Medical Research on Animal Models – Where Do You Stand?
Our self-aware cousins
This weekend I heard an incredibly interesting story on NPR's This American Life titled "Almost Human Resources" (Act 3). The story was all about the issues surrounding chimpanzees in the human world surpassing their usefulness and how we should care for them. Apparently this now includes retirement homes with TVs.
This story, along with a recent tangential debate over at Southern Fried Science and PETA's "sea kittens" campaign, sent my mind down a familiar path - one that anyone working in biology inevitably travels from time to time: the ethics of animal research for science.
There have been myriad writings, books, movies, discussions, and laws surrounding the practice of using animals for research. I'm sure most of us in the science world have come to very similar conclusions on the subject, though we may vary widely in the details.
Nonetheless, I'm very interested to hear where YOU, my readers and my fellow scientist peers, currently stand on the subject. I would like this post to be interactive.
First, I'd like to give my own thoughts.
In general, I view all living things as uber-complex organic robots (humans included). All life is amazing, precious, and beautiful - from bacteria to humans - but I still see us all as robots, running our nearly unfathomable genetic programs, developmental processes, and higher-level emergent programs of conscious and sub-conscious thought.
Mirror Test
At the same time, I feel - for no rational reason really - that consciousness and self-awareness inherently grant those that harbor them the right to live relatively free from human induced suffering. This is a feeling. We all feel it, at least for humans. We feel the immorality of conducting experiments on other human beings (though this was not always the case). Why? Because it's...just...wrong.
It's for this reason that I'm completely opposed to any medical research on chimpanzees or any great apes. There is no doubt that our great ape cousins share many if not most of our own emotional and sensory perceptions, as well as similar intellectual abilities (similar in type - not necessarily degree). For all intents and purposes, I see them as people. Not human people. Not anthropomorphized animals. But sentient to semi-sentient beings.
It's hard to measure degrees of self-awareness and know whether another creature has it. But the classic mirror test is one simple way to find when the answer is a clear yes. As of right now, great apes, dolphins, elephants, and at least one bird species, the magpie, have passed the test and shown that they have some understanding of "self."
If a creature can have any understanding of what is being done to "them," I am completely against it. Recently Orac at Respectful Insolence posted on the discontinuation of using dogs for teaching surgery techniques. He caught some flak from a few commenters for showing an emotional relief that dog use was being halted - at least partially because he loves dogs. As if any decisions on the use of other beings for our own benefit could be arrived at using only reason!
No - we as humans place some inherent value on consciousness, on self-awareness. Dogs may or may not be "self-aware" as defined by behavioral scientists. They can't pass the mirror test, but anyone who has had a dog knows that they clearly experience something akin to guilt, and a whole host of emotions similar to those of our own (I'm being careful here not to anthropomorphize). They know when they have done something wrong.
As any behavioral biologist, psychologist, or cognitive neuroscientist knows, there is no clear dividing line between conscious being and automaton. What about rhesus monkeys and the other more "primitive" primates? I personally feel that much monkey research - particularly those studies on the cutting edge of such diseases as A.I.D.S. - are critical right now. However, I also know that I could never be one to perform such studies. There is a mental hypocrisy here in my own mind. I would feel wrong performing primate research. But I support it to a limited extent.
But for some animals, it seems clear when they are well beyond that gray fuzzy line. Xenopus frogs, as far as any observation or measurement can tell, are much too dumb to have any sort of self-awareness. The same can be said of mice or rats. They simply do not have the cognitive capacity - the hardware - to generate emergent properties like self-awareness as we know it. It seems more than clear to science, I believe, that these creatures are fuzzy automatons. I have performed studies (using incredibly regulated and humane methods) using these creatures, and I have no qualms about it, so long as the use of animal models are absolutely critical to the study at hand. Hundreds of thousands of lives have been saved or vastly improved by such studies. Few people alive today (in America at least) can imagine what the state of human health would be without mice and rat studies.
And just to go one level further "down" the evolutionary ladder, consider fish.
Fish are NOT "sea kittens." We understand at least at a basic level what overall types of brain structures and neural pathways are required for higher cognition. Fish do not have these structures. They are insanely complex, from a genetic standpoint. They are beautiful. They are unimaginably important to the ecosystems of the earth. But they are still slimy scaly robotic automatons incapable of "suffering" in any human sense.
And invertebrates? Well, they're clearly organic machines. Would any of you really argue otherwise?
