Homology

Homology in biology describes similarity of structures.  This can be clearly seen in many cases.  For example we have four limbs, as do all vertebrates.  We have a nervous system, which in all essentials is again found in all vertebrates.  We have hair which is found on many vertebrates.  Human beings bear live young and suckle them, and the structures that are necessary for this are also found in other mammals.

For Darwinians the implication of this is that structures have a common origin in ancestors.  The argument is that since apes have hair we are descended from them; since vertebrates have four limbs they are all descended from some hypothetical four-limbed ancestor.  However we now know that this hypothesis is not factually justified, although it may be intuitively 'obvious'.  As Sir Arthur Conan Doyle pointed out, there is nothing so dangerous as an obvious fact.

Examples of homology are very common.  For example vertebrate pentadactyl limbs all have the same basic pattern, only being modified for each individual animal.  The same bones are in the same relative positions but shaped for particular purposes.  The human hand is designed for grasping, a mole paw for digging, a horse leg for running, a porpoise paddle for thrusting in water, and a bat's wing for flying.

Finer examples are the hind feet of a kangaroo for bounding over the plain, and koala feet for gripping branches.  Bandicoots, being ground dwellers, have the second and third digits very thin and bound together in a skin.  Each organism has the same basic pattern modified for very different ends. (Denton 1985 p144)

Homology has always been the 'obvious' evolutionary 'proof'.  The argument runs that only if animals developed in a sequence were we likely to get the sort of similarities we now see.  It has actually been argued that this is because a Creator would not feel bound to stick to a basic plan, to which the natural and immediate response is, 'Why ever not?  Don't we do exactly that when making things?'

Unfortunately this argument of the Evolutionists is illogical.  It is not immediately obvious why a God shouldn't keep to a pattern.  We do exactly that in the designs of transport vehicles.  A careful study of vehicles will reveal very similar patterns of structures.

It is important to note that Darwin never claimed to provide proof of evolution (Encyclopaedia Britannica 1981) or of the origin of species by homology, but he did claim that evolution provides a convincing explanation of facts otherwise (he believed) difficult to explain.  The argument for homology, he claimed, is the more telling because the bones can be seen to have been modified as the animals 'developed'.  Homologous resemblence is 'the main concept of evolutionary comparative anatomy' (Penguin Dictionary of Biology 1961), for without an underlying homologous resemblence in the fundamental design of dissimilar organs then evolution would have nothing to explain.

However, we have moved on somewhat from Darwin's limited knowledge.  In the first place, if homology has any meaning in our present understanding of cell biology, homologous structures would have to have homologous genes.  They would also have to follow homologous embryonic development.  We now know that neither is the case, in fact.

All vertebrates have similar structures: therefore they must have a common ancestor.  The assumptions, therefore, are that all vertebrates must develop

1. from the same ground tissues
2. by similar biochemical pathways and processes

While it is true that all vertebrates have similar structures, a study of homology actually provides incontrovertible evidence that they could not have had common ancestors.

1. homologous organs do not always develop from the same ground tissue:
   
   Blastula formation (the first tiny ball of embryo cells) and the blastulas themselves are different in amphibians, reptiles and mammals.  Gastrulation (a process in which the ball develops a hollow) diverges even more.  The actual gastrulae themselves may be homologous, in the sense that the structures may look similar, and the same basic ends are met, but the means whereby this occurs differs in remarkable degree in the three classes.  The stages in the three are so dissimilar that, were it not for the fact that we see the apparent homologies in the adult animals, we would not even put them in the same phylum (vertebrates, in this case). (Denton 1985 pp115, 145-7)


2. homologous organs cannot develop by similar biochemical pathways and processes:
   
   The failure to find a genetic and embryological basis of homology was stated by Sir Gavin de Beer.  Homologous structures are often specified by non-homologous genes, and the concept of homology, he said, can seldom be extended back into embryology (de Beer 1971).  In fact he is hedging his bets here.  We have found no examples where homology can be extended back into embryology.

After gastrulation the sites from which homologous structures are derived are different in the different classes.  The alimentary canal (the gut), for example, is formed from the roof of the embryonic gut cavity in the cartilaginous fishes (sharks), from the floor of the embryonic gut cavity in the lamprey, from roof and floor in frogs (amphibians), and from the lower layer of the embryonic disc, the blastoderm (a totally different area again), in birds and reptiles.  There is, therefore, no common site that provides this fundamental structure.

