Why do men have nipples? - A great question with a simple answer

Human bodies are amazing things.  You have eyes that enable you to detect sunsets, ears to hear a symphony by Beethoven, touch receptors to register the feel of an exquisite silk, and a nose to detect the awe inspiring smell when your youngest son releases the built up gas from a recent digestive experience.  In addition to those external things, you have organs that are all squished into your body cavities that enable you to do all sorts of amazing things including breathing, moving, and, to continue a train of thought, producing the gas that enables your youngest son to fart.  

The amazing things the human body can do are normally not limited to one sex.  Obviously some things can only be done by women including providing the nourishing space where babies to grow.  Other things are the only available to men like, well, finding humor in fart jokes after their 39th birthday.  Some body processes, like producing milk, are normally reserved for women but can be performed by men should those men have the inclination, money, and access to a willing medical professional.  While not likely, this is entirely possible due to the fact that men and women produce the same hormones in their bodies, just not normally in the same concentrations.  Change the concentration of a few of those hormones and interesting things can happen.

This explains not only the unusual situation of male lactation, but also a few other things as well.  For example, you may have noticed that men have nipples.  Upon noticing this you may have also noticed that, unlike women, they don't feed babies with them (unless, as previously noted, they really want to).  Once you have noticed both of these things (and by the way, congrats on all the noticing) you will probably have asked yourself the age old question of "why are they there if they don't normally serve any function?"

I have heard many different responses to this question ranging from the completely scientific and well researched to the less scientific and more bro-tastic, "Dude - chests would look weird without them so, you know...nipples."  The real reason men have nipples actually quite simple:  because women have them.

That isn't a very satisfying answer, so lets get a little more detailed.  To start with, lets work with an analogy.
My sons have a Lego set that allows them to build 3 different things.  Follow one set of instructions and you get a car, another set and you get a boat, while yet another produces a plane.  All of the pieces for each structure are included in the kit and you simply have to selectively ignore the pieces you don't need.  One of the slick parts of this set is that the first bit of the instructions for all three things is the same.  It is only after page 20 that you are forced to make a decision as to which structure you are going to build.      

The different human sexes are built pretty much the same way as our Lego set.  Lots of individual pieces are assembled and come together to make up a human and males and females are essentially two separate structures built from the same kit.  For the model with the uterus, continue on to page 21.  For the model with the dangly bits and the fart jokes, skip to page 45.

To put it slightly more scientifically, the vast majority of instructions (genes) used to build a male are also found in the female body and vice-versa.  To drive this point home a bit, in the not too distant future it is going to be technically possible to make male and female clones of yourself.  Flip a few genetic switches in the developing embryo and viola, female clones from male donors.            

Two things of note before we continue.  First, all humans have 46 chromosomes - half come from mom and half from dad.  Due to some interesting genetic and evolutionary issues that are well beyond the scope of this post, the half that come from mom are pretty much identical to the half that come from dad.  This means that both moms and dads carry around instructions for how to build nipples.

Second, all babies start out female and it is only with a well timed release of testosterone that the baby begins its transformation into a little dude.  So to go back to the Lego analogy, the first 20 pages of instructions for human babies includes building the basic body parts and setting the stage for a little female baby.  Everyone, even Dwayne "The Rock" Johnson, started off as a little girl.   

"What does this have to do with nipples?" you might ask.  Should you ask that question I would respond, "Everything."

You see, ever since mammals evolved to produce milk from modified sweat glands, nipples have been a fairly important part of the body plan for most of them.  And because half the genes to produce those nipples come from mom and half from dad, this means that dads had to carry around nipple genes whether they wanted to or not.  Since the dads had these nipple genes and it seems the nipple building directions were in the first 20 pages of our instruction manual, they built nipples even though they were not going to produce any milk.

These genes and their nipple instructions are only part of the reason men have them.  The other part is evolution.  To oversimplify a relatively complex concept, evolution works by natural selection.  This means that those critters that are well adapted and have no major physical issues will survive better and make more babies than those with severe problems.  In the grand scheme of things, nipples are not really that detrimental to the male who has them which means that he is still going to survive and he is still going to get to mate and have babies.  If he survives and has babies, his babies will then also have nipples.