However, with all of the above being said, I often think about how barbaric people were only a generation ago (or sometimes less), and I wonder which of my beliefs will be considered equally barbaric by the next generation. As Richard Dawkins mused in "The God Delusion," perhaps animal rights is the issue upon which our generation will be judged to have sinned. Perhaps our ancestors will cringe at our actions (while praising the 500 year lifespans our research has given them - kidding).
What do you think? Take these polls and leave your comments below.
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Under the Sea 3D – A Stellar Review
Under the Sea 3D
This weekend my wonderful wife arranged a date night for us. And how awesome does it make her that it consisted of the single most breathtaking documentary I've ever seen - "Under the Sea 3D," a stroll through the evolution of life at the NC Museum of Natural Sciences, followed by a heaping plate of crab legs at the 42nd St. Oyster Bar in Raleigh? (no the irony of that last part is not lost on me - but hey - I loves me some crab legs!)
This post is both a review and a shout out to everyone who has not seen "Under the Sea 3D" at your nearest IMAX to immediately drop what you are doing and go watch it (check out its nifty flash site as well).
I'm not being overly hyperbolic here - this film (directed by Howard Hall) is utterly stunning.
There is basically no narrative in this film. But for what it wants to accomplish, I don't think any documentary I've watched has achieved its goal so succinctly.
The film begins with nothing more than sequence after sequence of mesmerizing coral reef habitats and creatures. It's narrated by Jim Carrey (who is great - I found myself forgetting that it was even him most of the time - there were no characteristic Carrey antics here).
But the key to this film is in the fact that the footage itself leaves you begging for more. Everyone in the theater watched in wonder - their mouths forced open by the alien creatures - usually only realizing later that they've been slack-jawed like goons for five minutes. The three dimensionality is pulled off to such a great extent that the creatures seem like they are moving and living mere inches from your face. I have never been scuba diving (and can't due to my marine unworthy inner ear), but I have been snorkeling - and I consider it one of the most amazing experiences of my life. That being said, the detail in this film far exceeded any real-life ocean experience I've had.
Each of the reef scenes is so filled with action - shrimp scuttling in the background, various fish doing their things, corals waxing and waning in the current - that you literally will want to watch it again just to focus on different aspects of each scene (not to mention the fact that the IMAX screen fills your entire field of view - it's impossible to see it all in one sitting).
Aside from the imagery which is hands down among the best I've seen, the conservation message is presented in the absolutely perfect way for its target audience (basically - everyone in the world and especially kids or the uneducated). Conservation or the ills facing the marine world are not even mentioned until your mind has been boggled by the crazy critters of the sea.
Only after bringing you into a state of constant awe does Jim Carrey begin hinting that things aren't alright. The message ramps up to the inevitable images of dead reefs, bleached by ocean acidification. However, I don't think it ever became overly preachy.
Under the Sea 3D
In fact the conservation message ended on an overly optimistic high note (overly from a scientific perspective) but one necessary if we ever want the general populace to care. Basically it paints the current state of the conservation movement as a hopeful paradigm shift in human society. It plainly states that humankind is beginning to realize its mistakes and that most people are coming around. Whether or not this is true is irrelevant because it leaves you thinking "hey, caring about CO2 and the oceans and biodiversity is the normal smart thing now. I want to be part of the informed and enlightened crowd. I want to care too."
In other words it doesn't just say "The oceans are screwed. It's all our fault. We should all be ashamed of what we've done." It says "the better angels of human nature are trying to turn it all around. And they are giving the world hope." And because of the tone, one cannot help but naturally want to be one of those better angels.
For you marine biologists, the very simple message will seem quaint. But I'm sure you will understand the necessity of this sort of film serving as an initiator for conservationist thinking.
I honestly believe that every person on the planet should watch this film. Especially the children.
Oh, and did I mention that there are TONS of cuttlefish in it?
Don't even think for half a second that the following trailer comes close to doing the 3D beauty justice!
Flatfish Eye Development – Video Update
If you haven't read my piece on Flatfish Eyes & Recapitulation Theory, you should check it out. For those of you who have read it, I updated it with the following AMAZING morph animations of flatfish development that I somehow missed before (much thanks to Adrian Thysse, FCD of Evolving Complexity for pointing these out to me).
WANDERING EYES from Science News on Vimeo.
FROM FRY TO FISH from Science News on Vimeo.
Adaptation of the Week – Flatfish Eyes & Recapitulation Theory
Most biologists at one time or another in their training have learned of the 19th century theory expounded upon by Ernst Haeckel called "Recapitulation Theory".
