Posted: February 18th, 2022

Homework help – Discuss skeletons of a chimpanzee and a human being,|Biology

Homework help – Discuss skeletons of a chimpanzee and a human being,|Biology

As all good teachers know, students will work much harder for extra-credit points than at the assigned

task. I like to take advantage of this convenient trait in my introductory course on evolution. Once my

students (non-science majors at a midwestern land-grant university) understand the basic terms, I offer

additional points for answering the questions I really want them to investigate. Find a dozen differences

between the skeletons of a chimpanzee and a human being, I challenge them; tell me how a human

female skeleton differs anatomically from a male. The male and female skeletons I display are

exemplary in their difference, and since most students should be able to guess what that difference is if

they don’t already know, I usually feel confident that the final answer is a giveaway. I say “usually”

because seven years ago, the first time I taught the course, I got a surprising answer that still crops up

with alarming regularity. Five minutes into the lab period, a young woman announced that she could

answer the question without even examining the human skeletons.

I waited silently for her to explain that the female pelvis is shaped slightly differently from the male’s,

with a larger opening for childbearing. That part was the giveaway. The real purpose of the exercise was

to make her prove her conjecture with measurements to translate the theory to practice. I also wanted her

to explain why this sexual dimorphism-that is, this sexually determined physical difference-is not nearly

so pronounced in nonhuman primates, such as chimpanzees.

She spoke: “Males have one fewer pair of ribs than females.”

I was totally unprepared for her answer. My mandible dropped. After a moment’s reflection, I realized

she must be referring to the biblical story in which God creates Eve from one of Adam’s ribs. My

student was someone who believed in the literal truth of the Bible, and it was her religious belief, not her

previous knowledge of human anatomy, that made her so sure of her answer. This was going to be a

challenge.

I believe just as firmly in religious freedom as I do in the scientific search for understanding. Thus,

while I adhere rigorously to teaching the best science and showing how scientists recognize it as the

best, I never insist that students believe scientific results. On the contrary, I encourage them to be

skeptical-as long as their skepticism is based on logic and evidence. Scientific results, in my view,

should be compelling because the collected observations and experiments leave room for only one

possible rational explanation. To insist that students accept my word (or the word of any scientist) about

any fact would undermine the one thing that makes science different from all other belief systems. The

acid test of science is the personal one of convincing yourself that you perceive what everyone else

perceives, whatever reservations you may start with. The evidence should be so compelling that it

convinces even the most serious skeptic-as long as that skeptic retains an open mind. Even more

important, science must admit what it does not or cannot know. Questions are what drive science, not

answers. A teacher who insists on blind obedience might well crush some budding Darwin who sees a

higher and more compelling truth about nature than the current dogma admits.

But in this instance, I was dealing with a pretty bare-bones case. The skeletons stood there as mute

models of reality. Pedagogical ideals notwithstanding, I saw little hope of enlightening my young friend

without attacking her religion outright.

I stalled for time. “Have you actually counted the ribs?” I asked. She admitted that she had not. “Well,

since this is a science class,” I admonished, “let’s treat your statement as a hypothesis. Now you need to

test it.” So off she went to the back of the room, full of confidence that God would not let her down. The

breather gave me a chance to plot out what I hoped would be an enlightened, and enlightening, approach

to the crisis her assumption had precipitated. Science and religion are not mutually exclusive topics.

How might it be possible to learn about how evolution works and continue to believe that God created

Eve from one of Adam’s ribs?

“Are you sure those are male and female skeletons?” My cocksure friend was back, looking a little

puzzled.

“They’re the bona fide item,” I answered. “Not only did they come so labeled from the company from

which they were bought, but certain anatomic features that I have verified myself lead me to conclude

that the labels are correct. But I’m glad you asked. Skepticism is a very useful scientific tool, and

scientists do sometimes make mistakes. Not this time, though.”

“Yes, but the skeletons have the same number of ribs,” objected my student.

I agreed. “Why did you expect otherwise?” Best to get the argument out in the open. As I had guessed,

her information came from the Bible, via Sunday school.

“But what does the Bible actually say?” I asked. Surely there had to be some way out of this mess.

“That God took a rib from Adam to create Eve.”

“One rib or two?”

“One,” she replied without hesitation.

“Don’t forget that ribs come in pairs,” I prompted her. “Oh!” I could almost hear her mind whirring. “So

men should be missing only one rib, not a pair — is that what you’re saying?”

“I don’t know.” I shook my head. “Why should they be missing any?”

“Well, if God took a rib from Adam, wouldn’t his children also be missing a rib?”

”All his children?” I countered. “Boys and girls?”

My young friend thought for a moment. “Oh, I see,” she said. “Why should only males inherit the

missing rib-why not females, too? That’s a good question.”

“I have a better one,” I pressed on, a full plan of evolutionary enlightenment now formulated in my

mind. “What kind of inheritance would this missing rib represent?”

