anatomy

The diversification of the first primates from other early mammals took place partly because the ancestors of the primates came to inhabit a unique environment --- the trees. These early primates developed many features to allow them to move quickly in this arboreal habitat. Early primates evolved the ability to direct and focus both eyes on objects, called binocular vision, which allowed them to accurately judge distances to branches and other objects. They also developed grasping hands and feet, with opposable digits --- thumbs and big toes. The fingertips were broader in these early primates, to apply greater grip strength to branches, and instead of claws primates developed wide nails. All of these features helped primates to succeed as arboreal specialists.

• Primate adaptations to arboreal environments include binocular vision, opposable digits, enlarged fingertips and nails.

Many primates today continue this arboreal existence, spending large proportions of their time off the ground and in the trees. Moving in an arboreal environment has the obvious risk of falling out of the tree if everything does not go exactly right. Yet primates are virtual trapeze artists, masters of the arboreal environment. Smaller monkeys and lemurs can rush headlong from branch to branch, seemingly mindless of any risk of falling, because of their unrivaled arboreal skills. These primates bridge immense gaps by leaping from one tree to another, limited only by the acceleration of gravity. Even large primates like chimpanzees and orangutans can rapidly scale trees and move effectively from one tree to another. These arboreal skills are made possible by a large suite of adaptations, many of which originated very early in primate evolution.

But even though all primates are climbers, some of them have developed more specialized adaptations to other kinds of movement, or locomotion. These specialized forms of movement are adapted to different kinds of environments. Some primates are excellent at moving terrestrially, on the ground. Others are good at climbing tall vertical trunks and branches, or at swinging from one branch to another. Species who use these special forms of locomotion have consequences in their skeletons and muscle configurations.

Vertical clinging and leaping

Many prosimians, including tarsiers and many lemurs, use a form of locomotion called vertical clinging and leaping. These kinds of primates live in habitats where they climb relatively small tree trunks or bamboo. Sometimes they leap between these vertical supports. Other times, especially for the larger lemurs like sifakas, they leap along the ground. Their legs are relatively long compared to their arms, so that terrestrial movement can be more effective by leaping than by running on all fours.

This form of locomotion gives their bodies a very distinctive shape --- especially tarsiers, who are masters of this pattern. Tarsier legs are longer than their bodies and forelimbs put together, and they especially have very long foot bones. The name for these bones are tarsals, giving these unique little primates their name.

Below-branch locomotion

Some monkeys and all apes are adapted more to hanging beneath branches than running atop them. This suspensory style of locomotion is essential for larger arboreal primates, which can move above only the largest branches. Hanging below branches enables large primates to climb into the forest canopy, often on smaller branches than could support them from above.

A skeletal adaptation to suspension includes relatively long arms with very mobile shoulder joints. In contrast to quadrupedal monkeys, whose arms are limited in their range of motion, apes can move their arms through nearly a complete 360 degree circle. Also, apes' trunks are relatively flat from front to back, with the shoulders mostly alongside rather than in front of the spine. Putting the arms at the side requires long and strong collarbones --- called clavicles --- that serve as a strut supporting the shoulder musculature. Also, apes have extremely long fingers, useful for hooking onto branches quickly. Their thumbs are quite small, as they do not usually grip with their thumbs onto branches while hanging from them.

• Brachiation is arm-over-arm swinging.

Sometimes apes swing arm over arm from one branch to the next, a locomotor pattern called brachiation. Gibbons and siamangs are a brachiation specialists, but all living hominoids have the skeletal anatomy to enable them to brachiate. Brachiation conserves the energy of forward movement by treating the body as a swinging pendulum. This is a very efficient way to travel for medium-sized primates, combining energetic conservation with speed, and working with gravity instead of against it.

• Quadrumanous locomotion uses all four hands and feet to move among branches.

Large-bodied hominoids brachate less than gibbons. Branches of sufficient size to support the entire weight of a large ape are rarely located close enough to brachiate between them. Also, the pendular motion is less efficient for larger primates, because their arms just cannot be as long in proportion to the size of their bodies. Instead, when in the trees, the living great apes grip branches with three or four of their hands and feet at once. This allows large apes to support their weight on relatively small branches in the canopy. The masters of this kind of locomotion are orangutans, who move through the forest canopy moving one hand or foot at a time to a new support branch. This kind of locomotion, as if an animal was using four hands equally, is called quadrumanous locomotion.

Knuckle-walking and fist-walking

• Knuckle-walking allows quadrupedal walking in chimpanzees and gorillas, whose hands are adapted to suspension.