The theory's thesis: "Ontogeny recapitulates phylogeny." Don't worry - it's not as complicated as the biological jargon might imply.
Ernst Haeckel's Drawings (1892 Romane copy). Note: Haeckel oversimplified these drawings. I use them here as a simple illustration of the concept of developmental similarity.
The idea boils down to a simple one - one that seemed to make sense in light of the fact that the science of developmental biology had only just begun from a systematic standpoint. The idea: if you watch an organism develop from an embryo to an adult, you can watch it slowly move through the evolutionary steps that had created it. That is: development repeats evolution.
So a human embryo would first look like a fish, then a reptile, then a mammal, and finally a human. Of course, we now know that in a literal sense, the theory is completely and utterly wrong. No stage of human development, or of any other organism, correlates with a discrete step in evolution.
We are never fish. (Though we do have embryonic tails).
However, that doesn't mean that there aren't kernels of truth to the idea, if applied loosely. Take the most famous and classic example: embryonic human gills. You may have heard yourself that humans have gills as embryos. Unfortunately this claim arises from misconception and incomplete understanding of developmental biology. Humans do not - ever - have gills. But as embryos we do have "pharyngeal arches." These are little bumps around what you might consider the neck area of a developing embryo (see Haeckel's drawings above). And these little mounds of tissue do in fact remarkably resemble similar mounds found in fish - mounds that in fish develop into gills (Note: Haeckel vastly oversimplified these drawings. I use them here as a simple illustration of the concept of developmental similarity. See: http://zygote.swarthmore.edu/evo5.html. Thanks Bjørn!).
One of the amazing aspects of developmental biology that much of the public does not generally understand is that evolution does not occur by adding new organs, appendages, or tissues to adult animals (whether through gradual steps or not). Evolution works by slowly sculpting the early embryonic clay of an organism.
Fish evolved these gill pouches as embryos - pouches that could then be sculpted into gills. As evolution waltzed and hopped along at its geological pace, genetic mutations began to change how these little mounds were sculpted, such that now in humans, these arches are sculpted into various parts of the face and head. A genetic program was already in place to control the shaping of the pouch. All that natural selection did was slightly tweak that program. For example, instead of a group of cells moving one direction, they moved another. Instead of becoming blood vessel cells, they became cartilage or bone cells.
Thus, while we now understand that we are not witnessing evolution in miniature during development, we are seeing pieces of our evolutionary history - little remnants that remind us of our relationships to our ancestors and also help inform us on what morphogenetic processes underlie evolution.
Which brings us to our adaptation of the week: the freaky asymmetric eyes of the flatfish.
Flatfish (www.seawater.no)
Most people have probably seen a flounder - one member of the flatfishes. They have adapted to lie amongst the silty ocean bottom, hidden from predators and prey, flat on their sides. For a normal fish this might be maladaptive - they would constantly have one eye buried in the sand. Of course the negative of being one-eyed might be offset by being much more camouflaged and undetectable.
Luckily for the flounder, the eye that should be buried in the sand has moved around its forehead so that both eyes are on one side.
The flatfish eye served as one line of attack against natural selection back in the day - and Darwin himself didn't quite know how to answer the charges. Evolutionary gradualism would predict that through successive steps, the eye slowly moved upward toward the forehead and eventually to the other side of the face. But what advantage could a slightly moved eye give, if it still was on the wrong side? Alternatively, as Richard Goldschmidt postulated in the 1930s and 40s, perhaps a single monstrous freakfish was born with both eyes on one side, and this allowed it to lie flat without losing half its vision. It could have then survived and had lots of little freak fish babies of its own.
So how did the flatfish become the strange creature it is now? Let's first look at the developmental biology of the flatfish eye.
WANDERING EYES from Science News on Vimeo.
FROM FRY TO FISH from Science News on Vimeo.
Flatfish Metamorphosis (Schreiber 2006)
It's been know for quite some time that flatfish larvae look like perfectly normal, symmetrical, and upright fish. The picture to the right is from a study by Alexander Schreiber in the Journal of Experimental Biology from 2006. As you can see, at early stages the larvae are normal, but progressively tilt and become horizontal as one eye moves across the face. He also showed in this study that eye movement and flattening behavior occur independently during development - but that's a much longer story.
Alright, so one eye gradually moves across the skull during development. What about during evolution? Do we have any clues as to the steps involved? Well, as many biologists know, the fossil record has now answered the question for us.
