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Piecing Together Ancient and Fragile Clues

Posted by | March 28, 2009

A younger friend of mine bought a house nearby that was built in the 1880s. The structure had many shortcomings, like the ceiling over the main room that had quite an alarming sag. The new owner threw himself into major remodeling work as soon as he took possession of the place, partly just to prop the old place up.

Digging into the attic my friend unearthed layers of newspapers from the turn of the last century. When I stopped by the house, my generous contribution to all the hard work being done to reinforce rafters and crossbeams was to sit in a corner and read the old papers.

Newsprint doesn’t age well. Some of the newspapers dug out of the attic have fallen apart into a few pieces, while others have crumbled into hundreds of individual bits. One arresting story I could read fully on some big fragments of paper was about several murders at a farmhouse. It was clear from the story the authorities didn’t have a full theory about what had happened at the farmhouse – and perhaps they never understood it.

Modern forensic scientists could go to work on the old tragedy if they had physical evidence like bloodstains preserved from the scene 130 years ago. But the scientists would have to go about some of their work differently than they would on a recent murder case, and therein lies an interesting story.

Just as newspapers can crumble to pieces over time, so can living molecules. DNA is a long string of chemicals, a bit like a chain that’s been wrapped around a pole again and again. Over time, links of the chain are likely to fall apart.

Let’s say you had a bloodstain from a historic murder scene, or even better from a geologist’s point of view, some blood and fur from a beast that lived in the Ice Age. How could you gather genetic information about the material if the long DNA chain is now missing some links?

The answer lies in the fact that DNA is in each and every cell that’s present in the sample.  It’s a bit like having many, many copies of the exact same issue of an old newspaper. If I had many copies of the same newspaper from a specific date a century ago, I could likely recreate each and every word that was published on that day, just by piecing together different parts of the paper from all the copies available.  A particular copy of the paper might be missing the top corners of page A2, but some other copies would have that corner intact. So, using some information from a number of different copies, I could piece together the whole paper.

Using the same logic, scientists have developed ways to use a computer to compare all the broken bits of the DNA chain they analyze from different cells in the same sample. The computer, so to speak, lays out the short pieces of chain, looking for sections that are identical to other sections. By doing many such comparisons, the computer can recreate the whole long sequence, just as I could do with many issues of the same historic newspaper.

Professor Brian Kemp of Washington State University investigates matters like the DNA of 10,000-year old human remains found in Alaska using this type of approach. In previous studies, Kemp has linked the DNA from ancient human remains to those of native people living in areas from California to South America. What we find in both modern and ancient samples are variations on the same basic theme: the DNA of tribal groups here in North America have much in common with people native to Siberia and Mongolia. That fits nicely with the archeological evidence that North America was peopled by folks who came from northeast Asia during the waning stages of the Ice Age.

Investigating prehistoric human migrations through ancient genetic evidence is just one of the avenues now open to us by piecing together many fragments of copies of preserved DNA.  Such work was unthinkable just a generation ago, but it’s now becoming routine.

Stay tuned for more news from prehistory.