Kelly Knudson on How Chemistry Helps Archaeologists Learn about People in the Past

really what excites me I think is what has driven my work for a long time and it's what got me excited about archaeology as a kid which is understanding or trying to understand what people's lives were like in the past. Not the big picture work but what individual humans were experiencing. Hi, welcome to the Science Fair podcast. I'm your host, Susan Keatley. I'm a PhD chemist, writer, and I love talking to scientists. On the Science Fair podcast I aim to bring you conversations with scientists doing fascinating cutting-edge work on all kinds of interesting phenomena ranging from physics to chemistry to biology and even the nature of science itself. In this third season of the podcast every other week two episodes will come out. On Mondays there will be a shorter 10-minute episode linking the scientists research to what's happening in the classroom and then on Thursday the full-length interview. So come along and tune in for some Science Fair. We are so lucky to have Kelly Knudsen as our guest this morning. Kelly is a professor of anthropology in the School of Human Evolution and Social Change at Arizona State University and she also directs the Center for Bioarchological Research and the Archaeological Chemistry Laboratory. Kelly, thank you so much for coming on the show today. Thank you so much for having me. It's really a pleasure to be here, Susan. So I would love to start with what led you to study and pursue archaeological chemistry. Absolutely. So my journey to get to archaeological chemistry was really it had a lot of twists and turns which is perhaps not surprising because this is such a small specialty. When I was a kid, as soon as I learned about archaeology I loved it. I thought it was amazing that you could know what people were doing in the past and I loved the intimacy of it. I was interested in the people not, you know, the pyramids. I was interested in the people who built them and how they lived and what their lives were like. But I didn't know that it was possible to be an archaeologist as a job. I certainly didn't know any archaeologist except for Indian concepts. Exactly. You saw things in the movies but that was not an actual career path. That was a man with a whip. It was not what I wanted. So I went to when I went to college, I went to Ballot College which is a small liberal art school and I was going to be pre-med because I had always liked science and I thought medicine sounded interesting. I like the idea of helping people and so I was majoring in chemistry and math. Not necessarily because I loved them both but because I had the most credits in chemistry and math. So I was double majoring and going to go to medical school. And then I was really fortunate because I was able to go on a field school in Chile and I did it just to get social science credit honestly. And my choices for my social science credit were either doing an archaeological field school in Wisconsin in the summer when I knew it would be hot and humid or going to South America where I had never been and experiencing something completely new and different. So I thought well obviously I'll go to Chile. That sounds much more fun than digging in Wisconsin during the summer. And that's really where it all started because it was on that project where I first got excavation experience and first started to realize that you could do both chemistry and archaeology together. And I was really fortunate because the faculty I worked with were very encouraging instead of saying well that's strange and we don't know anyone who does that. They said great let's figure it out. Let's see how you could make this work as a career. So after that summer in Chile I came back and dropped the math major so I could just take archaeology and chemistry courses. And that's really where it started. And then after that I went to graduate school at the University of Wisconsin at Madison which at the time was the only archaeological chemistry program that was located within an anthropology department. And that was really important to me because it was the questions about people in the past that I really wanted to answer and I wanted to use chemistry as a method as a way to answer those questions. And now I have my own archaeological chemistry laboratory at Arizona State and I get to combine archaeology and chemistry and work with students and teach my classes and it's absolutely wonderful. But I had no idea that this is where it would end up when I just liked archaeology as a kid. That is so neat. And when you went on that field school was were there any archaeological chemists on the site? There weren't. And again this is why I'm so fortunate that I had encouraging mentors because I was starting to put together, oh you can combine chemistry and archaeology and I designed a soil chemistry project. And in retrospect I don't know why I thought this was doable. Like no one told me I had read articles about people doing this and thankfully my professor said yeah sure that's great. Take some soil samples and take them back to Wisconsin and analyze them in the lab. And we were trying to understand what people were doing in the agricultural fields. That is incredible. So for full disclosure to listeners Kelly and I were in graduate school together in this archaeological chemistry laboratory and I think that at one point I knew that story but it's fun to relive it again now. And it just shows how an experience like a summer program can have such a huge impact on your trajectory. You