My father was never certain of his own age. Though he had a birth certificate from India that recorded his date of birth as May 17, 1911, his parents had lied about his age, claiming he was older than he actually was so they could enroll him in school early. The fabricated birthdate became instantiated in the official records.
Anthropologists face a similar uncertainty problem with Y-chromosomal Adam. Determining when the male progenitor of humanity lived is fraught with all sorts of difficulties, as a new study reported by researchers from the University of Houston and Johns Hopkins University attests.1 This difficulty has wide-ranging implications for evolutionary and biblical models for humanity’s origin.
Finding Adam’s Birthday
When RTB put forth its human origins model in Who Was Adam? (2005), the most comprehensive analyses of Y chromosome variants returned a date for Y-chromosomal Adam between 50,000 to 60,000 years ago. Studies conducted in 2011 and 2013 made use of a larger portion of the Y chromosome and rare Y chromosome variants to estimate dates of 142,000 ± 16,000 and between 101,000 and 115,000 years ago, respectively. Two other studies published in 2013 reported that the last ancestor of all modern humanity lived between 120,000 and 156,000 years ago and 180,000 to 200,000 years ago, respectively.
These studies are significant because they bring the dates for Y-chromosomal Adam in line with the dates for mitochondrial Eve (most recently determined to have lived about 150,000 years ago). This finding means that it is reasonable to think that Y-chromosomal Adam and mitochondrial Eve were part of the same population. I have argued that Y-chromosomal Adam and mitochondrial Eve can be viewed as the biblical Adam and Eve. For this proposal to be feasible, the genetic male and female progenitors had to live at the same time.
Prior to these recent studies, a large discrepancy existed between the dates for Y-chromosomal Adam and those for mitochondrial Eve. This difference prompted some evolutionary biologists to claim that Y-chromosomal Adam never “knew” mitochondrial Eve. In other words, if Y-chromosomal Adam and mitochondrial Eve lived at different times, then they must have been part of distinct populations, a scenario incompatible with the traditional reading of the biblical account of human origins. Eliminating this troubling discrepancy would improve the scientific credibility of the biblical account of human origins.
However, another study published in 2013 by scientists from University of Arizona created commotion when its authors reported a date for Y-chromosomal Adam between 237,000 and 581,000 years ago, with the most likely age at 338,000 years. Once again, a possible discrepancy posed a potential problem for a biblical human origins model.
At the time the University of Arizona study was published, I pointed out that the date they proposed for Y-chromosomal Adam was about 200,000 years earlier than the first appearance of modern humans in the fossil record––raising questions about the result’s validity. I also noted that careful consideration of this study’s approach to calculating the date of Y-chromosomal Adam reveals that the mutation rate the investigators employed to calibrate the molecular clock was slower than that used in previous studies.
If they had used the same mutation rate as other researchers did, their estimated date for Y-chromosomal Adam would have been closer to 209,000 years ago. The researchers based the mutation rate they used on whole-genome sequence comparisons of people living today, but this type of calibration is probably invalid.
The new work by scientists from University of Houston and Johns Hopkins University affirms the very points I made several months ago. Not only did these researchers highlight the discrepancy between the genetic data and the fossil record, but they also pointed out that the University of Arizona scientists used flawed calibrations.
Other problems were also identified. For example, the University of Arizona team used unusually long generation times, compared DNA sequences of unequal length, and employed unusual statistical methods in their analysis. All these issues contributed to the much-older date for Y-chromosomal Adam. The team from University of Houston and Johns Hopkins University reanalyzed the data gathered by the team from University of Arizona with these concerns in mind. They generated a date for Y-chromosomal Adam of about 208,000 years in age, which is in line with other studies. Once again, it looks like Y-chromosomal Adam lived at the same time as mitochondrial Eve.
One key lesson from this episode identifies how susceptible molecular clock analysis can be. In this case, two teams achieved radically different results from the same data set. Molecular clock analysis is sensitive to the mutation rates used for clock calibration, the data set used for the calibration, generation time employed in the analysis, DNA sequence length used in the comparisons, and the specific statistical methods employed.
In reality, we will likely never know the precise date for when Y-chromosomal Adam (and mitochondrial Eve) lived. The dates from molecular clock analysis must be treated as crude estimates at best, but they should not discourage investigation of these dates and inquiry of such weighty matters as humanity’s origin.