228. The Baseline Scenarios -- part 4

I've defined the Hypothetical Baseline Population (HBP) as "that population from which the ancestors of either the Pygmies or the Bushmen, or both, diverged, at some point during the Paleolithic era." I will now proceed to clarify this definition, being as specific as I can.

Let's begin by reviewing some of the phylogenetic trees we've already seen, gleaned from recently published papers by some of our leading population geneticists. First, a tree based on mitochondrial DNA (mtDNA), representing the female lineages of selected African groups, as produced by Sarah Tishkoff et al, in their 2007 paper, History of Click-Speaking Populations of Africa Inferred from mtDNA and Y Chromosome Genetic Variation:

(For this and all other diagrams, I suggest right-clicking and selecting "Open link in new window," to get a larger, clearer view.)


The oldest, deepest clades (i.e., branches) are on the left, with the more derivative, thus later, clades toward the right, as suggested by the treelike structure of the diagram itself. Each letter-number combination at the tip of each branch (e.g., L0d, L0k, L0f, etc.) stands for what is called a "haplogroup," a term representing related sets of "haplotypes," i.e., genetic markers that have proven to be especially diagnostic for this type of research. The numbers stand for the percentages of each haplogroup found in the sample for each of the population groups listed (by language name) in the leftmost column.

Under L0d, the leftmost (thus deepest) clade, we see that the highest percentages by far are found among the !Xun/Khwe, !Xun and !Kung, three Bushmen groups. It is probably significant that the most isolated and thus least acculturated group, the !Kung, has the highest percentage of L0d: 96%. Moving to the right, we see that the next haplogroup with high percentages is L1c, under which we find Mbenzele (97%), Biaka (or Aka) (77%) and BaKola (100%), three Pygmy groups, all representing the Western Pygmy population. Moving farther to the right, we see another high percentage (55%) under haplogroup L2, representing the Mbuti, or Eastern Pygmies. We see that significant percentages for the Mbuti are also found to the left, under two deeper clades: L0a (30%) and L5 (15%).

Note further that no other high percentages are to be found until we reach L4g, where we find the Hadza and Sandawe, two other hunter-gatherer groups. Note that all the other African groups included in this survey are split among several different haplogroups, suggesting that their populations are the result of genetic admixture, reflecting thousands of years of interconnection with other groups. The two rightmost symbols, labeled simply M and N, represent the deepest clades of, believe it or not, literally all the remaining branches of the mitochondrial tree found thus far in the rest of the world. No non-African population is represented by any of the clades labeled "L," the very deepest clades worldwide, though the M and N clades are clearly branchings from L, as indicated in the diagram.

Moving from the female to the male line, as represented by the Y chromosome (found only in men), we see a phylogenetic tree produced by a group calling itself "The Y Chromosome Consortium," as published in a 2002 paper, A Nomenclature System for the Tree of Human Y-Chromosomal Binary Haplogroups:

While group names are not included in this diagram, the accompanying text informs us that the deepest clades (at the top) are occupied almost exclusively by either Bushmen or Pygmies, with Bushmen classified under either A or B and almost all Pygmies (Aka and Mbuti) under B. With the exception of one Zulu individual, none of the other groups included in the study belongs to either of the two deepest clades, A or B.

Finally, let's have a look at a phylogenetic tree based on nuclear markers, combining both male and female components, also produced by Sarah Tishkoff, with a somewhat different team, as part of a comprehensive study recently published in Science: The Genetic Structure and History of Africans and African Americans (2009):

(For simplicity sake, I've reproduced only the bottommost segment of a larger diagram.) This tree, based on a study of "1327 nuclear microsatellite and insertion/deletion markers," is especially easy to interpret as it's organized according to populations rather than haplogroups, with the population names printed on the right. Once again we find Bushmen (!XunXhoe and "San" -- actually !Kung) occupying the very deepest clades, with Pygmies following closely on their heels -- though this time the Eastern Pygmies (Mbuti) rather than the Western Pygmies (Bedzan, Bakola, Biaka and Baka) occupy the next deepest clade.