probably didn't think it would. You know it was just sort of a social science credit. Absolutely. And I think both kind of getting into the field was hugely beneficial and also having people and mentors who said that's interesting. I have no idea how that would work but sure give it a try. Instead of no we don't know anyone who does that. We don't have an archaeological chemist on the project. Just go excavate and stop talking about the soil samples. Right. Right. That's wonderful. So walk us through what your life as a professor and lab director is like now. So as a faculty member at Arizona State which I love I've been here since 2005. One of the things I love about my job is that I have a really nice mix of things that I'm doing at any given point in time. So one of my jobs is to direct the archaeological chemistry laboratory which has projects from all over the world with collaborators. We've analyzed about 12,000 archaeological samples. Every sample gets a sequential number and we're somewhere in the 12,000s. So the work itself is exciting and interesting. Part of it is boring because as lab director I do a lot of budgeting and administering the grants that fund our research and making sure and see recently I had to replace the band-ays in the first aid kit because they were expired and a lab safety invest not investigation of when they came to look at the lab to make sure that we were following all the safety protocols. So things like that are boring but the big picture is really fun and exciting. And then I combine that with my teaching responsibility so I teach everything from large undergraduate classes on things like death and dying or applying anthropology to small specialized classes in state biogeochemistry and anthropology together. And I work with students in the classroom and in the laboratory which I really enjoy. So the lab has students who are undergraduates who are working in the lab. For course credit we call them research apprentices. Sometimes we have high school students in the lab working as interns, lots of graduate students and then postdoctoral scholars. So I really enjoy that mix of things for me that's a really good fit because I love the science but I also love working with students. And when did you take on the responsibility of lab director? Was that right away when you started as a professor? It was. It was right away. So when I was hired the lab was being my first step was to design the lab and get everything up and running. And so I did not anticipate sitting in a lot of meetings looking at things like the shape of the outlet for the particular piece of it. We needed in the lab. It sounds like building a house. Yes I've never built a house but I imagine it's the same level of detail. Right. And I mean you didn't that's not what one studies so much in graduate school. Did you feel like being in the archaeological chemistry lab at Madison prepared you a little bit for that or how did you figure out how to do it? How did you learn how to do it? I think being in the lab at Madison was hugely helpful just because I got to see day in and day out the decisions that were made in a laboratory. So when we were there, Jim Burton was the person who the scientist who was working on all the day to day kind of get your hands dirty in the lab work and the dead price was the lab director. And so between the two of them I really got to see not just what had to happen, but the thought process behind it, which I really appreciated. So it wasn't a lab where you just followed protocols. It was a lab where you had to think about and justify, okay, well, this is why this particular step is important. And if it's not, and if you can't justify it, then you need to change it and figure out a better way to do it, or a more streamlined way, or more cost-effective way. And so participating in those discussions and really thinking about why we do the thing as we do was really helpful in then setting up my own lab and figuring out, okay, given the instruments we have, the space we have, the questions we want to answer, what makes sense? Yeah. So today we are going to do a deep dive into a particular research project by Kelly and her collaborators. And the results were published in a paper in PNAS, the Proceedings of the National Academy of Sciences. And the paper was called "Fisting" and the evolution of cooperative social organizations, circa 2300 BP in Paracas Culture Southern Peru. And in the research, you and your team use chemistry to help answer the question of how extensive a particular feast was. That is, did it draw more of a local crowd or more of a regional crowd? And this, I understand, is important because feasting is an indicator of social complexity. And the distance people travel to the feast hints at the nature of social networks at this time. I also feel like our timing is amazing because many people gathered last week for a feast. And there were local and regional feasts. And maybe one day someone will be looking at our artifacts from feasting. But so I would love it if first Kelly, you could walk us through the basic idea of using chemical element isotopes to trace where an object is from. Absolutely. And thanks for explaining the article so nicely. So archaeologists are often really interested in understanding where artifacts and the foods, the animals that people were eating, where they're coming from. And sometimes it's really obvious when you just based on excavation. For example, in the Andes where I work, there are certain plants, certain animals that would only be coming