Introduction to Homology
Since 1859 the phenomena of homology has been cited by evolutionary biologists as providing one of the most powerful lines of evidence for the concept of organic evolution and to this day, the phenomena of homology has remained a mainstay argument for evolution (Denton, 1986).
In biology, two or more structures are said to be homologous if they are alike. In traditional biological thinking homology of say bone structure in the limbs of different mammals is associated with assumed shared ancestry - see quote from Darwin below.
In genetics, homology is used in reference to similar (or otherwise, e.g. non-homologous) DNA sequences, and again the assumption tends to be that sequences that are homologous share ancestry. Homology can also be used to describe similar protein sequences.
A common example of genetic homology is that we share about 98% of our genes with apes or chimpanzees. This is discussed later and also on the video clip DNA: Human & Chimpanzee DNA (1:23 mins) from the Christian Answers web site.
Below are some quotes from Darwin's The Origin of Species where he presents arguments for homology being linked to natural selection:-
Chapter 13 - Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs.
"We have seen that the members of the same class, independently of their habits of life, resemble each other in the general plan of their organisation. This resemblance is often expressed by the term `unity of type;' or by saying that the several parts and organs in the different species of the class are homologous. The whole subject is included under the general name of Morphology. This is the most interesting department of natural history, and may be said to be its very soul. What can be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include the same bones, in the same relative positions?
Nothing can be more hopeless than to attempt to explain this similarity of pattern in members of the same class, by utility or by the doctrine of final causes. The hopelessness of the attempt has been expressly admitted by Owen in his most interesting work on the `Nature of Limbs.' On the ordinary view of the independent creation of each being, we can only say that so it is; that it has so pleased the Creator to construct each animal and plant.
The explanation is manifest on the theory of the natural selection of successive slight modifications, each modification being profitable in some way to the modified form, but often affecting by correlation of growth other parts of the organisation. In changes of this nature, there will be little or no tendency to modify the original pattern, or to transpose parts. The bones of a limb might be shortened and widened to any extent, and become gradually enveloped in thick membrane, so as to serve as a fin; or a webbed foot might have all its bones, or certain bones, lengthened to any extent, and the membrane connecting them increased to any extent, so as to serve as a wing: yet in all this great amount of modification there will be no tendency to alter the framework of bones or the relative connexion of the several parts. If we suppose that the ancient progenitor, the archetype as it may be called, of all mammals, had its limbs constructed on the existing general pattern, for whatever purpose they served, we can at once perceive the plain signification of thehomologous construction of the limbs throughout the whole class".
However, "Similarity (‘homology’) is not evidence for common ancestry (evolution) as against a common designer (creation). Think about a Porsche and Volkswagen ‘Beetle’ car. They both have air–cooled, flat, horizontally–opposed, 4–cylinder engines in the rear, independent suspension, two doors, boot (trunk) in the front, and many other similarities (‘homologies’). Why do these two very different cars have so many similarities? Because they had the same designer"! Taken from Human/chimp DNA similarity. To top
Anatomical homology.
A well know example of anatomical homology linked to macroevolution is that given by Darwin above, e.g. "What can be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include the same bones, in the same relative positions"? What Darwin is basically saying is Why do virtually all vertebrate forelimbs have the same basic "pentadactyl" (five fingered) design, if not due to evolution?
In his book Evolution, A Theory In Crisis, the author Dr Michael Denton devotes a complete chapter to homology and discussed this issue in depth. His arguments against anatomical homology being related to shared evolutionary ancestry are in part summarised as below, and also later, when considering the homology of Cytochrome C protein sequences between different species.
"One of the most commonly argued proofs of evolution is the pentadactyl limb pattern, i.e. the five-digit limbs found in amphibians, reptiles, birds and mammals. However, they develop in a completely different manner in amphibians and the other groups. To illustrate, the human embryo develops a thickening on the limb tip called the AER (apical ectodermal ridge), then programmed cell death (apoptosis) divides the AER into five regions that then develop into digits (fingers and toes). By contrast, in frogs, the digits grow outwards from buds as cells divide". From Ostrich eggs break dino-to-bird theory.
