# Introduction to Paleoanthropology/Bones



## Bone Identification and Terminology

### Skull

**Cranium**: The skull minus the lower jaw bone.

**Brow, Supraorbital Ridges**: Bony protrusions above eye sockets.

**Endocranial Volume**: The volume of a skull\'s brain cavity.

**Foramen Magnum**: The hole in the skull through which the spinal cord
passes.

-   In apes, it is towards the back of the skull, because of their
    quadrupedal posture
-   In humans it is at the bottom of the skull, because the head of
    bipeds is balanced on top of a vertical column.

**Sagittal Crest**: A bony ridge that runs along the upper, outer center
line of the skull to which chewing muscles attach.

**Subnasal Prognathism**: Occurs when front of the face below the nose
is pushed out.

**Temporalis Muscles**: The muscles that close the jaw.

### Teeth

**Canines**, **Molars**: Teeth size can help define species.

-   Gorillas eat lots of foliage; therefore they need very large molars,
    they also have pronounced canines.
-   Humans are omnivorous and have smaller more generalized molars, and
    reduced canines.

**Dental Arcade**: The rows of teeth in the upper and lower jaws.

-   Chimpanzees have a narrow, U-shaped dental arcade
-   Modern humans have a wider, parabolic dental arcade
-   The dental arcade of Australopithecus afarensis has an intermediate
    V shape

**Diastema**: Functional gaps between teeth.

-   In the chimpanzee\'s jaw, the gap between the canine and the
    neighboring incisor, which provides a space for the opposing canine
    when the animal\'s mouth is closed
-   Modern humans have small canines and no diastema.

## Using Bones to Define Humans

### Bipedalism

Fossil pelvic and leg bones, body proportions, and footprints all read
\"bipeds.\" The fossil bones are not identical to modern humans, but
were likely functionally equivalent and a marked departure from those of
quadrupedal chimpanzees.

Australopithecine fossils possess various components of the bipedal
complex which can be compared to those of chimpanzees and humans:

-   A diagnostic feature of bipedal locomotion is a shortened and
    broadened ilium (or large pelvic bone); the australopithecine ilium
    is shorter than that of apes, and it is slightly curved; this shape
    suggests that the gluteal muscles were in a position to rotate and
    support the body during bipedal walking
-   In modern humans, the head of the femur (or thigh bone) is robust,
    indicating increased stability at this joint for greater load
    bearing
-   In humans, the femur angles inward from the hip to the knee joint,
    so that the lower limbs stand close to the body\'s midline. The line
    of gravity and weight are carried on the outside of the knee joint;
    in contrast, the chimpanzee femur articulates at the hip, then
    continues in a straight line downward to the knee joint
-   The morphology of the australopithecine femur is distinct and
    suggests a slightly different function for the hip and knee joints.
    The femoral shaft is angled more than that of a chimpanzee and
    indicates that the knees and feet were well planted under the body
-   In modern humans, the lower limbs bear all the body weight and
    perform all locomotor functions. Consequently, the hip, knee and
    ankle joint are all large with less mobility than their counterparts
    in chimpanzees. In australopithecines, the joints remain relatively
    small. In part, this might be due to smaller body size. It may also
    be due to a unique early hominid form of bipedal locomotion that
    differed somewhat from that of later hominids.

Thus human bodies were redesigned by natural selection for walking in an
upright position for longer distances over uneven terrain. This is
potentially in response to a changing African landscape with fewer trees
and more open savannas.

### Brain Size

Bipedal locomotion became established in the earliest stages of the
hominid lineage, about 7 million years ago, whereas brain expansion came
later. Early hominids had brains slightly larger than those of apes, but
fossil hominids with significantly increased cranial capacities did not
appear until about 2 million years ago.

Brain size remains near 450 cubic centimetres (cc) for robust
australopithecines until almost 1.5 million years ago. At the same time,
fossils assigned to Homo exceed 500 cc and reach almost 900 cc.