from the Amazonian rainforest. And so if you see, say a parrot or certain lowland crops on the coast of Peru, you know immediately, okay, these were traded in. They're from the Amazon. Great. You're done. You don't have to do any more analyses to learn that. You can start to put together trade networks or social relationships just based on the species themselves. But in other cases, we don't. These plants and animals or artifacts could come from a really wide area. And so we don't necessarily know where they're coming from. And we often care quite a bit because for example, in this case, we wanted to understand who was coming to these big feasting events at this large sunken mound site where a lot of people had to come together to build the site. But people don't seem to have been living there. And this is again, is over 2000 years ago. So it's quite early to be building something this big and impressive on the coast of Peru. So to understand how people are building these big sites, what they're doing, why the sites were important to them, and then how that leads to state formation in the area, we wanted to know where all of these objects were coming from. And the species were things that can grow all over. For example, there was cotton which grows beautifully all along the coast of Peru. It could be from a lot of different places. And just looking at the textiles that were excavated, we're not going to be able to understand necessarily where they're coming from. So in cases where archaeologists don't know where artifacts or foods or including plants and animals are coming from, it's really nice to have archaeological chemistry to help us understand that. So one of the things that I do and what we did in this paper was use strontium isotopes, which are found all over the world. They vary based on the age of the bedrock and the type of the bedrock. We look at two particular types of strontium to isotopes, strontium 87 and 86. And we know that worldwide every geologic system has these two isotopes and varying amounts on them or in them. And those isotopes, whether into the soil in a region, which means they get into the water into the plants that are growing in that soil and taking up that water into the animals that are eating those plants and then into the humans that are eating those plants and animals as well. And what's really nice about these particular isotopes of strontium 87 and 86 is that the amount of 87 to 86 in, say, a corn plant will be the same as those isotopic compositions in the soil in which that plant was growing, which are going to reflect the bedrock that that soil came from. And so they don't change as the isotopes move through the ecosystem. And is that not true for other isotopes? Yes, and that's a really good point. So other isotope systems we use specifically because they change. And that can be really useful. But for strontium isotopes, they don't change. And so it's really nice to be able to look at a plant or an animal or a human and look at the strontium isotopes in that organism. And then infer where that plant was grown or where their animal or that human grew up. And of course, that's a little simple. It's not especially when you start to look at animals that move over a wide area. Maybe they eat plants from three different geologic zones. Maybe like in the Andes, we have llamas and alpacas that are caravan animals that could be moving up and down the mountains. So it's not perhaps as simple as I've made it out to be, but particularly for plants that are growing in one area. Once you understand what the strontium isotope values in that area are, you can then infer where that plant was gruffled. Yeah. So using that basic idea, what particular objects did you focus on in this research? And what did you find from the isotopic evidence? So this particular work, my colleagues, Chip Stanish and Henry Tantanlian, excavated a site called Cerro de Hentil, that was, it's on the southern coast of Peru. And one of the things they found in their excavations, I mentioned this large platform mound with kind of a sunken court. And they found what looked like lots of evidence for feasting. So different objects that were left like textiles, ceramics, gourds, which are these bottle gourds that are really useful as vessels. And that can be decorated with fire and gravy and they're really beautiful. So one of the objects that they found, one of the artifacts, were a number of little bottle gourds, which can be engraved like this one. This is a little guinea pig. So this is a contemporary one. Yeah. Was just from Peru. But people 2000 years ago would take these bottle gourds and would grow them and then empty them out, use them for vessels, particularly for liquids, and then use the same kind of fire engraving, that pyro engraving to decorate the outsides. That's a bottle gourds, food, lot food like corn, llamas who were butchered and eaten, textiles made from cotton. So they were finding evidence both of feasting events in like the remains of food that was consumed. But they were also finding areas where people were leaving what looks like offerings, like of the textiles. And the reason that this starts to look like a feast instead of something that, you know, wishes, everyday food consumption, everyday meals. We see that we don't see a lot of residential, really, any signs of people living at the site. So it's not a site where you see this platform out in the middle and then houses all around it where people were living year round. Rather, it's this isolated site where it looks like people were