Mr. Michel Masson
Of course, I teach other subjects as well, such as Human Physiology, Physical Anthropology, and General Biology, but I taught anatomy first, I have taught it every year since I started teaching, and I have taught it more times than any other subject, so I guess you could say that I am an anatomist.
And I understand why they ask: anatomy is, after all, the original "dry bones" subject. It requires the rote memorization of a large number of facts about dead bodies, mostly in dead languages. Plus it is a static science, since human anatomy has changed little over the ages. It is not often that you will read of the discovery of a new organ in the human body. Why wouldn't anatomy be boring?
Yet anatomy is a science, and any science is more than just a body of facts. Science is a procedure, a procedure by which you examine facts to gain insights, and the science of anatomy is no exception. Through what perspectives can we view the facts of anatomy, and what insights will we gain? In this lecture I hope to present a few examples, to share with you the flavor of the science and some appreciation of its extent and explanatory power.
The first perspective is that of physiology, the study of function: the brain is a computer, the eye is a camera, the kidneys are filters, the lungs are a ventilation system, and the heart is a pump. And "form follows function", as they say. A solid organ like the brain could not be a pump, and a hollow organ like the heart would make a poor computer. So form does follow function, to a degree. But why is the heart a 4-chambered pump?
Perhaps that will be more obvious if we view the same facts through the perspective of comparative anatomy. Until the 17th century the function of the heart was unknown. It was obviously a vital organ, since if it stopped beating life was over. It was thus thought to be the source of "vital spirits" that maintained the life of other organs, and the seat of emotions such as love, since the heart beat faster in the presence of a beloved.
Then, in 1628, the English physician William Harvey applied the technique of comparative anatomy to the problem of the function of the heart. He looked at the simpler, slower hearts of living fish, amphibians, and reptiles, on the theory that a simpler heart would have the same function as a complex one, but that it would be easier to discern that function in a simpler form. Harvey ligated the vein connected to the simple two-chambered heart of a fish, and noted that the heart became blanched and empty of blood, and that when he released the ligature the heart filled with blood again. He ligated the artery connected to the heart, and observed that the heart became congested with blood, and that when he released the ligature the heart became normal in size within a few beats.
From these observations he concluded that blood entered the heart via the veins, left the heart via the arteries, and that the heart was a simple mechanical pump, pumping the entire blood volume of the animal every minute into the arteries. The blood traveled in a circle, returning via the veins to the first chamber, the atrium, then to the ventricle, whence it was pumped out again. His work was titled De Motu Cordis et Circulatione Sanguinis (on the motion of the heart and the circulation of the blood), from which we derive the modern expression "the circulatory system."
He then noted that in the slightly more complicated amphibians lungswere added, and there was a dual circulation. The amphibian heart pumped the blood both to the body and to the lungs and two streams of blood returned to the heart, the one from the lungs to the left atrium and the one from the body to the right atrium. In reptiles the ventricle was partly divided, perhaps to better separate the streams of blood, and in mammals the heart was completely divided, into right and left atria and right and left ventricles, producing a four-chambered heart. The fact that the entire blood volume was pumped out into the ever-smaller arteries each minute and returned through the ever-larger veins led Harvey to predict that there were tiny connections between them, too small to see. His prediction was confirmed some 60 years later when early microscopists discovered the capillaries, and the later discovery of oxygen supplied the reason for the dual circulation: the heart pumps blood first to the lungs to be oxygenated, then to the body, where oxygen is delivered to the tissues. Harvey did not intend to imply that fishes developed into amphibians and so forth; he was merely postulating a hierarchy of complexity in a static animal kingdom.
When Charles Darwin published his Origin of Species it became possible to view the same facts through a third perspective, that ofphylogeny, the evolutionary history of a species. The structure of the arm skeleton of a variety of vertebrates led Darwin to conclude the following:
But from whom? What was the common ancestor, and why the one-bone, two-bones, many-bones, five-fingers configuration common to them all? Back in the Devonian, some 350 Million years ago, the only vertebrates were fishes. Some were built along modern lines, flattened into vertical discs stabilized by membranous fins. Yet others were chunky, lay on the bottom, and supported themselves on two pairs of fleshy, muscular fins. These were the "lobe-finned" fishes, known from fossils and from the single living representative of the family, the coelacanth. Their fins were stiffened by bones: first one, then two, then many, spreading outward into a broad blade, a perfect configuration for a fin. Such fins could easily support a fish trapped in a tidepool, perhaps even allow it to "walk" to the safety of an adjoining tidepool. Thus did fins become limbs, and all vertebrate limbs have the bone structure of their antecedent fins.
