When news sites discuss the attempt by the leader of ISIS, al-Baghdadi, to form a new caliphate, they often mention his claim to be a “direct descendant of Muhammad.” This claim is often met with skepticism because, as this article in the New Republic puts it:
...because no one knows much about Baghdadi - certainly not enough to trace his lineage back 1,400 years to a preliterate society a thousand miles away - it’s hard (and in the Islamic State, probably fatal) to suggest he’s lying.
I am not at all skeptical of al-Baghdadi’s claim. In fact, if Muhammad has any surviving descendants, I would be extremely surprised if al-Baghdadi were not one of them. I would also be extremely surprised if I were not one of them. I would also bet a great deal, dear reader, that you are also a descendant of Muhammad, especially if you are of Eurasian decent.
These counter-intuitive notions follow from the findings of coalescent theory. The logic is fairly simple, as we look backwards on our family tree the number of our ancestors grows exponentially. We have two parents, four grandparents, eight great-grandparents, sixteen great-great-grandparents, etc. The thing about exponentials is that the increase, well, exponentially. Exponentials are relentless in their increasing. Also, as you look backwards in time, there are fewer people so number of people who could be your ancestor decreases exponentially. Eventually, you reach a time where not only are you are descended from every person who has an ancestor today, but a time where everyone alive today shares the same set of ancestors.1
A 2002 Atlantic article by Steve Olson describes work by Joe Chang who found that this point for all Europeans was only around 1400 (Muhammad died much earlier in 632).2 This analysis has been backed-up by a more recent genetic analysis as described by Carl Zimmer.
Chang’s paper assumed random mating, so there may be concern about isolated groups. A more recent paper looked at a model that assumed pretty conservative migration rates between areas of the world (they actually did two models – one simple and once complex and had similar answers). Both models support the notion that Muhammad is the ancestor of most humans alive today.
The paper uses the example of Tasmania, which was geographically isolated from Australia for thousands of years before European colonization in the 19th century. Although Tasmania was reproductively isolated well before the birth of Muhammad, there is no one alive today of pure Tasmanian descent. So if Muhammad has any descendants, it is extremely likely that all of today’s Tasmanians are all descendants of Muhammad.
In summary, if being a descendant of Muhammad is a requirement for being Caliph, it is either a very low bar or a very high bar. Either Muhammad has no descendants and, therefore, no one can be Caliph. Or almost all seven billion of us are Muhammad’s decedent and the requirement is trivial. In any case, the press should not treat al-Baghdadi’s claims with skepticism, but indifference.
1- I should probably point out that these are “genealogical ancestors,” the ancestors that you would find on your family tree. You would have many fewer “genetic ancestors,” that is individuals who you share genes with through descent. Because you have a finite amount of genetic material and it is transmitted in discrete chunks, not all of your genealogical ancestors have contributed to your genome.
2 – This also explains one of the major scientific problems with the plot of the da Vinci Code.