coming together to participate in these big feasting events, leaving basically the trash from the feasts, you know, pushing them into the corners as they were cleaning up, and then also leaving offerings of things that were important to them as part of these big feasting events. Right. And so what we analyzed then were remains of the plants, like the corn, the bottle gourds, the llama bones that people were eating. There's also, we looked at things like canes that were used to make baskets or mats. And I mean, really the fact that I can give you this list is pretty remarkable because these are all, yeah, I'm talking about. pieces of cotton fabric that's over 2000 years old. - And that's because the preservation is so good in this area? - Exactly. So this part of Peru, it's hyperarid, it's extremely dry. And so we have really excellent preservation of these organic remains. At a lot of sites that are 2000 years old, in other parts of the world, they're wet enough, for cold enough, for they have big temperature fluctuations where we just don't see this kind of organic preservation. So it's really remarkable that we can do this kind of work. - Yeah. - And so what did you find out about the objects? - So when we tried to figure out where all of these objects were coming from, of course the first step is to try to figure out what do we think strontium isotopes should look like in any given area? Because of course, if the strontium isotopes, if you're interested in two different areas and the strontium isotopes are the same, even though the areas may have been culturally significantly different to people, if the isotopes don't look different to us, we're not going to see anything. And so thankfully the Andes do have quite a bit of variability in strontium isotope ratios. It's not perfect, but we do see enough variability that we knew that if these objects, these plants, these animals were coming from outside the area right around the site, they would look different in isotopic space than not. So that was the first step. And I have done that. I've been doing this for years in Peru to kind of create an isotopic map of what we would expect different regions to look like. And this is where it gets a little strange because I use guinea pigs to do that. (laughing) - Because I remember. - I remember this. - Very small, yes. So I started in graduate school collecting guinea pigs from all over the Andes. And that sounds strange, but it's because they're small mammals who eat food from a very constrained area. So I could go around and talk to farmers who were raising guinea pigs and ask them exactly what these animals were eating and exactly where the fields were, where they were growing the alfalfa that could feed these animals. And they're short lived. They tend to not move around very much. They're usually three to six months old when they're killed. And they're very, very common because they're a food source in the Andes. And I apologize to listeners who may have had guinea pigs as pets as children and maybe didn't know this, but in the Andes they're a food source. And so it's very, very easy to find them in this part of the world where people are raising them for feasts. - Is there an advantage to using guinea pigs over just soil samples, either a scientific advantage or like a logistical advantage? - Yes, absolutely. And I would say both. And so the guinea pigs are nice because they, in their three to six months of say eating alfalfa from one field system and drinking water from one constrained area, they're providing an average of that area. The soil samples can be much more variable. So to get good coverage with soil samples and a really good understanding of what's happening, you need a much larger number of soil samples. - That's interesting. It's almost like, in this case, the precision of soil is a negative. - Yes, exactly. And you also, it's nice to be able, what we talk about is bioavailable strontium because I don't actually care about the strontium that's in the rock. I care about the strontium that's getting into the plants and the animals and the humans. - Of course. - And so guinea pigs are a nice way to start to get at the bioavailable strontium. - Right. Because the guinea pigs might be reflective of the objects that you're then seeing later. - Yes, exactly. And so we have hundreds of guinea pig samples from around the Andes that we use as a proxy. And we have, I started out with guinea pigs, but later we have looked at soil samples, at water samples to compare what these values are giving us from different sources, and they do work well. So we still use guinea pigs as proxies. So based on these guinea pig data, and a larger understanding of the geology in the area and soil samples and water samples, we had a good sense that if we looked at these archeological artifacts and plants and animals, we would see if they were coming from a larger area. So that was the first step before doing any kind of analysis because of course this is destructive analysis. We only use a very small portion of an archeological material, but the processing in the laboratory and then analyzing it on the mass spectrometers does destroy the small sample that we use. So we try to do everything we can to retain as much information, to document it, and then to only do this kind of analysis if we can't get the information any other way. So when we looked at the samples, like I said, we looked at cotton, at corn, the gourds, which were used as vessels, not as food sources themselves for this particular species, materials that have been