In a well-known study that was published last summer in Nature and received much media attention, Matt Friedman showed findings from a series of fossils delineating a clear gradual evolution from symmetrical to asymmetrical flaltfishes. (For excellent in-depth coverage looking at this study and the debate over flatfish evolution, see one of my favorite science bloggers, Ed Yong at Not Exactly Rocket Science, and also see the popular science writer Carl Zimmer at The Loom, and GrrlScientist at Living the Scientific Life).
Flatfish Evolution (Friedman 2008)
The evolution of the flatfish eye seems to mirror what we see during development. Thus, here we have a case of ontogeny appearing to recapitulate phylogeny quite wonderfully. There are many excellent examples of this throughout the biological world, though few that show such incredible similarity between the two processes of development and evolution. Nonetheless, this isn't really evolution we're watching during flatfish development - we're merely seeing how slight changes of the developmental programs are themselves responsible for the changes we see over time through evolution. Generally speaking, earlier developmental processes appear much more similar across varied species than later processes.
Development is in fact one of the primary constraints against evolution.
So while you were never a fish, you still showed remnants of fishy development during your own development. For it was these fishy developmental process that allowed the evolution of your own.
Scientific References:
- Schreiber AM. Asymmetric craniofacial remodeling and lateralized behavior in larval flatfish. J Exp Biol. 2006 Feb;209(Pt 4):610-21.
- Friedman M. The evolutionary origin of flatfish asymmetry. Nature. 2008 Jul 10;454(7201):209-12.
Previous Adaptations of the Week:
Darwin and the Heart of Evolution
Happy 200th birthday, Charles Darwin!
Happy 200th birthday, Abraham Lincoln!
Happy 150th anniversary, On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life!
And here's to a happy Darwin Day and upcoming Valentine's Day to everyone else.
As a part of my own contribution to the Blog for Darwin campaign, I present to you "Darwin and the Heart of Evolution."
What do all four of the above events have in common, other than being events of celebration? The answer will become obvious, but as a clue, I will begin with an appropriate Valentine's question:
A Frog's Heart
Why do humans have hearts?
I can see it already – you’re rolling your eyes thinking, “Well duh…because we need a way to circulate oxygen, hormones, immune cells and other signals, and transport waste compounds and gases.”
Ahh, but you would be wrong. For the above describes only what a heart does – not why we have one. As I wrote a few days ago, evolution pays no attention to "needs." Species don't evolve because they "need" to adapt or change some trait. Natural selection is blind to all intention and desire.
Before Charles Darwin (and his buddy Alfred Russell Wallace) gave us the theory of natural selection, the above "necessity" explanation would have sufficed – with an added “because God designed it that way” just for good measure.
The genius, beauty, and simplicity of Darwin’s big idea was in how it utterly reshaped the manner in which all “why” questions about reality are posed and how their answers are understood. The Origin of Species laid the foundation for the complete upheaval of the very word “why.” In fact, when it comes to describing biology, astronomy, physics, geology, and every other empirical look into reality, the word “why” now means nothing more than the word “how.” The how is the why.
So again, I ask - why (how) do humans have hearts?
To answer this question we need to jump back about 500 million years ago into the ancient ocean. Based on the fossil record, this is a good date to pick, considering that worms don’t make great fossils; however, the exact date is not at all important for this discussion. Nor does it matter the exact species of worm-like creature we consider, or the exact details of the hypothetical time-traveling adventure upon which we will now embark.
Imagine it - we’re swimming now in the ancient ocean sometime after the massive explosion in the evolution of all sorts of strange ocean-dwelling invertebrate body forms (the Cambrian explosion). One of the many advantages that certain individuals of various species find is that their larger body sizes makes them better able to compete – up to a point. Once a small early worm-like species reaches a certain size, it finds that it cannot grow any bigger with its current body plan. This is because at this point, our hypothetical creatures do not have circulatory systems. They must absorb all their oxygen from the surrounding water. Any individuals born larger than a certain size can no longer get enough oxygen due to the oxygen not reaching deep enough into their tissues, and so they die (or are our-competed).
The Vertebrate Family
Now imagine an individual of this species is born with what others of its species would consider a defect (if they had brains with which to consider such a concept). This individual has certain cells that have formed a small simple tube-like structure. Perhaps it is only a vague cavity – or some extra space between its cells. Now when this individual swims around, contracting its primitive muscles, the fluid within its body spreads a little bit more and a little bit faster through this cavity or space.
