In this series, we explore the genetic evidence that indicates humans became a separate species as a substantial population, rather than descending uniquely from an ancestral pair.
In the previous posts in this series, we’ve examined several of the converging lines of evidence that support the conclusion that our lineage became human as a population – one that has not numbered below about 10,000 individuals over the last 18 million years or more. Not surprisingly, this conclusion is one that many Christians find difficult to accept, since it is commonly held that this scientific finding is incompatible with the Genesis narratives (though as we have discussed, there is good reason not to think so, given the limits of what science can say). As population genetics information and its implications for interpreting Genesis have become more widely known among evangelical Christians, some apologetically-minded organizations and scholars have attempted to cast doubt on these lines of evidence.
One such individual is the Rev. Dr. Vern S. Poythress, professor of New Testament Interpretation at Westminster Theological Seminary. In 2013 Poythress authored a lengthy article on human population genetics that has subsequently been adapted into a short book, Did Adam Exist?, in the series Christian Answers to Hard Questions. Here's a short video of Dr. Poythress introducing Did Adam Exist?:
In both the article and the book, Poythress argues against the ideas that humans and other forms of life share common ancestors, and that humans descend from a population, rather than a pair. Since Poythress is one of the few leading evangelical scholars to address these issues, and to do so from a scientific, rather than an exclusively theological perspective, we will take the time in this series to carefully examine his arguments.
Poythress’s two main scientific arguments can be summarized as follows:
1. Reports of human – chimpanzee DNA comparisons overstate the identity between our two genomes because they selectively focus on areas where high DNA identity is to be expected because of functional constraint. The true overall identity value is lower, and is difficult to explain if common ancestry is true.
2. Population genetics estimates of ancestral human population sizes can only report on long-term population size averages, and thus could not detect a bottleneck of two individuals.
These two arguments are not disconnected as they might seem at first glance. Poythress correctly understands that some methods used to measure human ancestral population sizes use the DNA of species closely related to humans (such as gorillas and chimpanzees) in their analyses. As such, for some methods – such as incomplete lineage sorting, as we have examined – knowing the correct pattern of species relatedness is important for the analysis.
From these arguments, Poythress concludes that what he sees as the “biblical view of human origins” – that Adam was created directly from dust, that Eve was created directly from his side, and that Adam and Eve are the sole genetic progenitors of the entire human race – remains scientifically credible. However, as we will see, these arguments do not hold up to scientific scrutiny. As such, Poythress does not have scientific support for his preferred interpretation of the Genesis narratives.
Common ancestry and human-chimpanzee DNA identity
Poythress’s first line of argument is to call into question the commonly-reported values for human-chimpanzee genome identity – values on the order of 96-99%, depending on how the analysis is done. As we heard in the video linked above, Poythress claims that these values are overinflated:
“For instance, the percentages go down to something like 70% if you actually take all the DNA, and not just the part that codes into proteins… And what about the 30% that doesn’t agree? Well, that’s problematic of where did that come from? That’s an awful lot when you think about it, to be different, if indeed there is common ancestry.”
In Did Adam Exist? the argument runs along similar lines (pp. 7-8):
“If the comparison focuses only on substitutions within aligned protein-coding regions, the match is 99 percent. Indels constitute roughly a 3 percent difference in addition to the 1 percent for substitutions, leading to the figure of 96 percent offered by the NIH… But we have only begun. The 96 percent figure deals only with DNA regions for which an alignment or partially matching sequence can be found. It turns out that not all the regions of human DNA align with chimp DNA. A technical article in 2002 reported that 28 percent of the total DNA had to be excluded because of alignment problems, and that “for 7% of the chimpanzee sequences, no region with similarity could be detected in the human genome.”
Even when there is alignment, the alignment with other primate DNA may be closer than the alignment with chimp DNA: “For about 23% of our genome, we share no immediate genetic ancestry with our closest living relative, the chimpanzee. This encompasses genes and exons to the same extent as intergenic regions.” The study in question analyzed similarities with orangutan, gorilla, and rhesus monkey, and found cases where human DNA aligns better with one of them than with chimpanzees.
At this point in the book, Poythress includes two questions for reflection, since the book is intended for use in small-group discussions:
What new issue have we discovered about the 96% “identical” genetic codes? How does this change the situation?
Does human DNA always align best with chimp DNA? How might this change your attitude to the first statistic given?
While the text of Did Adam Exist? left these questions somewhat open, the intent of the arguments (and the questions themselves) seems clear when considered alongside Poythress’s summary statement in the video above. Poythress is advancing an argument that the overall DNA identity between humans and chimpanzees is much less than the commonly-agreed value of 96-99%. His view, it would appear, is that around 30% of our genome has no similarity to chimpanzees – lowering the overall identity value to about 70%.
This is of course, mistaken – but it will take some effort to explain exactly why this is the case.
It’s also not exactly clear what the basis is for the argument—is it the 28% that had “alignment problems” in the 2002 study? The 7% in that same study that had “no region of similarity” to match to? The 23% of our genome that “share(s) no immediate genetic ancestry with … the chimpanzee”? While Poythress does not divulge exactly how he is calculating the ~30% dissimilar figure, these are the only sources he cites to this end. As such, it appears that he views these statements as the technical support for his argument.
These scientific statements—while accurate—do not support the conclusion that the human and chimpanzee genomes are only 70% identical, however. We will begin to unpack why this is the case in the next post in this series tomorrow.