Origin: A Genetic History of the Americas

This obsessive attention to sterile technique is only one reason why few
people want to work in our field. Another reason is how seldom you
succeed in actually getting DNA from a bone. While you can obtain huge
quantities of DNA from just a swab of a living person’s cheek, ancient
DNA is a completely different story. Damaged, fragmented, scarce, and
mixed with huge quantities of contaminating modern DNA, the molecules
are rarely ever present in detectable quantities within any given bone or
tooth, and the process of recovering them is extraordinarily difficult.
My confidence in my hands grew as I moved the cleaned, dried tooth to
a large cabinet on a nearby bench. This hood, made of clear plexiglass with
a hinged front that opened just enough for me to slip my hands into, was
effectively an ancient DNA lab-within-a-lab. We had several of them
scattered throughout the room, each dedicated to a few stages of the DNA
extraction process. Segregating our activities to different enclosed spaces
provides yet another critical measure of protection against contamination.
But there was even more care that needed to be taken with the first step of
the extraction process; mechanically powdering the bone or tooth sample
could result in material getting all over the lab. This meant I would need to
operate inside an even smaller space within the hood: to be specific, a small
plastic glove box that would catch any loose powder. I snaked a drill
through one of the ports on the side and began to carefully work on the
tooth. The surface material of the tooth went into a small plastic tray, to be
discarded as likely contaminated. I widened the hole I was drilling and
scraped powder from the inside of the tooth into a second tray (previously
decontaminated by exposure to ultraviolet radiation). Taking the tray out of
the box, I weighed it on a scale: 0.025 grams—about half the amount of
material that I needed. I continued to drill into the tooth, trying hard not to
crack it as I excavated powder. This was my least favorite step in the whole
process—I was tense and utterly focused until I had tipped the white
powder into a DNA-free plastic tube.
I breathed a sigh of relief when at last I had extracted sufficient powder
to move forward. We try not to sample more of an individual’s remains than
is absolutely necessary for obtaining DNA, and there is tremendous
pressure not to ruin the tiny samples.
But the rewards are worth the pressure.
It’s amazing when you stop to think about it: This tiny extraction from a