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Case Study 9. Reading the Bones (1): Recognizing Bipedalism

Case Study 9. Reading the Bones (1): Recognizing Bipedalism

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Case Study 9. Reading the Bones (1): Recognizing Bipedalism



Fig. 1 AL 288-1

Australopithecus afarensis

skeleton “Lucy” (cast).

Source: GNU Free

Documentation License,

with permission



far more unusual to find more than isolated bones in the open, where they had been

covered in a streambed. It was a piece of humerus that first caught Johanson’s attention; then other bones began turning up. After many square meters of soil had been

sifted and the bone fragments painstakingly put together, 46 % of Lucy’s skeleton

was represented on one side or the other. The cranium exists only as a few fragments, but we are able to study much of the rest of the body in detail. Many of the

missing parts were filled in by another discovery at Hadar the following year of

fragments of 13 individuals, dubbed the “First Family.” They reveal a body more

like that of modern humans than like apes below the waist, but with more ape-like

features in the upper half. These and other fossils from Ethiopia and elsewhere in

East Africa were placed in a new species, Australopithecus afarensis.

One of the most striking aspects of Lucy is her small stature, just over a meter

(about 3.5 ft) in height. Even today she appears to be the smallest adult member of

her species yet discovered. Her upper limbs suggest ape-like proportions, with long



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How Do We Recognize a Bipedal Skeleton?



and strong limb bones, while the legs are intermediate in length. Her ribcage is also

more ape-like, conical so that it is much broader at the bottom.



How Do We Recognize a Bipedal Skeleton?

The human body is reorganized from head to toe to balance and walk on two limbs.

Although some of these specializations lie in soft tissues, such as the wiring of our

brains and coordination of muscles, there are many indicators to be found within the

skeleton (Fig. 2).



mastoid process

forward placement of the foramen magnum



lumbar lordosis



sacroiliac joint closer to acetabulum



sacrum wide,

narrows inferiorly



Ilium shorter, broader, and deeper

iliac pillar

ischium shortened



hip and knee capable of full extension

increased carrying angle of femur

femoral condyles elliptical

weight-bearing bones and joints expanded in volume



midtarsal conversion mechanism

metatarsophalangeal joints hyperextend

first toe robust, lengthened &

permanently parallel with other toes



calcaneal tuberosity

weight-bearing

longitudinal arch

lateral toes short



Fig. 2 Skeletal indicators of bipedalism on a human skeleton. Source: Modified from Brehms

Tierleben, Small Edition 1927



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Case Study 9. Reading the Bones (1): Recognizing Bipedalism



The human spine is weight-bearing and there is a gradual increase in size of the

vertebrae from superior to inferior. The spinal column achieved its upright position

in part by bending the lower spine, creating a curvature called the lumbar lordosis.

This develops as a child learns to walk and is made permanent as the lumbar

vertebrae became wedge-shaped. The sacrum is wider at the top to support

weight and wedges between the parts of the pelvis. Lucy’s bones show this wedging. The skull needs to balance on top of the spine. The occipital condyles, where

the skull contacts the first vertebra, are brought to a more central position.

Muscles of the back of the neck that balance the weight of the face now attach on

the underside of the skull instead of its posterior surface. These changes are seen

consistently in australopithecines, although their heavy faces and jaws would have

prevented the head from balancing as easily as does ours.

Perhaps the most conspicuous changes occurred in the pelvis. The mammalian

pelvis is elongated and aligned with the spine. The femur intersects it at right angles

when the animal is standing quadrupedally. Muscles arising from the anterior portion of the pelvis, the ilium, can pull the limb forward at the hip, in the action of

flexion; those arising from the ischium behind the hip draw it back, or extend it, in

the action we associate with pushing off. By elongating of the pelvis, an animal has

increased the leverage and power of those muscles. When the body is reoriented to

an upright position, these relationships change. The thigh now lies parallel to the

axis of the spine. Hip flexors must gain power by being placed further away from

that axis anteriorly, and the extensors must have an origin dorsal to it. The ventral

blade of the ilium bears the attachment of iliacus, an important muscle for hip flexion. The dorsal surface anchors the gluteal muscles, which play the lesser role of

abduction of the hip in quadrupeds, but become extremely important in humans to

maintain balance. The iliac blade becomes broader to support a greater size of the

muscles and reorients by curving anteriorly, so that the gluteal surface is now facing

laterally. This brings the upper part of the pelvis into its familiar funnel shape. There

is no longer any advantage to be gained by a long pelvis parallel to the spine, and

there is actually a cost of balancing it. Therefore, the human pelvis has been greatly

shortened from top to bottom and made deeper from front to back and, most critically, by bringing the sacroiliac joint closer to the acetabulum.

