Bone Terminology and the Defining of Humans
Bone Identification and Terminology
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 centre 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.
Canines, Molars: Teeth size can help define species.
- Gorillas eat lots of foliage; therefore they are chewing all day and have great canines
- Humans are omnivorous and have small, more generalized 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 neighbouring 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
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; 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 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.
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.
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.
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 artefact 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.
In this chapter, we learned the following:
1. Humans clearly depart from apes in several significant areas of anatomy, which stem from adaptation:
- 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.