ARCHAEOLOGICAL CHEMISTRY—EXPLORING HISTORY WITH CHEMISTRY - Chemistry: Notable Research and Discoveries - page 180

nuclear DNA would have survived, so primers for mitochondrial DNA
sequences were chosen.
The operation was a success. As reported in “Molecular Genetic
Analyses of the Tyrolean Ice Man,” written by Oliva Handt, Svante
Pääbo, and many other researchers and published in Science in 1994,
the researchers found mtDNA fragments that are closely related to peo-
ple living today in parts of central and northern Europe. Pääbo and his
colleagues also reported a lack of success when they used primers for
nuclear DNA sequences. Even long mitochondrial sequences were rare;
as the researchers noted in their report, “The degradation of the DNA
made the enzymatic amplification of mitochondrial DNA fragments of
more than 100 to 200 base pairs difficult.”
Skeptical observers might wonder if the DNA found in ancient
bodies or bones are actually contaminants. PCR amplifies any DNA
in the sample, whether it came from the artifact or not. DNA intro-
duced into the sample by humans who handle the artifact is a common
headache for ancient DNA research, particularly for bones, which are
porous. Bone porosity increases about four or five times after death,
which provides a rough field test for archaeologists to gauge the age of
a bone—old bones are much more porous. But the increased porosity
means that these bones soak up solutions and chemicals, some of which
can contain DNA. For the bones of ancient humans, including the Ice-
man, in which the DNA sequence is expected to be quite closely related
to modern humans, it may be difficult to decide if the DNA is ancient
or if it is a modern contaminant.
Pääbo and his colleagues took extraordinary precautions in their
analyses, comparing their results with controls that did not contain any
Iceman samples. The researchers showed that several sequences found
in the samples were contaminants, but Ötzi's DNA was slightly differ-
ent and, as expected, more degraded because of age. As an additional
control, the researchers performed several experiments in a different
laboratory, showing that the DNA they found was not a contaminant
contributed by their own equipment.
Another study of Ötzi's DNA is more controversial. In 2003, the ar-
chaeologist Tom Loy at the University of Queensland in Australia tested
some of the Iceman's equipment, including his knife, arrows, and coat.
Loy had noted signs of blood, and when he used PCR on the samples, he
found not one individual's DNA, but four. This evidence suggests that