Archive for development

Episode 8: Face the Face

Posted in Individual Podcasts and Transcripts with tags , , , , , , , on February 16, 2012 by Dr. Matt Bonnan

Podcast Teaser: Of all the vertebrate animals, only mammals have muscles of facial expression … why?

Transcript: You are more than just a pretty face.  Your face contains facial muscles that allow you to express and emote to fellow human beings and, some recent data indicates, even to your dog.  But in e-mail, Facebook, and other electronic media that is text-based, we often encounter first-hand how important facial expressions are and how often we are misinterpreted without these visual cues.  To prevent ourselves from being misunderstood, we have given electronic birth to the now ever-present smiley-face emoticons.

But did you ever stop to think about all of the vertebrate animals that lack muscles of facial expression?  Think about it: when is the last time a fish winked at you?  When have you seen an alligator genuinely smile?  What about a frog frowning and expressing deep sorrow?  You haven’t, of course, because most vertebrates except mammals lack facial muscles.

I often ask my students this question: why do mammals have muscles of facial expression?  The usual answers revolve around communication – that mammals are good communicators and need muscles of facial expression to get their various emotional points across.  It’s a nice hypothesis until you consider that a substantial amount of evidence from genetics, brain structure, and the fossil record show quite convincingly that the earliest mammals were nocturnal, and probably spent a good deal of time in hiding from dinosaurs.  As the old Monty Python saying goes, in the darkness nods are as good as winks to blind bats.

But we have another conundrum aside from why facial muscles evolved in mammals, and that is where did these muscles come from in the first place?  Evolution, like a lazy engineer, often doesn’t invent new structures but instead borrows, steals, and augments anatomy from already existing anatomical architecture.  One good thing about muscles and tracing their evolution is that the nerves that supply them with the stimuli to twitch and pull are very conservative.  In other words, no matter how mother nature sculpts the muscles of vertebrates into different forms and functions, the same old chemoelectrical supply lines come along for the ride.  So, we can trace the nerves and their branches to diverse muscles and muscle groups in various vertebrate animals, and using thorough comparative studies we can then determine which muscle groups are related from sharks to shrews.

So, what do comparative studies tell us about facial muscles in mammals?  During embryonic development, the jaws develop from an arch of cartilage that folds forwards.  Behind the jaw arch, a second arch develops called the hyoid arch.  Just as there are muscles that develop with the jaw arch that help close the jaws, so there are muscles that develop with the hyoid arch that accomplish similar ends.  In a shark, the hyoid arch muscles help compress the throat and push struggling prey towards the stomach.  This function of the hyoid arch muscles continues into most other vertebrates, and part of this muscle group is often called the constrictor colli.  These hyoid arch muscles are all innervated by the same nerve, and so we can follow their development very precisely in all vertebrates.

In mammals, the lower hyoid arch muscles do something very fascinating during development: they expand from the hyoid arch onto the face!  It is these muscles that become our facial muscles.  In fact, since these muscles were first identified in human cadavers long ago during the early days of anatomy, the nerve that innervates them is called the facial nerve.  This always throws my students for a loop because it does seem rather odd that the throat-constricting muscles in a shark would be innervated by the facial nerve.

So, we know where the facial muscles are coming from, but we still haven’t tackled why during mammal evolution their lower hyoid muscles would have expanded onto their face.  The answer seems to be rooted in that most fundamental of mammal products: milk.  When most mammals are born, they instinctively search for the mammary glands of their mother, often contained within teats, and begin to suckle.  To suckle effectively, one needs to form the mouth into a gasket around the teat to create the appropriate suction for extracting the life-giving fluid contained within.  To assist baby mammals in obtaining milk from their mothers, there seems to have been strong selective pressure for the expansion of muscles previously associated with swallowing and throat compression.  So far as we can tell, milk production and the evolution of facial muscles go hand-in-hand.

Once a foundation of facial muscles was established in mammals, the way was paved for smiles, winks, frowns, and smirks.  But it is fascinating to consider that what has become an indispensible part of human expression, began with a simple sip of milk.

References / Further Information

Episode 4: A Brief History of Meat

Posted in Individual Podcasts and Transcripts with tags , , , , , , , , , on October 14, 2011 by Dr. Matt Bonnan

Many of us enjoy eating meat, but few of us pause to think about how important its pre-meal form, skeletal muscle, is for vertebrate life.  Or why you eat different parts of fish and tetrapods for that matter.

