Genoscoping the Time Scale of Racial Diversity

According to the most commonly accepted estimate, modern humans (Homo) originated in Africa about 2 million years ago. The first efforts at reconstructing human evolution from genetic data were made by Cavalli-Sforza and colleagues in 1964 from data on nuclear genes; DNA collected from 15 populations on five blood group genes (ABO, MN, Rh, Diego, and Duffy) comprising a total of 20 alleles was used for this purpose.3 This study and later research indicated a greater number of genes and a more balanced sample of populations that was as

New Guinean & Australian ' Pacific Islander Southeast Asian

Northeast Asian Arctic Northeast Asian Amerind European

Non-European Caucasoid African

African

02 01 0 Genetic Distance

150,000 yrs -1 Time line

Figure 8.1 Human phylogenetic tree. With permission from Princeton University Press.

close as possible to what may have been the aboriginal set of populations were necessary for evolutionary analysis. In another study in 1988 by Cavalli-Sforza and associates, these shortcomings were eliminated; 42 aboriginal populations incorporating 120 alleles grouped into "clusters" representing nine populations were drawn from the literature for analysis at the world level. The final sample, which was obtained on data culled from 1950 populations, yielded the human phylogenetic tree shown in Figure 8.1.13 This analysis placed the separation of African from non-African populations at about 150,000 years ago. Estimates based on other criteria vary somewhat. Thus an estimate based on 30 autosomal microsatellite polymorphisms placed the major split in human phylogeny at about 156,000 years ago,14 whereas comparisons of genetic distances^ (Figure 8.1) and archeological time data suggest that Africans separated from non-Africans about 92,000 years ago, and the next major split, separating Northeurasians from Southeast Asians, occurred about 40,000 years ago. Approximately 35,000 years ago Caucasoids split off from a cluster that included Northeast Asians and Amerinds.13

It is notable that Chinese populations are clustered with Mainland and Southeast Asians while Japanese and Korean populations cluster with Northeast Asians; hence the gene pool of Japanese and Korean populations would probably resemble each other more closely than Chinese. Additionally, there are

{Cavalli-Sforza et al.3 refers to D as the genetic distance used in all phylogenetic analyses. D is defined by the following equation:

D = -log(1-d) = -t/2N where t is time in generations, N is effective population size, and d = (x - y)2/[2P(1 - P)]

where x and y are gene frequencies and P is the ancestral gene frequency estimated by the mean gene frequency for all populations being considered.

Table 8.1 Racial Variation of Taste Blindness: Frequency of Phenylthiourea Nonstasters15

Population

Number tested

Nontasters (%)

Hindus

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