laboratory

The bones that make up the shoulder are the scapula, clavicle and humerus.

The humerus is the upper arm bone, with a ball-shaped head at the proximal end. The scapula is a flat, triangular bone in humans. The most prominent parts of the scapula are at its lateralmost angle where it articulates with the humerus. Here, the bone bears a shallow, bean-shaped depression called the glenoid fossa. Two projections, the acromial and coracoid processes, extend beyond the glenoid fossa providing attachments for some of the muscles and ligaments of the shoulder and upper arm. The clavicle articulates with the acromial process and extends toward the midline of the torso, with its medial end articulating with the superior part of the sternum.

What to do: Examine the scapulae of different kinds of primates. You'll find that primates with different locomotor patterns have rather different scapula morphology.

Monkeys and prosimians that are mainly quadrupeds have relatively long and narrow scapulae. Their shoulders are adapted for forelimb movement anteriorly and posteriorly, but not especially to the side or above the head.

By contrast, apes and humans have scapulae that are very triangular in shape. The shoulder joint is more mobile in these primates, with the arm able to move freely to the side and above the head.

The mobility of the scapula is also related to the shape of the trunk. Monkeys have a deep trunk that is relatively narrow from side to side, while apes and humans have a shallower trunk that is wider from side to side.

Note: This page will change, as your inquiry assignment progresses. Keep checking back here.

Humans, living apes and monkeys are grouped together as anthropoid primates. The anthropoids share a common ancestor that lived sometime more than 55 million years ago. Paleontologists have found fossils of ancient anthropoids that lived very near that ancestor, and also ancient anthropoids that were related to tarsiers, the next closest branch of the primate phylogeny.

Among the anthropoids, Old World monkeys (cercopithecoids) and hominoids are closer relatives, and New World monkeys (ceboids) are more distantly related. In other words, cercopithecoids and hominoids form a group that descends from a common ancestor within the anthropoids. We call this group the catarrhine primates.

In this inquiry-based laboratory project, you will use evidence from two recent primates to infer the probable anatomy of their common ancestor. Together with your laboratory group, you will be assigned two recent primates. Every group will receive a different pair of primates. Some of these will be New World monkeys, some will be Old World monkeys, and some will be hominoids -- that is, apes or humans. The pairs of primates will have one thing in common: their true common ancestors were ancient anthropoid primates.

How can scientists reconstruct the anatomy of a common ancestor, if they haven't necessarily discovered fossils of that species?

Every inference about an ancestor is a hypothesis. If we see that two descendants of the ancestor are similar in their anatomy, we can begin with the hypothesis that the ancestor was also the same as those two descendants. For example, humans and squirrel monkeys (a New World primate) both have fingernails instead of claws. We can hypothesize that our common ancestor also had fingernails.

We test this hypothesis in several ways. We can look at other descendants of the same ancestor. Our common ancestor with squirrel monkeys was the same species as the common ancestor of gorillas and squirrel monkeys, and the same as the common ancestor of baboons (an Old World monkey) and howler monkeys (a New World monkey). Not only humans and squirrel monkeys but also gorillas, chimpanzees, baboons, macaques, guenons, langurs, howler monkeys, spider monkeys, and all other monkeys and apes have fingernails. It seems very unlikely that all these species would have evolved fingernails by coincidence or in parallel with each other. The hypothesis that they inherited fingernails from their common ancestor seems very well supported by this evidence.

We can also look at species that are more distantly related. Such species give an outgroup for our phylogenetic comparisons. Lemurs, tarsiers, and lorises all have fingernails also -- although lemurs and lorises have instead a single claw, called a grooming claw on one finger. It appears from the evidence that not only the common ancestor of all anthropoids, but also the common ancestor of

Looking at an outgroup is especially important in cases where two living descendants of the same ancestral species are different from each other. Macaques have a tail. Humans don't. Did our common ancestor -- the ancestor of the catarrhines -- have a tail or not? Looking at just catarrhine species doesn't help us determine whether human ancestors lost a tail or instead macaque ancestors gained one. All living apes lack tails, all living Old World monkeys at least have some tail. But a look at an outgroup helps enormously. New World monkeys have tails, as do lemurs and tarsiers. These outgroups suggest that the ancestors of catarrhines had a tail and that the ancestors of the apes lost their tails.

For this inquiry, you will consider two major areas of anatomy. In week 2 of the laboratory, you will examine teeth. The number and anatomy of teeth vary among anthropoids, and you will develop and test a hypothesis about the number and anatomy of teeth in the common ancestor of your two primates.

In week 3 of the laboratory, you will examine body plan, including the number and types of vertebrae and the anatomy of the forelimb. You will develop and test a hypothesis about the body plan in the common ancestor of your two primates.

You will develop these hypotheses and work as part of a laboratory group. In week 4 of the lab, you will present your findings. One of the best aspects of this inquiry is that different groups may arrive at different hypotheses, depending on which species of recent primates they have examined. As you come together with other groups to discuss your findings, be prepared to compare the evidence from different groups to see whether it confirms or rejects your hypotheses about the common ancestor.

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.

Many of the differences between Neandertals and modern humans can be found in the face and jaw. Neandertals had relatively tall faces, and substantial prognathism of the midface. To describe more fully: Neandertal faces were tall from the chin to the browridge, and they extended far forward relative to the ears.

These aspects of facial anatomy are reflected in the Neandertal mandible. The part of the mandible that includes the alveoli for the roots of the teeth is called the corpus. The corpus tends to be thicker and stronger in Neandertals than in most living people. It also tends to be taller, with a greater distance between the inferior border of the mandible and the teeth.

At the front of the mandible is the mandibular symphysis. In modern humans, there tends to be a projecting triangle of bone, which we call the chin, but in technical terms is known as the mental eminence. Few Neandertal fossils have a chin. Most, like earlier hominins, have a slightly receding mandibular symphysis.

The part of the mandible that stretches upward from the corpus to connect to the temporal bones is called the mandibular ramus. The shape of the Neandertal tooth rows is basically the same as in the human jaw. But the mandibular ramus is relatively more posterior, so that there is a gap between the third molar and the anterior border of the ramus. This gap is called a retromolar space, and it reflects the strong midfacial prognathism of the Neandertal skull.

What to do: This station has several Neandertal partial mandibles, from the site of Krapina, Croatia. There is one early modern human mandible from Skhul, in present-day Israel. These are comparable in age (Krapina is 120,000 years old, Skhul is around 100,000 years old). Compare these to the recent human mandibles at the station and consider how these Neandertals fit relative to human variation.