Lucy’s pelvis looks much like that of a modern human at first glance. The ilium

is short and broad. Although the iliac blades have some curvature toward the front,

they do not have the full depth from front to back as the modern pelvis. Nonetheless,

they are clearly going to be more effective balancing an upright torso than powering

a quadrupedal one. In fact, they flare much farther laterally over the hip than we see

in humans.

At the knee, our limbs must be brought together to best support our center of

gravity. Since the hips are widely spaced to ensure an adequate birth canal, the

shafts of the femurs must slant toward the midline, forming a distinct angle with

the vertical. A relatively small section of the shaft attached to the knee joint is

sufficient to identify a bipedal individual. Lucy’s species meets that criterion.

Similarly, the size of the femoral head and condyles are enlarged to distribute

greater forces at the joints at both ends of the bone. Those joints are capable of



How Did Lucy walk?



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full extension in us, unlike in other animals, so that we can stand without using

muscular effort to support them.

The human foot has additional adaptations for walking. Because it consistently

bears more weight and greater stresses, the joints are more rigidly supported and

their mobility is reduced relative to what we find in a climbing ape. Instead of being

thumb-like, the first toe is sturdier and longer and fixed alongside the other toes. The

toes themselves are much reduced in length, having given up most of their grasping

function. They are capable of hyperflexing (bending upward, as when we stand on

tiptoe). The calcaneus, the heel bone, has an extra point of contact on the ground to

distribute weight and improve balance. Bones of the foot in general—indeed most

of the weight-bearing bones of the body—have increased in size disproportionately

compared with those of other mammals to better absorb and distribute the shocks of

walking and running. Lucy’s foot is very incomplete, but other fossils indicate some

of these adaptations were present. The first toe is larger and was held alongside the

others (as also indicated by a remarkable set of 3.6 million-year-old footprints from

Laetoli in Tanzania). The other toes are reduced in length, though still longer than

human toes. Joints show some restrictions in range of motion and the bones are

enlarged. Yet, the foot is not wholly human.



How Did Lucy walk?

How anthropologists answered this question in the past depended on whether they

focused on the similarities or the differences with our own anatomy. Some of the

earliest researchers still needed to argue that Australopithecus was our ancestor and

thus emphasized indicators of human-like walking. As Johanson put it, “Here was an

ape-brained little creature with a pelvis and leg bones almost identical in function

with those of modern humans” (Johanson and Edey 1981). This culminated in

C. Owen Lovejoy’s analysis of Lucy and other remains that concluded she not only

had a very human-like gait, but also differed in ways that further enhanced balance.

Although skeletal differences were evident, australopithecines had clearly departed

from an ape-like anatomy in a number of ways. This could not have been possible if

their overall behavior and ecological niche had not also been changing. The anatomy shows the outcome of selection for bipedal standing and walking. Selection, of

course, would have been more intense where the mechanical demands of efficient

balance and locomotion were greater. The upper body could comfortably have

lagged behind.

Critics of this approach, most notably Jack Stern and Randall Susman, asked

how Lucy could have walked like modern humans if she was so different from them.

The ape’s upper limbs are adapted for climbing. They are long, strongly muscled

and have elongated grasping hands. Stern and Susman argued that Lucy’s upper

body indicates climbing was still important for her. Differences from humans in the

lower limb are also ape-like in direction. The bones of the toes, although intermediate in length, are curved, like those of more arboreal primates. They concluded that



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Case Study 9. Reading the Bones (1): Recognizing Bipedalism



australopithecines still spent vital periods of time in the trees, perhaps for feeding or

sleeping, and thus possessed a unique repertoire of both climbing and walking.

The two sides of this debate assumed different perspectives, from different ends

of the evolutionary pathway that our ancestor was taking. It is inevitable that just as

anatomy may pass through an intermediate stage not like either ancestor or descendent, so behavior and function may, too. One must also be cautious not to assume

that an evolutionary path runs the straightest route.