Sir Gavin De Beer in his book Homology: An Unsolved Problem says "Homologous structures need not be controlled by identical genes and homology of phenotypes does not imply similarity of genotypes".
To investigate this subject in more depth, please follow some of the links at the bottom of this page such as The Invalidity of Morphological Homology, or read Michael Denton's bookEvolution, A Theory In Crisis.
DNA homology. DNA homology is also known as DNA relatedness. DNA relatedness compares the DNA of different species, e.g. one could compare the DNA relatedness of say apes, monkeys and man. If DNA of similar species is similar, evolutionists claim that this is added proof of evolution. The DNA of one species (say of an ape or monkey) has changed over time (by processes such as mutation and allelic exchange) to another species (say a human).
Human genome sequencing project. The recent human genome project has led to about 85 to 90% sequencing of the human genome but only two chromosomes have been fully sequenced (Nature, Genome special, June 2000, pp983-985; New Scientist, 1 July 2000, pp4-5). The quote below from the web site of Nature magazine shows what a colossal work it was to sequence as much as has been sequenced.
"Twenty years from conception to completion in draft form; the work of thousands of scientists from around the globe; over 90% coverage of our 3.2 gigabase genome. Everything about the Human Genome Project is large scale, including the 62 page report of the International Human Genome Sequencing Consortium which lies at the heart of Nature's genome coverage".
Having completed the project so far, much has been made not of the complexity of our DNA, but the similarity of our DNA to that of less complex animals, and the fact that we apparently only have twice as many genes as worms or flies.
"The papers published today give us for the first time a near complete set of human genes, which will form the basis for innumerable future investigations. It is estimated that there are only 30,000 to 40,000 genes, confirming the predictions made from the sequencing of chromosome 22, which was completed by the Sanger Centre in December 1999. This number, lower than once thought, is only twice as many genes as found in much simpler animals such as worms and flies. Many of the new genes in humans seem to be involved in organising how other genes work".
Much has also been made of the fact that we have many genes similar to those found in other living organisms. For example, studies have concluded that 75% of human genes or close variants exist in worms, that "we share half our genes with the banana" and that 98.4% of human DNA is similar to that of ape DNA. (New Scientist, 1 July 2000, pp4-5)!
However, it should be known that when comparisons are made between the human genome and other genomes, only short pieces of DNA are compared. Also, to date the entire ape or chimpanzee genome has not been sequenced (Creation News 2000, Vol. 14, No. 2). Sequencing whole genomes of living organisms is horrendously expensive (as one can imagine in view of the time to sequence most of the human genome). To date (2000) only the genomes of some bacteria and viruses, yeast, a weed named Arabidopsis and a worm named C. elegans have been totally sequenced and scientists are close to finishing the sequencing of the Fruit fly and mouse in the year 2000 (Creation News 2000, Vol. 14, No. 2).
So, on the basis of DNA homology, are we a little bit worm, a little bit banana, mostly ape and a little bit human? It is perhaps well to remember the size of the human genome of 3,200,000,000 base pairs as shown in the above quote from the Nature web site. Although we may comprise of only 30,000 to 100,000 (high estimate) genes, each gene can comprise hundreds to thousands of base pairs, and it is the unique sequence of base pairs that makes up a gene. It can take only one change in the coding sequence of one gene to completely alter or inactive the function of that gene.
Also, one should consider that similar gene sequences do not necessarily mean similar functions. Sir Gavin De Beer in his book Homology: An Unsolved Problem says "Homologous structures need not be controlled by identical genes and homology of phenotypes does not imply similarity of genotypes".