To sum up a bit, human embryos are basically identical until 8-9 weeks of development.  At this time the genes that determine maleness will turn on in males and those same genes will stay off in females.  During this initial 8-9 weeks of development the basic body shape happens including two arms, two legs, head, heart, internal organs, skin (nipples), etc.  It is only after this part that embryos branch off to become male or female.  So men have nipples because they have genes that produce them and these genes and their resulting superfluous nippleage don't seem to get in the way of survival or mating.

By the way, "Superfluous nippleage" would make a great name for a punk band.  

As a quick aside, a few other interesting things can happen due to the similarities in hormones and genetics. One of the most common issues is male breast tissue.  This normally develops during puberty and can come as quite a shock to boys who were not exactly expecting breasts, even if they are super small.   No one expects this because no one ever tells boys that this is even a possibility.  The sex talk in 6th grade and health class in high school often tell you that you will develop a deeper voice, hair in several novel places, and muscles due to hormone surges.  They inconveniently leave out the whole, "you might get small breasts" thing, probably because no one wants to tell a 14 year old boy that he and his 11 year old sister are experiencing something in common.

The male breast tissue (normally called "breast buds") is often sensitive and can be painful if you happen to be a goalkeeper and someone hits a soccer ball into your chest at approximately 400 miles an hour.  That may or may not have happened to me in high school, but here's a hint:  It definitely did.  Typically you will notice a is a small lump under one or both nipples which is completely unnoticable by everyone except, of course, the young man in question.  They develop because during puberty the normally delicate balance of hormones can be thrown out of whack resulting in a little bodily confusion.  Your male body recieves a dose of some hormones and thinks to itself, "Hey - I've got these nipples here, may as well do something with them."  After a few weeks, months, or years, the hormomes will go back to normal and the breast tissue will disappear leaving only fond memories.

Human bodies are amazing, strange little things, aren't they? 
    

How and Why did Fish Start Walking? A Short Evolutionary History of Limbs.

So, let’s say that you are a fish.  You have gills which work amazingly well for taking oxygen out of the water and helping you not die, and you have fins which help you swim around and avoid predators, also helping you not die.  Your body is streamlined and quite good at cruising through the water and you have paired fins at your head end and tail end which help you change direction quite easily.  These are amazing adaptations which came about through millions of years of evolution, and they are still very helpful to those critters that didn't head off down the path to living on land. 

If we travel back in time to the end of the Devonian period (around 385 million years ago) we would find that there were two types of fish.  One group, the ray finned fish, was very popular and had thin, beautiful fins which were very helpful for swimming in the open water.  They are still the most common type of fish.  The second, much smaller group, were called lobe finned fish, and they spent most of their time hanging out on the floor of lakes, rivers, and streams rather than cruising open bodies of water.  Despite being fish, these lobe finned critters had adapted to a life of bottom feeding and were not nearly as good at hunting or evading predators in the open water.  Their bodies were less streamlined and more flattened because flat is good if you are a bottom feeder.  The lower your profile the less likely you are to be eaten, and being eaten is never good.  

Eusthenopteron (right), a critter that lived about 385 million years ago, is a great example of this flat, lobe-finned critter.  It had a low profile and tended to spend all its time hanging out in shallow seas.  Its fins were still somewhat adapted to swimming, but it is currently thought that it had the ability to "crawl" along the underwater rocks.  The bones of its fins are the blueprint for the bones of your arms and legs and are thought to be the start of the evolution of all tetrapods, or 4 limbed animals.  You can see the pattern in the picture at the top of this post with one bone (pink) attaching at the shoulder of the fish, 2 bones (blue and yellow) attaching to the pink one, and lots of little bones below those two which would be similar to the fingers of later tetrapods. 

Tiktaalik (left), discovered by a team from the University of Chicago led by Neil Shubin, is a 375 million year old critter which is even more amphibian-like.  Shubin sometimes jokingly refers to it as a "fishapod" because it still has characteristics of fish but is well on its way to having 4 limbs like a tetrapod.  It is a great example of a transitional fossil.  It is more flattened than Eusthenopteron and spent most of its time in the shallow rivers during the late Devonian period.  The bones of its limbs have evolved to become more arm- and leg-like, but the main difference between it and its other, more fishy relatives, is that it had the ability to rotate the lower bones in the fin like a wrist.  And, based on the fact that it has a sturdy shoulder structure, this critter definitely used these limbs to support its weight.  Another aspect of this fishapod is that it could move its head from side to side and up and down, something that fish cannot do but is a characteristic of tetrapods.