The two interpretations made different assumptions about the chronology of

evolving bipedalism. Lovejoy’s model assumes that Lucy represents a snapshot of

her species, a moment in time in a lineage that was constantly changing. As his colleague Bruce Latimer argued, if adaptations for climbing had been relinquished,

climbing must have been less important for her than for her ancestors. And if her

immediate ancestors were better climbers, her immediate descendants would surely

have become better walkers. Lucy herself was not necessarily a fully adapted expert;

she was merely working toward something better.

In contrast, Stern and Susman began with the assumption that any species would

have to be well adapted to have survived at all. Lucy is better understood not as an

undeveloped human, but as a unique and very interesting hominin to be understood

on her own terms. The perspective of later discoveries supports Stern and Susman.

We can now date australopithecines back to 4 Ma ago and as recently as one and a

half million years ago. Although we cannot analyze their bodies through this time

period in as much detail as we can Lucy’s, we have to acknowledge that the australopithecine pattern of locomotion existed significantly longer than ours has been.

It represents a successful strategy in its environment, not merely a way station on

the road to becoming human.

Compared to living primates, Lucy is indeed unique anatomically. As anthropologists accepted this, several attempted to determine just how those differences

would have affected the way she walked. Her hips were unusually wide, her toes

long, and her hands swung heavily beside her thighs. It is tempting, but frustrating,

to compare this to modern human gait and judge it less efficient. The difficulty lies

not in our ability to discern bipedalism or describe the full range of behaviors

observable in humans and apes, but in our inability to comprehend a truly different

suite of movement appropriate for a different skeletal structure and ecological

context.

To complicate matters, recent discoveries in East and South Africa tell us there

is much more diversity of body design and locomotor patterns among the hominins

than previously assumed. The Ardipithecus ramidus skeleton published in 2009

putatively combined terrestrial bipedalism with arboreal above-branch quadrupedalism. Skeletons of Australopithecus sediba (2011) and Homo naledi (2015) present different unique combinations of primitive and derived traits that are not simply

awkward versions of modern humans. Isolated limb and foot bones from sites in

East Africa and the mostly undescribed “Little Foot” skeleton from Sterkfontein are

further stretching our understanding.



Additional Reading



73



Questions for Discussion

Q1: Is bipedalism the most important of the characters that differentiate humans

from other animals? Is “most important” the best way to select a trait to define

our lineage?

Q2: Lovejoy interpreted Lucy’s skeleton differently than did Stern and Susman.

Could they both be correct? If not, how does one mediate such a disagreement

to determine which is correct? Is either of the two models falsifiable?

Q3: Should we expect our ancestors at some point in time to show intermediate

anatomy corresponding to a semi-erect posture and inefficient gait?

Q4: Is every other form of bipedalism less efficient that ours?

Q5: What might we learn from the discovery that there were many other versions of

bipedal body designs?

Q6: Why has no other species, including baboons which evolved in a savanna habitat,

become bipedal?



Additional Reading

Berger LR et al (2015) Homo naledi, a new species of the genus Homo from the Dinaledi Chamber,

South Africa. eLife 4, e09560

Clarke RJ, Tobias PB (1995) Sterkfontein Member 2 foot bones of the oldest South African hominid. Science 269:521–524

Johanson D, Edey M (1981) Lucy: the beginnings of humankind. Simon & Schuster, New York

Langdon JH (2005) The human strategy: human anatomy in evolutionary perspective. Oxford

University Press, New York

Latimer B (1991) Locomotor adaptations in Australopithecus afarensis: the issue of arboreality.

In: Coppens Y, Senut B (eds) Origines de la Bipédie chez les Hominidés. CNRS, Paris,

pp 169–176

Lovejoy CO (1988) Evolution of human walking. Sci Am 259(5):118–125

Lovejoy CO (2009) The great divide: Ardipithecus ramidus reveals the postcrania of our last common ancestors with African Apes. Science 326(73):100–106

Stern JT, Susman RL (1991) “Total morphological pattern” versus the “magic trait”: conflicting

approaches to the study of early hominid bipedalism. In: Coppens Y, Senut B (eds) Origines de

la Bipédie chez les Hominidés. CNRS, Paris, pp 99–111

Susman RL et al (1984) Arboreality and bipedality in the Hadar hominids. Folia Primatol

43:113–156

Zipfel B et al (2011) The foot and ankle of Australopithecus sediba. Science 333:1417–1420



Case Study 10. Reading the Bones (2):

Sizing Up the Ancestors



Abstract To identify a fossil mammal or to describe a new species, paleontologists

like to have good specimens of the skull, especially the jaws and teeth. However,

when they want to know what an animal looks like, they need to have more of the

skeleton. Unfortunately, it is rare to have both skull and limb bones of the same

individual, and it may be some time before scientists can reconstruct the body with

some confidence. Paleontologists can apply the tools used commonly by forensic

anthropologists to reconstruct stature and body proportions from individual bones

to give a better picture of the size and proportions of the australopithecines and

early Homo. The bodies of early hominins did not evolve as quickly as had been

believed.



