Box 111 continued

tilizes the egg does not. This has led to the intriguing concept of "Eve," the first group of African women whose mitochondria is still (with base pair changes) within all of us. "Eve," however, should not be considered a single woman; rather, there probably were thousands of "Eves" that gave rise to modern humans. Nevertheless, mitochondrial DNA sequences fit well with the idea of a founder effect at the root of the human evolutionary tree.

DNA sequence comparisons were also made on the DNA from the Y chromosome. As we know, this chromosome is found only in males and thus transmitted only through fathers. Studies of the differences in sequence on the Y chromosome among different ethnic groups are similar to those obtained with mitochondrial DNA. Here also, scientists have interpreted this evidence as African "Adams" from whom humankind is derived. These "Adams" and "Eves" migrated out of Africa to Europe and Asia and finally to the Americas. The thousands of years during which the first humans traveled to all corners of the world allowed for natural selection and genetic drift to occur. Natural selection worked at the level of skin color and general body build. Random genetic drift and founder effect explain DNA sequence variations that do not seem to have selective advantage or disadvantage. Thus, Y-chromosome studies also support an African founder effect for the human family.

In addition to revealing the origins of Homo sapiens, mitochondrial DNA sequencing may shed some light on the fate of an extinct branch of the human family. It is known that the modern humans that colonized Europe and the Middle East (they are often called Cro-Magnons, after the place in France where their fossils were first discovered) encountered other humans, the Neanderthals (Homo sapiens neanderthalensis). These Neanderthals were well adapted to the cold climate prevalent during the last Ice Age. However, as the climate warmed up, Cro-Magnons progressively took over the Neanderthals. The latter became extinct about 30,000 years ago. A question that has intrigued paleontologists is how Neanderthals became extinct. Two theories are proposed: one that Cro-Magnons eradicated Neanderthals through warfare, while the other theory holds that Neanderthals and Cro-Magnons interbred, to result in continued on next page

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