Science moves closer to Adam and Eve?

Yesterday, the journal Science published a study providing evidence that humans are descended from very small population. The authors detect a bottleneck lasting about 100,000 years with an average effective population size of about 1280. They date this to about 813,000 to 930,000 years ago, placing it before the divergence of Neandertals and Denisovans from modern humans. This coincides with a large gap in the hominin fossil record. They suggest it is the period in which the human chromosome number of 23 originated.

Such a bottleneck has not been detected before. Effective population sizes did not fall below 10,000 in previous genome analyses. The authors of the new study provide simulations to show that their software is better at detecting bottlenecks than older software.

A few years ago, I was involved in an extensive discussion with other Christian biologists on whether the (then current) estimates, which never dropped below 10,000, disproved the hypothesis that human descend from a single couple. Representatives of the organisation Biologos argued that they did. I argued that they did not, because the methods used were simply unable to detect short sharp bottlenecks. Eventually, a measure of consensus emerged. We agreed that genomics does not rule out a single couple as the sole progenitors of humans. The organisation Biologos adopted a new position on the issue: that Adam and Eve are only ruled out in the last 500,000 years but not before that.

The methods used in the study published in Science yesterday are similar to the older methods in that they also cannot detect short sharp bottlenecks. They rely on the assumption that the human population size was stable over time windows lasting many generations, in order to calculate an effective population size for that time window. Thus, a bottleneck of two is not ruled out by their methods. In some ways, the single-couple hypothesis becomes more plausible given the new evidence for a prolonged bottleneck with an average effective population size of about 1280.

The new study, and the discussion going on around it, are also helpful reminders that no studies estimating past effective population sizes should be taken as absolute truth. The authors begin their study by saying “ancient population size history of the genus Homo during the Pleistocene is still poorly known” and “a new approach is needed to improve the inference accuracy of population size history.” This was of course true in 2018, when I was previously discussing this issue with Christian biologist. But this caveat was not as clearly stated in the scientific literature at that time, which made it hard to persuade some layperson onlookers that caution was needed. It is an unfortunate feature of the scientific literature that publicly accessible critiques of methods are often only available once a new improved replacement is found.

Christians must be cautious about how they interact with studies exploring past human effective population sizes from genomes. Such methods are not able to either prove or disprove the hypothesis of Adam and Eve. But none-the-less it is fascinating to see the science appearing to move towards, and not away from, this hypothesis.



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