Dead horse in Pompeii was actually a donkey.

17 November 2010

Interview with

Susan Gurney, Cambridge University

Diana -  Now, to mysteriously extinct horses in Pompeii.  In 1987, five equine skeletons were excavated from the famous site, but one was a little unusual.  The first set of researchers to analyse the DNA of the misfit, determine that its DNA was indeed quite different from that of the other four skeletons.  So different that it seemed quite unrelated to surviving modern horses, but Susan Gurney together with Peter Forster and colleagues reanalysed the data and found that actually, this alien horse was a donkey...

Susan -   So the remains were taken from an excavation in Pompeii from a Street in Pompeiihouse which is called the Castiamanti house which is the house of the chaste lovers and it's so called because of the friscos depicting romantic scenes which were found at the house.  And this house was owned by a baker, a wealthy baker and so, it's thought that the stables could have horses and donkeys in them because donkeys and mules were used within bakeries to pull the mill.  So, it would make sense that possibly, one of these equine skeletons belong to a donkey.  And so, the town of Pompeii is quite important archaeologically because it was covered by ash from the eruption of Mt Vesuvius in AD 79.  So by excavating the town, archaeologists are looking at what life was like back then and it means that in terms of genetics, we can also see what animals, what species of animals, were present at that time and how they differ to the species that exist today. 

Excavation was performed initially in 1987 but the DNA analysis of the skeletons, the equine skeletons, didn't take place until 2004.  So they extracted DNA from the remains of five skeletons which were found in the stables, and they were wanting to find out how closely related were they to modern horses.  And so, with one of the remains that was quite interesting because part of the DNA sequence didn't match any of the DNA sequences that we have for existing modern horses, so the Italian group who did the work concluded that it was possible that these horse remains were actually from an extinct horse breed which we don't see today.

Diana -   And how likely could this have been?  I mean, how similar or dissimilar was the DNA from a potential extinct horse?

Susan -   Well, because there were five equines found in the stable, you would assume that the DNA should be quite similar between all five of them but one of them had a very high number of additional mutations.  So, it should immediately have looked slightly odd that if you're looking at ancient horses, you've got a sample of five from a period of time from AD 79 that they should be fairly similar, but one of them definitely stood out as being different from what they concluded were horses.  Four horses.

Diana -   And how did this problem land on your doorstep?  How did you get involved?

Susan -   Well I'm doing a research project with Dr. Forster who's also part of Cambridge University and we're looking at horse profiles.  So we were looking at all of the published data on horses and we've also generated some of our own so we were actually looking at the ancient DNA sequences which have been published when we came across this very interesting sequence, and it was when we looked at it further that we've realised that actually it wasn't an extinct breed as had been published but was actually a mix of a donkey sequence and horse sequence.

Diana -   Oops!  Okay, so how could this have happened?

DonkeySusan -   Well there's three possibilities:  it could be a contamination that took place during the excavation,  the bones could've been mislabelled, it could've been a contamination that took place in the laboratory when they were actually extracting the DNA and doing the laboratory processes, or it could have taken place while they were actually writing the paper or during the computer simulation of the work - so trying to assemble the DNA sequences, they accidentally assembled a donkey to a horse.  So there's three possibilities.

Diana -   So how did you actually go about  disentangling the problem and what kind of methods did you apply to it?

Susan -   Well we had the initial DNA sequence that the researchers have published and what we did was we used an alignment program- you choose a reference sequence which we chose as a horse or a donkey and then you align the DNA sequence that you have so it then identifies where the mutations lie.  So initially, we did it with the horse sequence and found that all the mutations were at the beginning of the sequence which they'd examined.  Whereas at the end of the sequence, it actually matched the horse quite well and then we thought, well it doesn't look like a horse.  What are the other possibilities?  So we changed the reference sequence to be a donkey and we found that the mutations which appeared at the beginning of the sequence actually disappeared because they matched that of a donkey, but then we found that mutations at the end of the sequence appeared because that was in fact actually a horse.

Diana -   Okay, so this shows the kind of problems that can happen with ancient DNA. But are there any interesting things that have turned up about this donkey, now that you've got it?

Susan -   Yes.  So although it's not an extinct horse, it's still of interest because if we can confirm that the remains are from an ancient donkey, it would actually be some of the first evidence of the donkey lineage actually being in Italy.  There are two donkey lineages - the Somali donkey and the Nubian donkey, and within across Europe, we see both of these occurring in many of the European countries.  But in Italy, it's predominantly the Somali donkey lineage which this is an example of, so it could be actually the earliest date of a donkey actually being in Pompeii in Italy from this lineage.

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