Page 89 - The Restless Earth Fossils

Fossils

pigment melanin occurred only about 8,000 years ago. The muta-
tion reduced the amount of melanin in the iris of the eye, pro-
ducing the first blue-eyed individuals. Scientists made their claim
based on studying certain DNA that is easy to track: mitochon-
drial DNA. Mitochondrial DNA, which is located in the energy-
producing organelles (mitochondria) of women’s egg cells, can
be tracked from mothers to daughters. The amount of time that
has passed since a mutation occurred can be estimated based on
changes in the DNA near the mutation site. The more variation
in DNA around the mutation site, the more time has passed.
DNA on the Y chromosome of men passes only from father to
son. This DNA, too, can be tracked fairly easily. The dumpy little
Y chromosome partners with an X chromosome to make a guy a
guy. (Women have two X chromosomes.) The Y does not carry a
lot of genetic information, but errors in the genes it does carry
are uniquely male.
Geneticists like Bryan Sykes at Oxford University have been
tracking female ancestry following mutations in mitochondrial
DNA. Geneticist Spencer Wells and others have been tracking Y
chromosome DNA. Studies from both groups have come up with
similar results. The most genetic variety and the oldest muta-
tions come from African populations—implying that we are all
Africans. Other mutations occurred as humans spread across the
planet. By sampling isolated populations from all over the world,
they have been able to create maps of human migration and
evolution. Secondly, their studies indicate that every single mod-
ern population of human beings alive today descended from a
population that lived in Africa about 200,000 years ago—in other
words, they are our grandparents 10,000 times removed.
About 125,000 years ago, a period of favorable climate
allowed humans to migrate out of Africa again. Archaic Africans
occupied territory in what is now modern Israel and Lebanon and
may have encountered Neandertals. In fact, caves in this region
contain the remains of both species, usually in distinct layers, yet
some skeletons show hints of mixed character traits. Then the cli-
mate deteriorated again, either forcing these people back to Africa

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