Universal common ancestry
The idea of universal common ancestry is the idea that all life on Earth is related via a single family tree. Accordingly, it supposes that human beings, as well as animals, plants and every other form of life on Earth are related. People and monkeys are thus distant cousins.
While universal common ancestry (UCA) is a keystone of the naturalistic theory of evolution (NTOE), it is not exclusive to evolution. Some Intelligent design advocates, including Michael Behe, have no quarrel with UCA.
Creationism vigorously rejects this idea, because it contradicts a straightforward reading of Genesis, which reports that God created each living "according to its kinds," and that God created mankind in His Image, having a superior position and dominion over the rest of God's creation. Creationists also believe that the scientific evidence makes universal common ancestry appear highly improbable or impossible. They believe instead that life originated in a number of unrelated forms, as understood by Baraminology.
Universal common ancestry is an ancient idea. Many ancient pagans held that humans were related to animals. Anaxagoras, for instance, argued that humans are related to fish, but became humans because the water became too crowded, and the fish kicked humans out of the water. Many ancient American Indian cultures also believed humans were descended from animals, and some commemorated these beliefs through totem poles.
More recently, in the 18th and 19th centuries, the idea of universal common ancestry regained popularity in Europe. In 1745, French mathematician and scientist Pierre-Louis Moreau de Maupertuis (1698-1759) wrote in his book Venus Physique:
- "Could one not say that, in the fortuitous combinations of the productions of nature, as there must be some characterized by a certain relation of fitness which are able to subsist, it is not to be wondered at that this fitness is present in all the species that are currently in existence? Chance, one would say, produced an innumerable multitude of individuals; a small number found themselves constructed in such a manner that the parts of the animal were able to satisfy its needs; in another infinitely greater number, there was neither fitness nor order: all of these latter have perished. Animals lacking a mouth could not live; others lacking reproductive organs could not perpetuate themselves... The species we see today are but the smallest part of what blind destiny has produced..."
- "[P]robably all of the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed."
- "The whole history of the world, as at present known, ... will hereafter be recognised as a mere fragment of time, compared with the ages which have elapsed since the first creature, the progenitor of innumerable extinct and living descendants, was created."
- "When I view all beings not as special creations, but as the lineal descendants of some few beings which lived long before the first bed of the Silurian system was deposited, they seem to me to become ennobled."
Bases for belief
Belief in common descent is largely derived from the similarity among forms of life. For example:
- All cells are bound by a membrane
- All cells store heritable information as DNA.
- All cells adhere to the same basic mechanism of gene expression.
- All cells utilize the same metabolic pathways.
- All cells have ribosomes to produce protein.
From these similarities, an evolutionist infers that all life is related through one original life form.
Creationists argue that the inference of common ancestry cannot reasonably be drawn from similarity, for two reasons:
- Similarity is not meaningful evidence of common ancestry
- Differences which cannot be explained by evolutionary mechanisms are powerful evidence against common ancestry.
To illustrate the first objection, it stands to reason that if a designer were creating several lifeforms, he would probably use similar designs, because the designs are functional. Just as all cars have design similarities, including four tires, an internal combustion engine, a steering wheel, but obviously are not related; diverse species may simply share common design characteristics. Thus creationists argue that just because humans and plants both have cell membranes is no more evidence of common ancestry than it is of common design.
The second objection includes incidents of irreducible complexity and specified complexity to point to the unlikelihood that all life forms diversified through variation and selection alone. They point to issues such as:
- The transition from non-self-reproducing chemicals to self-replicating cells: the process of self-replication requires an enormous number of components be present in the cell for it to function at all, and none of the components required for self-replication serve any purpose unless all are present simultaneously; therefore it is unreasonable to conclude that abiogenesis occurred without design.
- The transition from asexual reproduction to sexual reproduction: male reproductive organs are incredibly complex, and serve no purpose unless female reproductive organs (which are equally, if not more, complex) already exist. Therefore the sexes could not have evolved in a step-wise fashion, because neither set of reproductive organs would serve any purpose unless the other set already existed.
- The evolutionary explanation is that living things share common features because the DNA that produces those features has a common ancestry. Creationists point out that in some cases, common features are caused by different genes, thus refuting that explanation.
For the above reasons, creationists conclude that similarity does not provide meaningful evidence of common ancestry.
A Retrovirus is a virus that incorporates itself into an host cell's DNA, and programs the host cell to produce copies of the virus. Endogenous retroviruses (ERVs) are retroviruses that infected an organism's germ cells, and were thus passed to the next generation, and incorporated into the organism's genetic line over time.
Researchers estimate that 1% of the human genome is occupied by ERVs, constituting approximately 30,000 different ERVs. Some of these ERVs are shared by humans and apes, and are found in the same place in the respective genomes. 
From these facts, evolutionists argue that these ERVs are shared by humans and primates because they were inherited by a common ancestor.
Evolutionists gloss over two important facts when making this argument: First, many retroviruses can infect both humans and apes. The most notable of these is HIV, which is widely believed to have originated as SIV in chimpanzees, but can also infect humans, apes, and monkeys. It is entirely possible, therefore, that humans and apes were independently infected with the same virus. Second, some retroviruses have been shown to have highly targeted insertion points, meaning that the virus selects very specific segments of the genome for insertion. Consequently, it is entirely possible that the same virus infected both humans and apes, and targeted the same location. This seems especially plausible in light of the fact that humans and apes have tens of thousands of endogenous retroviruses in their respective genomes -- at least a few of the retroviruses are likely to infect both humans and apes at the same location.
Additionally, scientists have determined that some endogenous retroviruses are indispensable to a species' life or reproduction. If the retrovirus is advantageous in some way, that would explain how the retrovirus spread to the entire species. It also presents the possibility that retroviruses were used as part of the creative process, as retroviruses are often used in genetic engineering today, to introduce new genetic material to cells.
Common DNA-based genes in diverse species
The most persuasive argument for common ancestry is existence of common DNA-based genes in diverse species. But there are at least three other plausible explanations -- parallel evolution, horizontal gene transfer, and creationism.
- Parallel evolution: Some people who believe in evolution deny universal common ancestry. They point out that there is still no good theory for how DNA-based life could have arisen from non-life. If it happened once, it may have happened many times. Some people say that separate evolution might have had an entirely different genetic code, but no one knows. It may well be that DNA-based life evolved from non-life many times, and the descendants of that life are living today.
- Horizonal gene transfer: It is also known that genes sometime migrate somehow from one species to another species, so having a common gene is not proof of common ancestry. Maybe there is no universal common ancestor, and important genes were spread by horizontal gene transfer.
- Creationism: Creationists argue that common genes in diverse species is no more evidence of common descent as it is of common design.
- Alan D. Gishlick, Alan D., Icons of Evolution? Darwin's Tree of Life
- Sverdlov, E. D. (2000) "Retroviruses and primate evolution." BioEssays 22: 161-171. 
- Lebedev, Y. B., Belonovitch, O. S., Zybrova, N. V, Khil, P. P., Kurdyukov, S. G., Vinogradova, T. V., Hunsmann, G., and Sverdlov, E. D. (2000) "Differences in HERV-K LTR insertions in orthologous loci of humans and great apes." Gene 247: 265-277.
- Citation: (2004) Retroviral Gene Vectors Show Clear Target Preferences. PLoS Biol 2(12): e443 doi:10.1371/journal.pbio.0020443 
- "Researchers Discover that Sheep Need Retroviruses for Reproduction," Texas A&M AgNews, 11 Sep 06.