What might account for this later and rapid expansion of hominid brain
size? One explanation is called the \"radiator theory\": a new means for
cooling this vital heat-generating organ, namely a new pattern of
cerebral blood circulation, would be responsible for brain expansion in
hominids. Gravitational forces on blood draining from the brain differ
in quadrupedal animals versus bipedal animals: when humans stand
bipedally, most blood drains into veins at the back of the neck, a
network of small veins that form a complex system around the spinal
column.

The two different drainage patterns might reflect two systems of cooling
brains in early hominids. Active brains and bodies generate a lot of
metabolic heat. The brain is a hot organ, but must maintain a fairly
rigid temperature range to keep it functioning properly and to prevent
permanent damage.

Savanna-dwelling hominids with this network of veins had a way to cool a
bigger brain, allowing the \"engine\" to expand, contributing to hominid
flexibility in moving into new habitats and in being active under a wide
range of climatic conditions.

### Free Hands

Unlike other primates, hominids no longer use their hands in locomotion
or bearing weight or swinging through the trees. The chimpanzee\'s hand
and foot are similar in size and length, reflecting the hand\'s use for
bearing weight in knuckle walking. The human hand is shorter than the
foot, with straighter phalanges. Fossil hand bones two million to three
million years old reveal this shift in specialization of the hand from
locomotion to manipulation.

Chimpanzee hands are a compromise. They must be relatively immobile in
bearing weight during knuckle walking, but dexterous for using tools.
Human hands are capable of power and precision grips but more
importantly are uniquely suited for fine manipulation and coordination.

### Tool Use

Fossil hand bones show greater potential for evidence of tool use.
Although no stone tools are recognizable in an archaeological context
until 2.5 million years ago, we can infer nevertheless their existence
for the earliest stage of human evolution. The tradition of making and
using tools almost certainly goes back much earlier to a period of
utilizing unmodified stones and tools mainly of organic, perishable
materials (wood or leaves) that would not be preserved in the fossil
record.

How can we tell a hominid-made artifact from a stone generated by
natural processes? First, the manufacturing process of hitting one stone
with another to form a sharp cutting edge leaves a characteristic mark
where the flake has been removed. Second, the raw material for the tools
often comes from some distance away and indicates transport to the site
by hominids.

Modification of rocks into predetermined shapes was a technological
breakthrough. Possession of such tools opened up new possibilities in
foraging: for example, the ability to crack open long bones and get at
the marrow, to dig, and to sharpen or shape wooden implements.

Even before the fossil record of tools around 2.5 Myrs,
australopithecine brains were larger than chimpanzee brains, suggesting
increased motor skills and problem solving. All lines of evidence point
to the importance of skilled making and using of tools in human
evolution.

## Summary

In this chapter, we learned the following:

1\. Humans clearly depart from apes in several significant areas of
anatomy, which stem from adaptation:

-   bipedalism
-   dentition (tooth size and shape)
-   free hands
-   brain size

2\. For most of human evolution, cultural evolution played a fairly
minor role. If we look back at the time of most australopithecines, it
is obvious that culture had little or no influence on the lives of these
creatures, who were constrained and directed by the same evolutionary
pressures as the other organisms with which they shared their ecosystem.
So, for most of the time during which hominids have existed, human
evolution was no different from that of other organisms.

3\. Nevertheless once our ancestors began to develop a dependence on
culture for survival, then a new layer was added to human evolution.
Sherwood Washburn suggested that the unique interplay of cultural change
and biological change could account for why humans have become so
different. According to him, as culture became more advantageous for the
survival of our ancestors, natural selection favoured the genes
responsible for such behaviour. These genes that improved our capacity
for culture would have had an adaptive advantage. We can add that not
only the genes but also anatomical changes made the transformations more
advantageous. The ultimate result of the interplay between genes and
culture was a significant acceleration of human evolution around 2.6
million to 2.5 million years ago.

