Why did the researchers wear protective clothing and why do they write so emphatically that they did so?

Because They needed to minimize the chance of introducing their own mitochondrial DNA to the sample.

Mitochondrial DNA could be easily introduced from the contact with human skin. If the researchers do not wear protective clothing, the friction that made during the research could compromise the DNA sample.

The researchers wear protective clothing and they note so emphatically what they did so because they needed to minimize the possibility of introducing their own mitochondrial DNA sample.

The mitochondrial illustration is the structures in the cell that transform the energy from food into a form that cells can use. Mitochondrial DNA has 37 genes which are important for normal mitochondrial function.