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A Journey to a New Land

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Paul Goldberg, Ph.D.

Professor of Archaeology, Department of Archeology, Boston University

DNA in coprolites

So, one of the objects, let’s say, that people have studied, particularly in the Western part of North America, are coprolites which are fossil remains of excrement. And typically, they’re found in the Western part of North America because the conditions are ripe, so to speak, for their preservation. And normally they’re found in dry caves and so the remains get mummified and preservation is very good.

In the past, typically, people have used these coprolites, particularly human coprolites, as indicators of past diet. And so, what they have done in the past is picked samples of the coprolites to segregate them and then sort of deconstruct what they contain, such as seeds and stems and whatever else is in there, and tried to, on the basis of the materials that they pull out of the coprolite, tried to reconstruct what the people ate in the past. And some of the earliest coprolites go back several thousand years in the Great Basin area.

Now, more recently, with greater technological ability, people have started to think well maybe we can do other types of analyses to study these coprolites and get other kind of information that’s more sophisticated than has been done previously which was, as I say, based on just the remains inside the coprolite based on desegregated specimens.

One of the examples that is very current these days, was some human coprolites that were recovered from Paisley Cave, in Oregon, where, not only did they recover these coprolites from the deposits, but they also applied several different types of analyses among which was DNA. And based on the DNA analysis that was done by a group in Denmark they claimed in May of 2008 that these coprolites have human DNA which they ascribed to not to contamination (which you can imagine something that sits in the ground for so long can be contaminated by the people who excavated or the people who have come back into the cave and so on). And so, at the same time of doing the DNA analysis, they actually dated several of these coprolites and they came out with dates between twelve and thirteen thousand years ago. And so the claim was made in science that these represent the earliest human coprolites in North America and that these show clearly without a doubt that we have again pre-clovis occupation of this part of the world.

Interestingly enough, I received a specimen to analyze, using soil micromorphology. Essentially, what I did was, I took a piece of the small coprolite and imbedded it in a polyester resin and sliced it up and made a thin section about thirty microns thick, or grounded to the thickness of about a piece of paper which would be about thirty microns thick. I put it under the microscope and, low and behold, what I saw in the microscope was not typical at all to a human coprolite that I’ve seen a number of them from the Old World. And instead this looked just like a herbivore that I’ve seen something that could be something like a camel or other kind of grazers that would eat grasses and things like that. It had these phytoliths which are siliceous remains of plant cells that are quite stable and do not break down very much. So they were in this coprolite. And we analyzed this thing and we claimed and wrote into science a little note saying, while based on what we could see, this is not a human coprolite.

For the moment we haven’t really found out exactly how the original authors responded to our claim, but it raises a whole issue of when you analyze something. And this is where the science comes in. More and more these days science exploits these high tech techniques like DNA analysis. But it might be a little bit better first to say, well gee what am I actually dealing with rather than just take the sample, grind it up, extract the DNA out of it and then analyze it and get an analysis of something that we’re not even sure if it’s human. And, in fact, when this paper came out there were a number of researchers that said that the DNA that’s in the sample is contamination, not so much by the people who analyzed it, but by subsequent occupations inside the cave where people came and either urinated or defecated in the site and as a result the DNA that they actually measured in Denmark is not the DNA at the time that this coprolite was formed in the digestive gut of what we think is a herbivore.

So, I guess the moral of the story is that evidence that seems to be locked tight, in a way, when you kind of balance it out and try to say, well gee first what do we actually have here, and it’s a similar kind of strategy that we try to adapt in many of the archeological sites that I’ve worked on, is let’s see what we have in the sediment itself, before we go on and spend a lot of effort and time and money to analyze something, with whatever technique, before we know actually what we’re analyzing in the first place because otherwise it can lead you down the garden path.