Origin: A Genetic History of the Americas

two hours, the machine would cycle the temperature of the tubes,
progressing from very hot to separate the DNA strands, to cooler, which
allowed the primers to bind to the single stranded DNA, and then gradually
becoming warmer again to allow the regions bracketed by the primers to be
copied. If there was any DNA present, I should go from those few scraps of
broken strands to millions of copies by the late afternoon. It feels like a
miracle every time it works.
The PCR reaction serves two purposes: It screens for the presence or
absence of DNA in the samples and negative controls, and it also provides
the researcher with some preliminary information about this person’s
maternal ancestors.
Today I was targeting DNA from this ancient person’s mitochondrial
genome. Mitochondrial DNA (mtDNA) is found within the small structures
in living cells that make cellular energy. These structures have their own
circular genomes, made up of just 16,569 DNA base pairs (7). Because we
inherit our mitochondria only from our mothers (sperm doesn’t contribute
any mitochondria during fertilization except in very rare cases),
mitochondrial DNA lineages are a record of maternal ancestry. That is to
say, your mitochondrial sequence is the same as your mother’s, and her
mother’s and her mother’s. Not only does it provide this record, but regions
of the mitochondrial genome also accumulate DNA changes slightly faster
than the nuclear genome, making it a sensitive and accurate witness to
recent evolutionary events. The gradual accrual of these changes is a sort of
metronomic tick on a millennial timescale.
More ancient DNA studies have been done on mitochondrial DNA than
on any other part of our genomes, because there are hundreds to thousands
of mitochondria within each of our cells (as compared to just two of each
chromosome). Mitochondrial DNA is thus much more likely to be
recovered from ancient bones or tissue than nuclear DNA, which only has
two copies per cell.
Mitochondrial DNA has taught us a tremendous amount about human
history. But unfortunately, its strengths are linked to limitations. Because
it’s exclusively maternally inherited, this genome provides only a tiny
glimpse of an individual’s ancestry. You can, for example, test the
hypothesis that a group of people buried together are maternally related
using ancient mitochondrial DNA. But you won’t be able to tell if they