The vertebrate forelimbs come from trunk segments 2-5 in the newt (amphibian), 6-9 in the lizard (reptile), and 13-18 in man (mammal)!  On this basis the vertebrate forelimbs are not strictly homologous at all!  The development of the kidney follows a similar disparate development in the major classes, none being developmentally homologous.

De Beer describes the case of the eye lens of frogs.  This develops from the epidermis (the skin) overlying the optic cup (the hole in the skull in which the eye eventually sits), and the problem is that it always develops in exactly the right place and fits perfectly.  It has been discovered that the development is 'organised' by certain tissues, notably the optic cup itself, which induces the epidermis to develop in just this way, and only in this place, and only to a certain (correct) size.

At least, this is true of Rana fusca, the common frog.  But in Rana esculents, the edible frog, and one very closely related, the lens develops perfectly even when the optic cup is removed.  The lenses are undoubtedly homologous in the structural sense, but they differ completely in the mechanism by which their development is carried out.  Apparently there are many other examples of this sort of thing.

In true vertebrates the spinal cord has an organiser, but the tadpole larvae of the tunicates (supposedly ancestral to vertebrates) has no organiser.

De Beer points out that insects provide even more bizarre examples of this.  In metamorphosis the larval contents usually liquify and then reform from imaginal buds or discs (starting points) within the larval wall.  Logically these would be closely similar in position and effect in all related insects, but they are not.  In some, indeed, dissolution does not even occur.  The ways in which the adult organ systems are formed are bewilderingly diverse in different species and there is not common pathway.

In the Coleoptera (beetles) it is the foregut which contains the starting points, and there is no liquefaction: larval cells renew their activity throughout the gut area.  In Lepidoptera (butterflies) and Diptera (flies) there is complete liquefaction and dissolution, and, again, the imaginal rings proliferate cells at the ends of the new fore and hind gut.

In Coleoptera the excretory tubules are rebuilt from special cells in the larval tubules.  In the Hymenoptera (Stone Flies) tubules are replaced by new tubules developing from the tip of the hind gut.  These fundamental structures show no developmental, and therefore logically, no genetic homology, even though they are considered homologous structurally.

The Spermatophyta consist of the conifers and the flowering plants, and are grouped together on the basis of the homology of their seeds.  Each seed consists of an enclosed egg or ovule, with its associated food store, the endosperm.  This is the common structure on which the classification of these organisms is based.  Yet the methods of formation of both differs markedly in the two groups, rendering developmental (and therefore genetic) homology invalid.

The mammal heart starts as a single tube, develops into a double tube, reverts to a single tube, then develops again into a double tube (White 1989).  This is unique in vertebrates.

Darwin defined homology as 'that relationship between parts which results from their development from corresponding embryonic parts' (Darwin 1872) and it is exactly this which is wholly untrue of homology (De Beer 1971).

Sir Alastair Hardy said that "The concept of homology is absolutely fundamental to.......evolution - yet......we cannot explain it at all in terms of present day evolutionary theory." (Hardy 1965)

There are many other difficulties.  A typical one is that both fore and hind limbs in a vertebrate are homologous.  The detailed correspondence of the limbs is very striking, but by no stretch of imagination did they develop from one another.  If we invoke evolution then they must have evolved separately, from the pectoral and pelvic fins of a fish.  Further, these virtually identical structures, by evolution, must have been formed by millions of incredibly similar tiny advantageous mutations.  Is there really an adaptive necessity for all the known structures in the pentadactyl limb in the precise relationship which they show?  Denton believes this to be highly unlikely. (Denton 1985 p152)

Perhaps many cases of homology are merely analogous and show no descent from a common origin.  If that is the case then we are faced with a breathtaking idea: that many of the structures which appear to have developed under selection pressure were inevitable.  We have to accept that designs of structures are in some way part of the fabric of our universe!  Some are actually beginning to argue this, which must underline the depth of frustration which Evolution imposes on them.

As biological knowledge has accumulated, common genealogy as an explanation for origins moves further away from us as a reasonable explanation of the facts.  There is simply no biological explanation for homology.

Closely bound up with the concept of homology is the idea of embryonic recapitulation.  Although this has long been discredited as a proof of evolution it still turns up like a bad penny, particularly in school textbooks.  If this proves anything it is that school textbook writers don't keep up with the literature, and, worse, don't care about probity and intellectual rigor.  What a terrible example to the young!