Estimating Body Size for Australopithecus

The estimation of stature from a skeleton or individual bones is a standard tool of

forensic anthropology, where the determination of age, stature, sex, and ancestry

may assist in the identification of an individual. The simplest method is to take the

length of a single bone, such as the femur, and determine the correlation between

that measure and body height for a population. It is not difficult to derive predictive

equations for males and females. Within a measurable range of error, this method

should allow us to predict stature for any member of that population. For example,

Trotter and Gleser produced the following formula for calculating stature (in centimeters) in white American females from the maximum length of the femur:

STATURE = 2.47 FEM + 56.60 ± 3.72

This equation is entirely empirical—that is, the constants are calculated from a specific sample population and will be slightly different for any other sample. Moreover,

they only apply reliably to individuals from the sampled population and in the size

range of the original sample. In the above equation, the mean stature of the sample

population was 160.682 cm with a standard deviation of 7.508 cm. If another



© Springer International Publishing Switzerland 2016

J.H. Langdon, The Science of Human Evolution,

DOI 10.1007/978-3-319-41585-7_10



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Case Study 10. Reading the Bones (2): Sizing Up the Ancestors



population is substantially different in body size or composition because of age,

ancestry, or nutrition, the appropriate equation will also be significantly different.

Therefore all such extrapolations require caution.

The first comprehensive study of australopithecine postcranial remains was

undertaken by John Robinson. He had available to him the Sts 14 skeleton and a

number of isolated bones from the South African caves. The skeleton, representing

Australopithecus africanus, included a distorted pelvis, a crushed femur, and a number of vertebrae and ribs. The pelvis and the light build of the bones in general suggested the individual was a female. Using the Trotter and Gleser equation for the

length of the femur, Robinson calculated a height of 130 cm. The vertebrae were

also consistent with a height between 122 and 137 cm. From the lightly built bones,

he estimated a body weight of 18–27 kg.

These calculations must be put into perspective. They might be reliable if Sts

14 were a modern Euro-American female. However, Australopithecus africanus is

clearly not a modern human, and the femur length and therefore the estimated body

size were well below Trotter and Gleser’s population sample. This introduces a

significant, but unavoidable, degree of uncertainty into the estimate that is not

encompassed by the numbers.

Robinson also had a partial humerus that seemed disproportionately long and

robust for the femur. He assumed the humerus came from a male and that male

australopithecines were larger than females. This is the pattern observed widely

across Old World monkeys and apes and, to a lesser extent, in modern humans.

Sexual dimorphism would account for only part of the discrepancy, however, and

Robinson suggested the australopithecine upper limb was proportionately longer,

another example of the fossil representing an intermediate form between apes and

humans. Although the pelvis was unusually broad and the femur was slender, he

nonetheless concluded that lower limb length was of human proportion.

The second species sampled was Paranthropus robustus. Parts of two femora and

a distorted coxal bone were more strongly built than those of A. africanus. Although

neither they nor other bones enabled him to calculate body height, Robinson concluded the robust species was slightly taller and substantially heavier than the gracile species. His estimate was stated as a broad range: 137–152 cm and 70–90 kg.

These projections were consistent with expectations based on skulls and teeth and

helped anthropologists paint a more complete picture of this phase of our ancestry

while waiting for more complete material. Robinson’s image of robust australopithecines was gorilla-like, and gorillas are the largest of living primates. This perception was undoubtedly influenced by the better-developed chewing apparatus, which

he assumed was for a vegetarian diet, and especially by the sagittal crest atop the

cranium. He therefore saw in the partial and distorted pelvis evidence of a more

quadrupedal ape-like locomotion.

Additional material to improve our interpretation of these species became available

over the next 20 years. By 1991, there were five partial femora of A. africanus. Henry

McHenry applied a variety of forensic correlations to the bones and produced results

similar to those of Robinson, with stature estimates of 110–142 cm. Again, one must

be aware of the limitations of applying human standards to a smaller nonhuman species.



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