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Feedback 2009
The human umbilical vesicle (‘yolk sac’) and pronephros—Are they
vestigial?
Published: 2 May 2009(GMT+10)
After Larsen
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.
Thank you,
André Z
Hi André
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).]
Photo stock.xchng
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.
Embryonic development
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.
Embryonic kidney development
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
Yolk sac
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
Embryonic heart
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
Endogenous retroviruses
The term “endogenous retroviruses” is inherently misleading—see
the ‘Endogenous retroviruses‘
section within the article Junk
DNA, asteroid impacts, and supernovas.
You said you have read our two articles related to ERVs. We have previously published
articles refuting the general “shared mistakes” claim by evolutionists:
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
CMI–Au
Further reading
Recommended Resources
References
- 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.
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