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metabolism, which is the prime function of mitochondria in the cell. Because these enzymes have a
very particular structure, decided by their amino-acid sequence, mutations in the genes which alter
the amino-acid sequence almost always diminish or destroy the enzyme activity. The individuals
who are unfortunate enough to experience these mutations in their mDNA usually die. Aerobic
metabolism is such a vital part of life that we cannot tolerate even the slightest malfunction. The
genetic result is that because these individuals rarely live long enough to have any children, the
mutations are not passed on to future generations. If all mDNA mutations behaved like this, we
would never find any genetic differences between individuals and it would be quite useless as a
guide to the past because everybody’s mDNA would be the same. However, fortunately for our
purposes, not all mDNA does code for these vital metabolic enzymes.
Approximately 1,000 of the 16,589 DNA bases in the mDNA circle have a different function
altogether, one that does not depend on the precise sequence. This stretch of DNA is called the
‘control region’ because it controls the way mDNA copies itself during cell division. Fortunately
for us, part of this control region comprises a stretch of 400 bases whose precise sequence is
unimportant. It is really just a piece of genetic padding. It must be there and it must be 400 bases
long for the control region to work properly, but it doesn’t seem to matter what these 400 bases
actually are. This is the complete opposite to the parts of mDNA that code for the metabolic
enzymes, which, as we have seen, need to have a very particular sequence. The vital consequence
for us of this tolerance in the DNA sequence of the control region is that when a mutation happens it
doesn’t affect the performance of the mitochondria at all. Instead of killing the individual who
carries it, the control-region mutations just carry on unnoticed through the generations, and we can
find them.
During our work in Europe it was the mDNA sequences that we found in the control region that
showed us that there were seven principal groups. Within each group, everybody shared a
particular set of control-region mutations. The notation that we used to describe these mutations
was as simple as we could make it. We chose one particular sequence as our ‘reference sequence’.
If we use the metaphor of DNA as a word, then the reference sequence is its standard spelling. The
sequence we chose as the standard was the one we most frequently encountered in Europe. If a
particular mDNA sequence differed from the reference at the 126th base of the 400 in the control
region, then it was denoted simply as 126. If there was another mutation at the 294th position, then
the notation became 126, 294. We found a lot of people who shared this particular combination of
mutations and they formed one of our seven groups. In other groups there were different sets of
‘signature’ mutations. However, within the groups like the one defined by mutations at 126 and 294,
there were plenty of other mutations as well. While about a third of people within the group had
just the bare minimum of 126, 294, the rest had one, two, three or even more additional mutations.
By looking for the signature mutations it was fairly easy to place any individual DNA into one
of the seven groups. Occasionally we would find individuals where one of the signature mutations
had changed back to the original reference, but on the whole it was quite straightforward. But what
did these groups actually signify? It had to mean that everyone within the same group must be
related to one another through their matrilineal ancestors, which was the line we were following
with mDNA. If two people in the same group had been able to follow their maternal ancestry back
in time through their mothers and their mother’s mothers and so on, at some point they would
converge. There would have been a woman living in the past who was the common ancestor of