Chimpanzees and gorillas, when they are on the ground, walk quadrupedally using the proximal finger joints of their hands instead of the palms of their hands. They use this style of locomotion, called knuckle-walking, because of their unique forelimbs. These apes have long forelimbs relative to their hindlimbs, and their hands are very long with strong tendons for curling the long fingers into a powerful hook. These hands are very well adapted to suspension in the trees, but they are not capable of being extended with the palms toward the ground, which is the way most other primates walk quadrupedally. So instead of walking palm-down, they use their knuckles.

A few skeletal features of living chimpanzees and gorillas appear to be adaptations to knuckle-walking. Because the arms are held in a locked position while supporting the body, unlike the somewhat flexed position usual in quadrupeds, the proximal end of the ulna is more U-shaped, built for supporting the humerus mainly from below. Likewise, the joints of the hands are more limited in motion, and relatively incapable of being extended backward at the wrist and knuckles. It is not clear whether these unique anatomical consequences of knuckle-walking evolved in parallel in chimpanzees and gorillas, or whether they represent homologies inherited from a knuckle-walking common ancestor of gorillas, chimpanzees, and humans.

Although many orangutans spend little time on the ground in their natural habitat, they can walk quadrupedally. But unlike chimpanzees and gorillas, orangutans do not walk on their knuckles when they are on the ground. Instead, they curl their hands inward, walking on the outside of their fists. This fist-walking accomplishes the same goal as knuckle-walking --- it allows walking on all fours without facing the palms of the hands toward the ground.

In this course, you will be working extensively with skeletal anatomy. The skeleton provides the primary evidence about our evolutionary history. Skeletal evidence is a limited source of information about biology, but soft tissue evidence is fragile and does not persist long even in curated museum contexts. So a disproportionate fraction of our knowledge about anatomical variation comes from the skeleton.

Fortunately anthropologists have been very clever in finding evidence that connects skeletal anatomy to behavior and other aspects of biology. Nowadays bone and teeth provide some of the strongest evidence about diet, development and health of ancient human and primate populations. We are even getting new genetic evidence from bone and teeth, including the complete genomes of archaic humans.

Knowing the skeleton is an essential skill in biological anthropology. Most students will enter this class with a basic knowledge of the bones of the skeleton, and this lab station should help remind you about the parts you probably already know.

Basic divisions of the skeleton

The skull, or cranium sits atop the spine. The rest of the skeleton, everything from the neck down, is called the postcranium, or postcranial skeleton

The skull itself is a complicated structure made up of 26 cranial bones plus the mandible. Except for the mandible, these bones mostly are fused together so that they do not move. The joints between most of the cranial bones are borders where the bones knit together, called sutures. You will learn most of the major bones of the cranium in this class. For now, be sure to remember the mandible.

The teeth are rooted in the mandible and the bones of the face, called the maxillary bones, or maxillae. The teeth are the only part of the skeletal system that come into direct contact with the environment. They are not bone, but are instead made up of hard calcified tissues called dentin and enamel. The teeth are small but contain a vastly outsized fraction of information because of their long persistence in the fossil record as well as their close relationship to development and diet.

The postcranial skeleton can be roughly divided into the appendicular skeleton, which includes the arms, legs, hands and feet, and the axial skeleton, which includes everything else.

The long bones

The major bones of the arm and leg are called the long bones. These are variations on a common theme: A long shaft with two ends, each of which forms a movable joint, or articulation with another bone or structure. The long bones are all paired bones, meaning that each individual has both a left and right. The anatomy of the each bone enables us to identify whether it came from the right or left side of the skeleton.

The bones of the leg include the femur, tibia and fibula. The femur is the thigh bone, the tibia is the shin bone, and the fibula is a thin bone at the outside of the leg, mainly noticeable because it forms the outside of the ankle joint.

The bones of the arm are the humerus, ulna and radius. The humerus is in the upper arm, the radius and ulna are the lower arm bones. These two bones rotate around each other, and are mostly obvious at the wrist and elbow joint. The ulna is the bone that is most prominent on the back of the elbow. The radius is the lower arm bone that lies nearer the thumb, the ulna is nearer the pinky side of the hand.

The axial skeleton

The spinal column makes up the connection between upper and lower parts of the skeleton. It is made up of 24 vertebrae in most people. Twelve of the vertebrae connect to twelve pairs of ribs. These numbers vary within humans, and between humans and other kinds of primates, and that variation will be the subject of a lab.