And so, from the most prosaic of sciences-anatomy-comes a most radical conclusion: We were once fish! Not "we were once apes"; though this latter possibility upset many people of Darwin's day, it is so obvious that Darwin didn't feel he had to point it out in the Origin. No, Darwin was implying that we were once fish. If you do not think that is a radical hypothesis, you have lost your sense of the ridiculous. Try telling it to a 5-year old and see what she says! We were once fish, indeed!
Such a radical hypothesis demands a greater degree of proof, so let's examine the facts of anatomy through a fourth perspective, that ofontogeny, the development of the individual, from egg to adult. During the 19th century many biologists examined the embryos of vertebrate animals to find that vertebrate embryos passed through several stages, the earliest of which strongly resembled those of fish. In 1866 Ernst Haeckel formulated his "Biogenetic Law":
Ontogeny recapitulates phylogeny.
Unfortunately he meant this quite literally: that we, in our individual development, are first fish, then amphibians, then reptiles, then mammals, then man. When it became quite clear that this was not the case, that our earliest embryos are not free-swimming, gill-breathing fish, his Biogenetic Law was rejected by biology and the idea the ontogeny recapitulates phylogeny fell into disrepute. Too much so, perhaps, since the phenomenon of early resemblance of embryos remained. Darwin had remarked upon it thusly:
More recently Stephen Jay Gould has explored the idea in his excellent book Ontogeny and Phylogeny. It is still a useful concept, I feel. Let's look at some specific examples.
First, the heart. The human heart begins as a simple beating tube, which then divides into a two-chambered, fish-like heart. The atria are divided from above by a complex series of septa, and the ventricle is then divided into two by a septum that grows up from the bottom, just as the fish heart evolved into the amphibian heart, then the reptile heart, then the mammalian heart. So what Harvey had viewed as a simple hierarchy of complexity was also a temporal sequence replayed in the development of each mammal, and very probably the sequence of evolution as well. And some of the serious birth defects of man, such as interatrial and interventricular septal defects ("blue babies"), can be seen as a failure to complete a complex series of ontogenetic events, a failure to fully recapitulate.
Next, the "gills". As the human embryo develops, it first looks like a sort of worm, though with a stiffening rod down its back and "slices" of muscle down its sides. At this stage it resembles Pikaia, the earliest-known protochordate from the Cambrian of 500 million years ago. Then, as it grows, it develops the slits and arches that would become gills, were this to be a fish embryo. In humans these arches progressively transform, the first becoming the upper and lower jaws, the second the hyoid bone, the third the larynx, and the slits close, all but the first. The first "gill" slit is retained, to become. . . . the ear! If we had not been fish, we wouldn't have ears! Internally the transformations are more interesting and more profound; but let me first set the stage:
Fish, like this basking shark, both feed and breathe by means of apharynx, a portion of the neck that is perforated by slits, so that water and food can enter the mouth, with food entering the gullet while water exits via the slits. Hobart. M. Smith, in The Evolution of Chordate Structure, wrote:
Internally, the ventral heart pumps blood forward through a ventral aorta, sending it up through 6 pairs of aortic arches between the slits to the dorsal side, where it is sent anteriorly to the head and posteriorly to the body, as shown in this drawing by my colleague Larry Jon Friesen. As the blood in the arches passes by the water exiting via the slits it gains oxygen, allowing the shark to breathe as well as eat. These are the gills of the fish.
Humans have an asymmetrical heart and aorta, as shown in this computer image produced by my nephew, Mark Ferrer's son William Ferrer. How could this strange, asymmetrical setup be derived from the symmetrical six-arched fish pharynx? Try this series of transformations: First, the first arch disappears and extensions grow toward the arms from the fourth arch. Next, the second arch disappears, as do the connections between the third and fourth arches. Then the third arch splits down the midline, the fifth arch disappears, and "sprouts" appear off the sixth arches. Lungs develop on these sprouts, the connections between the right fourth, fifth, and sixth arches disappear, along with the superfluous right aorta, and the adult circulatory pattern begins to show itself. From fish to man, once again!
