Sunday, October 16, 2011

Archaeological Genetics

Approaching archeology from a science background, I am constantly on a quest for the deeper scientific explanation for any observed behavioral change. It makes sense to me, for example, that people might want to settle down and build communities or farms; a stable controlled food source and a community to aid in protection seem like attractive options, and we see that while some communities did remain with a hunter/gatherer lifestyle, others domesticated cattle or began agricultural practices. While the development of these cultural practices and the archaeological records they leave behind are incredibly interesting, I can't help but wonder, how did these environmental changes influence human morphological or genetic composition?

A relatively common occurrence in human evolution, particularly in Saharan Africa tribes, is the domestication of cattle, sometimes even the sacred reverence of cattle, such as the cattle burials found at Nabta Playa.1 This domestication and semi-worship of cattle introduced several sources of food, through meat and dairy products. Interesting in itself, as it shows the emerging control of homo sapiens over their own environments, this domestication is even more interesting for the subsequent genetic changes it has induced in human populations.

Most mammals can not digest dairy products as adults; homo sapiens are no exception. The gene coding for the enzyme that digests milk products, lactase, is expressed after birth in order for infants and young toddlers to break down their mother's milk, but gradually decreases expression after weaning. This is not the case, however, in traditionally pastoralist populations, such as those in northern Europe and Africa .Through mtDNA transcription studies, as well as genomic sequencing of those with lactase persistence, the coding gene has been found to be dominant in pastoralist societies2. The high prevalence of this lactase persistence phenotype (~90%) compared to non pastoralist populations (~1-20%) suggests natural selection for the genotype that produces lactase well into adulthood, allowing pastoralists to exploit the dairy provided by their cattle, further equipping them for species survival. This relationship between cultural practice and biological fitness suggests that human culture has gradually outfitted the human population for greater survival.

Another example of this cultural influence on human fitness is the emergence of malaria resistance in Africa. In Western Africa, ancient agriculturalists began clearing rain forests for yam farming land as early as 8000 BC.3 This deforestation allowed for pools of water to form where they otherwise would not have, providing the perfect breeding ground for malaria carrying mosquitos. The gene that confers resistance to malaria, when possessed by a heterozygous person (a person who has two differing copies of the same gene), allows for normal phenotype (physical characteristics) with resistance to malaria, but when possessed homozygously (a person has two identical copes of a gene), this gene causes sickle cell anemia, a deadly disease of the blood. Because of its lethal capabilities, this gene is extremely rare in most human populations, thus rarely causing any sort of trouble, but in agricultural communities in warmer tropical climates where malaria carrying mosquitos are found, natural selection has favored a heterozygous genotype for the sickle cell anemia gene, making the gene, and subsequently, the disease, incredibly prevalent in those select populations4. Again with this example we see a genetic advantage selected for based on cultural mediation of environment. These genetic modifications suggest that though humans have found ways to selectively manipulate their surroundings for thousands of years, high cognitive functioning does not a modern human make. Homo sapiens still underwent, and are probably undergoing, relentless natural selection based on the self imposed changes within their societies.

This relationship between culture and genetics is not always so clear, though there are several more examples mostly relating to diet, skin tone, disease resistance, and sexual reproduction, which are the most survival related characteristics, but the reverse analogging of genetics can also be an effective method in tracing these genetic selections. Through genomic sequencing, a gene called RSEN/PSEN1 has been found to be more prevalent in African Yoruba societies than elsewhere.5 This gene cleaves amyloid plaques in the brain; the plaques that are the main effectors of brain function loss in Alzheimer's disease. In these Yoruba populations, Alzheimer's disease is exceedingly rare. Knowing what we do about gene-culture co-evolution in small populations, we can look at these populations with genetic resistance to Alzheimers and possibly extrapolate the historical environmental or cultural changes conferring this genotype. Further archaeological research must be done to propose any logical theories regarding this resistance to Alzheimers. Both disciplines, evolutionary genetics, and archaeology depend on each other to construct informed theses about the migrations and evolution of peoples across the world, and to answer the questions about ourselves that we desperately seek the answers to. Who are we? And why are we the way we are? We know why some peoples have malaria resistance and the ability to drink milk, but these genetic clues are merely a small piece to the infinitely large puzzle of human evolution.

1Wendorf, F. and R. Schild 1998, Nabta Playa and its role in Northeastern African Prehistory, Journal of Anthropological Archaeology 17: 97-123.

2Tishkoff, S, et al., Convergent adaptation of human lactase persistence in Africa and Europe, Nature Genetics, 2007, 39:1


4Laland et al., Cultural Niche Construction and human evolution, Journal of Evolutionary Biology, 2001, 14:1:22-33.

5Voight BF, Kudaravalli S, Wen X, Pritchard JK, A map of recent positive selection in the human genome, PLoS Biol, 2006, 4(3): e72.

1 comment:

  1. Mary, this is a great pairing of the archaeological issues we've discussed in class around pastoralism together with recent genetic work on a range of issues related to pastoralism.