Exploration 8: Deeper messages in the conventional ancestral tree of human groups
The original Tishkoff diagram of human ancestry is certainly easier to read than the reticulating web of exploration 6, let alone the web overlaid with aprons in exploration 7. We could try to remedy this by helping audiences to become familiar with the graphic conventions and by using technology like the slide show to display the branching and replacement of ancestral aprons with those of their descendants. In this concluding post of the series I argue that it is important for all to work on being able to read reticulating webs because of an undesirable message built into the simpler branching diagram.
To expose this message, consider the horizontal links in the Tishkoff diagram, which represent gene flow between branches, that is, admixture. The branching pattern can be extracted from the genetic data only because these flows are not so large as to obscure the genetic mutations or other differences that arose over time after each branching. Indeed, to ensure that this is the case, some studies of human genetic variation involve data from the special subset of people who live in the same place as, say, all their great-grandparents. The reticulating web with aprons likewise relies on a branching pattern that can be discerned despite the potentially confounding effects of gene flow. Still, the aprons remind us of variation around the mid-point of each group—variation that may well have been enlarged by gene flow.
Now, there are some branching patterns that are subject to minimal or no gene flow, namely branching of species or higher taxa (taxonomic groups) from ancestral taxa. We are all familiar with such evolutionary trees. The first example is for the classes of vertebrates; the second is for liverwort species.
Our familiarity with these trees invites us to think—even if subconsciously—about human genetic ancestry as if the branches are like separate species. There is a long history of scientific arguments that human races are separate species, or that the branches of the human tree achieved human status at different rates. As Desmond and Moore (2009) have shown in Darwin’s sacred cause: How a hatred of slavery shaped Darwin’s views on human evolution, the debate was especially heated during Darwin’s adult life. Darwin’s view of descent from a single common ancestor was a minority view, discredited to some extent by its association with literal interpretation of the bible’s account of Adam and Eve, but more so by its association with anti-slavery movements. Yet, the debate did not disappear with the 19th century. Carleton Coon, a physical anthropologist who died in 1981 after a long career as a professor at Harvard and University of Pennsylvania, wrote in 1962 that Homo erectus evolved into Homo sapiens five separate times “as each subspecies, living in its own territory, passed a critical threshold from a more brutal to a more sapient state”. (http://en.wikipedia.org/wiki/Carleton_S._Coon#Polygenism) The multiregional hypothesis is a more recent variant.
Ideas about multiple origins for humans are not the only way that biology can be invoked to explain or even justify a hierarchy of human races. However, to the extent that we want to distance ourselves from such views, it can only help to do the work to depict genetic relationships among humans in ways that allow simultaneously for similarity, diversity, and admixture at the same time as we depict ancestry.