Podcast Teaser: I don’t know about you, but I enjoy a good steak, especially fillet minion.  In fact, many of us enjoy eating meat, but few of us pause to think about how important its pre-meal form, skeletal muscle, is for vertebrate life.  Unless you injure your skeletal muscles, you barely notice them – of course, if you’re a body builder, you probably notice them a lot more.  But the contractions of skeletal muscles across the joints in your skeleton do everything from keeping you upright to preventing nasty falls.  Believe it or not, meat is so universal among vertebrate animals that muscles in one area in a fish do very similar things in the same area in your body.  This is because, long ago and 540 million years away, our common ancestor developed two important traits …

References / Further Information

Transcript Available Upon Request.

Episode 3: How Do You Make a Snake?

Posted in Individual Podcasts and Transcripts with tags , , , , , , , , , , on October 11, 2011 by Dr. Matt Bonnan

It seems only fitting that a podcast series called Along the Backbone should discuss the formation of the backbone in one of lengthiest vertebrates: snakes.

Podcast Teaser: Snakes are lizards.  More specifically, snakes are limbless, eyelidless, earless lizards with megakinetic skulls and well-developed salivary glands that often produce venom.  Among the many standout features of snakes, perhaps the most fascinating is how these vertebrates routinely develop a body that will have 120 or more rib-bearing vertebrae and no limbs.  It turns out that a simple but profound difference in the timing of the expression of developmental genes called HOX genes renders snakes limbless, whereas an increase in the frequency of another set of clock-like genes generates their amazing number of vertebrae.

References / Resources:

Transcript available upon request.

Episode 2: Why Pedigrees Are Not Just For Dog Shows

Posted in Individual Podcasts and Transcripts with tags , , , , , , , , , , , , , , , on October 10, 2011 by Dr. Matt Bonnan

When we talk about reconstructing the vertebrate family tree, we are also trying to develop a pedigree of relationships. This pedigree, in turn, helps us understand how anatomical changes have taken place over time.

Transcript of Episode 2: Why Pedigrees Are Not Just For Dog Shows

Our family has a miniature pinscher named Squirrelbella.  You can ask my six-year old daughter about that name.  Squirrel, for short, is no show dog, and so as much as we love this little tail wagging, occasionally yippy carnivore, we do not have pedigree papers for her.  In the world of dog shows, pedigrees are important to establish the blood line of the dogs which end up being winners.  To make future money, you as a serious breeder would want to have offspring that descended from the winners and not the losers.  In short, you want a reliable pedigree: a family tree.

When we talk about reconstructing the vertebrate family tree, we are also trying to develop a pedigree of relationships.  Formally, such a pedigree of relationships is known as a phylogeny.  A phylogeny is a hypothesis of relationships, based on data, which is open to testing and which predicts the shared common ancestry of various groups.  This means that any phylogeny is not permanent – it will either be supported, modified, or rejected when new data surface.

If our goal is to understand how vertebrate form and function has changed in different groups, or even to define what a vertebrate is, we have to understand the order and appearance of these skeletal features.  For vertebrates, we typically reach an understanding of changing function by comparing the anatomy of several related groups to see what has changed (what traits are special to our group of interest) and what has remained the same (what traits are present in all vertebrates).  This requires an understanding of the relationships of vertebrates.

We establish vertebrate phylogeny on traits, the attributes or features of particular animals.  Simply put, a phylogeny is a hypothesis of relationships established using trait data.  But traits are not self-evident – they are not labeled neatly on fossils or written on the tags of pickled museum specimens.  So, how do we select informative traits?

The simplest approach to reconstructing vertebrate phylogeny would be to survey as many animals as possible and then compare the distribution of similar traits.  We could then tabulate up all the similarities, and group together vertebrates which share more traits in common.  For example, we group all known vertebrates together because they all possess specialized back bones called vertebrae.  We could continue comparing similar traits, divvying up the vertebrates into smaller subgroups possessing more exclusive shared traits.  Fish have gills, amphibians have a moist skin, reptiles are scaly, birds have feathers, and mammals have hair.  By doing this, we would eventually have a phylogeny, a hypothesis of relationships among the living vertebrates.

Yet, not all similar-looking traits are related to common ancestry.  For example, a shark and a dolphin both have a stream-lined body form with fins.  At face value, we might conclude that these traits were evidence that sharks and dolphins shared a recent common ancestor.  However, on closer inspection, we would begin to notice some large discrepancies.  The skeletal structure of the shark is cartilaginous whereas that of the dolphin is bone.  A shark’s skin is rough and covered in tooth-like scales, yet that of a dolphin is smooth and overlies a layer of blubber.  Sharks breathe using gills, but dolphins have lungs and must surface occasionally to take in fresh air.  Dolphins nurse their young on milk from mammary glands while shark pups must fend for themselves.

