Biology Reference
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that is almost certainly discrepant since the 95% confidence interval was
extremely broad. Using 30 microsatellite loci to construct a phylogenetic tree for
14 different human populations, Goldstein
et al
. (1995) estimated that the time
since divergence of African and nonAfrican populations was 156 000 years (95%
CI, 75 000-287 000 years). The above estimates are broadly compatible with those
derived from polymorphisms associated with the
CD4
(12p12-pter) gene
(Tishkoff
et al
., 1996), microsatellite data (Nei and Takezaki, 1996) and protein
polymorphism data (Nei 1995; Nei and Takezaki, 1996). It must be remembered,
however, that these results simply reflect the time elapsed since the most recent
common ancestor for the sample population rather than the most recent common
ancestor of all humans. Small populations and/or population bottlenecks
(Ambrose, 1998) will have served to obscure the actual timing of the 'origin' of
modern humans.
As to the place of origin of modern humans, Wainscoat
et al
. (1986) claimed that
the relative frequencies of haplotypes of the
-globin (
HBB
; 11p15.5) gene in
African and nonAfrican populations provided evidence for a migration out of
Africa by a fairly small population. Further evidence for a recent African origin for
modern humans comes from the observation that African populations have a ~20%
greater microsatellite sequence diversity as compared with Asian and European
poulations (Armour
et al
., 1996; Bowcock
et al.,
1994; Jorde
et al
., 1997; 1998; Nei
1995; Perez-Lezaun
et al
., 1997; Tishkoff
et al
., 1996). Studies of mitochondrial
DNA have also shown that there is greater genetic diversity between African popu-
lations than among Asian or European populations (Comas
et al
., 1997;
Merriwether
et al
., 1991). Indeed, these authors showed that genetic variation
among humans on all continents are subsets of the variation present in Africans.
However, some polymorphism lineages do not show deep branches for African pop-
ulations which has made the Out of Africa hypothesis somewhat contentious (Jorde
et al
., 1995; Harding
et al
., 1997). The higher level of genetic diversity manifested by
African populations may simply be a reflection of their greater population size over
the last million years (Relethford and Harpending, 1995).
Y chromosome variants appear to be more highly clustered geographically than
those of mtDNA (Cavalli-Sforza and Minch 1997; Ruiz-Linares
et al
., 1996;
Underhill
et al
., 1997). One explanation for this difference could be that male
migration has been more limited than that of women (Seielstad
et al
., 1998).
About 84% of human genetic diversity exists as differences between individuals
within populations but the remaining 16% can be used to distinguish between
populations (Barbujani
et al
., 1997). By comparison with apes, the extent of the
genetic variation exhibited by modern humans is relatively low (Ferris
et al
.,
1981; Jorde
et al
., 1998; Li and Sadler 1991). This lack of genetic diversity is likely
to be a reflection of long-term small population size [before the introduction of
agriculture 10 000 years ago, the entire human population probably did not
exceed 100 000 and is thought to have been around 10 000 for most of its history
(Erlich
et al
., 1996; Harpending
et al
., 1998; Takahata, 1993; Zietkiewicz
et al
.,
1998)], the effects of past population bottlenecks and the explosive population
growth particularly during the last 10 000 years (Ambrose, 1998; Cavalli-Sforza
et
al
., 1993; Harding
et al
., 1997; Knight
et al
., 1996; Reich and Goldstein, 1998;
Xiong
et al
., 1991).
