This blog is the fourth piece in a series by Darrel Falk and David Kerk. The previous entry is found here.
In one of our previous essays we made the prediction that if evolution is true, and ancestral species do give rise to descendant species by a process of “descent with modification,” then we should be able to find fossil transitional forms which display body characteristics clearly showing evidence of this process. Out of many possible examples, we then examined one specific instance (evolution of whales) where this expectation has been resoundingly confirmed. The process of fossilization, however, is overwhelmingly biased toward hard mineralized body parts (bones and shells) – traces of soft body parts are very rarely preserved. But if evolution is true, then each of us should retain, in our own bodies, “buried fossils” in the form of the soft tissue of our organs, which show evidence of descent with modification. In this and the following essay we will examine the human heart, and the great arterial vessels which emerge from it. We will see that in those structures is the clear record of an evolutionary process.1
Let’s start by recalling some aspects of the human heart with which you are probably familiar. The human heart (like that of all mammals) has four pumping chambers – two atria (which receive blood coming into the heart) and two ventricles (which receive blood from the atria, then pump it out to other destinations). Furthermore, functionally the human heart works by using two pumping “circuits,” one served by each side of the heart (a set of one atrium and one ventricle), which work in parallel. The right side of the heart receives “spent” blood from the body and pumps it to the lungs. The left side of the heart receives “fresh” blood from the lungs, then pumps that to the rest of the body (Figure 1).2 There is a set of large vessels (arteries) which leave the heart, and are vital in carrying blood to the appointed destinations.
How do the human heart and this set of great vessels develop? One might think that the most straightforward way to do this would be for a mass of primitive tissue to hollow out into a tiny four-chambered structure with the appropriate connections, then progressively enlarge them as the entire embryonic and fetal body grows. However this is far from the case. Instead there is an intricate (at first sight perhaps bizarre) set of stages and transformations which are shown in Figure 2.3
Figure 2: Early stages of human heart development
The first stage is the formation of an elongated tube with thickened walls. This occurs near the beginning of the fourth week. Blood comes in at one end, passes successively through several structures to the other end, then it is pumped out. Note that there is only a single atrium, which receives the blood from the body and a single ventricle which sends the blood on its journey out of the heart. A couple of days later this tube-heart twists so that it becomes more compact in shape. Near the end of Week 4, the two structures at each end (the sinus venosus and the bulbus cordis) become absorbed into the atrium and ventricle, respectively.
Eventually, at the end of Week 6, the single atrium and single ventricle become divided by a partition, forming the final four chambered configuration we are familiar with. Figure 34,5 shows an internal view of the human heart as it nears completion of the constrictions that lead to four chambers.
Figure 3: Human Heart Just before its division into four distinct chambers (41 days)
Note at this point that the end that receives blood (the atria) has shifted to the top (the head end), while the sections that deliver blood to the body (the ventricles) have shifted to the bottom, just the reverse of the way that things had been earlier.
This set of transformations is altogether remarkable, and not at all what we would expect to be necessary to form a human heart and its major arteries. It is, however, even more amazing when we consider the fact that this process resembles in essential ways the process of development in all other animals with backbones (vertebrates). For example, the “tube heart” stage we described above for the early human heart corresponds closely to the early structure of the heart as a fish develops (Figure 4).6
There is thus only one atrium and one ventricle in the mature fish heart. In amphibians, such a “tube heart” stage also occurs in development, along with a very specific folding that also occurs in the fish. But now in addition, the single atrium becomes divided into two. The ventricle, however, remains as a single chamber. In reptiles, the same sequence of changes again occurs (with the same looping that characterizes the fish heart), but now in addition to the atria being divided, there is some division of the ventricle into two (variable in different types of reptiles, but complete in crocodiles, for example).
Thus the human heart at its different stages of development resembles closely that of fish, amphibians, and reptiles during some stage of their development. Are these just isolated “factoids” or can we account for them in some more meaningful way?
If we reconsider the development of the heart, this time in an evolutionary context, then all of this suddenly and dramatically makes sense. If evolution is true, then fish were the first vertebrates that appeared in the history of life. Indeed, fossil evidence indicates that fish predominated about 400 million years ago, which has sometimes been called the “Age of Fishes.” If evolution is true, then amphibians appeared later (fossil evidence shows this began about 400 million years ago), and reptiles appeared still later (around 300 million years ago based on fossils). Furthermore, if evolution is true, each major vertebrate group evolved through “descent with modification” from a pre-existing group. Thus amphibians evolved from fish, reptiles from amphibians, and mammals from reptiles. Thus if evolution is true, and all of this is so, then we might expect that as a member of a later group of organisms develops before birth, it would use aspects of the “developmental program” (i.e. the “instructions”) that were already in place for its ancestral organism. Why reinvent the wheel and develop a complex structure from scratch when it is possible to modify a previous structure to suit instead? When viewed in this light, it becomes quite sensible that the human heart passes through stages during its development which would be similar to those seen in a fish, amphibian, and reptile.