The basic argument was that the embryos of man and other vertebrates are so similar that it is almost impossible to tell them apart, and this means that the embryos of man and the 'higher' animals are 'recapitulating' their genetic past.  That is to say, they are supposed to be going through the development which they went through when they were lower animals, and this is supposed to give us strong indications that man evolved from 'lower' animals.

The gentleman who first proposed this was one Ernest Haeckel in 1868.  I find it significant the Haeckel's name is so tarnished that people like Denton don't even mention him.  Subsequently, and quite quickly by scientific standards, Haeckel's proposition was debunked.

Ernst Haeckel's drawings were declared fraudulent by Professor His in 1874 at which time Haeckel confessed.  Even Haeckel's confession was fraudulent and misleading, "a small portion of my embryo-pictures (possibly 6 or 8 in a hundred) are really (in Dr Brass's [one of his critics] sense of the word) "falsified" - all those, namely, in which the disclosed material for inspection is so incomplete or insufficient that one is compelled in a restoration of a connected development series to fill up the gaps through hypotheses, and to reconstruct the missing members through comparative syntheses.  What difficulties this task encounters, and how easily the draughtsman may blunder in it, the embryologist alone can judge."  (Haeckel 1909)

See this site for a more detailed run-down of the fraud.  The paragraphs above and below are quoted direct.

In 1997, Michael Richardson (embryologist at London's, St. George's Hospital Medical School) exposed Haeckel's fraud in a published paper (Richardson 1997).  He shows that the early embryonic stages of 39 different creatures including the fish, the turtle etc., are nothing like the same.  Richardson and his colleagues compared the embryos of 50 vertebrates to Haeckel's drawings.  They say that Haeckel left out some features, such as the budding limbs that some embryos have, while adding others, such as an excess of vertebra-like "somites".  Dr. Michael Richardson called Hackael's biogenetic law as "one of the worst cases of scientific fraud".

The 'gill slits' are depressions between the corresponding arches which support the blood vessels necessary to supply the forepart of the body.  In the human embryo they are never perforated and there are no gills.  The respiration of the human embryo is through the placenta, and therefore there is no functional relationship whatsoever between the gill slits of the fish and the visceral furrows of the human embryo.

"The visceral pouches of embryo reptiles, birds and mammals bear little resemblence to the gill slits of the adult fish.  Anyone who can see can convince himself of the truth of this.  All that can be said is that the fish preserves its visceral pouches and elaborates them into gill slits, while reptiles, birds and mammals do not preserve them as such but convert them into other structures such as the Eustachian tube, the tonsils and the thymus glands." (de Beer 1958)

There is another issue with respect to homology here.  If the gill slits are truly homologous, then why do they not produce homologous structures in the adult organisms?  If they do not produce this, and they don't, then we cannot reasonably claim them to be homologous in any sense of the word.

"A fancied resemblence between two living things does not begin to prove that there is a physiological relation between them.  Even when the similarity is real, as it is between the harelip of a baby and the lip of a rabbit, it does not indicate that they are physiologically connected, but rather that the laws of embryonic development are similar." (Van Haitsma 1941)

References:
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Darwin, C., 'The Origin Of Species' (1872) 6th Ed (1962) pp434-5
de Beer, G., 'Embryos and Ancestors' Oxford 3rd Ed (1958) p52
de Beer, G., 'Homology, an Unresolved Problem', (1971) OUP)
Denton, M., (1985) 'Evolution, a theory in crisis', Adler & Adler, Maryland
Encyclopaedia Britannica (1981) 15th Ed, Macropedia vol 7, p8
Haeckel, E., (1909) letter to M�nchener Allegemeine Zeitung, 'an international weekly for Science, Art and Technology', January 9, 1909.  Recorded in 'The Truth about Haeckel's Confession', The Bible Investigator and Inquirer, M.L. Hutchinson, Melbourne, March 11, 1911, pp. 22-24
Hardy, A., 'The Living Stream', (1965) Collins, p213
Penguin Dictionary of Biology (1961) p114
Richardson, M., (1997) 'Anatomy and Embryology', Vol.196 (2), pp 91-106
Van Haitsma, J.P., 'The Supplanter Undeceived', Calvin College (1941) p153
White, A.J., (1989) 'Wonderfully Made', Evangelical Press, Darlington