hewes, G. W. 1961 Food transport and the
origin of human bipedalism. Am. Anthrop.,
6 3 : 687-710.
1964 Hominid bipedalisni: Independent
evidence for the food-carrying theory. Science,
146: 416-418.
Kohler, W. 1959 The Mentality of Apes. Vintage Books, New York.
Kortlandt, A. 1962 Chimpanzees in the wild.
Sci. Am., 206: 128-138.
Le Gros Clark, W. E. 1955 Fossil Evidence for
Human Evolution. Univ. of Chicago Press,
Chicago.
Napier, J. R. 1964 The evolution of bipedal
walking i n the hominids. Arch. Biol. 7 5 : 673708.
Nissen, H. W. 1931 A field study of chimpanzee. Comp. Psychol. Monog., 8: 1-122.
Reynolds, V., and F. Reynolds 1965 Chimpanzees of the Budongo Forest. In: Primate Behavior. I. DeVore, ed. Holt, Rinehart and
Winston, New York, pp. 368-424.
Robinson, J. T. 1963 Adaptive radiation i n the
Australopithecines and the origin of man. In:
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Howell and F. Bouliere, eds. Viking Fund Publications in Anthropology, No. 36, Aldine, Chicago, pp. 385-416.
1968 The origin and adaptive radiation
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Hominisation, 2nd. ed. G. Kurth, ed. Gustav
Fischer Verlag, Stuttgart, pp. 150-175.
___ 1970 Australopithecine
Posture and
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Schaller, G. B. 1963 The Mountain Gorilla.
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Sigmon, B. A. 1969 Anatomical Structure and
Locomotor Habit in Anthropoidea with Special
Reference to the Evolution of Erect Bipedality
i n Man. (Unpublished doctoral thesis, Univ.
of Wisconsin).
Sigmon, B. A,, and J. T. Robinson 1967 On
the function of m. Gluteus maximus in apes
and i n man. (Abstract) Am. J. Phys. Anthrop.,
2 7 : 245-246.
Van Lawick-Goodall, J. 1968 The behavior
of free-living chimpanzees in the Gombe Stream
Reserve. Animal Behavior Monographs, vol. 1,
part 3 , pp. 161-311.
Washburn, S . L. 1950 The analysis of primate
evolution with particular reference to the origin
of man. Cold Spring Harbor Symposium. Quant.
Biol., 15: 67-78.
Washburn, S. L., and F. C. Howell 1960 Human evolution and culture. In: Evolution after
Darwin. Vol. 2, the Evolution of Man. S. Tax,
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Wescott, R. W. 1967 Hominid uprightness and
primate display. Am. Anthrop., 69: 738.
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