Not only does this sequence explain the phylogenetic transformation from fish to man; it is also seen in human ontogeny. And failure to faithfully recapitulate this sequence can lead to serious birth defects, like Tetralogy of Fallot. Thus is our fate dependent on our history.
What is the end result of evolution? There are some general trends. Lineages tend to develop larger, more complex, more specialized forms over time. But then when the world changes, as it eventually must, these large, specialized species are left with inadequate resources in a world for which they are no longer adapted. The result is extinction; 99% of all species that have ever lived have suffered this fate, and evolution is a dead-end street leading inexorably to extinction. Is there no way out?
Perhaps there is. If in its development each individual creature passes through earlier stages of its phylogeny, and if those earlier stages were smaller and less specialized, perhaps stopping ontogeny early would be a way of turning back the clock, of reversing evolution. This process is called neoteny, or paedomorphosis: the retention of juvenile features into adulthood. The classic example is the axolotl, an amphibian once considered a delicacy by the Aztecs. In most salamanders the ontogenetic sequence is that of aquatic egg, aquatic tadpole with external gills, metamorphosis, in which the gills are lost and lungs and limbs develop, followed by the emergence of a 4-legged air-breathing adult, in itself an example of ontogeny recapitulating phylogeny.
But what of a tadpole swept into an underground cavern? If it metamorphoses it loses its gills and, without an air supply, drowns. If it were merely to retain its juvenile gills it could survive, as a blind, albino, yet gilled axolotl, living off food swept in by the current. And such forms have evolved, separately, in countless caverns around the world. Saved from extinction by neoteny.
Now, where did we, Homo sapiens, come from? Structurally and intellectually our nearest relatives are the "great apes", the orangutan, the gorilla, the common chimpanzee, and the dwarf chimpanzee, or bonobo. Yet they are hairy, small-brained knucklewalkers and we are naked, erect, large-brained bipeds. Such marked differences must have taken a very long time to evolve. This is the "Early Divergence of Man" theory, the theory in favor when I first started teaching. It held that the great apes form a closely related group, the Pongidae, from which we separated 20 million years ago.
In 1967, however, 2 Berkeley radicals, the anthropologist Vincent Sarich and the biochemist Alan Wilson, dropped a bombshell: their study of ape and human proteins concluded that the orangutan split off first, and that humans, gorillas, and chimps shared a common ancestor as recently as 8 million years ago. Studies of a number of proteins showed that, out of 2633 amino acids, only 19 amino acids differed between chimp and man, a difference of 0.72%. We are more than 99% identical to chimps! Microscopic studies showed that human and chimp chromosomes are strikingly similar, and DNA studies confirmed that our DNA is more closely related to that of chimpanzees than chimp DNA is to gorilla DNA. So the Pongidae are not a coherent group, and humans must have evolved rather recently from the African apes. We are, in the words of the title of Jared Diamond's recent book, The Third Chimpanzee. This theory, the "Late Divergence of man" theory, is now almost universally accepted.
But where, why, and how could we have evolved so rapidly from a chimp-like ancestor? Could we possibly have come such a distance in so short a time? One scenario, dubbed the "East Side Story" by the French physical anthropologist Yves Coppens in a recent Scientific American article, goes like this: Africa was once belted from sea to sea by a tropical forest, the home of large, hairy, knuckle-walking apes much like today's chimpanzees, well-adapted to a forest life of eating fruit. Ten million years ago tectonic forces began to tear Africa apart, opening the East African Rift System. As the rift opened, volcanic action raised a chain of mountains that cut the narrow east side of Africa off from the wider west side, and created a "rain shadow" effect that caused a gradual drying-out of the east side, with a consequent replacement of the tropical forest by open savanna grasslands.
