What ancient DNA reveals about Mayan sacrifice culture

A genetic study has uncovered some fascinating details about the life and death of those put to ritual sacrifice in the ancient Mayan city of Chichen Itza. Modern day Chichen Itza is located in the Yucatan peninsula in Mexico, and in its heyday, around 1300 years ago, was a site of major significance for the Mayan people. At that time, ritual sacrifice was a somewhat common means of providing nourishment for their gods and goddesses. And now, a new study of ancient DNA remains found at the site have revealed more about the demographics of those selected for sacrifice, but also what happened to the genetic composition of the indigenous population in the millennium since.

Aylwyn - My understanding is they were extracted in the usual way for ancient DNA, which is from the Petrus bone, which is a bone in the inner ear which has a high concentration of DNA. It's quite a dense bone and therefore more resistant to sort of the leaching out of DNA into the environment. And I think that was what they did, because they did mention that for some of the samples, that bone wasn't found and therefore they weren't able to get DNA and

Will - Shivani, I feel like this might be a bit of a study into the bleeding obvious, but how can we be sure that these are sacrificial remains?

Shivani - The site in which these burial remains were found is just one of many Mayan sites where these human sacrifices are made. As studies progress and we understand more about the Mayan culture, we know that human sacrifices, whilst not as common as we believe, were part of their kind of giving back to the gods, and they believed in the importance of nourishment. So we do see other very similar kinds of mass burial sites in South America at least.

Aylwyn - I don't know if it's true for all of them, but I guess the fact is that you find a large number of children all in one place, which is suggestive of that not being just a normal burial site. So I think that some of it is inferred due to the religious nature, and we know that ritual sacrifice was an important part of Mayan culture.

Will - And I suppose being found in a cenote, it's unlikely that they all went down there and just died by accident.

Shivani - Yeah, these were all found in underground caverns. So we know, I think, that the Mayas believed there was a connection with Gods in these caverns. And also like Aylwyn said, there were many similarities between the remains. So there seemed to be a preference for twins and a preference for children. And I think they were all male. That can not be by coincidence. They're clearly selected. And, you know, twins play a really important role in Mayan culture, and one of their main religious texts has this concept of hero twins who sacrifice themselves for the betterment of the world. So twins are a recurrent theme we see in many burial or sacrifice sites for the Mayans.

Aylwyn - You do raise a good point though, Will, in that one of the opportunities that arises when you have a new source of evidence, like genetic evidence is, is to keep an open mind with regard to some of the interpretations that have been made based on other evidence and to say, 'well, you know, is this also consistent with those stories?' But it is also true, maybe not here, but certainly in other sites around the world, that we have discovered things through genetics that maybe don't necessarily agree with every aspect of the story that people had proposed before. So it's worth asking those basic questions.

Will - So much like the Rapa Nui study of last month, this is a good bit of DNA confirmation of a previously posited archaeological theory.

Aylwyn - I think in this case. Yes.

Will - As you said, it revealed that the preference was for young males and even twins due to the ideological preference of that being a more potent sacrifice. Did it reveal anything about how they lived before they died, though?

Aylwyn - A lot of their diets had been quite similar too, and I think the interpretation of that was that they had been eating a diet as part of a ritual in preparation for sacrifice of something. I mean, one wants to watch out with interpretations like that because that's something where it would seem there's a number of possible explanations.

Shivani - I mean, it would also make sense if children were to be living in the same area where a certain type of crop is grown, that everyone's diets would be similar. So I suppose it's hard to make the distinction of whether the diets are preselected for ritualistic purposes or everyone's just eating the same thing, because that's what there is.

Aylwyn - Yeah. We should maybe clarify also what we mean by isotope data. That's another kind of molecular data that one can get from ancient remains, not genetic data. Essentially you're looking at the particular types of molecules that you find, and there are different versions of molecules that are often characteristic of different origins in the ecosystem or in diet. And so that actually does give you an insight into what people have been eating and into their environment that they've been living in. So that's been a part of archaeological science for a few decades. And so it's quite nice now we get these studies that really combine sources of evidence of various different types, including genetic evidence to really build up a picture.

Will - I suppose a good secondary side effect of this study is that you get a genetic map of the individuals, the indigenous people that lived in that area kind of pre and post colonization as well. Do these remains show that the modern day populations have had any significant alterations to their genomes in the time since?

Aylwyn - Well, I think the first thing to note is actually there's quite a lot of similarity between the nearby modern population and the ancient population of this ancient city. And we're talking about going back well over a thousand years, I think about 1200 years or something. And so there are quite a lot of similarities. So that suggests there's a lot of shared ancestry that a lot of the ancestors of present day people are similar to the ancestors of these people in the ancient city. There were a few changes. One for example being that these ancient individuals seem to have a little bit more genetic similarity to present day and ancient samples from the Caribbean. So there was obviously some shared component of ancestry there, which has since been either diluted or lost in the present day in the modern population.

