The human umbilical vesicle (‘yolk sac’) and pronephros—Are they vestigial?
Published: 2 May 2009(GMT+10)
This week we feature an enquiry from university student André Z of New Zealand, whose biology lecturer teaches that the “yolk sac” (umbilical vesicle), the pronephros, and other human embryonic structures are vestigial, constituting evidence that humans evolved. André also asks about so-called “endogenous retroviruses” (ERVs). Below is André’s enquiry, followed by a response by Andrew Lamb and Jonathan Sarfati.
Hello, I study 2nd year biology (BSc/BA) at a university in NZ. I have searched your site and not found any (or much) relevant information from a creationist perspective on two common evolutionary arguments in particular.
- Embryo development—the human embryo has some structures which serve no purpose in adults and resemble the embryos of ‘simpler’ organisms, which do have a use for these structures. The pronephros ‘kidney’ is effectively the same as in simpler animals, but in humans it degenerates by the 6th week.
- The yolk sac is apparently vestigial, having very little purpose in humans—certainly my lecturer is fond of it as evidence for evolution.
- Related to the comment on the yolk sac is something my lecturer was rather keen on repeating; that an engineer would not design a system such as found in the embryonic blood circulations. The claim is that the ‘mixing’ of oxygenated/poorly-oxygenated blood in e.g. the embryo’s heart is rather inefficient—I assume the idea is that poorly oxygenated blood shouldn’t really be mixed back in with oxygenated blood after circulation.
- Endogenous retroviruses—I have read the two articles on your site on these, and I am no expert on the details, but the extreme similarity in the location of some ERVs in the genetic sequences of different creatures such as chimps and humans, and a pattern of differences in these ERVs (or their surrounding genetic sequences) which fits evolutionary phylogenies, seems to support a common ancestry of different mammalian species. Is there a story in our genes, when we examine specific examples of similarity (rather than broad similarity biochemically which is arguably required to an extent for nutrition purposes and such)?—i.e., I am not convinced of the strength of your arguments concerning molecular similarity and would appreciate any further comments or hints.
Many vestigial arguments like those your lecturer pushes are based on the long-discredited theory of embryonic recapitulation, supported by the forged diagrams of German Darwinist Ernst Haeckel (1834–1919). More recent research shows that even the embryonic similarities that appear in many biology textbooks were actually based on Haeckel’s forgeries.
There are just too many anomalies for the recapitulation idea to work: The “tail” in the human embryo does not mean that we descended from tailed animals. In fact, the human embryo also has a post-anal gut. Does this mean that we descended from an animal with such a thing?
Some of the numerous examples of embryonic development which are contrary to the supposed evolutionary sequence are: the mammalian heart forms before the circulatory system, the teeth form before the tongue, and the whale embryo never has a four-legged phase.
Therefore, since embryonic recapitulation is utterly defunct, any argument based on it should not trouble anyone.
Another common evolutionary claim is that the pharyngeal arches of the human are vestigial gill slits, but these pharyngeal arches are neither gills nor slits! Refutations we have published of this claim can be found by entering “gill slits” in the search field near the top right of our website. [Update: according André,
Gill slits were discussed by my lecturer and the Haeckel-esque simplistic story which has previously been attached to them was debunked by him; it is clear that the pharyngeal arches do not develop into slits in humans (though the possibility that they may sometimes ‘break through’ seemed to be left open).]
Potentially helpful resources re human embryology include:
- Does the human fetus temporarily develop gills, a tail, and a yolk sac?, largely adapted from Gary Parker, Embryonic Development, pages 54–63 in: Creation: Facts of Life.
- Alex Williams, Abortion argument unravels, Creation 27(4):16–19, September 2005.
- Jerry Bergman and George Howe, “Vestigial Organs” Are Fully Functional, Creation Research Society Books 1990.
- Andrew Lamb, Human tails and fairy tales, 1 September 2007. See especially the quotes from embryology textbooks in the References section at the end of this article.