used to make baskets. One of the things that I thought was interesting, there were little tiny wooden bottle stoppers. That was one of the objects that we were able to analyze. And when we looked at all of these, what we found was that there are quite a few that look like they're local to that area. But not exclusively, there's a significant percentage over 25% that are coming from geologic regions outside of the site itself. And so we interpreted those data as evidence for much larger social networks and larger groups of people who were coming into the site periodically for these large feasting events and bringing things with them, because they were bringing the food, drinks, clothing that they were wearing, special objects that they were going to leave at the site. So they were bringing everything with them, coming to the site, coming together from lots of different areas, and then going back to their homes. - Wow. Was that surprising to you? - I think if you had asked me before we started doing the work, it would have been tempting to say, "Oh, it'll be really interesting if they have these large social networks." But I also think in all the years that I've worked with people who lived in the past, I've learned to not predict, (laughs) because it's often when people are doing things that I don't predict, it tends to be where the research gets really interesting. When I find things that really make no sense, and then I have to go down a completely different path to understand what I think could have been happening in the past. - Yeah. So was this surprising for you, these data? - I think these data were exciting, but not necessarily surprising, and that I think the idea that people were bringing large networks together, makes a lot of sense when you think about the long-term development of people moving from being hunter-gatherers in small social groups, which is how we existed for the vast majority of our species, and then moving into these larger sites, and eventually to these really complex, socially stratified states and even empires, where you have large groups of people coming together in really complex ways. And so the site, like, instead of the Lentil, is early enough that you're seeing the beginnings of that, right? You're seeing people building these magnificent platform mounds and coming together periodically from a really wide area, and they're not working in a large stratified state yet, but you can start to understand how that could happen. When you start to have large groups of people coming together, working together to build these sites, and then dispersing, then maybe a few generations later, they start to spend more time together. You have more diverse groups of people coming together and not leaving after the feast, but instead staying together and building even larger and more complex sites. So from that perspective, I think it's exciting, but not surprising, right? So in this particular paper, I'm curious to think about how the chemistry, the chemical evidence, and then the more traditional archaeological evidence, how they complement each other. I think we've touched on it a little bit already. I also wonder, I wonder if there was ever a time when the archaeological data and the chemistry data were conflicting, and like, what did you do in that situation? In this case, the different lines of evidence really came together beautifully, but we wouldn't, for example, we would have never thought that this was evidence for feasting just based on the, if you just had, say, a list of species. If you said, okay, we had the site, and here are the plants and animals that they were eating, and they were using baskets, and they were using gourds, and they were wearing cotton textiles. That could be a site where people live year-round, a site where people lived seasonally, and maybe while they were growing crop. and they went somewhere else. It could be a large site, it could be a small site. But when you start to look at the archaeological record and see that all of these plants and animals and artifacts are found in a place that's a large sunken platform mound with no other evidence of habitations, then it starts to make more sense and it starts to look like a feasting event. So we wouldn't have interpreted the archaeological data very differently, had it been found in a different context. And so in this case, you mentioned the hypotheses that we came up with before the hypotheses made a lot of sense because they were based on the archaeological data. But if we didn't have that, we really wouldn't be able to say much at all. I would have a list of plants and animals and I would have said, they came from a variety of places on the Southern Proving Coast. But without the archaeological context, that would have meant very little. Right. You have no sense of why they were there in the first place. And then without that, you can't get to these interesting questions about social complexity and its evolution. Exactly. And that's why looting can be such a problem because you're removing artifacts from that archaeological context. And so then you do just have a list of these ceramics, these textiles, these plants. And once a site's been looted, you really have no way of reconstructing those data. Right. So I think that's an important point. And then you would ask about what happens when different lines of evidence don't match up. And that's complicated. And it's where I think things start to get really interesting. Like I tend to like those situations where I'm like, oh, I had no idea this was going to