Mutation Rates as a Creationist Clock

By Paul Bechard

Phylogenetic trees established by measuring mutation rates in chromosomal and mitochondrial DNA and extrapolating back to the age of a common ancestor have recently come under criticism. Critics question how much we know about both chromosomal and mitochondrial DNA (mtDNA) mutation rates and whether mtDNA characteristics, such as maternal inheritance and non-recombination, are always true. Scientists in the field have brought up these questions, and some Creationists, such as Carl Wieland and Dr. D. Russell Humphreys, have seized upon them as a way to discredit the theory of evolution and an old earth.

DNA, the string of nucleic acids found in and essential to all living cells, contains more than just the information necessary to code for various metabolic proteins. DNA holds information capable of delineating its own history. Not only can DNA show relationship between two individuals or species, it can provide the information necessary to estimate how long ago the two individuals or species in question shared a common ancestor. The method by which DNA is duplicated and passed on to daughter cells makes it ideal for tracing this history.

Ideally, after a cell has replicated its DNA, each chromosome will have produced another identical chromosome. For the most part, this is true; the machinery used to replicate DNA strands is exceedingly accurate. However, the machinery occasionally makes a mistake. The likelihood that a mistake will be made in the same spot the next time replication occurs is incredibly small. Thus, the error will be passed along to subsequent generations of daughter cells. Over time, DNA will accumulate more and more errors, which have two uses for genetic researchers. The first is that they can be used to establish relatedness between individuals or species. The second is that assuming a constant rate of error, or mutation, the amount of time required for a certain number of mutations to accumulate can be calculated, and therefore, the time since the two species diverged can also be calculated.

One method of determining the existence and approximate age of a common ancestor to all humans lies in the mutation rate of mtDNA. The DNA found in mitochondria has several advantages over nuclear DNA. First, mtDNA is inherited maternally. A direct maternal inheritance easily allows relationships to be established. Also, mitochondrial genes do not undergo a shuffle with each generation as nuclear chromosomes do (Zimmer 2001:32). This means that the differences between the mtDNA of a mother and her offspring can only be due to random mutation, which, as stated above, can be used to establish relatedness when the rate is taken to be constant.

The current mutation rate for human mtDNA was established using estimates that place the human and chimpanzee common ancestor at five million years ago. The differences in mtDNA and protein between all the great apes are then used to calculate a mutation rate for mtDNA. The currently accepted rate suggests about one mutation every 300 to 600 generations, or one every 6,000 to 12,000 years (Gibbons 1998:28). In a study conducted by Allan Wilson, researchers gathered samples of mtDNA from people around the world. After sequencing, they discovered that all native Europeans shared unique markers, as did all native Asians. Wilson and colleagues used these markers to construct an evolutionary tree in which African lineages formed the lower-most branches, suggesting humans descended from an African woman. They also used the accepted rate of mtDNA mutation to calculate that this woman lived about 200,000 years ago (Zimmer 2001:33). The assertion that the researchers had found a common female ancestor to all living humans earned her the title "Mitochondrial Eve".

The term "Mitochondrial Eve" has stirred much debate, and may be interpreted in the wrong sense. Scientists do not believe that this woman is the Biblical Eve. Fossil evidence shows that humans existed throughout much of the world at this time. Therefore, this woman is only one of our common ancestors, as many other women existed at the same time, and may have contributed to our nuclear DNA. However, some Creationists claim that not only is the age for Mitochondrial Eve too great, but also that this woman is the Biblical Eve who gave rise to the rest of humanity. The Creationist website Answers in Genesis states, "The biblical creationist would conclude that the one woman suggested by the mitochondrial data is almost certainly the real Eve." These Creationists also claim that the mtDNA mutation rate must be much higher than what is currently accepted, and point to several studies that have shown the mutation rate to be much higher than originally calculated.

Indeed, Gibbons and Rokas et.al. cite a few studies in their respective articles which show mtDNA mutation rates to be as high as one every 25 to 40 generations, or one every 1,200 years (Gibbons 1998:28; Rokas et al. 2003:411). Using this rate, the mitochondrial Eve would be approximately 6,000 years old. These studies use the occurrence of heteroplasmy, a condition in which offspring inherit two different mtDNA sequences from the mother, to calculate this higher rate. Since heteroplasmy is the result of mitochondrial DNA mutation, the frequency of this condition in human populations gives insight to the rate at which mtDNA is mutating. Although these studies have brought into question how much is known about the dynamics of mtDNA, a 6,000 year old mitochondrial Eve is not scientifically proven or accepted, contrary to what Dr. D. Russell Humphreys, a creation scientist, claims. According to his article, Impact #384, "measurements of the mutation rate of mitochondrial DNA recently forced researchers to revise the age of "mitochondrial eve" from a theorized 200,000 years down to possibly as low as 6,000 years." However, the article from which that information was taken directly states that no one believes that the mitochondrial Eve is truly only 6,000 years old (Gibbons 1998:29). More research needs to be done to clarify the issue of mtDNA mutation rate.

In an effort to help clarify the debate about mitochondrial Eve, another group of researchers applied the same concept described above to mutations in the male Y chromosome. Although it was much more difficult to analyze this chromosome due to its much greater size compared to mtDNA, the researchers claim to have had great success. They report tracing the Y chromosome found in all human males to an African man living 100,000 to 200,000 years ago (Gibbons 1997:805). This "Y chromosome Adam" correlates nicely with the age and origin of mitochondrial Eve, which they claim strengthens both hypotheses.

Though more research must be done on the occurrence of mitochondrial mutation, recombination, and inheritance pattern before any definitive conclusions are drawn about the age of mitochondrial Eve, an estimated age of about 170,000 years is generally accepted. Evidence from studies of Y chromosome Adam supports the age and origin of mitochondrial Eve. Regardless of the age debate, two important facts remain. First, fossil evidence shows that humans had proliferated through much of the globe at this point in time, and therefore, mitochondrial Eve could not be the Biblical Eve. Second, though scientists and creationists disagree on what the evidence shows to be true, both agree that mutation rates are a reliable method for establishing how long ago humanity shared a common ancestor.

Sources:
  1. Gibons, Ann. 1997. Y Chromosome Show That Adam Was an African. Science. 278: 804-805.
  2. Gibbons, Ann. 1998. Calibrating the Mitochondrial Clock. Science. 279: 28-29.
  3. Humphreys, Russell. 2005. Evidence for a Young World. Acts and Facts, Impact Article #384, Institute for Creation Research. http://www.icr.org/index.php?module=articles&action=view&ID=1842.
  4. Rokas, Antonis; Ladoukakis, Emmanuel; Zouros, Eleftherios. 2003. Animal Mitochondrial DNA Recombination Revisited. Trends in Ecology and Evolution. 18: 411-417.
  5. Wieland, Carl. A Shrinking Date for 'Eve'. 2005. 13 October 2005 http://www.answersingenesis.org/docs/4055.asp. Copyright: 2005. Accessed: Oct 13, 2005.
  6. Zimmer, Carl. 2001. After You, Eve. Natural History. 110: 32-35.