Introduction

Survey of the Living World is a course that considers the variety of life on earth and the many different kinds of organisms that inhabit the planet. At the cellular level, eukaryotic organisms have a large number of structural and chemical features in common, and these are evidence of their descent from a common ancestor in the deep past. The huge variety of eukaryotic organisms reflects their diversification and evolution in response to the changing variety and diversity of environments during approximately the last billion years of earth history.

The unifying theme of the course is evolution-the change of populations of organisms through the ages-the idea of descent with modification articulated by Charles Darwin in 1859. What were ancestral populations of organisms like? How do groups of similar organisms living today relate to each other and to extinct groups that lived in the past?

Materials and Methods

In order to uncover relationships among organisms, it is necessary to make careful observations of the parts of their bodies and the functions of these parts. Thinking about the possible functions of body parts is something a paleontologist does when examining fossil remains of organisms that can no longer be observed alive. By comparing similarities and differences among bones and teeth of various vertebrate animals, it is possible to make educated guesses or hypotheses about their relationships: which ones are recently divergent branches (very similar to each other, e.g., a housecat and a lion) of the family tree and which branched much longer ago in the past (very different from one another, though basic similarities are present, e.g., a cow and a whale).

What are suitable materials for study? Anything that can be observed can be compared-from DNA nucleotide sequences to bones and behavior. Skeletons and teeth are especially useful because they are the parts of vertebrates most likely to be fossilized and are thus available for comparing living animals with those of the past.

Overall Appearance of Skeleton

The axial skeleton, the vertebral column, runs the length of the body from anterior to posterior. The pectoral and pelvic girdles are the parts of the skeleton from which the appendages (arms and legs in humans) are suspended; together, these are the appendicular skeleton.

Compare several features, especially the position of the appendages in relation to the body. Is there a difference in support requirements for animals that live in water (aquatic) vs. animals that live on land (terrestrial)? Which appendages are attached to the axial skeleton in such a way to propel the axis of the body forward? What does a fish use for propulsion?

Observe that some terrestrial organisms have appendages positioned directly underneath the body (mammals). Others have the appendages positioned at the sides of the body (amphibians and many reptiles). Which one do you think is most efficient (would use the least muscle energy) in locomotion?

Comparison of Appendages

Compare the appendages of a fish, frog, alligator, mammals, and bird. Which limbs look most similar overall. What about the limbs are different? Do the limbs serve the same functions in all of these kinds of animals? Even though differences are obvious, are there any similarities? Compare two different terrestrial animals' hind limbs; can you match them bone for bone? Might the similarities support the hypothesis that the earliest vertebrates were fish and that terrestrial animals evolved from them?

Comparison of Skulls

Look at the skulls. Group them by similarities, and note the differences. What criteria can you observe that suggest relationships?

1. Look particularly at the teeth. Consider the number, type, shape, and position.

   Compare the teeth of various animals with the teeth in your mouth. Are the teeth of the alligator similar in size and shape throughout its dentition? Are they similar to your teeth? Your teeth are grouped as incisors in front, pointed canines, bicuspid premolars, and finally complex molars; can you make the same kind of groupings for the alligator's teeth? Do the upper and lower teeth in the alligator close together (occlude) in the same kind of relationship as your teeth? What is tooth replacement like in an alligator and in a mammal?

   Look at any two mammal skulls available. Can the teeth of these animals be grouped into incisors, canines, premolars, and molars? Do the upper and lower occlude in a specific relationship as your teeth do, or are they like the alligator.

   Form and Function: Do you see evidence from tooth shape and size in mammals that the toughest jobs (those that require the greatest force) are done with the posterior teeth? Think about a nutcracker and the crushing force generated near the hinge. Do you use your own teeth the same way no matter what you are eating? Is there a difference in the way you bit into a peach and the way you bit into a whole carrot? Review the information in your textbook about diets of animals: what is a carnivore, a herbivore, an omnivore? Which kind of diet to humans have? What are the teeth like in a carnivore, e.g., a cat, vs. a herbivore, e.g., a cow?

2. Look at the eye sockets. Which mammals have the eye sockets facing forward? In most, the eyes face somewhat laterally. What happens to vision, when the fields of both eyes overlap?

3. Are there any other features you notice that suggest relationships?

Things to Think About

Is it reasonable to think that when things look very similar they might be related? Under what circumstances might this not be the case?

If we look at a few individual specimens of a few kinds of vertebrates and then make general statements (inferences) about the groups from which they came (common ancestors), what errors may we be making? What is meant by a representative sample? How can one be obtained?

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