happen. And it makes no sense to me. No, we need to figure it out. And maybe generate more data. So for this project, we didn't come up with this. But there have been other projects. One that comes to mind is working in Northern Chile. We're based on the clothes that people were buried in. These are a series of cemetery sites that date from about 2000 to 1000 years ago. And based on the clothing that people were buried in, the pottery that they used, some of the metal objects that they used, archaeologists for a long time thought that they were immigrants coming from a site called Tuanaco, which is up in Bolivia, which is about a 40 day care of antrip away from these oasis sites in Northern Chile. And the oasis sites are near lots of mineral deposits. They're also oasis. So they're water in the middle of one of the driest deserts in the world. So it made sense that people would have maintained these connections between this large site in Bolivia and the sites down in Chile. But what we found when you actually start to-- when we started to look at where the people were from, there were really substantial mismatches between what they were buried with, where they had actually grown up and the clothing that they were wearing. So we tend to think about, say, migration as being very straightforward where one person moves, and they bring their clothes, and they bring the objects that they use every day, and then they move to this new place. But we didn't see that at all. We would see people who grew up in Northern Chile, who were using some ceramics or some ritual objects from Bolivia and others from Argentina. And it was really complex when you started to put it all together. And it didn't make sense initially to me, because I had thought, well, if someone is buried with things from Bolivia, they're from Bolivia. Right? They're buried with things from Argentina. They're from Argentina. And instead, we saw this much more complex and mixing. And what we think is people who were advertising ties with different places that might not reflect their biological ties but might reflect other aspects that were important to them, trading relationships or maybe aspirational relationships. Yeah. I mean, it's just like what people do today. It's so interesting. Exactly. Well, and that's it. I love that story, because it's not saying, like, if these things can flick, well, one of them must be right. So we'd better get more data to make sure one of that you're saying, they could both be right. And it actually is just a different story than we originally thought. Absolutely. And I think it also reflects just how the science was being conducted. So initially, a lot of these sites were excavated by a parish priest. So he was very interested in archaeology in the '40s and '50s, but was not trained as an archaeologist. And then after the sites were excavated, different specialists would come in. So a specialist who focused on ceramics would look at the ceramics and say, OK, this is what I see. And we have these immigrants based on the ceramics. And then someone else would say, OK, I've looked at the textiles. And these are the immigrants based on textiles. But they hadn't looked at everything altogether. And when you start to look at it together and realize that the people who are immigrants based on ceramics are not the same people who are immigrants based on their physical bodies or what they're wearing, then it gets complicated, but really interesting. Yeah. Oh, that's so neat. I really like that. And it just shows the complexity of humans, like the idea that you could be buried with an object that-- I love how you use the word aspirational. Yeah, it does make people who were living one and 2,000 years ago feel very close to us. Yeah. So I would like to move on to some more reflective questions about archaeological chemistry. How has the field changed since you began studying it in graduate school? So one of the things that I think is really exciting is the technique, especially using strontium isotopes, was very new when I first started to work on it. So I've been able to see the field really mature. And one of the things that's been exciting is to see how much more sophisticated our understanding of the isotope mixing and then how the isotopes are moving in the environment. But also these maps. So like I talked about the guinea pigs and trying to understand these maps. Now scholars are using-- just making beautiful predictive isotopes using GIS and looking at making maps that are much more sophisticated and much more fine tuned than our previous maps. And so I think that's been really exciting. And what excites you the most about what you do? What keeps you going? So I think, especially as we were talking about these very intimate decisions that people were making in the past about how they portrayed themselves and how they lived their lives, what kind of relationships they had with other people. Really what excites me, I think, is what has driven my work for a long time. And it's what got me excited about archaeology as a kid, which is understanding or trying to understand what people's lives were like in the past. Not the big picture work, but what individual humans were experiencing and recognizing, of course, that we're never going to understand perfectly. We might not get particularly close, but we're getting closer. And then to take that really intimate data and try to link it with the bigger questions while also keeping a sense of that scale of one individual human, one life and what that was like in the past is really