Following on the heels of Tiktaalik comes another of the transitional critters that helped pave the way for tetrapods to take over land.  Acanthostega (right) lived about 365 million years ago and was, most would agree, more salamander-like than fish.  It had definite limbs - no fins for this one - but due to the structure of the bones it was unable to put any major weight onto those limbs.  It was still, like Tiktaalik, mostly aquatic, but the size and strength of the shoulder bones and the fact that its pelvic (hip) bones were attached to the spine instead of free floating suggest that it relied entirely on these limbs for movement.  Baically, it "walked" along the bottoms of the rivers and streams, only occasionally venturing out of the water. 

Around 360 million years ago, Ichthyostega (left), came around.  At a little over 4 feet in length, this is one of the largest of the transitional critters we are going to discuss.  It was one of the first of the transitional tetrapods to be discovered and for quite a while it was the only transitional tetrapod we had.  It has a fishy tail but everything else about it is amphibian.  It had lost its gills, and its tail fin is significantly smaller than would be necessary for swimming.  The bones in the front and rear limbs show the "one bone, two bones, little bones, fingers" pattern that is characteristic of all tetrapods, and its shoulder, pelvis, and backbone are so robust that scientists agree that this thing definitely spent quite a bit of time on land.  It most likely only used its front two limbs to walk around.  It is still not considered an amphibian, but it is the most amphibian-like of all the transitional critters we have seen so far.

Eventually, around the same time Ichthyostega was cruising around, the amphibians - the first true tetrapods - evolved.  

Based on the evidence, it is obvious that all land based animals with 4 limbs started out in this manner.  We, along with dogs, cats, squirrels, whales (yes, whales), dinosaurs, birds and other animals with 4 limbs owe our existence to a group of critters that, for some reason, left the water and invaded land.  Why would this happen?  Wasn't the water full of things to eat and places to live and other fishy things to mate with?  Yes, it was - that isn't the problem.  The real problem is that, from time to time, the "things to eat" category included fish that would rather not be eaten.  Predators were everywhere and some of them, like the armored Dunkleosteus (right), were massive and not terribly picky about what they ate.  

Imagine that you are a small lobe-finned fish living about 380 million years ago.  Even though your large fins are constantly mocked by the other, more svelt and popular ray-finned fish in the area, you are perfectly content living near the edge of the water, crawling around on the rocks, searching for food.  You come across a tiny shrimp and are about to dig in to what will most likely be the best meal of your young life when out of the corner of your eye you see a shadow.  This shadow might be nothing, a piece of floating wood or one of those ray-finned meanies, but it might be one of those massive predators that the Devonian is so chock full of.  You decide not to chance it and run away, but since you are so small compared to the predator you won't get far before your latest meal becomes your last meal and you are killed and eaten in some horrible way.  What does a small lobe-finned fish do?  The only thing that you can do - scamper into shallower water to avoid this predator.  

You scoot to your left to begin the hopefully life-saving journey away from the predator, but NO!  It has seen you!  Crawl little lobe-fin, crawl!  So you put on a little burst of speed and pull out your trump card.  Your stronger fins - the ones the other fish make fun of  because they are so large and weird - allow you to leave the water to avoid being lunch.  You scamper up out of the shallow water and onto land just as the predator makes its move, and due to the fact that you are no longer in the water the predator makes a quick change of direction and eats one of the ray-finned fish that were unable to get out of the way.  You perch on the rock for a second to gloat and make sure the predator is gone and then dive back in, ready to resume your search for a little shrimpy snack.

This predator avoidance strategy is one of the more likely reasons for tetrapod evolution.  If you are not in the water you are not going to be eaten by anything in there.  As it turns out, until amphibians evolved and truly started hanging out on land, most land animals were more like millipedes and other insects and therefore were not really something the first tetrapods needed to worry about.  At this time, land was the safest place to be if you could get there.

This is just part of the story of how our ancestors got up and changed the world.  I'll do a post on lung evolution in the future to help round out the major transformations needed for life on land.  

You are Awesome. A Quick Evolutionary History of Your Accomplishements.

You are a winner.  I'm not just saying that to garner your affection, although I am not against that, but rather because it is the truth.  You are a winner because, to begin with your most recent amazing achievement, as a sperm you beat out an estimated 300,000,000 other sperm for the coveted title of being you.  This race was long and arduous and, while other sperm made fateful errors and turned the wrong direction at the fallopian tubes or were ensnared by the vaginal mucous before even reaching the uterus, you persevered and ended up fertilizing that egg.  So you got that going for you, which is nice.

