1. Carefully examine the drawings of the bones shown in Figure 1 on the next page. Look for similarities among the various animals.
a. Color each part of the human arm a different color using the given key above Figure 1.
b. Describe the function (job) of each set of bones in the table below:
These structures are formed in similar ways during embryonic development and share like arrangements; however, they have somewhat different forms and functions. They are called homologous structures.
PART 2: ANALOGOUS STRUCTURES
1. Examine the butterfly wing and the bird wing shown in Figure 2 above.
a. What function (job)
do these structures share
b. What supports a bird’s wings? (What is it made of?)
c. What supports a butterfly’s/insect’s wings? (What is it made of?)
d. Do the differences in these structures suggest that birds and insects (butterflies) share a “recent” common ancestor? (Remember: Bird & insect wings are analogous structures!)
Some apparently unrelated animals have organs with similar functions, yet are very different in structure and form. These structures are called analogous structures.
PART 3: VESTIGIAL STRUCTURES
Gradual changes have occurred through time that have in some cases reduced or removed the function of some body structures and organs. The penguin's wings and the leg bones of snakes are examples of this phenomenon.
1. The cave fish and minnow
shown in Figure 3 below
are related, but the cave fish is blind
a. Explain why eyesight is not an important adaptation to life in a cave.
b. Does the overall appearance of the cave fish and minnow suggest common ancestry? Explain your reasoning (give specific examples)?
Organs or structures that have lost their function in the organism and become reduced in size (because of efficiency) are called vestigial structures.
Read the list of human vestigial structures
shown in Table 1.
a. In the table below, suggest a probable function (job)
for each structure and explain why it is vestigial (why it isn’t used/needed anymore)
. Use the “Useless Body Parts” list at the end of this lab (or given as a separate packet) to help you.
Analysis and Interpretation Questions
Probable Function (job) in the Past
EXPLAIN why they’re vestigial
(Why is it no longer used/needed?)
Don’t just say, “It’s not used/needed anymore.”
coccyx (tail bones)
extrinsic ear muscles
1. Explain why the homologous structures in Part I
are evidence of evolutionary relationships
2. Is there evidence of a “recent” common ancestor
between the structure of a human’s leg and a fly’s leg
? (Hint: Think about what they are made of.) Explain your reasoning.
3. List TWO DIFFERENT human structures NOT USED on Table 1 above (from the “Useless Body Parts” sheet attached) that are vestigial and explain why they are vestigial.
Useless Body Parts
By Jocelyn Selim
DISCOVER Vol. 25 No. 06 | June 2004 | Biology & Medicine
In the first chapter of The Descent of Man
, Charles Darwin identified roughly a dozen anatomic traits that he gleefully described as “useless, or nearly useless, and consequently no longer subject to natural selection.” The list included body hair, wisdom teeth, and the coccyx superfluous features that served as Exhibit A in his argument that humans did not descend from “demigods” but rather from a long line of fur-insulated, plant-chewing creatures that sported tails.
Darwin’s catalog of oddities was far from complete—our bodies are littered with parts we don’t need. Some are vanishing leftovers from our pre-hominid ancestors
, such as muscles useful for walking on all fours or hanging from trees that appear in various atrophied forms. Others are by-products of a natural redundancy inherent in human sexual development, including nipples on men and the tiny vestigial sperm ducts lurking behind the ovaries of women. Then there are curiosities that, having outlived their apparent usefulness, linger simply because there’s no real reason to leave: What good or bad is hair on the little toe—or even the little toe itself?
Nearly a century and a quarter after Darwin’s death, science still can’t offer a full explanation for why one outdated anatomic trait lingers in the gene pool and another goes. Modern genomics research has revealed that our DNA carries broken genes for things that seem as though they might be useful, like odor receptors for a bloodhound’s sense of smell or enzymes that once enabled us to make our own vitamin C. In a few million years
, humans may very well have shed a few more odd features. So look now before they’re gone.
The nasal sinuses of our early ancestors may have been lined with odor receptors that gave a heightened sense of smell, which aided survival. No one knows why we retain these perhaps troublesome mucus-lined cavities, except to make the head lighter and to warm and moisten the air we breathe. A cross section of a skull from the collection of Matthew Cryer, a doctor and dentist whose Internal Anatomy of the Face was published in 1901, is housed at the Mütter Museum at the College of Physicians of Philadelphia. Cryer most likely used the skull to analyze the anatomy of paranasal sinuses
, which come in four sets: frontal (forehead), maxillary (beneath the cheeks), and ethmoid and sphenoid (behind the nose). In animals with an acute sense of smell, the sinuses are largely lined by olfactory tissue.
These fused vertebrae are all that’s left of the tail that most mammals still use for balance and communication. Our hominid ancestors lost the need for a tail before they began walking upright. The human coccyx is highly variable but is generally composed of three to five vertebrae. On rare occasions, infants are born either with no coccyx at all or with tails. While some have suggested that the coccyx helps to anchor minor muscles and may support pelvic organs, surgically removing it has no discernible effect on health.
EXTRINSIC EAR MUSCLES
This trio of muscles most likely made it possible for pre-hominids to move their ears independently of their heads, as rabbits and dogs do. We still have them, which is why most people can learn to wiggle their ears.
Early humans had to chew a lot of plants to get enough calories to survive, making another row of molars helpful. Only about 5 percent of the population has a healthy set of these third molars.
A common ancestor of birds and mammals may have had a membrane for protecting the eye and sweeping out debris. Humans retain only a tiny fold in the inner corner of the eye.
A small folded point of skin toward the top of each ear is occasionally found in modern humans. It may be a remnant of a larger shape that helped focus distant sounds.
This small muscle stretching under the shoulder from the first rib to the collarbone would be useful if humans still walked on all fours. Some people have one, some have none, and a few have two.
This long, narrow muscle runs from the elbow to the wrist and is missing in 11 percent of modern humans. It may once have been important for hanging and climbing. Surgeons harvest it for reconstructive surgery.
Bundles of smooth muscle fibers allow animals to puff up their fur for insulation or to intimidate others. Humans retain this ability (goose bumps are the indicator) but have obviously lost most of the fur.
This narrow, muscular tube attached to the large intestine served as a special area to digest cellulose when the human diet consisted more of plant matter than animal protein. It also produces some white blood cells. Annually
, 300,000+ Americans have an appendectomy.
Brows help keep sweat from the eyes, and male facial hair may play a role in sexual selection, but apparently most of the hair left on the human body serves no function.
Often mistaken for a nerve by freshman medical students, the muscle was useful to other primates for grasping with their feet. It has disappeared altogether in 9 percent of the population.
Lesser apes use all their toes for grasping or clinging to branches. Humans need mainly the big toe for balance while walking upright.