“TREE” fig. 4; modified tree with proposed admixtures; “TRELLIS” fig. 3
HHS Public Access Published online 2013 May 16
Biological Races in Humans
6. A Trellis or a Tree?
The imagery of recent human evolution is dominated by evolutionary trees of human populations. Human populations are shown again and again as separate branches on an evolutionary tree, related to other human populations by splits that occurred at specific times in the past. Even papers that document genetic interchange among human populations, such as the recent papers on admixture with archaic populations (Green, et al., 2010; Reich, et al., 2010), place human populations on an evolutionary tree with only weak arrows indicating isolated events of admixture that minimally violate an otherwise tree-like structure (see Figure 4, adapted from Reich, et al., 2010). In particular, as is typical of the human population genetic literature, Africans are portrayed in Figure 4 as having “split” from the rest of humanity a long time ago with not one episode of genetic interchange being portrayed since that ancient “population separation” (Reich, et al., 2010, pg. 1058).
Contrast Figure 4 to Figure 3, which also depicts recent human evolution. All aspects of Figure 3 are supported by explicit hypothesis testing and statistically significant inferences. Indeed, as shown in this paper, our evolutionary history has been dominated by gene flow and admixture that unifies humanity into a single evolutionary lineage, as shown by the trellis structure and arrows of expansion that overlay upon, not replace, earlier populations. This finding does not mean that all human populations are genetically identical. Past founder events, isolation-by-distance, and other restrictions on gene flow ensure that human populations are genetically differentiated from one another, and local adaptation ensures that some of these differences reflect adaptive evolution to the environmental heterogeneity that our globally distributed species experiences. However, most of our genetic variation exists as differences among individuals, with between population differences being very small. In every case in which treeness has been tested for human populations, it has been rejected. In contrast, the evolutionary trees found throughout the human genetic literature, such as that portrayed in Figure 4, are simply invoked. There is no hypothesis testing, even though treeness or multiple lineages are testable hypotheses.
Simply invoking conclusions without testing them is scientifically indefensible; yet, that is the norm for population trees in much of the human evolution literature.”
Many of the papers that portray human population trees caution in the text that the populations are not truly genetically isolated, but this makes the tree portrayal even less defensible as the authors are knowingly portraying human evolution in a false fashion. Moreover, it is socially irresponsible. Scientific papers on human genetics and evolution often attract much attention in the popular media, and it is the pictures and figures from these papers that are primarily transmitted to the general public and not nuanced text.
That is certainly the case for Figure 4, which has appeared in various versions in many newspapers and websites (e.g., http://ufomaniacs.blogspot.com/2010/12/ancient-humansthe-denisovansinterbred.html). The message of these figures is both that there was some admixture with archaic human populations in the past and also that Africans have been separated from the rest of humanity for a long period of time with no genetic interchange. Africans in these figures are clearly presented as a distinct lineage of human evolution; that is, a separate race. This pictorial conclusion has been definitively falsified as indicated in this paper, so figures such as Figure 4 mislead, not educate, the public about our scientific knowledge of human evolution and race.
Scientists should take seriously what their work communicates to the general public. If they applied the most straightforward concept of science, the idea that hypotheses should be tested whenever possible, then human evolutionary trees such as Figure 4 would disappear and would be replaced by trellises that emphasize the genetic interconnections among all humans on this planet. Humans are an amazingly diverse species, but this diversity is not due to a finite number of subtypes or races. Rather, the vast majority of human genetic diversity reflects local adaptations and, most of all, our individual uniqueness.