Another important point with embryology is that the needs of the developing embryo are as important as those of the adult. The “tail” ensures that there is an adequate blood supply to the developing leg buds in the embryo. The development of the kidneys is an example of this (see below). Just as many temporary structures such as scaffolding, ramps, rubbish chutes, portaloos, etc. are needed on a construction site, but are superfluous once the building is completed, so too it is reasonable to expect there to be temporary structures needed by a growing organism, that may no longer be needed by the fully grown adult.
Still another point is that some structures develop only when induced by other structures. An embryology textbook explains:
The needs of the developing embryo are as important as those of the adult.
“Organs are formed by interactions between cells and tissues. Most often, one group of cells causes another set of cells or tissues to change their fate, a process called induction. In each such interaction, one cell type or tissue is the inducer that produces the signal, and one is the responder to that signal. … Examples … include … gut endoderm and surrounding mesenchyme to produce gut-derived organs, including the liver and pancreas, limb mesenchyme with overlying ectoderm to produce limb overgrowth and differentiation; and endoderm of the ureteric bud and mesenchyme from the metanephric blastema to produce nephrons in the kidney [more below]. Inductive interactions can also occur between two epithelial tissues, such as the induction of the lens by epithelium of the optic cup.”1
The book goes on to explain, “Cell-to-cell signaling is essential for induction, for conference of competency to respond, and for cross talk between responding cells.” Then it explains these complex biochemical processes.
Induction explains another favourite evolutionary “proof”: teeth in embryonic baleen whales, supposedly proving that they evolved from toothed whales. But Louis Vialleton (1859–1929), who was Professor of Zoology, Anatomy and Comparative Physiology at Montpelier University, southern France, argued:
“Even though the teeth in the whale do not pierce the gums and function as teeth, they do function and actually play a role in the formation of the jaws to which they furnish a point d’apui on which the bones mold themselves.”2
Douglas Dewar (1875–1957), a prominent British creationist who strongly refuted evolutionary arguments around WW2, supported Vialleton’s argument in several ways:
- the embryonic teeth are very different in disposition, form and number from the toothed whales
- why would toothless whales acquire extra teeth, then scrap them and replace them with the new structure of baleen plates;
- there is a parallel example in humans, where microcephalic individuals with very poor or non-existent teeth development suffer from receded jaws. These poorly developed jaws are due to “a deficiency or actual total failure of development of the dental germs, the effect being that the investing jaws likewise fail to execute their normal growth and evolution.” 3
With these principles out of the way, we’ll now tackle the “yolk sac”, pronephros (plural pronephroi), embryonic heart, and “endogenous retoviruses” in turn.
Formation of the pronephric kidney lays the foundation for the induction of the mesonephric kidney, and it in turn lays the foundation for the induction of the metanephric kidney.—Larsen’s Human Embryology
The above embryology textbook points out that the pronephros serves an important role as an inducer, as explained above:
“Formation of the pronephric kidney (i.e., pronephros) lays the foundation for the induction of the mesonephric kidney (i.e., mesonephros), and it in turn lays the foundation for the induction of the metanephric kidney (i.e., metanephros). Hence, formation of a pronephric kidney is really the start of a developmental cascade leading to the formation of the definitive kidney.”4
Also, Dewar suggested that the pronephroi have a function in the very early embryo, and its “simple” structure and positioning are appropriate for this function:
