late pleistocene population interaction in western europe

difficult to detect when dealing with human remains (Hofreiter et al. 2001; Pusch and
Bachman 2004). Due to its sheer numbers in humans (more than 1,800 copies per cell),
mtDNA is more likely to be preserved in archaeological remains than nuclear DNA,
which possesses only a few copies per cell (Avise 1994). Our discussion will therefore
focus on mtDNA. As we will see, however, mtDNA does not provide a full picture of the
molecular evolution of humans and needs to be contrasted with information from other
regions and genomes.
In humans, mtDNA consists of a closed circular molecule composed of ~16,569
base pairs, which represents only a small fraction compared to the three billion base pairs
found in the nuclear genome (Avise 1994). With few exceptions (Howell et al. 1996),
individuals are generally homoplasmic, that is, they possess a unique mtDNA sequence
repeated in all bones and tissues. mtDNA molecules are generally stable, but on some
occasions may vary in size (Avise 1994). Insertion or deletion changes are rare in
mtDNA. However, transversions, and especially transitions, both grouped under the term
nucleotide substitution, are more frequent (Li and Grauer 1991). Research on modern
human origins has generally focused on the mtDNA D–loop or displacement loop, a non-
coding region associated with replication that shows a very high mutation rate, and
therefore often is referred to as the hypervariable control region (Avise 1994).
Because it does not have or has inefficient repair enzymes to correct for
substitutions, mtDNA accumulates mutations at a rate 10 to 20 times faster than nuclear
DNA (Avise 1994). This is one reason why mtDNA has attracted the interest of
researchers, the second being that mtDNA is, with rare exceptions, strictly maternally
inherited. This means that, contrary to nuclear DNA, which reshuffles variation during
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