K. Owens and M-C. King:
Genomic views of human history
Science 15 (October 99) 286: 451.
- Molecular genetics has begun to revolutionize the study of human evolution.
Analysis of human genomes now offers the possibility of understanding movements
and events of more recent human history, and analysis of records written in
human DNA can complement historical analysis of records written by human observers.
- Human migrations: Every present-day population retains clues to its ancient
roots, and common ancestries can be confirmed and human migrations traced
by comparing DNA frequencies of present-day populations. Early migrations
of modern humans out of Africa have been traced by analysis of DNA sequences;
more recent human migrations have been followed through genetic trails as
well. An example is the application of statistical analysis of classical polymorphisms
to the question of ancient migrations within Europe. One important question
concerning migrations in general is whether males and females migrate in the
same ways. Genetic analysis of sequences of mitochondrial DNA (mtDNA) and
Y chromosome markers carried out in the past few years suggest that the migration
rates of males and females have been dramatically different for much of human
history, with higher migration rates among females than among males. When
females relocate to the birthplaces of their spouses, children are born close
to the birthplaces of their fathers but males. When females relocate to the
birthplaces of their spouses, children are born close to the birthplaces of
their fathers but further from the birthplaces of their mothers. Most individual
females do not move far, but over hundreds of generations, the genetic effects
of their movements accumulate, leading to the observed migration patterns.
- Genetic perspectives on cultural history: Genomic analysis can reveal the
historical demography of cultures with ancient roots, and also indicate how
current populations are related to each other, including the extent and timing
of their contacts. An important historical question, for example, concerns
the movement of people and genes along ancient trade routes. The Kazakh, Uighur,
and Kirghiz populations of central Asia live along the Silk Road, the trade
route between Europe and Asia that flourished between approximately 200 B.C.
and 400 A.D. Analysis of mitochondrial DNA sequences of these populations
suggests that they are descended from people moving from Europe to Asia and
vice versa more than 2000 years ago, albeit long after the early human migrations
out of Africa. Y chromosome variation in part parallels language differences
among these populations, whereas mitochondrial DNA variation does not. Y chromosome
data from central Asia and from other regions of the world suggest that genetic
differences at linguistic boundaries are due primarily to male rather than
female isolation. Genetic evidence also supports the oral tradition that the
Lemba, who are now Bantu-speaking people of southern Africa, derive from Jews
who migrated from the Middle East to Yemen 2700 years ago, and from Yemen
to southern Africa 2400 to 2000 years ago. More than 50 percent of Lemba Y
chromosomes carry haplotypes that are common among Jewish populations but
absent in their African neighbors.
- Genetics, history, and race: Of importance is the fact that genetic differences
of populations from different continents represent only approximately 10 percent
of human genetic diversity: no major genetic discontinuities across populations
have been observed. Most human genetic variation antedates the migration of
modern humans out of Africa. The possibility that human history has been characterized
by relatively homogeneous genetic groups ("races"), distinguished by major
biological differences, is not consistent with genetic evidence. Variation
in traits, including skin color, popularly used to identify "races", is likely
due to straightforward mechanisms involving limited numbers of genes with
very specific physiological effects. Of course prejudice does not require
a rational basis, let alone an evolutionary one, but the myth of major genetic
differences across "races" is nonetheless worth dismissing with genetic evidence.
Contact: Kelly Owens, Univ. of Washington Seattle 206-543-8992.