Laboratory Notes for BIO 1003

© 30 August 1999, John H. Wahlert & Mary Jean Holland

Observing Similarities and Differences

© 2 July 2000, Joan Japha and John H. Wahlert


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.


The characters (skeletal structures you observe) in two different organisms are homologous if there is evidence that they are shared because they were present in a common ancestor. 

Example: The arms of vertebrates that live on land have an endoskeleton with shoulder blade, humerus, radius and ulna, wrist, and hand. This was inherited from a common ancestor. Insects that live on land have “arms” with an exoskeleton that makes a single row of jointed parts; they had a common ancestor. Conclusion: Land vertebrates and insects did not inherit their limbs from a common ancestor. The structures are made differently though they have a similar functions.  

Basic Vertebrate Homolgies

All of the vertebrates available in lab share characters that are homologous and indicate that they are descended from a common ancestor that had these characters.

  • Their skeletons are made of bone, not chitin (arthropods) or calcium carbonate (mollusks).
  • They have an axial skeleton—vertebral column.

  Bilateral symmetry (left/right) with head and tail ends is probably more primitive—an earlier ancestry with other groups.

Vertebrate arm bones are homologous

All arms have the same pattern—humerus, radius and ulna, wrist, and hand—except for the fish at the lower right.

tetrapod arms and fish fin


bird skeleton

Two structures in different organisms are analogous if they serve the same function. They may also be homologous. Analogy, by itself, is not evidence of descent from a common ancestor. 

Example: Birds, bats and bugs all fly. Their wing membranes are analogous and push against the air to give lift or to glide. But the structure of the wings is entirely different; they are not homologous. The wing membrane in birds is formed by feathers and supported by the arem. The analogous membrane in bats is skin that stretches between elongated fingers. Differences in basic structure indicate that flight was not inherited from a common ancestor.

Comparison of bird and bat wings
bird wing bat skeleton

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 to which the appendages (arms and legs in humans) are attached; together, the girdles and limbs are the appendicular skeleton. .

reptile skeleton, dorsal view

Examine the skeletons of a number of different animals such as the alligator, monkey, cat, dog, fish, frog, snake, etc.

Arch of vertebral column - fighting gravity

The two sides of the arch lean toward the keystone in the middle. It’s hard to squash an arch with downward pressure.

arch wtih keystone

Land-living vertebrates with four legs (tetrapods) have an arched vertebral column—front and back lean together toward a middle vertebra. The arch resists the pull of gravity.


An egg works two ways; the hen sits on the arch, but the hatching chick bursts up through it. hen sitting on eggs

Locomotion, moving from place to place, is important in all animals; can you discover from their skeletons how different animals move?

Connection of limbs to axial skeleton

The bones of the limbs are firmly but moveably attached to the pelvic and pectoral girdles by articulations, ligaments, and muscles.

The pelvis is joined tightly to the vertebral column. Hind legs push the animal forward. reptile pelvis
Forelimbs are not directly connected to the axial skeleton. The body is suspended in a sling of muscle between the dorsal edges of the scapulae (shoulder blades). The forelimbs keep the nose off the ground and keep pace with the speed of the hind limbs.

Many animals have freed up the forelimbs for manipulating the environment. Think of humans and kangaroos.
reptile pectoral girdle

Comparison of Appendages

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?

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?

Limbs and Life

The rat is a generalist and has equal lengths of upper and lower arm portions.

rat arm skeleton

Fossorial (burrowing) life

The mole digs with its clawed, shovel-like forepaws. Arm bones have flaring processes for attachment of lots of muscle. Hind limbs are small and kick away the loosened soil.

mole skeleton, dorsal view

Running on tiptoe

In a fast–running animal, such as a deer or a gazelle, the legs are made longer by adding the hand and fingers and foot and toes to the length. Not only does a long limb sweep through a greater arc than a short limb, but each bone segment moves at the same time, so the arc is completed much more quickly than a single bone could achieve.

artiodactyl forelimb

And then there are snakes

Snakes don’t have legs; they undulate and push body segments against the ground. Did they ever have legs?

Boas and pythons have remnants of the pelvis and femurs. Early fossil snakes with short legs have been found.

snake skeleton

But what about fish?

The limbs (fins) of fish are used for steering. The axial skeleton itself is the organ of propulsion. Fish don’t need legs to hold them up against gravity. They are buoyant in water, which is much denser than air.

fish skeleton

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.