Additionally, even before you won the sperm race, you were descended from winners.  Your direct ancestors were all strong enough and fast enough to be able to survive until it was time to mate, and when it was business time they were attractive enough to actually get a mate.  Had even one of your ancestors been a bit of a dud with less than adequate skill in the survival department you wouldn't be here now.  You are the product of over 3.8 billion years of evolution.

But this planet of ours, where that evolutionary process happened and continues to happen, seemed, at times, to be downright antagonistic to the squishy things that were attempting to flourish on it.  From volcanos belching lava and tiny bacteria spewing forth nasty toxic gasses to meteors impacting the surface and making life significantly more interesting than it already had been, there have been times when it was incredibly difficult to be a living thing.

Life, and therefore your ancestors, almost come to an abrupt end on five different occasions in the past.  It was during these Mass Extinction events that your ancestors survival skills were really put to the test.  The first of these occurred about 450 million years ago and wiped out 60-70% of all species.  The second, around 370 million years ago, resulted in the deaths of about 70% of all species.  The third, the Permian extinction event, is the subject of the rest of this post and will be discussed in detail in a bit.  The fourth occurred around 200 million years ago and wiped out 70-75% of all species including many of the competitors of the dinosaurs, helping them rule the land for another 140 million years or so.  The fifth happened about 65 million years ago and is the one most people know about.  It was responsible for not only the deaths of all of the non-avian (not birdlike) dinosaurs but also the extinctions of around 75% of all species.

The third event, the Permian extinction, occurred around 250 million years ago and is known as the Great Dying due to the fact that almost 95% of all species went extinct. Your great-great-great (x140 million or so) grandparents, the reptiles, had evolved only about 70 million years prior and were attempting to make a go of it on this crazy death planet.  The reptiles almost immediately split into two main types, the synapsids and the diapsids.  These were sort of like sibling groups in that while they were similar in a lot of ways, there were also some key differences, the most obvious of which, assuming you have access to these things, is the number of skull holes (or, should you wish to impress people at a tea party, temporal fenestra) each had.  As in all sibling rivalries there was some competition, and there was a winner and a loser.  The winner, at first, were the synapsid reptiles which grew to fairly decent sizes and were essentially the kings and queens of the Permian period.  Some, like Dimetrodon, could reach 15 feet in length with a massive sail-like crest on its back that allowed it to regulate its body temperature.  These huge reptiles are often mistaken for dinosaurs because of the large size and scary teeth, but they predated dinosaurs by around 70 million years.

This sibling rivalry lasted, with the synapsids in the ascendency, until the Permian Extinction event.  The synapsids, and indeed almost everything else, were basically wiped out during this time, and this opened up the door to their rivals, the diapsids.  The diapsids had bided their time while their synapsid relatives dominated, staying small and inconspicuous.  After the Great Dying the diapsid reptiles began their dominance, leaving the synapsids to the fate of small stature and inconspicuity, assuming that is even a word.  Spell check doesn’t have a problem with it so I am leaving it in.  

The diapsid reptiles eventually went on to evolve into such hit animals as the pterosaurs, crocodiles, dinosaurs and, eventually, birds.  The synapsids hung back and began the process of evolving to survive.  They stayed small and adapted to the cooler, darker night conditions.  They developed hair to insulate themselves and assist in temperature regulation.  They developed more refined jaw bones and modified ear bones to aid in hearing.  They began to produce milk from glands on their stomach to feed their young, and when the time was right, roughly 65 million years ago after the Cretaceous mass extinction event, evolved into all of the groups of mammals that we have today.  Dogs, cats, platypuses, squirrels, elephants, whales, primates and eventually you - all of them descended from that one group of reptiles that refused to die when almost everything else did.  

And that is why you are a winner.  Your ancestors survived everything that this cranky old Earth and solar system could throw at them and came out on top.  You are #1.  

Or, to put it like one of my college biology professors:

Your 3.5 Billion Year Old Family Tree

One of the coolest bits about teaching students evolution is that they get to learn that they are related to every living thing.  For example, Dunkleosteus, the critter pictured above, is your 185 millionth great-grandparent.  If you take your family tree even further back you can find that you and that oak tree outside share an ancestor from about 1.4 billion years ago. 