Our little worm leads a happy life, finding mates (or perhaps reproducing asexually) and leaving an ocean full of cavity-containing offspring. It seems self-evident to us now, but Darwin found himself surprised at the amount of variability in traits throughout the animal kingdom. All populations vary; thus, some of our worm’s children are a little bit bigger than their siblings. And some of these worm children will have inherited papa worm’s fluid cavity, which meant that they could survive with a slightly larger body than those without the primitive vessel, due to the oxygen distributing power of the fluid filled vessel.
Thus began the evolution of the heart. By a series of easy to imagine steps through thousands or millions of generations, the cavity became slightly more developed, eventually forming an actual tube. I would like to note here that the above scenario is strongly supported by much embryological, anatomical, and genetic data. However, I would like keep this simple and vague for the layperson.
Now, we move forward in time, though how far is unclear. Our little worms are now bigger worms, insect ancestors, and a myriad other small invertebrate species. Some of these species have evolved their tubes to have contractile regions - that is, a region of the tube than can actually squeeze and pump. Some, like our modern earthworm, have seven of these pumping “hearts”. Others, like the Drosophila fly, have only one heart - called a "dorsal vessel" (see the Drosophila larvae movie below).
The Fish Heart
We swim forward to 525 million years ago, just as the first fish appear in the fossil record. A lineage of the invertebrates has slowly morphed through primitive chordates (organisms with a nerve cord) to become the most primitive fishes. Along with the changes in many other body structures, the basic contractile heart and vessel system has itself become more complex. Instead of one contractile chamber, the fish heart has divided into two chambers: an atrium and a ventricle (and a stretchy region called the conus that isn’t contractile). The fish themselves then radiate over time, each lineage slowly accumulating many small changes, resulting in the gradual evolution of an ocean teeming with fish species – all with two-chambered hearts (see image at right).
Eventually, some fish species start shacking up near shorelines or in shallow ponds and lagoons. Some are born with thicker fins, which allow them to push along the bottom of the pools a little more quickly or lithely than others. They mate, and the process continues. Finally, one of them decides to just get it over with and leaps out of the water to land as a frog on four fully-formed legs.
The Amphibian Heart
Not really, but you get the picture.
We now see amphibious creatures roaming the shorelines like beastly salamanders. Their hearts have changed even further as other aspects of their bodies evolved to take in oxygen through lungs. Why did this happen? Because the changes that make it possible did happen. These shallow water-dwelling creatures began to develop vessel-filled outpockets on their esophagus, giving them the advantage of pulling oxygen from the air. In addition, the individuals with slightly better circulatory systems found their bodies better at all sorts of other things, such as regulating their bodies with hormones and getting rid of cellular wastes.
At this point, a series of further changes occurred in the amphibian heart. The atrium became two separate atria, either through a physical division of the one atrium, or through a duplication of the vessels coming into the heart. Thus, the frog ancestors developed three-chambered hearts, which were subsequently passed down to every frog currently inhabiting the earth (see image).
The Reptile heart
As time passed, the frogs began drying off their slime, sprouting scales and forked tongues, and inspiring instinctive reptilian nightmares in their prey. They became lizards. As the lizards moved fully to land and grew even larger, certain inherited variations in their hearts naturally worked a little better – thus natural selection continued the continuous sculpture of life. The ventricle began to separate into two chambers, much like the atrium had done in the amphibians. However, the ventricles didn’t fully divide. As one can see in almost every reptile on earth today, the ventricular division is incomplete – almost like a four-chambered heart, but with a hole between the ventricles (see image). However, I said that almost all reptiles have the pseudo four-chambered cardiac morphology; in fact, one branch of the reptiles went on to develop a fully-featured, true four-chambered heart: the crocodile - but that's a side story.
From some of the lizards the dinosaurs then sprung forth, populating the land from the small dark corners to the open plains. A short while later (a paltry 170 million years) most of the dinosaurs died off. Along with their distant crocodilian, lizard, and snake cousins, at least one dinosaur lineage and one reptilian lineage survived. We now call them birds and mammals, respectively.
Both the bird and mammalian lineages mirrored the path of the crocodile, completing the division between the ventricles (probably prior to their divergence). Natural selection has continued to sculpt our own mammalian hearts, resulting in marvelous structures such as the multiple different valve types, chordae tendenae ("heart strings"), and trabeculae (fibrous strings in the ventricle's interior).
The Bird and Mammal Heart
And with that, we have answered our initial question, in a massively oversimplified fashion. We have hearts because each change leading to our hearts conferred some small advantage to the individuals that inherited them (or at the very least, were not disadvantageous).