Shared functions - similar genes. Our bodies comprise millions of individual cells and each of these cells carry out complex biochemical reactions to perform the tasks relevant for that cell. At an external level we look very different from say a mouse or a banana. However, humans share the same environment as both the mouse and the banana and like the mouse and the banana we require oxygen, some common nutrients, minerals and water for our survival, repair and growth. Thus, there will be some commonality in the biochemical processes that go on within the cell of a human or a mouse or a banana.
Similar biochemical functions will require similar enzymes which in turn will require similar sets of genes to code for such enzymes. Thus, whilst we may look very different from say mice or bananas, we require many similar genes for functions at a cellular level.
Evolution of DNA. Taking the full range of supposed Evolution from a bacteria with a genome size of about 1,000,000 base pairs to about 1000x as many as that for a human, it would be truly amazing if all the complex sequence changes occurred by random mutational events followed by selection to produce the rich tapestry of life that we see. If these sequence changes did occur by random events, why should there be such similarity between different living organisms.
Radiation mutates DNA, but if you irradiate bacteria (or humans or fruit flies) you get damaged, not enhanced versions of the original. To top
Protein homology. Just as there can be homology between the way animals look or their bone structure (e.g. morphological or anatomical homology), and between their genes (e.g. DNA relatedness), so there can also be homology between the proteins of animals. However, once again there is evidence that this does not point to shared ancestry through evolution.
"The really significant finding that comes to light from comparing the proteins amino acid sequences is that it is impossible to arrange them in any sort of an evolutionary series." (
Cytochrome C. In his 1986 book, Evolution: A Theory in Crisis, Michael Denton has presented the most telling criticism of evolutionary trees based on protein sequence data. He analyses the percentage differences between the Cytochrome C molecules of different organisms to show that each group of organisms is equally isolated from any other particular group as shown in the table below:-
Percent amino acid sequence divergence between cytochrome C2 in Rhodospirillum rubrum and various eukaryotic Cytochrome C (taken from The Creation Explanation with permission requested).
If evolution were true, then the further organisms have evolved from bacteria, the greater change there should be in Cytochrome C.
| MAMMALS | BIRDS | TELEOSTS |
| Human 65 | Chicken 64 | Tuna 65 |
| Monkey 64 | Penguin 64 | Bonito 64 |
| Pig 64 | Duck 64 | Carp 64 |
| Horse 64 | Pigeon 64 | ELASMOBRANCHS |
| Dog 65 | REPTILES | Dogfish 65 |
| Whale 65 | Turtle 64 | CYCLOSTOMES |
| Rabbit 64 | Rattlesnake 66 | Lamprey 66 |
| Kangaroo 66 | AMPHIBIANS | |
| | Bullfrog 65 | |
| INSECTS | ANGIOSPERMS | YEASTS |
| Fruit Fly 65 | Mung-bean 66 | Candid Cruse 72 |
| Screw-worm 64 Silkworm 65 | Sesame 65 Castor 69 | Debaryomyces kloeckeri 67 |
| Tobacco Horn | Sunflower 69 | Baker's yeast 69 |
| Worm Moth 64 | Wheat 66 | Neurospora crassa 69 |
From the data in table above it is evident that the amino acid sequence of the Cytochrome c molecules of all of the species in all of the groups of organisms are equally isolated from that of the bacterium Rhodospirillum rubrum. Thus there is no basis in this data to indicate that any group is intermediate between other groups. All are equally isolated from all other groups. This data supports the biblical record of creation of each "kind" separate from all other "kinds." (taken from The Creation Explanation with permission requested).
Haemoglobin. Haemoglobin, the molecule that carries oxygen around the body in red blood cells is found in all vertebrates, but also exists in earthworms, starfish, molluscs, in some insects and plants and even in certain bacteria (Blanchard, 2002). However, when scientists examined the haemoglobin of crocodiles, vipers and chicken, they found that crocodiles were more closely related to chickens on the basis of similarity in haemoglobin than to their fellow reptiles (Blanchard, 2002).