Each shoulder girdle is composed of the scapula, or shoulder blade, and that clavicle, or collar bone. At the front of the chest is a flat bone called the sternum that connects ribs by means of the costal cartilages.

Finally, at the lower end of the axial skeleton is the pelvis. This structure is composed of three bones, the sacrum at the base of the spine, and the left and right os coxae or innominate bones. The pelvis is also the subject of an entire lab in this course.

Practice

That quick introduction will help to orient you toward the skeleton. Remember that each of the bones can be found within your own body, and for the most part you can feel them from the outside. In total, the human skeleton has more than 206 bones -- more because there are minor bones within tendons that vary in number in different people. Humans are variable, as you will discover during the course of this semester, and not everyone has the same numbers of bones or the exact same arrangement.

For anthropologists, Africa was a point of exceptional diversity between 2 million and 1.5 million years ago. In both East and South Africa, the fossil record presents evidence of several different hominin species. Some fossils belong to our own genus, Homo, and others belong to robust australopithecines.

These two forms seem like they should be easy to tell apart. Robust australopithecines had extraordinarily large mandibles compared to living humans. Consider:

• The main part of the mandible, which holds the teeth, is called the mandibular corpus. In robust australopithecines, this is often extremely thick and tall, with a large distance from the inferior border of the mandible to the teeth.
• The portion of the mandible that extends upward to articulate with the temporal bone is called the mandibular ramus -- with one on both left and right sides. The mandibular ramus of many robust australopithecines is exceedingly tall, reflecting the very vertically tall faces of these hominins.
• Robust australopithecines have hugely expanded premolars and molars, and greatly reduced incisors and canines. Early Homo has overall larger teeth than in living humans, but the proportions between the molars, premolars, incisors and canines is very much like people today.

However, despite these obvious differences, the mandibles of early Homo and robust australopithecines are not always so easy to tell apart. This station has several mandibles from robust australopithecines, mainly from Australopithecus robustus from Swartkrans and Kromdraai, South Africa. There are also several mandibles of Homo erectus here, and a handful of mandibles that are likely early Homo but not definitely H. erectus.

Can you tell them apart? Try seriating these from most humanlike to most robust australpithecine-like. Is there a clear dividing line between the two, or are there questionable specimens?

The hominid pelvis is much shorter than ape pelves, with muscle attachments reoriented for effective walking.

The most dramatic evolutionary change underlying human bipedality is the change in shape of the pelvis. The pelvis is rarely preserved as a fossil, but several partial pelves are available from australopithecines, including the "Lucy" skeleton and several partial pelves from later South African sites. The pelvis consists of three bones — the sacrum, which lies at the bottom of the spine and is composed of several fused vertebra-like elements, and the two os coxae, or hip bones. In early hominids, both the sacrum and hip bones are relatively short compared to apes. The upper portion of each hip bone, called the ilium, is short and curved compared to the long, flattened ilium of chimpanzees and other apes. The curvature places the attachment of the quadriceps muscle closer to the front of the body, allowing the muscle greater leverage in pulling the femur forward in an upright posture.

Lucy (AL 288-1) skeleton.

Although the ilia of Australopithecus were short from top to bottom compared to a chimpanzee, they extend more broadly to the side, resulting in a pelvis that is very broad overall. Lucy’s pelvic width was within the range of today’s women, despite her very small body size. As a result, her body was differently shaped from recent people — very broad for its short height.

The width of the pelvis affects the muscular requirements of walking. Whenever one leg supports the body, gravity tends to tilt the upper body away from the supporting leg. The muscles on the opposite side must counteract this force to prevent the body from falling over. These muscles attach to the lateral part of the ilium and to the femur, pulling the trunk upward around the hip joint. A wide ilium tends to make these muscles more effective, by positioning the point of force further from the joint. A long femur neck also helps, just as long handles on a pair of scissors greatly increase the force with which they can cut. The configuration of these muscles in australopithecines is more extreme than the condition found in living people.

A wide pelvis and long femur neck may have helped australopithecines to maintain a long stride with short legs. Two things add up to determine the length of a step: how much the leg swings, and how much the pelvis rotates. A wider pelvis rotates farther and thereby increases the length of a step. Another explanation is that widely spaced legs may allow a greater mechanical advantage for the muscles that draw the legs toward the midline. This configuration might help the style of climbing that requires the legs to clamp around a branch or trunk. This kind of climbing would be more necessary to bipeds who lacked the prehensile feet of living apes.