Homology is a shared design concept that is generally assumed by evolutionists to be the result of a shared ancestry and therefore providing support for the Darwinian theory of common descent. Evolutionists assert that if multiple organisms evolved from the same ancestor, they would possess structural similarities. In The Origin of Species, Darwin referred to the similar bones in the same relative positions in the hand of a man and several other mammals as specifically what the theory of common descent would expect.
Homologies may include structural (morphological) components, such as the forelimbs, which illustrate a similarity of construction overall, but are yet unique in other ways, providing varied functions. Similarities during embryo development or in molecular structures are also considered to be derived from a common ancestor and therefore homologous. Cellular homologies include the existence of common organelles in eukaryotes, and nucleic acid (DNA/RNA) as the universal genetic code. Homology (similarities) is used as one of the main proofs for macroevolution and the common descent of all life on Earth.
The concept of homology (as proof of Darwinism) is commonly taught as a matter of fact in public schools. Prentice Hall Biology, which may be the most widely used Biology textbook in U.S. public schools, describes homology as follows.
|“||By Darwin's time, researchers had noticed striking anatomical similarities among the body parts of animals with backbones. For example, the limbs of reptiles, birds, and mammals—arms, wings, legs, and flippers—vary greatly in form and function. Yet, they are all constructed from the same basic bones, as shown in the figure at right. Each of these limbs has adapted in ways that enable organisms to survive in different environments. Despite these different functions, however, these limb bones all develop from the same clumps of cells in embryos. Structures that have different mature forms but develop from the same embryonic tissues are called homologous structures. Homologous structures provide strong evidence that all four-limbed vertebrates have descended, with modifications, from common ancestors.||”|
In contrast, creationists view most "homologous" structures as a reflection that they were designed by the same creator. It is held that a brilliant and well functioning design would be applied to multiple organisms, much in the same way as human designers apply concepts.
- Main Article: Embryology
Since the time Charles Darwin published The Origin of Species, embryology has been used to support common descent. In fact, Darwin referred to embryological homology as the strongest single class of facts that existed to support his theories. A theory later put forth by Ernst Haeckel, known as the biogenetic law, asserted that the evolutionary history of an organism was recapped during embryo development. Although the biogenetic law is now discredited, in recent years, embryology has reemerged as a tool used by evolutionary biologists that attempt to establish phylogenetic relationships by identifying developmental similarities between taxonomic groups. Evolutionary Developmental Biology is a merging of developmental biology and evolutionary biology that is commonly known as "evo-devo".
Several decades before Darwin published the The Origin of Species, a German embryologist Karl Ernst von Baer had shown that the embryos of some vertebrates looked very similar during particular stages of development. This observation became known as von Baer's law. After reading a summary of von Baer's law in 1842, Darwin concluded that embryonic resemblances were a very strong argument in favor of common descent. In The Origin of Species (1859) Darwin stated "Community of embryonic structure reveals community of descent", and in a 1860 letter to Asa Gray stated that "embryology is to me by far the strongest single class of facts in favour of change of forms". Despite Darwin's use of embryology to support his views, Von Baer rejected the theory of common descent and did not regard embryological similarities as evidence for evolution.
Differences among organisms have traditionally been assumed to become more significant as development proceeds, and therefore earlier embryonic stages should resemble one another more than latter stages. Although these assumptions have persisted into modern times, it has been recognized by embryologists since Darwin that this doctrine of developmental biology "is not in accordance with the facts of development", according to British embryologist Adam Sedgwick (1894) who also stated: there is no stage of development in which the unaided eye would fail to distinguish between them with ease". Modern embryologists confirm this as well, and have further challenged the erroneous nature of these assumptions. Recent work has revealed that embryos are not more similar during the earlier stages of cleavage and gastrulation, but in fact, are most alike at the later stage illustrated by Ernst Haeckel in his infamous forgeries. In the gastrulation stage, fish show dramatic differences when compared to amphibians, and neither are similar to reptiles, birds, or mammals. Organisms begin life very dissimilar, then become somewhat alike during embryo development, then diverge again as adults. Evo-devos (evolutionary developmental biologists) refer to this pattern as the developmental hourglass.  William Ballard adds that only by semantic tricks and subjective selection of evidence can we claim that “gastrulas” of shark, salmon, frog, and bird are more alike than their adults.