Evolution as a Story - Part 1

I love studying evolution.  There are many reasons for this, but one of the most compelling is that it is the story of how we, as humans, came to be.  It is a story of adaptation and extinction, success and failure, and if told correctly it can help students to a greater understanding of just how cool this whole evolution thing is.  I mean, we are related to fish and oak trees for crying out loud.  How cool is that?  I plan on writing about how I tell the story of our evolution in a future post or two, but for right now I'd like to share a few things I have made which help me tell this story to my classes.

First, I have a short textbook that I wrote for use in my Zoology class which focuses on the evolutionary story of the major groups of organisms including invertebrates, fish, amphibians, reptiles, birds, and mammals.  Each 4-6 page chapter deals with a particular group of animals and explains the main points of their evolution in what I hope is a humorous yet educational manner.  This book is available for sale as a digital download at my Teachers Pay Teachers store should you be interested.  There you can download a free sample to see what it is like as well as purchase either individual chapters or the entire textbook, should you feel so moved.  And because I am such a stand-up kinda guy, for a limited I am giving away the evolution chapter of my textbook for free!    

To supplement the textbook I put together graphic organizers for the chapters on Reptiles, Birds, and Mammals.  I give these out to the students so they can use them to take notes while we discuss these topics.  Click the links to go to my store to download free higher resolution copies of these organizers.  (The images below are just JPEGs because I am relatively new to blogger.com and haven't figured out if it is possible to link files to posts.)  

Finally, I have made Prezis which use the graphic organizers as the main focal point to help the students follow the story we are discussing.  The links to the Prezis are below.  If you are a regular user of Prezi, feel free to make a copy of these presentations for use in your classroom.  If you are not a regular user of Prezi, it is an amazing, free, online presentation tool that I can't recommend highly enough.

Reptile Evolution Prezi Link

Bird Evolution Prezi Link

Mammal Evolution Prezi Link

New Video Series from PBS - Your Inner Fish

I am more excited than I probably should be for this new series from PBS.  "Your Inner Fish" is one of my favorite evolution books and now PBS and the author, Neil Shubin, have teamed up to make a 3 part series on the subject.  It is slated to start on April 9 at 10pm Eastern/9pm Central.  Check out the website for more information and a few videos.  

Major Transformations - Jaw Evolution

I like my jaws.  They are, perhaps, my favorite thing to use when I am chewing stuff.  They are also incredibly helpful in the whole talking department, and given that I am a teacher this is a fairly important adaptation for me to have.  Jaws are really swell.

But how did we come to have these neat little chompers anyway?  It turns out that the story of jaw evolution is fairly simple.  You see, once upon a time about 530 million years ago, there lived a group of critters named the Agnathans.  These were a strange sort of organism what with their long bodies and sucker faces and no paired fins, but they were some of the first fish to grace our fair ocean and for this we have to give them credit. 

They were a happy sort of fish (I assume) and lived their lives happily filtering their breakfasts, lunches, and dinners through their happy little sucker mouths.  The water would flow in through their open face hole and then out again through the gills.  This provided them with both food and oxygen which are good things to have if you want to not die.  And not die they did.  Or didn't.  Or didn't not do.  Or whichever combination of words means that they survived for quite a long time and can, in fact, be found alive today.  The relatively gross Lampreys and the downright vile Hagfish are both examples of Agnathan (jawless) fish.

Those little gills that they used to pull oxygen from the water are all feathery and frilly and, if taken out of the water, will collapse in a little less than frilly pile.  To combat this, these jawless fish used special cartilage bars called Gill Arches to give the gills a bit of backing.  These gill arches served the purpose remarkably and kept those little frilly gills open and extracting oxygen, but the relative stiffness of these gill arches could also be viewed as a bad thing.  Since they couldn't flex too much the fish had to rely on continuous movement or the occasional currents to keep water flowing over their gills.  

This constant motion was a major energy drain and could have been a bit of a problem for these early critters.  No matter how streamlined you are, constant movement is going to require more energy than simply hanging out.  If you will allow me a bit of gross oversimplification, after a bit of time a mutation or series of mutations in the genes that regulate the formation of these gill arches came about and, quite literally, changed the world.  The gill arches became hinged so that they could be a bit more flexy.  This added flexibility gave those critters with these mutations an advantage over their less flexy peers.  They could now pump their gill arches like a bellows and bring flowing water over the gills without having to move around much.

