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July
20, 2004 -- The archaeologists in "Coming Into America" mostly
rely on artifacts like tools to help piece together the history
of the first Americans. But in recent years, scientists have
begun turning to genetic evidence as they trace the earliest
human migrations around the globe.
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The
predominant theory is that modern humans evolved in Africa
and then migrated through the Middle East to Europe and Asia,
beginning some time between 40,000 and 60,000 years ago. What
was the route for early humans into the Americas? Scientists
compare the DNA of indigenous people in the New World with
that of people in Europe and Asia, looking for similarities.
The more alike two people's DNA, the more closely related
they are - it's as if tiny maps of our ancient origins are
hidden within our cells. Anthropologists are using genetics
to figure out where some of the first Americans came from,
and when.
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Mitochondria
-- like the one pictured above -- are found within every
cell that produces energy. They retain their own genes
from generation to generation.
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Mother's
side of the family
One group of scientists tracks maternal lineage through what's
called mitochondrial DNA (mtDNA). This is genetic material
found inside mitochondria - structures within every cell that
produce energy. What's useful about mitochondria is that they
retain their own genes from generation to generation. Their
genes aren't a mixture of those from both parents, like all
the other genes in our cells. Instead, mtDNA comes solely
from your mother, when some of her mitochondria are included
in the egg that eventually becomes you. Everyone, whether
male or female, gets their mtDNA from their mom, unchanged.
So
mtDNA isn't reshuffled every generation in the normal course
of inheritance. What does happen, though, is that over the
centuries small mutations take place regularly, that are then
passed on to future generations. This is the unique process
that scientists take advantage of to help figure out both
the age and origins of human populations.
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