The following is taken directly from the workbook of Mystery Fossil, Version 3.0. Mystery Fossil is an interactive CD-ROM that can be packaged with The Human Species for an additional $7, or, about the cost of most other physical anthropology texts alone. To order this package option, use ISBN 0-7674-1401-2. Detailed description of Mystery Fossil, Version 3.0.

The following questions will help you determine which species your Mystery Fossil best fits.

1. Is the dating the same as a known fossil?

For an overview of dating methods, see p. 16. For practice in using dating methods, try Exercise 2, p. 20.

2. Is there anatomical similarity to a known fossil?

Use the Anatomy Charts in "Mystery Fossil" to become familiar with the bones, teeth, and other anatomical features. Also, try Exercises 1 and 3.

Some morphological (that is, shape) similarities are particularly significant as clues to placing individuals in the same species. For an overview of the important morphological features, see the next section on p. 13.

Remember that individuals in a species exhibit a range of sizes and shapes due to sex, age, and individual differences, so the fossils do not have to be identical. You are looking for a "best match." To gain practice in seeing intraspecific (within-species) differences, try Exercise 4.

Examining Physical Features

Physical features of fossilized bones usually hint at their function -- that is, how a feature worked as a component of the hominid's entire biological system. In turn, function is a clue to the hominid's lifestyle and environment, important factors in deciding how closely related one fossil is to another.

Some physical features are not strongly linked to any function and are therefore not strongly acted upon by natural selection. They might change slowly, if at all, through long segments of the fossil record. When we find such a feature on two fossil bones, it is a good indicator that they share a common ancestor, or that one is the ancestor of the other.

Paleoanthropologists generally agree that hominid evolution has exhibited certain trends in the last four million years. You may apply the following anatomical trends as criteria to evaluate whether a fossil's features appear earlier or later in the evolution of humans.

Changes in the cranium. In the course of human evolution, the following changes occur in the shape and size of the cranium:

The volume of the interior of the cranium, called cranial capacity in cubic centimeters (cc) increases as the capacity for culture grows. An adult male chimp, our closest relative, has about 350 cc cranial capacity. A twentieth-century male Homo sapiens has about 1350 cc.

The cranium becomes more rounded, less football-shaped as its musculature decreases and the brain inside grows larger.

Ridges and crests running over the cranium from front to rear, where heavy jaw muscles attach, become smaller.

The frontal bone (forehead) becomes wider and higher as the forebrain grows, permitting more dexterity and complex language.

The supraorbital torus, or ridge of bone over the eyes, serving to keep the skull rigid during strong chewing action, grows smaller as tools replace teeth.

Ridges on the occipital bone, where neck muscles attach to the back of head, grow smaller. The occipital becomes smother and the back of the head becomes more rounded.

Changes in the face and teeth. Other changes in the skull occur in the face and teeth:

Molars are initially large for grinding, then grow smaller relative to incisors, which do tearing or holding, and lastly all teeth become smaller as they are replaced by tools and a diet easier to eat.

Initially prognathism is pronounced, that is, the large teeth and jaws make the lower face protrude. In later stages, prognathism is reduced as the entire facial area grows smaller compared to the growing cranial area.

As the mandible and teeth grow more delicate, a pointed chin appears and acts as a small buttress to reinforce them.

The outwardly flared posterior edge of the ascending ramus (the rear edge of the mandible's cheek section) and the outwardly flared inferior (lower) edge of the mandible, where strong chewing muscles attach, grow smaller.

The zygomatic bones, or cheekbones, become narrower and shorter as their importance in anchoring large chewing muscles diminishes with an increasingly easier-to-chew diet.

Changes in the entire skeleton. In addition to cranial changes, the following changes occur in the skeleton:

Through most stages of human evolution, the overall stature increases and limb bones become more robust. In later stages the robustness decreases but the tall stature is retained.

Upper limbs become shorter than lower limbs as bipedalism develops.

Finger bones straighten out as tree climbing becomes less important. Thumbs become longer and the hand more flexible to manipulate objects such as tools.

Here are some questions to help you evaluate the five competing phylogenies and select the one that best fits the fossil data.

1. Are the fossils correctly arranged in time? A first step in constructing a phylogeny is to arrange the fossils into a one-dimensional time line. For example:

2. Are the fossils from one time period correctly grouped by similarity of anatomy and ecological position? Make clusters of the fossils, generating a two-dimensional time line:

3. Are the clades correctly grouped? Clades are groups of species which share derived traits. Derived traits are new adaptations in the descendent not found in the ancestor, such as a prehensile tail developing on a monkey species whose ancestors didn't have one. In contrast, primitive traits are inherited from an ancestor, for example, a red face on a monkey species whose ancestors also have the red face. Of course, whether a trait is primitive or derived is always relative to the group of fossils you are analyzing.

Identifying clades by primitive and derived traits allows you to draw lines in the two-dimensional time line connecting ancestors to descendents. For example:

This figure is called a cladogram. Fossils e and f are connected to d, meaning they have inherited traits that were derived in d after a and that are not found in c or b.

Similarily, c has developed some novel, or derived, anatomical traits, more of which show up in g and h than in the other fossils.

We have connected d to a, meaning d shares traits with a that b and c don't have. These are traits derived in a, or perhaps derived in an ancestor of a after it separated from the lineage that b and c are in. ( We would have to go back to fossils earlier than those on the cladogram to find the common ancestor of a with b and c).

Fossils b and c must have an ancestor earlier than the fossils on the cladogram, because b and c share derived traits that d doesn't have.

Where are the descendents of fossil c? Perhaps there were none; c may be an extinct line, an evolutionary dead-end. How do we know c is not the ancestor of g and h? Because g and h inherited some derived traits from b which its otherwise close relative c doesn't possess.

Other Useful Clues

Other fossil characteristics provide clues that will help you classify the mystery fossil.

• Teeth can tell you information about diet and disease patterns as well as their use as tools.
• Larger cranial capacities, especially expansion of the forebrain, indicate more reasoning, planning, manual dexterity, and communication skills.
• To infer musculature from bare bones, look for ridges, flanges, knobby surfaces, and sharp angles to which the muscle could have firmly attached.
• The associated natural and cultural material reveal much. Fauna indicate land forms, vegetation, weather, and possible food supplies in the area. Artifacts indicate the level of manufacturing sophistication and dependance upon tools, art, clothing, and shelter.