The apes on the west side did not change much in 10 million years, and we know them today as the gorillas, chimpanzees and bonobos. The smaller number of apes trapped on the east side, however, had to adapt to the drying conditions, sunlight, heat, and a paucity of jungle fruits. Too large and too specialized to survive in such an environment, what was their alternative to extinction? Neoteny might have been the answer. Whereas the adult chimp is large, hairy, dependent on fruit, and too low-slung to see over the grass, perhaps an infant chimp would be smaller, requiring less food, would be less specialized, and thus better adapted to the new conditions. And might it not, as a consequence of neoteny, be larger-brained, more erect, and possibly more naked?
Notice that the chimp skull is more changed from the fetal condition than is the human skull, and that an adult human skull more closely resembles both the human fetal skull and the chimp fetal skull. In other words, humans are more fetal-like than chimps. Look at this picture of an infant and an adult chimpanzee; does it seem more likely that we descended from an adult chimp or, neotenically, from an infant chimp? Personally, I favor the latter scenario. I certainly think that it is a reasonable suggestion, and I present it to you for your consideration. It certainly is a favorite speculation of mine. Perhaps you wonder why.
When I first heard the suggestion that our origins might be neotenic I embraced the idea as an old friend. You see, I had been exposed to it before... When I was a child my father read to me every night, from children's books, from the classics, and from everything in-between. Of all those books my favorite was The Sword in the Stone, by T. H. White. For those of you who are not familiar with this work, I'll describe it briefly. It is a serio-comic account of the childhood of the legendary King Arthur. The young Arthur, known as "the Wart", is portrayed as an orphan being raised by a minor country nobleman; neither he nor Arthur knows that he is to be king one day, the day that he pulls the magic sword from the stone. To prepare him for his kingship a tutor is sent, the magician Merlin. Merlin educates the young Arthur by turning him into a series of animals, so that he can learn wisdom from their ways.
Really, the book is about how a young boy develops character, character enough to become a king. It affected me profoundly. In one of these episodes Merlin turns the young Arthur into a hedgehog so that he may visit the badger, a scholar of the history of life. The Badger tells him a story of the origin of man. I must caution you that this is a whimsical and apocryphal tale, as you will be able to see toward the end when they start playing around with the Divine "We", as used by and applied to God.
A charming explanation of the neotenic nature of man, don't you think? It is surely not scientific per se, but it was inspired by science. The art and literature of any people spring from the totality of their culture, and science is an increasingly important part of ours. In today's world you cannot be thought truly literate without knowing something of science.
Consider Stanley Kubrick's film adaptation of Arthur C. Clarke's 2001, A Space Odyssey. In one of the climactic scenes of the movie our hero is propelled at tremendous speed through space and time, eventually to see both the past and the future. To the strains of Richard Strauss' "Thus Spake Zarathustra", the camera pans from earth to the image of a giant earth-sized fetus, floating in his amniotic sac, quietly contemplating the world.
In this fetus our hero saw himself, since it had his face, yet I suggest that the allusion is to all of mankind, its neotenic origins and its future destiny in the peaceful, vulnerable, yet endlessly potential qualities of the embryo in us all.
These speculations are not facts, of course, but science only starts with facts. A fact is like a brick, useful but prosaic, and a pile of bricks is equally prosaic. But a pile of bricks can be assembled, to form an outhouse, or a library, or a beautiful cathedral. And a cathedral is more than just bricks; it is bricks plus vision, and its beauty lies not in the bricks, but in the vision. In just such a way are scientific theories assembled from facts; a theory can be thought of as facts plus the vision that assembles them, and a theory can also be beautiful. Darwin certainly felt this aesthetic side of science. Consider the very last sentence of the Origin, a book in which he had marshalled an incredible number of facts and assembled them into his theory of evolution by means of natural selection:
Henri Poincaré, the mathematician and philosopher of science, put it more generally and more succinctly when he wrote:
That is why I am a biologist: I'm hooked on the power and the beauty of biological thought.
That is why, to a biologist, going to work every day is a bit like going to a party. You say to yourself: "What new facts am I going to meet today, what interesting new ideas?" It gives us a sort of permanent grin.
And that, ladies and gentlemen, is why I never get bored teaching anatomy.
Darwin, Charles; The Origin of Species, 1859
Diamond, Jared; The Third Chimpanzee, 1992
Gould, Stephen Jay; Ontogeny and Phylogeny, 1977
Smith, Hobart M.; Evolution of Chordate Structure, 1960
White, T. H.; The Sword in the Stone; 1939