Reviewing all three diagrams, it's not difficult to see why I've attached so much importance to Bushmen and Pygmies in defining HBP. As should also be clear from the same diagrams, the ancestors of the Bushmen and Pygmies can also be regarded as the ancestors of every human now alive, which means that HBP represents the ancestry of not only Pygmies and Bushmen, but every single one of us as well. However, it's very important to understand what I mean in this case by "ancestry," because we have ancestors going back very far, not only to the earliest "modern" humans (Homo sapiens sapiens) but to the earliest archaic humans as well, the ancestors of Homo Erectus, Homo Neanderthalus and a host of other human and proto human groups, and beyond, to the ancestors (or more accurately ancestor) of every living thing that ever dwelt on Earth.

I have therefore defined HBP very specifically, as that population "from which the ancestors of either the Pygmies or the Bushmen, or both, diverged." We can see that founding, ancestral, population represented, however vaguely, in all three of the above phylogenetic trees, by looking in each case for the most fundamental clade, or "root," from which all the others stem. But in all cases this "root" is ambiguous, because it could represent the very earliest "modern" humans, or even some prior human group, closer to Homo Erectus; neither of which can really teach us very much, because, aside from some archaeological relics, we have no way, at present, of inferring very much at all about either them or their culture.

More useful in this respect is the following table, from Chen et al., mtDNA Variation in the South African Kung and Khwe—and Their Genetic Relationships to Other African Populations, 2000. (Again I'll remind you to expand this image by right clicking and selecting "Open link in new window".)

Though this study is somewhat outdated, the information in this table, based on estimates that still seem reasonable, is nevertheless useful. However, what I want to focus on is the basic idea, not particularly evident in the phylogenetic trees, that the various nodes on all the trees indicate points of divergence, i.e., "moments" in history (representing time spans from just a day or so to periods of many years) when the ancestors of one group separated from the group to which they had previously belonged, with which they had previously shared a common culture.

Thus, according to both the genetic evidence and common sense, there must have been a time when the first Pygmy group or the first Bushmen group separated off from a group that can now be considered, in retrospect, as the common ancestor of both. According to the genetics-based estimates offered by Chen et al, the earliest time of divergence may have, very roughly, been between 77,600 and 102,000 years ago, when, according to their best estimates, the Biaka (Aka) Pygmies diverged from the common ancestor. According to the same methods (admittedly very hypothetical), the !Kung Bushmen (aka the Ju'/hoansi) diverged from the same ancestral group 41,000 to 54,100 years ago, with the Mbuti and Senegalese diverging at a considerably later date, roughly 18 to 23 thousand years ago.

Since genetic evidence gleaned from so many different sources is not yet fully consistent in this respect, I hesitate to base HBP exclusively on such evidence, though the results are consistent enough to tell us a great deal. Nevertheless, the Chen table is useful as a point of departure because it encourages us to think not so much in terms of a common ancestor, but a very particular common ancestor, i.e., the very specific ancestral group from which the first of the Pygmy or Bushmen groups diverged, at some point during the Paleolithic, possibly 77,600 to 102,000 years ago, as estimated by Chen, possibly at some other date either earlier or later. The divergence would certainly show up in the genetic evidence -- though the methodologies are not yet at the point that they are producing fully consistent results. This doesn't really matter so much, however, because we also have considerable cultural evidence pointing in exactly the same direction, back in time to a moment of divergence from a common ancestral group. It is the cultural traditions practiced by the common ancestors at the moment of divergence that concern me here. This very specific ancestral group, as they were living at this particular time in history, at some particular place in Africa, is what I am referring to as the Hypothetical Baseline Population. And the culture of this group, at this particular time and place, is what I am referring to as the Hypothetical Baseline Culture.

(to be continued . . . )