“As the embryo must have a kidney to rid himself of waste products at an early stage, one has to be developed while the complicated adult kidney is being formed. Accordingly what is known as the pronephros or head kidney is first formed. This consists of a row of two or three nephridia on each side of the body. These nephridia are tubes, one end of which opens into the body-cavity and the other end into a common duct leading to the exterior. Each nephridium comes into contact with a bunch of tiny blood-vessels known as a glomerulus. From the blood in these the waste products of the embryo are taken up by the nephridia and so passed out of the embryo. As the embryo increases in size new nephridia are formed behind the first ones. These are of more complicated structure and are described as a second kidney, the mesonephros or middle-kidney. As the mesonephridia increase in number the pronephros gradually undergoes atrophy. A kidney of the mesonephros type suffices to carry off the waste products of comparatively simple organisms; in consequence in fishes it persists throughout life as the functional kidney. In some cases the pronephros also persists. The mesonephros is inadequate for the needs of organisms higher [i.e. more complex] than fishes, in consequence a far more complicated kidney—the metanephros or hind-kidney—develops behind the mesonephros. When this final kidney is ready to function, the nephridia of the mesonephros become absorbed, but their duct persists, being used to carry the male genital products. …
“The reason why the early embryonic kidney, instead of being converted into, is replaced by the adult kidney, thus appears to be, not that the embryo is compelled to recapitulate prepiscine and piscine stages, but that embryonic conditions require the kidney to be situated far forward—a position that would be inconvenient in the adult.”5
As yet medical research has not confirmed Dewar’s inferences about the function of the pronephros, but research has shown the pronephros to have the crucial function of inducing development of the kidney, as related earlier in this article. But the next stage, the mesonephros, does have kidney function, which would vindicate Dewar’s argument that it is designed for what it does and where it does it:
“Although there is evidence of urinary function in the mammalian mesonephric kidney, the physiology of the mesonephros has not been extensively investigated. Urine formation in the mesonephros begins with a filtrate of blood from the glomerulus into the glomerular capsule. The filtrate then flows into the tubular portion of the mesonephros, where the selective resorption of ions occurs. The return of resorbed materials to the blood is facilitated by the presence of a dense plexus of capillaries around the mesonephrous tubules.
“The structure of the human mesonephros is very similar to that of adult fishes and aquatic amphibians, and it functions principally to filter and remove body wastes. Because these species and the amniote embryo exist in an aquatic environment, there is little need to conserve water. Therefore the mesonephros does not develop a medullary region or an elaborate system for concentrating urine as the adult kidney does.”6
Evolutionists sometimes argue that the yolk sac of mammals is vestigial, being small and devoid of yolk, in contrast to birds and reptiles. However, an embryology textbook points out that it is vital to the embryo because of other functions associated with it.7
The so-called ‘yolk sac’ is the source of the human embryo’s first blood cells, and death would result without it!
As creationist biologist Dr Gary Parker points out, “The so-called ‘yolk sac’ is the source of the human embryo’s first blood cells, and death would result without it!” (Creation: Facts of Life, page 56). Even creation-hostile Wikipedia acknowledges its importance, saying “it functions as the developmental circulatory system of the human embryo, before internal circulation begins” (Yolk sac).
In fact, most embryologists no longer call it “yolk sac” but “umbilical vesicle”. Here is a relevant excerpt from a contemporary textbook:
Significance of the Umbilical Vesicle
Although the umbilical vesicle is nonfunctional as far as yolk storage is concerned (hence the name change), its presence is essential for several reasons:
- It has a role in the transfer of nutrients to the embryo during the second and third weeks when the uteroplacental circulation is being established.
- Blood development first occurs in the well-vascularized extraembryonic mesoderm covering the wall of the umbilical vesicle beginning in the third week (see Chapter 4) and continues to form there until hemopoietic activity begins in the liver during the sixth week.
- During the fourth week, the endoderm of the umbilical vesicle is incorporated into the embryo as the primordial gut (see Fig. 5-1). Its endoderm, derived from epiblast, gives rise to the epithelium of the trachea, bronchi, lungs, and digestive tract.
