Do genes record history?

Luca Cavalli-Sforza has asked, "can the history of humankind be reconstructed on the basis of today's genetic situation?" (Cavalli-Sforza and Cavalli-Sforza 1995:106). Well might we ask, given that identification of ancient DNA from bone is a technique still in its infancy, having so far yielded few data significant for the questions addressed in this book (Paabo 1999). The question of whether genetic data drawn from the blood, hair, or saliva of modem sample populations can be used as direct witnesses for the histories of whole populations who lived many millennia ago is a very fundamental one. A mind-modeling of what might have happened in a Neolithic situation involving population growth will quickly show why.

If we begin with a Neolithic source population, dependent upon farming and with a healthy rate of demographic growth, then during every generation a few members of the group will wish to seek new land, with their families, sometimes close to the source region, sometimes further afield, depending on the varied realities of geography and environment, social competition, and desire for founder status. If allowed, as for instance in situations where early farmers were surrounded by hunter-gatherers rather than other equally territorial farmers, the wave of advance will spread out gradually, yet continually, in theory involving interbreeding with surrounding huntergatherer populations, perhaps mainly via forager female/farmer male parenthood if the ethnographic record is any guide. There will be a continuous watering-down of the original farmer genetic profile as the frontier spreads away from the source region.

But now impose upon the foundation genetic edifice that has resulted from such a farming dispersal several subsequent millennia of continuing mutation, natural selection, and genetic drift. Impose also cultural and natural events such as invasions, massacres, epidemics, and natural disasters, and we might wonder how the source genetic configuration could ever possibly survive at all to be still traceable into the present. That such tracing can occur at all is quite remarkable, and in part due to the careful attention paid by geneticists to both the clinal geography of multiple genetic markers studied in combination (e.g., Ammerman and Cavalli-Sforza 1984), and to the phylogenetic analysis of non-

recombining mitochondria) DNA and Ychromosome lineages as they spread through space and mutate through time.'

Currently, geneticists engage in considerable debate about the historical significances of their data, especially of the trans-continental clines that are often visible in both the nuclear and non-recombining genetic systems. To understand the genesis of these clines, it is necessary also to be aware of the roles of natural selection (especially through climate and disease), and the factors of chance in reproductive strategies that can have a major effect on the survival of lineages.

For instance, Ronan Loftus and Patrick Cunningham (2000) note, actually in this instance discussing the mtDNA of African cattle:

in a population where females leave on average one surviving daughter per generation, any single mother has only a two percent chance of contributing her mtDNA to a population one hundred generations later.

Thus, some of the mitochondrial lineages that characterized a population at source can disappear through genetic drift as the generations pass by, ultimately to be replaced by other lineages that have either mutated or have become incorporated through intermarriage. Such lineages need have no bearing on the ultimate origin of the core population. These stochastic processes will be enhanced in small populations that tend to undergo periodic isolation, as for instance in small islands or rugged terrain.

In fact, mtDNA lineage distributions and mutation ages have to be handled with extreme care when the history of a whole human population is at stake, rather than just that of the lineage itself. Molecular clock forms of dating, whereby mtDNA and Y-chromosome lineages are given mutation ages according to assumed rates of nucleotide sequence mutation through time, can be particularly contentious. This issue is far too complex for further discussion here, but many geneticists have misgivings about the accuracy of such dating methods (e.g., Bradley and Loftus 2000:248; Cavalli-Sforza 2003:85). Perhaps it is not surprising that Erika Hagelberg (2000:5-6) has recently stated that genetic data "cannot provide clear-cut evidence of historical events ... We cannot simply look at DNA of so-called native peoples and expect to

reconstruct the past."

Nevertheless, geneticists are currently making many grand claims for the histories of populations, including those populations attached to language families, so we turn now to relevant early farming situations where genetic or skeletal data can reflect usefully upon issues of demic versus cultural diffusion. The focus is quite heavily on Neolithic Europe and Austronesia, because of the high intensities of debate and publication in these regions.