So now we have flexible gill arches.  You might be asking, What does this have to do with my ability to eat a cow?  Actually, it has everything to do with your ability to eat a cow, or celery, or anything else for that matter.  You see, it was these flexible gill arches that eventually became the jaws.

In another gross oversimplification, over time the front gill arches became more flexible and pushed forward a bit.  The flexible gill arches in the front linked up with the mouth and gave the fish a tool for trapping food.  Now the fish could flex the rear arches to bring water into the mouth and over the gills, and it could flex the front gill arches to snap completely shut to catch other critters for food.  The jaw was now basically complete - an apparatus for taking oxygen out of the water was turned into a weapon.

For a more complicated version of these events complete with fancy science words and a lot more detail, go here. 

Teaching Evolution Part 1: Increasing teacher knowledge

Last night at the Creation Museum, Bill Nye the Science Guy debated Ken Ham the Young Earth Creationist on the question "Is creation a viable model for origins?"  I don't want to spoil it for you because that ruins the fun of watching, so I'll give the answer in Spanish:  "No."

The debate over creationism and evolution has been going on for quite a while, and the legality of teaching creationism in public schools has been addressed by US courts at a variety of levels.  Thankfully, the courts have almost always sided with science and said, in no uncertain terms, that you cannot advocate for religion in public school classrooms, and creationism amounts to doing just that.  

That doesn't make evolution any easier to teach, though.  While many students are more than happy to learn about where they came from there are those that offer a bit more of a challenge.  Some students state that they don't believe in evolution but they are willing to listen and memorize things for the test.  Some refuse to participate when the evolution unit rolls around.  Others push it a bit further and engage in debates with their teachers and fellow students, often coming in with prepared remarks often given to them by parents of pastors.  Believe me - I have been given my fair share of pamphlets, Jack Chick books, and prayer cards over the years and I live in an area where this is not a major issue.  My colleagues in unnamed southern states that rhyme with “Malabama” and “Mennessee” have had whole groups of students form prayer circles in class, turn their desks around and read their bibles, and engage in mass walk-outs to protest the teaching of evolution.    

So given the fact that this subject can be controversial, how is a teacher to deal with students that find it difficult to accept the theory of evolution?  It isn't an easy thing to do for a wide variety of reasons, and in a series of posts I am going to attempt to address this by sharing the strategies that have worked for me over the years.  You may find that you have similar issues, or, should your issue not be covered here, feel free to leave a comment and I’ll tackle it in a future post.  

So lets get out of the gate here with the thing that is simultaneously the easiest and most difficult thing to address:  Teacher knowledge of evolution.

In this day and age of Common Core, state mandated testing, RTI, reading and writing across the curriculum, and the many other initiatives teachers have to deal with on a regular basis, most teachers are highly educated in pedagogy (which totally sounds like something you could be arrested for) but not as educated in the content of our discipline.  Because almost all of our inservice time is taken up learning how to implement these initiatives there isn’t a whole lot of time for us Biology teachers to further educate ourselves on content knowledge.  We all come out of college with a bachelors degree which is essentially the equivalent of a black belt in martial arts.  It seems like it is a real accomplishment, and it totally is, but it basically just proves that you have the ability to learn things and you are now ready for the next level.  It does not necessarily mean that you have mastered anything.  

As an example, I felt that I had a really good grasp of cellular respiration because I did well in cell and molecular biology and passed my tests and did well in the lab and was, in general, a ferociously amazing person.  Sometimes it physically hurt to be as awesome as I was.  When I first taught cell respiration to my students I realized that I was lacking a bit in the general knowledge department.  “Where do those NAD+ molecules come from?” inquired one student.  “Why does adding electrons to it suddenly make it NADH?” asked another.  “What, exactly, causes a cell to switch from aerobic to anaerobic respiration?” demanded yet another.  “Do you pick your own clothes out because those pants don’t go with that shirt,” said a 4th who had clearly stopped giving any semblance of a crap about cell respiration.

Aside from explaining that, yes I do pick out my own clothes and yes these pants did go with this shirt, thank-you very much, I couldn’t answer these questions.  I knew what NAD+ and NADH were, and I also knew what anaerobic and aerobic respiration were, but I had no idea how to address the specific things these kids wanted to know.  It was only after doing a bunch of research and asking more experienced teachers that I learned the answers.  Evolution is similar but can be so much worse.  When have you ever been told by a student that they don’t believe in cellular respiration?  