    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?

    Teeth are clues to animal diets

    When you compare the teeth of various animals in the lab, you see distinctly different designs.

    • All the teeth in the alligator and the fish are the same (termed homodont). The mouth is a trap, and these pointed teeth hold and subdue prey. They can be replaced as they fall out.
    • Mammals have a heterodont dentition: incisors in front nip and crop, pointed canines puncture, bicuspid premolars slice, and finally complex molars chew. The working surfaces of upper and lower teeth fit together to slice and shear; this fit is called tooth occlusion. These teeth are replaced in sequence, so no function is temporarily lost. The tooth categories and bones that bear them are mammal homologies.

    Bite force

    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?

    As in a nutcracker, the highest force is close to the pivot, the jaw joint. When you look at dentitions, including your own, the teeth that exert the most force are at the back of the jaw. In some mammals the incisors, farthest from the joint, are tiny and used mostly for grooming.

    nutcracker wtih walnut

    Carnivore vs. herbivore

    Carnivore skulls are designed like scissors with the pivot in line with the teeth. Cutting edges are above and below the pivot.

    Herbivore skulls are designed like these pliers with the pivot above the teeth. The teeth all meet at once and grind up plants.

    scissors slip-joint pliers

    Carnivore: Cat

    Herbivore: Cow

    cat skull, lateral view cow skull, lateral view

    Slicing teeth in the carnivore

    Cutting edges, which align above and below the pivot, slice meat.

    Upper and lower teeth occlude precisely to cut. The jaw joint has little room for wiggle since a mal–occlusion could crack these teeth, which are crucial for survival.

    Your teeth have no edges that can be damaged, and you can slide your jaw from side to side.

    cat carnassial teeth

    Biting gape

    Carnivores have a huge gape for biting prey. Can you open your mouth this far? Does a cow open its mouth so wide?

    [This is my cat Tiger.]

    cat with mouth agape

    Herbivores: Horse and cow

    Horses have upper nipping incisors. Eyes face obliquely to the sides. Teeth are tall and occlude for grinding. Cows lack upper nipping incisors; lower incisors bite against a tough upper pad. Eyes face obliquely to the sides. Teeth are tall and occlude for grinding.
    horse skull, lateral view showing jaw joint cow skull, lateral view showing jaw joint

    Horse and cow teeth point to separate ancestry

    Horse premolars and molars have complex folds of enamel, the hardest tissue in the body, resist wear. horse upper molars and premolars
    Cow premolars and molars have crescent shaped ridges of upper and lower teeth that shear across each other. cow upper molars and premolars

    Tooth size and occlusion

    Cow as example: Upper teeth are stationary. Muscles raise and move the jaw thus bringing teeth into occlusion.

    Crests of the lower teeth sweep through valleys of uppers, and crests of upper teeth press into valleys of the lower dentition. Red lines indicate direction.

    Hard enamel ridges break down the food. The moveable lower teeth are not as wide, cheek to tongue, as the uppers against which they chew. upper and lower teeth of cow showing tooth occlusion

    Primates – large brains, eyes forward

    The jaw joint is high relative to the occlusal plane of the teeth. What does this suggest about the common ancestor of the primates?

    Gorilla Human
    gorilla skull with jaw joint indicated human skull with jaw joint indicated

    Gorilla dentition

    Tooth occlusion: upper and lower teeth have cusps that fit into basins of opposing teeth. These have mortar and pestle crushing action and are similar to human teeth. The enamel is thicker and stronger than in human teeth. The jaw joint allows side to side movement.

    gorilla, upper teeth

    Gorilla skull

    A ridge on the top of the skull and another at the back make a broad fossa or channel on each side. The ridges are surfaces for attachment of huge temporal muscles that will powerfully raise the jaw in chewing. Humans don’t have any large ridges and the comparable temporal muscle is much smaller. gorilla skull showing temporal muscle ridges

    Skulls of large herbivore and primate compared

    Both are designed as herbivores; the cow has a long face to crop grass. 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? The cow’s eyes face obliquely to the side for nearly 360–degree vision. The cow moves in a 2–D world and needs to spot predators. The human’s eyes face forward giving 3–D vision. Human ancestors presumably lived in trees, a 3-D world. The cow has a brain that is smaller compared with head size.

    cow skull, lateral view human skull, lateral view

    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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    Last updated 8/14/2022    (JHW)