Monday, September 3, 2012

Hunter-Gatherer Study Inconclusive

Alice C. Linsley

Although the oldest human fossils date to about 3.8 million years, modern humans arose in Africa around 200,000 years ago. One theory says that the archaic human populations died out. A recent study published in the journal Cell casts doubt on that theory. Apparently, Neanderthals had company in Africa.  As early as 100,000 years ago humans were already moving out of Africa.

DNA samples along the paths of migration of early populations out of Africa reveal genetic drift. The farther from Africa, the more genetically similar the human populations. No geographic origin outside of Africa accounts as well for the observed patterns of genetic diversity. There is little doubt that humans originated in Africa. (Further investigation needs to be done to confirm migrations routes, as in the case of the Nilotic Ainu who are at the center of Cavalli-Sforza's genetic distance chart.)

The Cell publication “Evolutionary History and Adaptation from High-Coverage Whole-Genome Sequences of Diverse African Hunter-Gatherers” focuses on three hunter-gatherer populations: the forest-dwelling Pygmies of Cameroon and the Click-speaking Hadza and Sandawe of Tanzania.

Hadza warriors
Photo: Sarah Tishkoff

The publication covers the genomes of five individuals from these groups and highlights yet again that the genetic diversity of African populations is greater than anywhere else on Earth. The authors discovered over 3.4 million genetic variants, or SNPs, of which five million are previously unknown. They report:

We sequenced the whole genomes of five individuals in each of three different hunter-gatherer populations at 60× coverage and identify 13.4 million variants, substantially increasing the set of known human variation. We found evidence of archaic introgression in all three populations, and the distribution of time to most recent common ancestors from these regions is similar to that observed for introgressed regions in Europeans.

The "Click" or Khoisan language family is very old. Genetic data show that the Sandawe and southern African click speakers share rare mtDNA and Y chromosome haplogroups. The common ancestry of the Click populations dates back 35,000 years. DNA studies indicate that the common ancestry of the Hadza and Sandawe populations dates back to15,000 years. These findings suggest that at the time of the spread of agriculture and pastoralism, the Click-speaking populations were already isolated from one another which explains linguistic divergence among the respective Click languages.

Baka Pygmies of Cameroon

Not surprisingly, the Pygmy population has multiple highly differentiated loci on genes on chromosome 3 which affect growth and pituitary function associated with height. This may account also for the early age of reproduction among the Pygmies.

Hypothetical branching shows theoretical common
ancestor of African populations and chimps

SNPs not found in any modern populations can be explained by the practice of endogamy which is common in Africa, and especially characteristic of hunter-gatherer communities. Others speculate that these represent an unknown, yet-to-be discovered human population that might be a sibling to Neanderthal humans. Co-author of the study Joshua Akey of the University of Washington in Seattle states, "Fossils degrade fast in Africa so we don't have a reference genome for this ancestral lineage... one of the things we're thinking is it could have been a sibling species to Neanderthals."

Richard Klein, paleoanthropologist at Stanford University, considers this speculation “irresponsible” because there is no fossil evidence for this mysterious hominid. Klein excavated at South African Stone Age sites and his research on hunter–gatherers who lived more than 50,000 years ago makes him an important authority on the subject. He believes that the claim of archaic and modern human interbreeding “is a further example of the tendency for geneticists to ignore fossil and archaeological evidence, perhaps because they think it can always be molded to fit the genetics after the fact.” Klein urges skepticism about such claims "when they are so clearly at odds with the fossil and archaeological records."

What strikes me about this research is how little importance is given to marriage and kinship structures among these populations. To speak of "interbreeding" without knowing the kinship structure is to shoot in the dark. For example, do the forest Pygmies of Cameroon have a kinship and marriage structure like that of the Mbuti Pygmies of the Ituri rainforest in the Congo? Their skin tone and languages are different, but are they genetically kin? Clearly, further investigation is needed. Such questions must be answered to gain an accurate anthropological history of Africa.

Mbuti Pygmies
Notice the red skin tone. Their language is in the Central Sudanic group.

It is conceivable that at one time these forest dwellers lived as one population or in proximity to each other and were related by blood and marriage. Their geographical separation can be explained by the practice of sending away younger sons to establish their own territories. Geographical separation does not stop intermarriage between the clans, except in the cases of catastrophic events that might cause isolation. Kinship structures have been shown to be highly resistant to change.

Luigi Cavalli-Sforza's distribution of African Pygmies

The common ancestry and languages of the Hadza and Sandawe populations dating back to15,000 years further suggests that geographical separation did not interrupt kinship structure.

Perhaps it is time for paleoanthropologists, molecular geneticists and kinship analysts to start sharing information.

Related reading:  The Beginnings of Spoken LanguageBiblical Anthropologists Discuss DarwinSent Away Sons; The Marriage and Ascendancy Pattern of Abraham's People

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