Biology Reference
In-Depth Information
and mitochondrial genes, for example cytochrome c oxidase subunit IV ( COX4 ;
16q22-qter and mt genome; Lomax et al ., 1992).
Mitochondrial DNA variation has been very informative for the study of the
evolution of human populations (see Section 2.3.6). The interested reader is
referred to Stoneking (1996) for a review of the topic.
2.3.6 The evolution of human populations
Darwinian Man, though well behaved, at best is only a monkey shaved!
W.S.Gilbert Princess Ida (1884)
Discussion of the fossil record of early hominids is outwith the remit of this volume
and the interested reader is referred to Wood (1992, 1996) for readable préces.
Similarly, the origins of modern humans and the movement of human populations
have been well reviewed by a number of authors including Cavalli-Sforza et al .
(1994), Cavalli-Sforza (1998), von Haeseler et al . (1995), Jones, Martin, and Pilbeam
(1992) and Lewin (1998).
There are currently two different views of the origins of modern humans. The
first, which is not inconsistent with the fossil record, proposes that different pop-
ulations ('races') of Homo sapiens evolved independently from their ancestor Homo
erectus in different parts of the Old World ('multiregional model'). Migration of H.
erectus from Africa to the rest of the Old World may have occurred ~1 Myrs ago.
The alternative hypothesis is that H. sapiens arose once in Africa ('Out of Africa
model') and that the species may have been subjected at some stage to a severe
population bottleneck. With both models, the geographical separation of popula-
tions has led to the emergence of morphological differences although continued
gene flow between populations has served to ensure that human gene diversity
remains graded rather than discrete.
One of the most dramatic demonstrations so far of the utility of molecular
genetics to the study of human gene evolution has been the analysis of mitochon-
drial DNA (mtDNA) from a fossil Neanderthal specimen (Krings et al ., 1997).
Neanderthals and humans are considered to have shared a common ancestor
between 550 000 and 690 000 years ago. Since the Neanderthal mtDNA sequence
was found to be quite distinct from those of modern humans, it would appear as
if Neanderthals became extinct without contributing mtDNA to human popula-
tions, a finding consistent with the Out of Africa hypothesis.
The question of the age of the human gene pool has been approached by study-
ing variation associated either with the mitochondrial genome or with Y chromo-
some-derived DNA sequences (mtDNA exhibits a high rate of evolutionary
change and, along with most of the Y chromosome, is transmitted without recom-
bination). Studies of mtDNA have yielded dates of the order of 200 000 years ago
for the origin of modern humans (Hey, 1997; Horai et al ., 1995; Ruvolo et al .,
1993; Vigilant et al ., 1991), broadly consistent with estimates derived from Y-
chromosome-derived DNA sequence data (Agulnik et al ., 1998; Brookfield, 1995;
Dorit et al ., 1995; Fu and Li, 1996; Hammer, 1995; Jobling, 1996; Mitchell and
Hammer, 1996; Weiss and von Haeseler, 1996). Use of intronic variation in the
ZFX (Xp21.3-p22.2) gene yielded a figure of 306 000 years (Huang et al ., 1998)
 
 
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