Metaphors in science play an important role in communicating results from one field to scientists in other fields and to the general public. In some cases, however, metaphors are so successful and so appealing that they actually obscure rather than enlighten.
In human population genetics, it is a simple fact that all of the Y chromosomes present in the world today can be traced back to a single common ancestor–if you follow my paternal line (my father’s father’s father’s father, and so on) and your paternal line back far enough, eventually they will overlap. At some point, a population geneticist had the clever idea of calling this common ancestor “Adam”. This is a biblical allusion, of course, and it probably was good for a bit of amusement a couple of decades ago. But it’s time to retire this metaphor–not only because it confuses the public (see a nice series of posts by Melissa Wilson Sayres on this topic here) or scientists in other fields–but because it confuses even practicing human population geneticists!
I was reminded of this when reading over a paper by Eran Elhaik, Dan Graur, and colleagues critiquing work on the human Y chromosome phylogeny by Mendez et al. The basic question being debated is: when did the most recent common ancestor (MRCA) of all Y chromosomes exist? Mendez et al. claimed that this Y chromosome was present around 300,000 years ago, and Elhaik et al. claim they arrived at this number incorrectly.
The details of these papers are not relevant for this post. The key thing I want to point out is an underlying assumption, perhaps most clearly expressed by Elhaik et al., who write:
[Mendez et al.] estimated the time to the most recent common ancestor (TMRCA) for the Y tree to be 338,000 years ago (95% CI=237,000–581,000). Such an extraordinarily early estimate contradicts all previous estimates in the literature and is over a 100,000 years older than the earliest fossils of anatomically modern humans. This estimate raises two astonishing possibilities…
The implicit assumption here (the reason Elhaik et al. find the numbers “extraordinarily early” and “astonishing”) is that the individual carrying the most recent common ancestor of all human Y chromosomes (AKA “Adam”) should be an anatomically modern human. Amusingly, Elhaik et al. argue that to claim otherwise is analogous to claiming you have a unicorn in your backyard. But there is simply no reason that “Adam” must be a human. At the top of this post I’ve put a figure showing a hypothetical Y-chromosome genealogy superimposed on a hypothetical human phylogeny. In this (of course hypothetical) example, “Adam” existed well before the diversification of modern humans; this type of scenario is perfectly compatible with basic population genetic theory. From the point of view of population genetics, there is absolutely no reason that the common ancestor of all human Y chromosomes must have existed in an individual that we would identify as “human”.
So why would anyone make this assumption? Note that Elhaik et al. made a YouTube video describing their results; this video leads with a bit of religious iconography. It seems plausible that by calling the most recent common ancestor of all Y chromosomes “Adam”, population geneticists have confused themselves into thinking that “he” must have been human.