In 1978, Mary Leakey excavated the well-preserved,
75 foot long trail of hominid footprints. The footprints indicate at least
two hominids walked across the moist volcanic ash at the site of Laetoli.
The date for the footprints is 3.75 million years ago.
Features such as long curved fingers and toes, long upper limbs (arms),
short lower limbs (legs), structure of the knee joint, and pelvic orientation
lead to two conclusions: (1) A. afarensis was capable of efficient
climbing and may have spent time in the trees, and (2) A. afarensis
was bipedal on the ground.
A. afarensis may have walked with the knee and hip slightly bent,
suggesting that their pattern of walking would have been less efficient
than ours today.
To best set the stage for understanding
why some primates chose to move around bipedally, you may wish to read the
following:
The real issue is why? Why did a group of apes live in such a way that
bipedal movement was selected for through a process of natural selection?
The following readings may help you understand the possible answer to this
question. The answer is far from simple. Remember that
bipedality is a lousy form of locomotion in a sense. It's slower and more
awkward, and it puts hominids at greater risk of injury - especially as
early hominids experimented with it and were not very good at it! Something
must have favored the erect posture and two-legged movement. For read: BIPEDALITY
The following article from the New York
Times is another good place to explore this question: The New York Times on Bipedality
This may help put things into
perspective.....
WHAT DO THE FEETPRINTS TELL US?
A. afarensis was a short hominid. Lucy is estimated to stand
only about 4 feet tall. It is known that there are larger A. afarensis
individuals, probably males. Sexual dimorphism may explain the variation
in size most simply. A. afarensis males may have been 50% larger in size
than females. Differences in the size of footprints from Laetoli exhibit
this size range.
In this computerized image (the Laetoli foot print is on the left and a
human print on the right), you can see the distinctive big toe and heel
imprint. The human arch can also be seen. These footprints are very strong
evidence of habitual bipedality. Other evidence suggests that these hominids
did not walk exactly like we do however. Studies of the footprints from
Laetoli concluded that the big toe was in alignment with the other toes
and the mechanism of force and weight transfer through the foot during walking
was remarkably similar to that of modern man. However, the toes were longer
than ours. The darker areas reflect the greater weight bearing surfaces
of the foot.
The placement of the large toe in humans
is unique. In pongids, such as the gorilla, the big toe is positioned so
that it can be used in climbing and grasping objects. The big toe in pongids
is used in a manner similar to the thumb. In humans the big toe is parallel
with the other toes and is important for walking bipedally. In A. afarensis
the placement of the big toe is similar to us, making it possible for them
to be more efficient at bipedal walking than a chimpanzee.
A human heel is four times the size of a gorilla
heel and twice that of a chimpanzee.
This is because it contains a lot of spongy bone that absorbs the shock
of walking. Besides being smaller than a human heel, a chimp heel has a
thicker coating of bone around the outside. The A. afarensis heel
has only a thin outer layer like that a modern human heel, but is only partially
inflated with spongy bone.
Note
the differences between the chimpanzee pelvis and that of a hominid. The
iliac blade of the chimpanzee's pelvis is relatively longer than in A.
afarensis. The shortened blades found in A. afarensis indicate
that the structure of their hips was becoming similar to modern humans.
The shortened iliac blades suggest that Lucy had better control of her trunk
while in an erect posture than a chimpanzee.
This picture also illustrates the differences between the orientation and
shapes of the pelvis. Notice the position of the iliac blades. In A.
afarensis the blades are located on the side of the body, similar to
the human position. You can feel this bone if you place your fingers below
your waist, on the side, just above your hips. In chimpanzees the iliac
blades are located on the back of the body. The side orientation of the
pelvis improves the balance of the trunk during activities requiring a bipedal
posture.
An important feature of our bipedality
is that we are "knock-kneed". This means that the femur is at
an angle so that the two bones come close together at the knees. The angle
of the femur in A. afarensis is similar to the human angle. The human
like femur angle contributes to better bipedal walking than what is possible
by a chimpanzee.
There are significant aspects of the femur neck that differentiate chimpanzees
and hominids. The femur neck is that area that attaches this long leg bone
to the pelvis (hip). The femur neck of a chimpanzee is built for climbing.
This means that the bone cannot withstand stress when walking or running
on the ground. There is a dense ring of bone around the outside of the chimpanzee
femur that is suited to absorb force from compression but not tension. The
hominid femur has a substantially longer femur neck and it is specialized
for walking upright. The outer layer of dense bone is unevenly distributed
and thins at the top in hominids in contrast to chimpanzees who have the
dense ring around all of the bone. Longer femur necks are crucial to hominid
development. They enable muscle attachments to stabilize the hip while one
leg swings forward. As soon as one leg lifts off the ground, the muscles
fire and press the femurs' neck into the hip socket. This action relieves
the tension caused by weight bearing down on the top of the neck and focuses
stress along the bottom of the neck where bone density is maximized. The
spongy bone inside the neck absorbs shock that can reach the force equal
to three times the body weight on the leg as it supports the body when we
walk or run.
The angle of the femur in A. afarensis brought the legs under the
body. The comparison of the bottom portion of the femur shows how similar
the knee of A. afarensis was to that of ourselves. The larger flat
surface to this part of the femur enabled A. afarensis to transmit
weight through the extended leg and reduced pressure on the knee.
| Ask yourself why bipedality would have
been a successful form of locomotion? In addition, analyze the changes in
the physical structure that would have been necessary for this form of locomotion
to be efficient? |
The remains at Laetoli are truly
remarkable.
The hominid impressions were left in
soft volcanic ash shortly after it was deposited. A slight rain had moistened
the ash. After the hominids walked across this surface, the sun baked the
ash dry.
Volcanic eruptions and other forces
continued covering the footprints for over 3.75 million years.
Then, nearly four million years later,
erosion from wind and rain uncovered them so that they could be discovered
by the Leakey team in 1978.
|
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