Encyclopedia of Evolution.pdf - Online Reading Center

high priest, Aaron, brother of Moses. These males are known
as kohanim, and today their last names have such variants
as Cohen, Cohn, and Kahn. Researchers obtained DNA
samples (from cheek cells) from modern Jewish men who
identified themselves as kohanim, in Israel, North America,
and England. About half of these shared a DNA sequence
(a marker) in their Y chromosomes that had to come from a
common ancestor about a hundred generations back—which
was approximately the time that Aaron is believed to have
lived. This Cohen Modal Haplotype may very well have
come from Aaron’s Y chromosome. (A haplotype is a DNA
sequence that is not broken up by sexual recombination; in
many cases, haplotypes are genes, but in this case it is nearly
the entire Y chromosome. Modal refers to the most frequent
set of sequences found in this region of the chromosome in
these men.)
The Lemba tribe in southern Africa, though phenotypically
black, claimed a tradition of Jewish ancestry. In 1997
researchers studied Y chromosomes from men in this tribe.
Two-thirds of the Y chromosomes in this sample were of
Middle Eastern, rather than Bantu African, origin. And most
of the Y chromosomes contained the Cohen Modal Haplotype,
suggesting that they were not merely of Jewish descent
but of Jewish priestly descent. In contrast, the mitochondrial
DNA in this tribe was not of Jewish origin, strengthening the
suggestion that this tribe is descended from Jewish men, not
Jewish women.
Jefferson’s descendants. Many historians believe that
American president Thomas Jefferson produced offspring
through his slave and friend Sally Hemings. Researchers
found a DNA marker that was shared between known white
descendants of Thomas Jefferson and some black Americans
who also had a family tradition of Jeffersonian descent. While
most historians accept this as proof that Thomas Jefferson
had illegitimate offspring, some point out that the DNA
marker could just as easily have come from Thomas Jefferson’s
brother, who was also present at the places and times
when the offspring would have been conceived. This example
illustrates both the power and the limitations of DNA markers
in genetic research.
Markers are also used in cases of paternity identification
and criminal investigation. Markers, not usually being subject
to evolutionary forces, often change rapidly over time.
A major application of markers to evolutionary science is to
short-term studies of population genetics. Tracing the parentage
of individual animals and plants in populations helps
to reveal the patterns of mating and of reproductive success
in these evolving populations.
Further Reading
Ayres, Debra R., and Donald R. Strong. “Origin and genetic diversity
of Spartina anglica (Poaceae) using nuclear DNA markers.”
American Journal of Botany 88 (2001): 1,863–1,867.
Drayna, Dennis. “Founder mutations.” Scientific American, October
2005, 78–85.
Haak, Wolfgang, et al. “Ancient DNA from the first European farmers
in 7500-year-old Neolithic sites.” Science 310 (2005): 1,016–
1,021. Summary by Michael Balter, Science 310 (2005): 964–965.
Mars, life on
Olson, Steve. Mapping Human History: Discovering the Past through
Our Genes. New York: Houghton Mifflin, 2002.
Shreeve, James. “The greatest journey.” National Geographic, March
2006, 60–73.
Mars, life on Life-forms resembling the bacteria found on
Earth may have once existed on Mars. When the evidence
for this was first announced, some people considered it the
most important discovery in the history of science. The evidence
came from a Martian meteorite that was found in
Antarctica. Had the evidence been confirmed, it would have
indicated that the evolution of life from simple molecules
was not an isolated event on the Earth. If life originated
on two planets in the same solar system, it is quite likely to
have occurred many times on many other planets throughout
the universe.
Mars is the planet most similar to Earth, in temperature
and chemical composition, in the solar system. Jupiter
and Saturn consist largely of cold liquids, while Venus
is extremely hot. Some moons of Jupiter and Saturn are
more likely to have conditions suitable for the origin of life,
although they too are very cold. Scientists have long believed
that, if life ever existed outside of the Earth, it would have
been on Mars or on one of these moons.
In recent years, astronomers have presented evidence
for several planets orbiting other stars in the Milky Way
galaxy. The first photograph of such a planet was published
in 2005. Because these planets are usually too far away to
be directly observed, the evidence for their existence is usually
the gravitational force that they exert upon the stars
around which they revolve. From the movements of these
stars, the mass and orbit of the unseen planets can be calculated.
Most of these planets appear to be large and gaseous
and very close to their stars (“hot Jupiters”). However, this
does not mean that Earth-like planets are rare; small planets
like Earth are much less likely to be detected by such
methods. The presence of planets may also be inferred by
periodic slight decreases in star luminosity, perhaps produced
when the planet passes between the star and the
human observer.
The U.S. National Aeronautics and Space Administration
(NASA) sent two spacecraft that landed on Mars in
1976. These spacecraft sent back photographs and data, none
of which indicated life on Mars. A burst of carbon dioxide
production from soil was thought at first to indicate microbial
life on Mars, but scientists later decided that the carbon
dioxide could have been produced by an inorganic reaction.
Other spacecraft sent to Mars in 1997 and 2004 have sent
back data that reinforce the conclusion that Mars is lifeless at
the present time.
But was there life on Mars in the past? Mars was a very
different planet in the past than it is at present. When first
formed, it was warm and had liquid water. The evidence for
this comes from the following sources:
• Satellites orbiting Mars have photographed geological patterns
that appear to have been produced by water erosion
(see photo on page 260).