We all know the basics of evolution.  We know who Darwin was and all about his journey on the HMS Beagle.  We are aware of fossils and probably can name some of the cool ones like Archaeopteryx and Basilosaurus.  We can read and interpret cladograms and help the students determine a common ancestor.  But often our knowledge is lacking when it comes to the details that help transform evolution from a series of events and facts into a coherent story, and it is this story of life that makes biology amazing and comprehensible to the students.

As I mentioned before, addressing the lack-of-knowledge issue is both the easiest and the most difficult problem to tackle.  It is easy because all it takes is a some time to read books and articles, watch videos and debates, and discuss with your colleagues the things you are having trouble with.  It is also difficult because it quite a bit of time and, as already mentioned, time is not a luxury most of us have.    

To get you started on the journey, I have added links to many places where you can begin to enhance your understanding of evolutionary biology.  Some are great for classroom use while others might border on the NSFW (Not Safe For Work).  Take some time and enhance your knowledge of evolution. I'll update this list as I find/remember resources. If you have any ideas, chuck 'em into the comments and I'll add them in.

Talk Origins Archive - One of the oldest creation/evolution sites.  It has basically all of the information you will ever need to answer almost any question a student might have.  Spend enough time on this site and you will most likely never worry about teaching evolution again.

Ẅhat Next? - John Kyrk - An animated timeline of the earth showing basically everything that happened in the evolution of both the universe and the life in it.  Move the slider along and watch tiny animated atoms, fish, dinosaurs, primates and everything in between inhabit your screen.

Pharyngula - a blog by popular researcher/blogger PZ Myers.  Often irreverent and potentially offensive, but almost always worth checking out. (Occasionally NSFW for language),

Your Inner Fish - A book by Neil Shubin.  Starts off with a wonderful description of the work involved in discovering Tiktaalik and continues with an easy to read and totally informative look at how evolution has shaped the human body.  Should be required reading for anyone interested in evolutionary biology.

Understanding Evolution Website - a repository for teaching materials including activities and  labs, as well as resources for teacher education.  

Flatworm sex

Sex.  It's how babies are made.  Whenever a student asks about sex in class I always begin my answer the same way:  "When two people/lions/jellyfish/etc. love each other very much, the engage in a special hug...(pause)...and then babies come out!"  But the truth is that not only is love not always a part of it, other individuals are not always involved.  Critters can reproduce many different ways, and students are often surprised to find that the world of animal reproduction is incredibly diverse.  

Below is an excerpt from the Zoology/Evolution text that I wrote.  It deals with one of the many methods of reproduction and it involves flatworms, or to use their correct phylum name, Platyhelminthes.  The students have already read the previous section of the chapter dealing with the flatworm's incomplete digestive system and a few other physiological tidbits, but this is the bit that I normally get the biggest response from.
  

 Flatworms can typically reproduce sexually, but they are mostly hermaphrodites which means both types of sex organs are present in each organism.  This is a great adaptation and makes reproduction so much easier.  Let me explain.  Imagine that you are a male worm and in the mood for a little reproduction.  You cruise out into your little part of the puddle in the hopes that you might meet the female worm of your dreams.  You know the kind - all flat and sexy.  Anyway, your little auricles tell you that there is another flatworm a few centimeters to your left and you begin your long crawl over to her.  On the way over you are working on your lines, fixing your pharynx so it is not dirty from your latest dinner/poop, and, in general, getting ready to mate.  When you get over to her you begin the courtship process and are immediately pushed off because the love of your life happens to be rather more male than you had expected.  You might find that you enjoy this other male's company and that you even support the same wormy soccer team, but alas, tonight is about reproduction and another male simply will not help in this matter. 

So you move on to the next worm in the pond who also happens to be male.  The same is true of the next 3 worms you try your little wormy pick-up lines on ("Hey there - I couldn't help but notice the cleanliness of your mouth/anus and I was wondering...").  Hopefully you are starting to see that it can be a bit of a problem to find a mate if the population is split into males and females.  With hermaphrodites you don't have to find a female worm or a male worm, you simply have to find another worm.  All worms are potential mates.  Not that they will all be willing to mate with you, but the possibility is there.  This is especially helpful if you happen to be a parasite and are literally stuck to a space on the intestinal wall. 

It is a little shameless self promotion, but if you like this sort of thing and are looking for something that is entertaining and informative, check out the rest of the text that I wrote for my Zoology/Evolution class at my store on Teachers Pay Teachers.  Available here.