In class we had discussed the differences between Lamarckian evolution by transmission of inherited

body modifications and Mendelian inheritance through genetic material that is subject to mutations, but

my student missed the point of my question. I explained. “Essentially, Lamarck maintained that anything

that affects your body could affect your offspring. Lift weights regularly, and your daughter could

inherit a bigger and stronger body than she would if you never stirred from the sofa. Chop off the tails of

generation after generation of mice, and eventually you should end up with tailless mice. Make an

antelope put its neck out for high-growing leaves, and its distant descendants will be giraffes.

“The problem is that generations of Jewish and Muslim males have been circumcised, without any effect

on the presence or absence of the penile foreskin of later generations. Certain breeds of dogs have had

their ears and tails cropped for hundreds of years without affecting the length or shape of the ears and

tails of their offspring. In other words, Lamarck was wrong.

“In fact, if you recall from lectures, he couldn’t have been right. Nothing you do to change your personal

physiognomy, from lifting weights to having a nose job, will affect the genetic makeup of your

offspring.” As I re-explained these basic points, I realized that, lacking a problem to apply the

information to, my student had not yet understood the important differences between Lamarck’s and

Mendel’s theories.

“Look at it this way. Suppose you had an accident, and your right thumb had to be amputated. Would

you expect all your children, assuming you have any, to be born lacking a right thumb?”

“Of course not,” said my student. Then, after a pause, “Oh, I see. You mean that for the same reason my

children would have thumbs even if I didn’t, Adam’s children would have the normal number of ribs

even though God took one of his. Otherwise, it would be Lamarckian inheritance.”

“Right!” I said. ”And there is no creditable evidence to support Lamarckian inheritance. So you’ve

actually got several problems here. First, Lamarckian inheritance doesn’t work. Why should Adam’s loss

of a rib affect his children? Second, everyone has ribs, men and women alike. Ribs certainly aren’t a sex-

linked trait like excessive facial hair or a scrotum. So there’s no reason I can think of that Adam’s male

offspring but not his female ones should be missing a rib. If the sons were missing a rib, wouldn’t the

daughters be missing one, too?

“Third, there is nothing in the Bible that says exactly how many ribs Adam started out with, or how

many ribs we should have, is there? So you have no compelling reason to believe that in taking a rib

from Adam, God left all of his male offspring one short. That’s an inference, and a particularly poor one

since it relies on an outdated theory of evolutionary change. You don’t really want to use a discarded

evolutionary theory to prop up the Bible, do you?”

I was pleased to see that my ploy had worked. My student accepted my explanations with good grace

and an active intellect. Her religion was intact, but she was learning to think about her assumptions and

to reason a bit more like a scientist. She was soon back at the human skeletons counting and measuring

other bones. With some help, and a few broad hints (“How can you tell the difference between a man

and a woman from behind, if they are the same height and have equal-length hair?”), she finally realized

that the reason she wore a different cut of jeans from the men in the class was because she is built

slightly differently.

Most human females have a relatively wider pelvis than males because the human brain (even in a

newborn) is too large to pass through a narrow birth canal. Thus, one of the reasons sexual dimorphism

is so much more pronounced in humans than in most other primates is relative brain size. (“Don’t trust

me,” I told her, “check it — the skeletons are there!”) Bigger brains require wider hips for birthing.

By the end of the course, five more students had reported to me that they too knew without having to

look at the skeletons that women have more ribs than men. Some of them trotted off to count the ribs

and came back to report that they had verified their preconceived notion. I had to stand beside them and

count the ribs two or three times before they would believe that there really are the same number in the

two skeletons.

These days I’m better prepared than I was that first year. Sometimes I bring in an extra pair of skeletons

or a medical textbook with X-ray photographs of the chest, so that the students can count ribs to their

hearts’ content. I’ve come to expect at least 10 percent of the students in each class to tell me that men

and women differ in rib count. I have conducted surveys of nearly a thousand first-year college students

who either are non-science majors or have not yet declared a major. More than 25 percent report

believing that God created the Earth within the last 10,000 years and that man was formed in God’s

image exactly as described in the Bible. Another 50 percent report being undecided as to whether

evolution is a valid scientific theory or a hoax. Only about 20 percent enter my university having learned

enough about science and the evidence for evolution to consider it a valid scientific theory.

My college classroom numbers follow fairly closely those reported in recent national polls. A 1991

Gallup poll, for example, found that 47 percent of the respondents believed that God created man within

the last 10,000 years. Forty percent believed that man evolved over millions of years but that God had a

direct hand in guiding that process. Only 9 percent said man evolved without God’s direct intervention.

In many communities, such as mine, there are ongoing, active attempts to exclude evolution from the

public school curriculum. Lecturing on evolution is an interesting challenge under these circumstances.

Take nothing for granted, I counsel my students: that is what makes a scientist. I’ll make no bones about

it: anatomic differences are what drive evolution — and its teaching.

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