Shivani - I think it's interesting to note that during colonisation, the Mayans were, well, not only persecuted, but underwent a lot of natural selection because they were exposed to pathogens such as smallpox. So, by comparing the kind of the alleles with the remains in the present day population, it does actually support that the present day Maya have undergone a large degree of pathogen selection. You can tell by the overlapping of these particular alleles. So it's another incident of what we think to be true versus what is represented genetically. I think it's just interesting that, you know, things have advanced so much to the point they know which alleles correspond to which diseases. So there's a specific one quoted here, which is a binder for salmonella peptides. So I think as things become more sophisticated, not only can we say, oh yes, there's been natural selection and pathogen resistance, we can even say what to, and I think that's really interesting.

A study published in Nature has appraised the genetic impact that an extinct species of cattle has on our modern populations. The Auroch was a wild bovine that appeared in Europe about 650,000 years ago. It grew to a height of 1.8 metres and lived throughout much of the continent, before being driven to extinction due to hunting and habitat loss in the 1700s. But their genetic legacy lives on. The analysis of ancient auroch DNA has revealed when and how these wild beasts were slowly coveted into our more docile domesticated cattle.

Aylwyn - In a very straightforward sense, it's just a direct comparison. We know that modern cattle derive, and there are various versions of modern cattle too, but they all derive from domestication of ancient aurochs populations. And so you're right that we can do, and there were people, people did initially do studies of just looking at modern cattle and trying to tease out. Can we work out what the ancestral relationships between these present organisms are? But it's now we've got a thing to compare with, it just makes it a lot easier. You know, you're trying to do a jigsaw puzzle without looking at the box. It's a lot harder than when someone shows you what the picture's supposed to be. And to some extent seeing the ancient genomes from which the present day domesticated cattle derive that helps us solve that puzzle.

Will - What do we now know then, when we compare the two DNAs and what insight does it give us as to when humans started to domesticate them?

Aylwyn - Well it's quite an interesting picture actually, because today we have, broadly speaking, there are two groups. There's the Bos taurus, which European cattle are derived from and which are familiar to all of us in the fields around us in Europe. And then there are also Bos indicus, which is the cattle domesticated in South Asia. And all, or nearly all, Asian cattle are derived from that domestication. And it's believed, I think, still that those are the two main domestication events. And both of those derived from ancient wild aurochs. There were different, or populations around the world, just like other animals. And we can see ancient European aurochs and ancient Asian and north Asian aurochs. And this study was able to sequence DNA from various samples of all of these different aurochs species. And it seems that the aurochs, their common ancestor goes back kind of a hundred, few, couple of hundred thousand years that there's a sort of divergence of the aurochs into different groups in Asia and Europe, and then subsequently the European cattle. Those emerged from the domestication of some wild aurochs population, aurochs population in sort of the Middle East, western southwest Asia, that part of the world. And then spread from there. As Europeans moved out to the Middle East and brought farming techniques into the rest of the world, into Europe, et cetera, they brought their cattle with them.

Will - We've got the timeline then. Do we have any idea about the process itself? Because it's kind of fairly widely accepted now in the case of wolves that we managed to pick the most docile ones and kind of inadvertently, selectively breed from there. Was this a similar situation?

Aylwyn - I don't know that we know anything about how docile they were, and I suspect it's conjecture that they probably weren't. A wild oryx would've been quite a formidable beast. And so therefore there is some evidence from this study that actually the initial domestication event didn't involve that many animals, probably because it was somewhat risky business to try and go and capture one. And also, if you think about it, suppose I want to start domesticating some animal or group of cattle, maybe the thing I do is I somehow kind of put a fence around a whole herd, and thereafter that's my herd. And I start kind of trying to domesticate the offspring and so on. Actually, it seems like an easier approach, which is what was taken, was you just capture one female animal and you somehow tether that or keep it within your control, and then you allow that animal to breed freely, relatively freely with the wild population, with wild males. But then you've got control over that female and her offspring, and therefore you can build up your stock that way. And it seems that's what's happened. And therefore we can see evidence that the domestication is somewhat female dominated at the earliest stages. And that there's lots of ongoing interbreeding with the wild population. And that carries on throughout the early domestication period. There's a number of these similar domestication stories where we're trying to put it together and it's surprisingly hard. And one of the reasons why it was hard actually is that this pattern of ongoing with the wild population, you know, it's not like we take the whole wild population, domesticate that, and then there are no more wild animals. The wild animal continues to exist. And this pattern seems to be something that's actually the nature of domestication. I'm not sure that we've found any examples where there was this sudden domestication and a complete isolation of the domesticated crop or animal.