Eventually, it would occur to us that, more likely, the similar shapes of the shark and dolphin were not due to common ancestry but instead to a common environment: water.  Water is denser than air, and there are only so many “solutions” to swimming fast in it.  The shark and dolphin have converged onto a similar functional solution, the streamlining of their bodies and the possession of fins, to move fast in a dense medium.  So, overall similarity is not good enough: we must be able to distinguish between traits inherited through common descent and those that are due to convergent evolution.

Traits are not static.  Under the theory of biological evolution, traits are inherited and modified during descent from a common ancestor.  Traits, the key features we are establishing vertebrate relationships on, are plastic and malleable – to wit, they change.  Although the plasticity of traits seems at first to be a stumbling block to establishing vertebrate phylogeny, it is in fact a great asset.  This is because traits have states – that is, they have an original form and one or more variant forms inherited and passed down to different descendants.

The original form or state of a trait is referred to as primitive, whereas a trait is considered to be in a derived state if it has changed from this original condition.   It is the change from the primitive to the derived trait state which reveals evolution and pedigree.  For example, one type of trait is that of possessing appendages.  The appendages of the earliest jawed vertebrates were fins.  Therefore, fins are the primitive appendage state, and modern jawed fishes would be said to retain the primitive appendage condition.  In contrast, amphibians, reptiles, birds, and mammals have changed the original condition (a fin) into a new structure (a limb with digits).  So, in this example, fins are the primitive (original) appendage type, and limbs with digits are the derived (changed from the original) appendage type.  All vertebrates possessing the derived appendage state of limbs with digits are grouped together as tetrapods.  We would hypothesize that all tetrapods share a closer common ancestry with each other than any of these animals would with fishes.

The word primitive is often confused with something inferior or less developed.  It is important to note that all living vertebrates today actually possess a complex combination of primitive and derived traits.  To dispel you of equating a primitive trait with inferiority, imagine being thrown into a tank with a hungry shark.  Yes, the shark has primitive appendages (fins), and you have derived appendages (limbs with digits), but which of you will swim well?  Which of you will be the diner, and which the dinner?

But how does one determine whether or not a trait is in its primitive or derived state?  The answer is something called polarity.  Because trait states are changeable, they have polarity, or a direction of change from primitive to derived.  Polarity is an arrow of change pointing in the direction that a particular trait evolved.

Polarity is inferred in a number of ways.  In one approach called outgroup comparison, the trait state of interest is noted over a large number of animals outside the ones being studied.  If one particular state is widely distributed over these so-called outgroups, the researcher may conclude that this is the primitive state.  This reasoning would follow from evolutionary theory: all vertebrates should share certain general trait states, followed by different descendant groups that possess more exclusive trait states.  Therefore, commonality may indicate that a particular trait is in its primitive state.

Another approach to determining trait polarity would be to note when certain anatomical features appeared during embryonic development.  The idea here is that more general, foundational trait states should appear earlier in development, whereas more derived states of various traits should develop at a later time.  For example, all vertebrate embryos develop throat (pharyngeal) slits during development, but these are only retained as spaces for gills in fishes; the slits anneal and contribute to other structures in tetrapods.  Since all vertebrates develop these pharyngeal slits before these features transform into more specialized structures within each group, the retention of these slits in fishes would be considered primitive, and the annealed versions would be called derived.

The modern biologist or paleontologist thus collects trait state data and, using specialized software, analyzes the distribution of primitive and derived trait states.  The relationships of the vertebrates are determined by how many derived trait states they share in common, and this is translated into a branching diagram of relationships: the phylogeny.

The phylogeny is established on trait states that are independent of the chronological order of their appearance in the vertebrate fossil record.  This means that the fossil record can serve as an independent check or test against the phylogeny.  Since the assigned polarities are hypothetical, the chronological sequence of when certain trait states appear in the fossil record can either support or call into question whether something is indeed primitive or derived.  For example, the earliest jawed vertebrates in the fossil record possess fins, whereas it is not until much later that we see the first fossil vertebrates possessing limbs with digits.  This observation independently bolsters the assignment of fins to the primitive appendage state.  Through careful analysis of both traits and the fossil record, a reasonably consistent phylogeny of vertebrate animals has emerged, even if my little min pin doesn’t appreciate it.