- Primordial germ cells appear in the endodermal lining of the wall of the umbilical vesicle in the third week and subsequently migrate to the developing gonads (see Chapter 12). They differentiate into spermatogonia in males and oogonia in females.8
Here is a comment from another textbook:
“The definitive yolk sac remains a major structure associated with the developing embryo through the 4th week and performs important early functions. Extraembryonic mesoderm forming the outer layer of the yolk sac is a major site of hematopoiesis (blood formation; discussed in Ch. 13). Also, as described in Chapter 1, primordial germ cells can first be identified in humans in the wall of the yolk sac.”9
The reason why the mammalian embryonic heart is at first a simple tube is, not that mammals evolved from fishes, but that, as the mammalian embryo must have a functioning heart at a very early stage, the simplest possible heart is formed.—Douglas Dewar
The evolutionary lecturer claims design flaws, but I would challenge him to design a better system that develops from a single cell and keeps the creature alive. Once again, the “simple” heart is vital for the embryo at this stage of development. Dewar explains:
“The so-called fish heart and gill-arches have to be formed because the head region of the embryo from a very early stage onwards, requires a copious blood supply. This necessitates the early formation of a heart or pumping organ and a simple system of blood vessels. These have to be formed before there is time to develop the four-chambered heart necessary to the higher animal. …
“The heart develops as follows: Two tiny tubes are formed which run parallel. Those coalesce to form a single tube; the wall of the front part of this thickens and the thickened part becomes separated from the thinner hind part by valves. The heart is now an effective pumping machine composed of two communicating chambers … In fishes this type of arrangement persists throughout life, being suitable for a gill-breathing animal … Animals higher up the scale need a more complicated heart and in them the embryonic heart becomes three-or four-chambered … by the growth of a septum in one or both of the chambers.
“Clearly then, the reason why the mammalian embryonic heart is at first a simple tube is, not that mammals evolved from fishes, but that, as the mammalian embryo must have a functioning heart at a very early stage, the simplest possible heart is formed. As development proceeds the form of the heart changes to meet the increasing demands made upon it.”10
You said you have read our two articles related to ERVs. We have previously published articles refuting the general “shared mistakes” claim by evolutionists:
- John Woodmorappe, Are pseudogenes shared mistakes between primate genomes? Journal of Creation 14(3):55–71, 2000.
- John Woodmorappe, Potentially decisive evidence against pseudogene shared mistakes Journal of Creation 18(3):63–69, 2004.
ERVs act as promoters, starting transcription at alternative starting points, which enables different RNA transcripts to be formed from the same DNA sequence.
We find the arguments in these two articles compelling. [André informed us he had also read the instructive overview Junk DNA: evolutionary discards or God’s tools? Which likewise discusses ERVs]
Extreme similarity (homology) of component parts is to be expected if things have the same Designer—see Are look-alikes related? Indeed, in most cultures that have existed around the world, such similarities would bring great honour to a designer, demonstrating his complete mastery over what he had made—see Not to Be Used Again : Homologous Structures and the Presumption of Originality as a Critical Value.
Our most recent article on ERVs, and specifically on this topic, is:
Shaun Doyle, Large scale function of endogenous retroviruses Journal of Creation 22(3):16, 2008.
This points out:
Moreover, researchers have recently identified an important function for a large proportion of the human genome that has been labelled as ERVs. They act as promoters, starting transcription at alternative starting points, which enables different RNA transcripts to be formed from the same DNA sequence. … We’re not just talking about a small scale phenomenon. These ERVs aid transcription in over one fifth of the human genome!
Since the so-called ERVs clearly have a vital function, this is consistent with a design explanation.
Best wishes in your course.
Andrew Lamb and Jonathan Sarfati
- Sadler, T.W., Langman’s Medical Embryology, 10th Ed., pp. 7–8, Lippincott Williams & Wilkins, 2006; bold in original. Return to text.
- Vialleton, L. L’origine des Êtres Vivants [The origin of living beings] 1930, Librarie Plon, Paris. Return to text.
- Dewar, Douglas, The Transformist Illusion, Sophia Perennis et Universalis, pp. 171–172, 1957/1995. Return to text.
- Schoenwolf, G.C, et al., Larsen’s Human Embryology, Fourth Edition, p. 483, Churchill Livingstone Elsevier, 2009. Return to text.
- Dewar, ref. 3, p. 198. Return to text.
- Carlson, B.M., Human Embryology and Developmental Biology, 3rd edition, p. 35, Mosby, Philadelphia, 2004. Return to text.
- Carlson, Ref. 6, p. 131. Return to text.
- Moore, K.L. and Persaud, T.V.N., The Developing Human: Clinically Oriented Embryology, 8th edition, p. 134, Saunders Elsevier 2008. Return to text.
- Schoenwolf, Ref. 4, p. 58. Return to text.
- Dewar, ref. 3, pp. 194–195. Return to text.