In biology, homology is defined as any structure within a creature’s body that is similar in shape, placement, and/or function. This is considered different from analogy, where a structure serves a similar purpose by way of a structure that is similar in outward appearance. Evolutionary biologists often relate these similarities in body structure to point to the idea that all living organisms derived from the same original ancestor. It seems plausible or, in the evolutionist’s perspective, undeniably logical that if one animal has a limb or an organ similar in shape or function to that of another, though differing slightly in size, purpose, or ultimate necessity, it has a common ancestor that possessed the common trait and passed it down to both descendants. Evolutionists also see this as an explanation for the existence of vestigial structures in the body. A vestigial structure is any organ, bone, etcetera, that is shown to have no function whatsoever, is not necessary for the creature’s survival, or is useless to serve the purpose that a homologous structure in another creature would serve. Some morphological homology, or homology in the body, is developmental (meaning the particular body part developed from the same embryonic tissue during early life in the womb) while some is evolutionary (meaning that the particular body part evolved from that of an early, common ancestor and, as mentioned before, it was passed down to more than one descendant). 
In 1971, embryologists Sir Gavin de Beer, raised the question of homologous structure, which remains unanswered.
|“||But if it is true that through the genetic code, genes code for enzymes that synthesize proteins which are responsible (in a manner still unknown in embryology) for the differentiation of the various parts in their normal manner, what mechanism can it be that results in the production of homologous organs, the same "patterns", in spite of their not being controlled by the same genes? I asked this question in 1938, and it has not been answered. ||”|
Some homologous structures, such as the eyes of a human and the eyes of an octopus are very similar but are extremely distant on the evolutionary tree. Evolutionists, then, are quick to label these structures as analogous, though they are really more homologous, in order to save their theory. This is just another example of how evolutionary scientists only look for answers that line up with their dogma instead of considering scientific facts that could contradict it. It is unscientific to believe that such complex and similar structures could have come about separately, by way of chance mutations, at drastically different times. But, to say that they evolved at relatively the same time breaks down the whole theory of the order in which things evolved.
Wing structures are also pushed aside and labeled as analogous because according to Darwinism, these structures have to have evolved at four different times during history. The likelihood of such similar, yet separate, occurrences is incredibly minute because they do not make sense in the framework of either natural selection or mutation.
Mammals are another strong example of the flaws in evolution. Mammals are divided into three groups called monotremes, placentals, and marsupials. According to evolution, these groups all evolved and developed individually after appearing in history. The problem is that many animals among placentals that have counterparts, so to speak, that are marsupials, the North American Wolf and the Tasmanian Wolf for example. These counterparts have very similar, or homologous (though the common evolutionist would probably label them as analogous), bodily structures. Once again, it is incredibly unlikely that these very similar creatures could have evolved separately, while remaining completely unrelated to each other. 
To the evolutionist, molecular homology is perhaps considered the most substantial evidence of common ancestry. All organisms on Earth use same basic molecules for cellular processes: DNA as their heritable information-carrying molecule, and proteins as their molecular machines. Furthermore the genes and proteins that perform similar functions are frequently, though not always, very similar. In general, the more similar the molecules, the ancerstries are believed to be.
Examples of cellular and molecular homology include:
|“||DNA comparisons are just a subset of the homology argument, which makes just as much sense in a biblical framework. A common Designer is another interpretation that makes sense of the same data. An architect commonly uses the same building material for different buildings, and a car maker commonly uses the same parts in different cars. So we shouldn’t be surprised if a Designer for life used the same biochemistry and structures in many different creatures. Conversely, if all living organisms were totally different, this might look like there were many designers instead of one. ||”|
Because genes are believed to have a ancestral origin, if two animals have homologous morphological structures, then the genes that produced those structures should be homologous as well. Unfortunately for Darwinism, these genes are often not homologous. In a vertebrate embryo, a countable number of segments divide the tissue. Each segment or group of segments is destined to develop into a particular structure. Studies conducted on six kinds of vertebrates showed that the front and back limbs of each creature developed from different groups of segments (especially the back limbs).  Homologous organs are produced by non-homologous DNA and homologous DNA develop into non-homologous organs. Also, homologous structures are shown to go through completely different embryological stages of development, when they should be the same up to the point of the evolutionary difference. Pere Alberch wrote that these differing embryological stages in homologous structures are "the rule rather than the exception".
|“||Furthermore, there are some puzzling anomalies for an evolutionary explanation—similarities between organisms that evolutionists don’t believe are closely related. For example, hemoglobin, the complex molecule that carries oxygen in blood and results in its red color, is found in vertebrates. But it is also found in some earthworms, starfish, crustaceans, mollusks, and even in some bacteria. An antigen receptor protein has the same unusual single chain structure in camels and nurse sharks, but this cannot be explained by a common ancestor of sharks and camels.6 And there are many other examples of similarities that cannot be due to evolution.||”|
Which Does it Prove?