What about the set of arteries leaving the heart? There is a set of six paired arteries that are formed (each pair has a left and a right component), connected by a vessel on the belly side of the body (called the aortic sac in humans) and a pair of longitudinal vessels on the back side (each called a dorsal aorta). Figure 5 at right shows an idealized representation of the six arches.
Figure 67 below shows them as they actually exist at about six weeks into human development. The six paired vessels are not all present at the same time. The first two pairs at the head end appear first, followed by the last several, which appear in sequence moving toward the hind end of the body. The first two have disappeared by the time the last ones become prominent. In humans the fifth pair are either rudimentary or do not appear at all, depending on the individual.
It turns out that the third, fourth, and sixth pairs of vessels are particularly important. The third pair forms the carotid arteries which supply the head. The fourth artery on right side withers away, but the artery on the left side is important in forming part of the aorta, which is the main artery leaving the finished heart. The sixth artery on the right side also withers, but the one on the left side is important in the circulation of blood to the lungs (Figure 7).8,9
Why do the blood vessels emanating from the human heart develop in such a strange way? Why six arches, especially when some subsequently disintegrate? As we all remember, fish have gills to allow them to exchange gases while living in water. The gills develop from a set of structures called the “branchial arches” (“branchial” means gill). Most fish have six arches at some point in their development, each one containing a blood supply, muscle, cartilage and nerve. Each arch is supplied by an artery called a branchial arch artery, and there are initially six of them, arranged from the head back toward the tail. The heart is on the belly side of the body, and sends blood forward toward the branchial arches. The blood then passes through the six pairs of arch arteries, and collects in paired dorsal aortae on the other side, finally to go to the rest of the body. At first, in the embryonic fish, these arteries simply carry blood through this region (there is no gas exchange yet). In fact, the first two branchial arches and their arteries are diverted to support the development of structures in the head. As the fish matures, slits break through around each of the remaining arches, allowing the flow of water, and the vasculature of arches 3,4,5, and 6 form the functional gills, which then persist throughout life. The heart now sends blood first toward the gills, through them, and finally away from them, through the mature branchial arch artery vasculature (Figure 8).10,11
In an amphibian like a frog a process very similar to fish development occurs. Very early on, the branchial arch arteries merely carry blood through the arches, prior to the development of the gills. Then during the tadpole stage the gill vasculature develops from these arch arteries, and the gills become functional. Finally, as the mature frog develops, the arch arteries are again modified to supply the needs of a land-dwelling animal. The arteries of the 3rd arch supply the head and neck, those of the 4th arch supply the rest of the body, and those of the 6th arch supply circulation to the lungs (Figure 8). In reptiles, there is never any functional gill apparatus. Yet a similar set of branchial arch arteries develops, and supplies various body regions using the same arch arteries as in amphibians (Figure 8). We introduced the mammalian pattern earlier in this essay (see also Figure 8). Thus at each stage the successor vertebrate type, during its development, will reproduce structures much like those of its series of ancestor organisms during their own early development. This is comprehensible only if evolution is “adding new features on top of old.”
In the human, sometimes the set of paired transitional arteries which appear during development before their later transformation are called “aortic arch arteries,”12,13 since they join a ventral to a dorsal aorta. However, they are often called the “branchial arch arteries” to emphasize their evolutionary origins. They are a classic example of the retention of a structure which has lost its ancestral function (i.e. supplying blood vessels to functional gills) but is retained exclusively to be used as a building block for a more recently evolved structure. This is precisely how we would expect evolution to work.
Let’s summarize, and be very clear about what we have just observed. The structural characteristics of the heart and great arterial vessels amongst living vertebrates do not merely possess surface similarities. Two crucial points need to be emphasized here. First, the retention of “aortic arch arteries” (or “branchial arch arteries”) in non-aquatic vertebrates serves no respiratory function. They are merely connecting pipes. Their sole purpose is to be used as building blocks to construct modified circulatory elements which function in the species which possess them. But remember, in principle, such “building blocks” might have been constructed in any manner whatsoever. The fact that all living vertebrates retain a set of six arch arteries during their development is strong evidence that they have inherited this pattern of development from a common ancestor, one which did use these arteries to develop functional respiratory structures (i.e. gills).
Second, given a set of six arch arteries, there is no logical or structural reason why the 3rd artery must contribute to the carotid circulation, the 4th artery must supply blood to the body, and the 6th artery must contribute to the circulation to the lungs. Why not use different arch arteries for different final structures in various vertebrates? The conclusion is inescapable—successor organisms have inherited a set of instructions for development from ancestral organisms, and are not free to deviate readily from it. Rather, since evolution is a historical process, it is a necessity that the descendant organisms follow the same general pattern of development used by their ancestors.
The next blog in this series can be found here.