A recent paper by Daniel Shriner and Charles Rotimi in The American Journal of Human Genetics [1] has weighed in on the question of the origins of the genetic mutation that causes sickle cell disease. Sickle cell anaemia is a classic example of human evolution. While the disease is markedly debilitating, it persists as those who are carriers for the disease are relatively protected against malaria compared with those who do not carry the gene.
There are five sickle cell haplotypes, and it has been assumed that each haplotype originated from independent occurrences of the mutation. Shriner and Rotimi argue, based on an analysis of genetic data from 156 carriers of sickle cell disease that the mutation had a single origin around 7300 years ago. While we are in no need of further evidence to show that it is impossible for the entire human race to have originated exclusively from two people 6000 year ago, articles such as this not only serve to underline this fact, but also show why believers such as myself who work in medicine and the life sciences regard common descent and large-scale evolutionary change as facts as real as a spherical earth.
While there are popular summaries [2] of the paper available that are worth reading, the good news is that this paper is open access, and despite its necessarily technical nature, accessible to the educated layperson. The authors accessed whole-genome-sequence data from nearly 3000 people, and identified 156 sickle cell carriers. After identifying haplotypes, they performed a number of analyses that allowed them to infer a single origin for the allele, around 7300 years ago during the so-called 'Green Sahara' period. [2]
Given the considerable morbidity and mortality caused by the disease, and its emergence well before modern medicine, it is not unreasonable to ask why the trait was not eliminated by natural selection shortly after it emerged. The answer is a textbook example of evolution. The sickle cell allele is recessive, meaning that a person needs to inherit alleles from both parents for them to have the disease. Those with only one allele usually do not have the disease, but are protected from malaria. While those with two sickle cell alleles are plagued with sickle cell disease, those without any sickle cell alleles are susceptible to malaria, which also carries considerable morbidity and mortality. Therefore, there is a selective advantage to having a single copy of the sickle cell allele, something which glories in the name heterozygote advantage.
One would assume that Africans whose ancestors came from areas in which sickle cell anaemia was prevalent, but no longer live in areas where malaria was endemic would have a lower percentage of carriers given that there was no longer any selective advantage to being heterozygotic for the sickle cell allele, and this is indeed the case in the US, where the prevalence of sickle cell anaemia in African Americans is declining as compared with west Africans.
It goes without saying that yet again, the fundamentalist belief that the entire human race originated exclusively from two people living six thousand years ago and that disease and death in humans was unknown prior to Adam's sin is falsified by the evidence. The emergence of a genetic disorder approximately 1600 years before the creation of humanity is of course the main problem for fundamentalism on this subject. While fundamentalists could (and do) try to fudge the problem by declaring that creation occurred around 6-10 thousand years ago, which in this case would lessen the problem by allowing the creation of Adam to be pushed back before the appearance of the mutation, this attempted resolution betrays the fundamentalist axiom that the literal reading of the Bible is authoritative, given that summing the genealogies gives a date well under 10,000 years. One either embraces literalism consistently, or not at all.
A less obvious, but more devastating problem for fundamentalism lies in the map comparing the distributions of sickle cell disease and malaria. The correlation between the prevalence of malaria and sickle cell anaemia in Africa, Madagascar, the Middle East, and India is fairly clear. However, despite malaria being a problem outside these areas, sickle cell anaemia is not prevalent there.
For the fundamentalist, the appearance of death and disease as a result of Adam's sin has achieved the status of a first principle, which means that according to their logic, the sickle cell mutation appeared after Adam's sin, if not as a direct consequence of it. Given both their belief in universal human descent from Adam, and the fact that being heterozygous for sickle cell anaemia is protective against malaria, one would expect the sickle cell trait to be present in America, central and east Asia, and the Pacific. This we do not see. The problem is even more acute given the presence of sickle cell anaemia in south west Asia, where one would place Adam and Eve according to a literal reading of the creation narratives.
Conclusion
It is something of a mantra at this site that the evidence for evolution is beyond rational dispute, and I do admit to feeling more than a little weary after making this point several times. However, in an age where rational thought is in decline, and fundamentalist intolerance and anti-intellectualism rampant, it is one that does need repeating, in the hope that repetition may one day beget wisdom.
Conclusion
It is something of a mantra at this site that the evidence for evolution is beyond rational dispute, and I do admit to feeling more than a little weary after making this point several times. However, in an age where rational thought is in decline, and fundamentalist intolerance and anti-intellectualism rampant, it is one that does need repeating, in the hope that repetition may one day beget wisdom.
References
1. Shriner and Rotimi, Whole-Genome-Sequence-Based Haplotypes Reveal Single Origin of the Sickle Allele during the Holocene Wet Phase, The American Journal of Human Genetics (2018), https://doi.org/10.1016/j.ajhg.2018.02.003
2. Sarah Wild "Everyone with the sickle cell gene mutation descended from the same ancestor 7,300 years ago" Quartz March 12th 2018
3. Shriner and Rotimi, op cit., p 1.