Evolutionist, such as Theodosius Dobzhansky, see homology to be such strong proof of evolution, that creationism implicitly accuses God of creating the world to mislead students that it actually evolved.
|“||To be sure, some diehard anti-evolutionists still insist that homology means only that the Creator gratuitously chose to make homologous organs in quite unrelated organisms. This opinion may be said to be implicitly blasphemous: it actually accuses the Creator of arranging things so that they suggest evolution merely to mislead honest students of His works. (Dobzhansky 1959)||”|
A common ancestor seems to make sense when you are told that homologous structures developed from the same creature. Yet, you have factor in all of the flaws in the "proof" that homology supposedly provides. Mammals that are said to have evolved individually, but have homologous structures, or homologous organs that formed from non-homologous DNA all disprove this theory. Also, the presumption that a divine creator would always be completely original in His creations, does not take into account that a working model that is being reused shows signs of intelligence. There is plenty of proof to show that homology is an extremely poor basis for supporting evolution, but rather supports intelligent design.
The idea that homology proves that all living creatures developed from a common ancestor derives from the belief that a divine creator would be ultimately original and unique in his designs of every single creature. Though, just because God would have infinite power and ability for creativity, doesn’t mean that He is restricted to being original. Our creator devised a model framework that was effective, so why shouldn’t He use this framework multiple times? If you were designing cars for an automobile company, you may differ in your plans for the outward appearance of different models, but it would make no sense to try to rebuild the entire engine or rework all of the wiring, or even to remove the wheels and try to invent some other method of getting the car to move. To reuse an ingenious and perfectly capable design is a sign of intelligence. 
- Bergman, Jerry., Does homology provide evidence of evolutionary naturalism? Journal of Creation 15(1):26–33 April 2001.
- Darwin, C., The Origin of Species, 6th Ed., pp. 434-435, 1872.
- Prentice Hall Biology. 2008. p384. Kenneth Miller & Joseph Levine.
- Wells, Jonathan. The Politically Incorrect Guide to Darwinism and Intelligent Design, p.25-26. Regnery Publishing, Inc. 2006.
- Gilbert, Scott F. Developmental Biology: Evolutionary Embryology, 6th Edition. 2000. by Sinauer Associates.
- The Life and Letters of Charles Darwin Volume II, Page 61.
- Wells, p.26
- Sedgwick, Adam, 1894. "On the Law of Development commonly known as von Baer’s Law; and on the Significance of Ancestral Rudiments in Embryonic Development." Quart. J. Microscopy, 36, p. 36
- Wells, p.30
- Ballard W.W., "Problems of Gastrulation: Real and Verbal," BioScience, Vol. 26, No. 1., January 1976, pp.36-39.
- Sir Gavin de Beer. "Homology, An Unsolved Problem", 1971 (Oxford Biology Reader)
- Sarfati, Jonathan. Refuting Evolution 2 Chapter 6 - Argument: Common design points to common ancestry. Greenforest AR: Master Books, 2002. (p112)
- Sarfati, p113.
- Dobzhansky, T., Evolution; Genetics and Man, John Wiley & Sons, New York, pp. 227-228, 1959. p. 228.
- Homology/biology Wikipedia. www.wikipedia.org.
- Not to be used again Creation Ministries International. Journal of Creation. by James Patrick Holding. Volume 21(1) 2007. www.creationontheweb.com
- The Invalidity of Morphological Homology by Adnan Oktar. www.darwinismrefuted.com
- The Genetic and Embryological Impasse of Homology by Adnan Oktar. www.darwinismrefuted.com
- A serious problem for homology by Robert E. Kofahl, Creation 14(2):31 March 1992
- Argument: Common design points to common ancestry by Jonathan Sarfati. Refuting Evolution 2 (Chapter 6)
- Does homology provide evidence of evolutionary naturalism? by Jerry Bergman. Journal of Creation 15(1):26–33 April 2001.
- Homology: A Concept in Crisis by Jonathan Wells and Paul Nelson. Origins and Design 18:2, Fall 1997.
- Homology: A Problem, not a Proof, for Evolution By Jon A. Covey.
- Homology can't be evidence of ancestry if it is defined thus Response to Talk.Origins