important. And I think it's exciting too. Yeah. It's like the next best thing to a time machine. Yes, absolutely. Now we are going to move on to the high school science section. Yes. So in the Arizona State High School Science curriculum, in the chemistry portion of the learning standards, students are asked to explain how the structure of atoms relates to patterns and properties observed within the periodic table. So this question relates to your research when we talk about how strontium gets into bones. So I wonder if you could please tell us more about that. Absolutely. So we talked a bit about how strontium-- and especially the isotopes I look at are found all over the world. They're in bedrock and soil and water. And then they're incorporated into the bodies of animals through the plants that they eat, humans through the plants and animals that they eat, and the water that they drink. But I think a natural question is, so how does that happen? And is it happening to us right now? And that answer is yes, it is. Bone and animals and humans is made up of a mineral called hydroxyapatite, which is what gives us our hard brittle bones. But then it's also made of a protein called collagen, which gives us some flexibility as well. And so the hydroxyapatite is a crystallitis, and the strontium substitutes for calcium in that high calcium. hydroxyapatite, crystal lattice. So for most bones that we would analyze, if I have a llama bone, it's 2000 years old from Peru, most of that hydroxyapatite is going to have calcium, but sometimes strontium will substitute, and it's not a perfect substitution, but it's pretty good, and that it's able to substitute, because if you go down, if you're looking at the periodic table and you go down column two and you see strontium, you'll see that it's right underneath calcium, and that means that they have similar atomic radii, even though the atomic weights are different, which means that strontium, even though it's not calcium, it's not a perfect substitution, it can substitute for calcium in that hydroxyapatite crystal lattice. And in bone, most, for the most part, there's not a lot of reason to keep the strontium from doing that, and the only exception is really in when a baby is in utero, the placenta does discriminate against anything that's not calcium. For the most part, if there is strontium, it can substitute no problem. And if you go down the periodic, that column in the periodic table, you'll see that, it also, you can see substitution for barium because it's a similar atomic radius for magnesium and also for radium, which again, very different atomic weights, but similar atomic radii. And so all of these can substitute for calcium. You may have heard or read the radium girls, which is phenomenal book, and looking at the periodic table, that makes perfect sense because the radium that these women were ingesting when they were, say, putting glow in the dark paint onto watches, and they were ingesting it because they were licking the brushes 100 years ago. The radium was substituting for calcium, and that hydroxyapatite crystal lattice, and was being incorporated into their bones, which is why, unfortunately, as they had more and more exposure to radium, they were developing bone cancers and bone growth as radium, in this case, a radioactive element, or a radioactive isotope of radium was being incorporated into their bones in the same way that strontium, the non-radioactive isotopes of strontium are being incorporated into our bones every day, with the food that we eat. And so finally, what advice do you have for high school students interested in studying science? And especially for students interested in something very specific, like archaeological chemistry, would they start out, would you recommend starting out broad and specializing later, or getting right into that specific area? What do you think? - That's a great question. And I think there's no right answer, because I think especially when you're starting out and exploring what you're interested in, that's exactly what you should be doing, is exploring, and taking the courses, gaining the experience, and like you said, kind of listening and learning what's appealing to you, what's fulfilling, what's interesting, and maybe that's field work, or maybe that's bench science, or maybe that's physics, or chemistry, but you won't know that until you try them and see what you think. And I also think it's really important to just be open to different opportunities and different pathways. Certainly my pathway is not what I thought it would be, but I've loved it and it's worked out really well. And I think if I had been very focused on a particular pathway, instead of being open to letting that change, as my interest changed, and my knowledge and my experience changed, then I would have ended up in a different place, and it wouldn't have been as good of a fit for me. So I think just explore, if you wanna specialize, great, if you wanna stay broad, that's great too, but just exploring and being open to the messiness of science and what is appealing to you about science is really important. - Thank you, this was awesome, it was so fun. - That was Kelly Knutson, talking with us about how chemistry is so useful in archeology, and how she decided to become an archeological chemist, and her advice for high school students interested in science, especially very specific areas of science. Listeners, please consider filling out a survey so we can continue to bring you great content. 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