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 last post in this series, we began to address Poythress’s arguments that population genetics methods are not able to rule out the possibility that humans descend uniquely from a single ancestral couple. As we have seen, Poythress’s arguments are threefold:
- Population genomics methods report only long-term average population sizes. These methods could not detect a bottleneck to two individuals, even if one existed, since they report only long-term averages.
- Population genomics methods report sizes for more recent human populations, and do not address more distant history. As such, Adam and Eve may have lived further back in time than these analyses measure. This view is consistent with Scripture since the biblical genealogies may have gaps.
- The findings of population genomics are based on uniformitarian assumptions that may not in fact be true if allowance is made for miracles.
Having dealt with the first claim, and shown it to be unrealistic in light of how a population bottleneck would shape the genetic variation of a species, we can now turn to the second claim: that perhaps Adam and Eve lived deep in the past, beyond the ability of current population genetics methods to reliably assess.
There are at least two significant problems with this approach. The first is that population genetics techniques are perfectly capable of estimating human ancestral population sizes over a wide range of times in the deep past, and the results do not support Poythress’s claim of an ancestral pair deeper in prehistory. Secondly, the further back in time one attempts to place Adam and Eve, the greater the tension one must be willing to bear between the internal evidence from Genesis about their time and setting, and what we observe in the paleontological / archaeological record. We will discuss these problems in turn.
Population genetics and human prehistory
In the section of Did Adam Exist? Where Poythress discusses ancestral human population sizes, he is primarily interacting with my 2010 paper in Perspectives on Science and Christian Faith (PDF) that discusses the evidence for human population genetics and common ancestry. One study that I discuss in that paper uses linkage disequilibrium to estimate human ancestral population sizes between about 20,000 and 100,000 years ago, but deliberately excludes dealing with genetic variation that would estimate population sizes in the more distant past. Poythress notes this limitation, and uses it to frame an argument that perhaps Adam and Eve lived before this time:
the study indicates that there is an effective limitation to how far one can probe into the past. Information based on correlations between nearer locations on a chromosome probes further into the past, but the analysis always results in figures that represent a rough average over many generations in the human population. Consequently, the principal figures, like 3,100 for non-African populations and 7,500 for the African population, represent average populations over many generations. They say nothing one way or the other about whether the size decreased rapidly to two individuals in the more distant past.
Later in the book, Poythress makes a case that the Genesis genealogies do not exclude such a possibility:
William Henry Green did an extensive analysis of biblical genealogies and concluded that they may contain gaps. If they do, the gaps mean that we cannot use Ussher’s procedure of adding up the years in the genealogies to obtain a date for the creation of Adam and Eve. The Bible simply does not tell us how long ago it was. Thus, Adam and Eve may have lived further back in time.
Taken together, Poythress leaves the impression that human population genetics models cannot reach back far enough in time to locate Adam – and that locating Adam and Eve as our unique ancestors deep in human prehistory is compatible with the Genesis narratives.
The first problem with this argument is based on the same misunderstanding that we discussed in the last post. As we saw, a population bottleneck to two individuals will markedly affect the genetic variability of a species for tens of thousands of years or more. Here Poythress notes that one particular study reaches back to 100,000 years, and hypothesizes that Adam and Eve perhaps lie beyond that limit. If they did so, however, they would have to be tens of thousands of years removed from that limit in order for the data at 100,000 years to appear as it does (barring, of course, recourse to miracles).
Secondly, noticing that one study reaches back to 100,000 years is not the same as establishing that population genetics as a field cannot reach back further. Indeed, as more and more human genetic variation data becomes available, we are able to look at various time points in our prehistory with increasing accuracy. For example, one such study published in 2011 uses coalescence methods to estimate the population size of our lineage between 10,000 and 1,000,000 years ago. The results are in good agreement with prior estimates: back to 1,000,000 years ago, our population has never numbered below about 10,000 individuals. In fact, as one goes further back in time, our population size increases: the bottleneck to 10,000 between 40,000 and 60,000 years ago is the smallest population for our lineage over the past million years. Recently a very similar approach was applied to a larger data set, with the same results. While Poythress could not have read the more recent paper prior to writing, the 2011 paper was available for over a year before he wrote the text of Did Adam Exist? and published it in the Westminster Theological Journal. Contrary to the impression he gives his readers, human population genetics had already examined the time he implied was unknowable, and demonstrated that no bottleneck occurred in our lineage between 100,000 and 1,000,000 years ago.
Adam and his Genesis context
Aside from the population genetics data that span our prehistory back to one million years, there are significant biblical challenges with locating Adam prior to 100,000 years ago. Briefly stated, the Genesis narratives assume a cultural setting consistent with the late Neolithic period, with agriculture and animal husbandry present (and metalworking a mere few generations off). To push Adam back to 100,000 years ago or more is to place him at a time where there is no archeological evidence for domesticated plants or animals, and only possible hints of artistic expression or religious activity. What Poythress might make of these difficulties is left unsaid.
In the next post in this series, we’ll address Poythress’s third line of argumentation: that the uniformitarian assumptions of population genetics may in fact be incorrect due to miraculous events in our past.