Genes, peoples, and languages
L. LUCA CAVALLI-SFORZA
Department of Genetics, School of Medicine, Stanford University, Stanford, CA 94305-5120
The genetic history of a group of populations is usually analyzed by reconstructing a tree of their origins. Reliability of the reconstruction depends on the validity of the hypothesis that genetic differentiation of the populations is mostly due to population fissions followed by independent evolution. If necessary, adjustment for major population admixtures can be made. Dating the fissions requires comparisons with paleoanthropological and paleontological dates, which are few and uncertain. A method of absolute genetic dating recently introduced uses mutation rates as molecular clocks; it was applied to human evolution using microsatellites, which have a sufficiently high mutation rate. Results are comparable with those of other methods and agree with a recent expansion of modern humans from Africa. An alternative method of analysis, useful when there is adequate geographic coverage of regions, is the geographic study of frequencies of alleles or haplotypes. As in the case of trees, it is necessary to summarize data from many loci for conclusions to be acceptable. Results must be independent from the loci used. Multivariate analyses like principal components or multidimensional scaling reveal a number of hidden patterns and evaluate their relative importance. Most patterns found in the analysis of human living populations are likely to be consequences of demographic expansions, determined by technological developments affecting food availability, transportation, or military power. During such expansions, both genes and languages are spread to potentially vast areas. In principle, this tends to create a correlation between the respective evolutionary trees. The correlation is usually positive and often remarkably high. It can be decreased or hidden by phenomena of language replacement and also of gene replacement, usually partial, due to gene flow.
Which contains the
One reasonable hypothesis is that the genetic distance between Asia and Africa is shorter than that between Africa and the other continents in Table 1 because both Africans and Asians contributed to the settlement of Europe, which began about 40,000 years ago. It seems very reasonable to assume that both continents nearest to Europe contributed to its settlement, even if perhaps at different times and maybe repeatedly. It is reassuring that the analysis of other markers also consistently gives the same results in this case. Moreover, a specific evolutionary model tested, i.e., that Europe is formed by contributions from Asia and Africa, fits the distance matrix perfectly (6). In this simplified model, the migrations postulated to have populated Europe are estimated to have occurred at an early date (30,000 years ago), but it is impossible to distinguish, on the basis of these data, this model from that of several migrations at different times. The overall contributions from Asia and Africa were estimated to be around two-thirds and one-third, respectively.
Which doesn’t seem to fit the mt/Y DNA patterns, although to be fair L mt types don’t seem to thrive in a cold climate. Since he gives a 146,000 ya date for the first migration out of Africa, this second wave of expansion could have been a very long time ago. Possibly a double OOA might explain the total failure of Y chr dates to tally with the mt DNA expansion dates.
The first estimate gave a separation time of the first migrants out of Africa of 146,000 years ago, very close to the date obtained with the mtDNA full sequence. This was based on results with 30 microsatellites (5). More recent results (L. Jin, unpublished work) with 100 microsatellites gave an earlier date.
Also more humourously, but unlikely..
The Ethiopians genotype is more than 50% African. It is difficult to say if they originated in Arabia and are therefore Caucasoids who, like Lapps, had substantial gene flow after they migrated to East Africa, or if they originated in Africa and had substantial gene flow from Arabia, but not enough to pass the 50% mark.
I think the ‘ mixed expansion south from Egypt with some later Neolithic Arabian farmer’ is a more likely scenario.
I’ll admit to not reading the whole thing before posting it. I have a rotten headache and the kids are playing up. I’ll read it tomorrow.
And having had another look..
There’s this interesting map showing patterns of variation in Europe.
FIG. 2. Hidden patterns in the geography of Europe shown by the first five principal components, explaining respectively 28%, 22%, 11%, 7%, and 5% of the total genetic variation for 95 classical polymorphisms (1, 13, 14).
The first component is almost superimposable to the archaeological dates of the spread of farming from the Middle East between 10,000 and 6,000 years ago.
The second principal component parallels a probable spread of Uralic people and/or languages to the northeast of Europe.
The third is very similar to the spread of pastoral nomads (and their successors) who domesticated the horse in the steppe towards the end of the farming expansion, and are believed by some archaeologists and linguists to have spread most Indo-European languages to Europe.
The fourth is strongly reminiscent of Greek colonization in the first millennium B.C.
The fifth corresponds to the progressive retreat of the boundary of the Basque language. Basques have retained, in addition to their language, believed to be descended from an original language spoken in Europe, some of their original genetic characteristics. (From ref. 1, with permission of Princeton University Press, modified.)