Of course, all of these cumulative small changes in the shape of the vessels and hearts, ultimately involved millions of small changes in the genes that controlled the behavior, shape, and functions of the circulatory cells. Scientists have now discovered an incredibly large and complex network of such genes controlling development of the heart.
One of the most astonishing yet completely expected facts we have garnered through studying organisms from Drosophila to the African clawed frog (Xenopus) to humans is the discovery that every organism on this planet with some version of a heart contains the same or a similar set of genes to control heart development.
That’s right. Read it again.
Many of the genes involved in the formation of the relatively primitive “dorsal vessel” in a fly are versions of the same genes that initially form our own hearts. Think about that! Think about how massively more complex we are compared to flies (which are themselves insanely complex in their own rights). Think about the hundreds of millions of years that separate us from our most recent common ancestor with a fly. Yet your heart still uses many of the same genes and in the same ways during early heart development. Of course flies and humans have continued to evolve in parallel ever since our lineages split those hundreds of millions of year ago – we have both made countless changes and tweaks to our own cardiac programs and networks. Nonetheless, our hearts remain related.
In fact, if you watch heart development in an embryo, such as in the Xenopus movie below, you can almost see the course of heart evolution itself. Of course this isn't really ontogeny recapitulating phylogeny - but some of the evolutionary history behind cardiac development is at least evident.
Tbx20 expression in a frog larva heart
One example of a cardiac gene that I’m particularly familiar with, having received my doctorate studying it, is a gene called “Tbx20”. For this discussion, its exact function does not matter. Suffice it to say that when I began my studies, we had a clue that this gene was important in heart development. Why? Because flies have a copy of this gene, as do humans, mice, and every other heart-bearing organism we’ve looked at; furthermore, in each of these organisms this gene is “turned on” in the developing heart tissue.
I went on to show that when you prevent frog larvae from making the Tbx20 protein, they develop incredibly malformed hearts (see the videos below). This means that the Tbx20 gene is indeed important in making a heart. Other researchers later went on to show similar results in mice and flies. Finally, about two months before I finished graduate school, another group of researchers found that some humans born with congenital heart defects have mutations in the Tbx20 gene.
Normal African Clawed Frog (Xenopus) heart
African Clawed Frog (Xenopus) heart lacking Tbx20 protein
So here we have found in only a few years of research a single gene that supports the entire model of evolutionary theory. To rephrase the famous quote from Theodosius Dobzhansky, the existence of Tbx20 in controlling the development of the heart in organisms from flies to humans does not make any sense – except in the light of evolution.
Due to the rich evolutionary history behind the development of this complex organ, the genetic network has become incredibly complex, involving hundreds of genes in thousands of cells all working, moving, and functioning in precise coordination. The higher the complexity, the more things that can possibly go wrong. Unsurprisingly, congenital heart defects are among the most prevalent of all inherited diseases, resulting in about 9 babies out of every one thousand being born with some sort of cardiac abnormality.
I’m sure many of you were wondering how I would manage to tie Abraham Lincoln tie into all this. Although still hotly debated and unproven, at least some researchers believe that Abraham Lincoln may have been afflicted with a disease called Marfan Syndrome, a connective tissue disorder affecting the heart and many other organs. Other researchers believe that he had an unrelated disease. Regardless, it remains at least possible that President Abraham Lincoln was the inheritor of one of the billions of less advantageous variances in heart development that have presented themselves throughout the heart’s evolutionary history.
In summary, the heart of Darwin's theory of natural selection is the idea that evolution comes not through the "why." It comes through the how - through the accumulation of minute individual variations that spread like wildfire when they contribute an advantage. There remains no better demonstration of this principle than the myriad heart morphologies and functions we can trace today.
Each of you has most certainly inherited a cardiac variation, whether it be a major mutation in a gene, or a tiny change in one letter of your genetic code (a "single nucleotide polymorphism").
Who knows...perhaps yours is the one upon which an entirely new evolutionary history will be built.
So here’s to your own personal variation, and to the man who made our understanding of it all possible. We would have gotten there without him – but I doubt anyone could have rivaled the combination of his incredible intellect and beautiful prose.
Happy birthday Darwin!
_____________
Image credits
Frog heart photograph: Me
Phylogenetic tree: McGraw-Hill and Biology Corner (links to original source broken)
Drosophila heart tube movie: unknown
Heart diagrams: Oracle ThinkQuest Education Foundation
Cardiogenesis animation: Me
Frog heart movies: Me
Lincoln photograph: Visiting DC
Lincoln photo: