Webinar: Fossilized Footprints Reveal a Snapshot of Early Human Life

Webinar: Fossilized Footprints Reveal a Snapshot of Early Human Life
Aired July 15, 2020

Briana Pobiner:
Hi, everybody. Welcome to our program. We're excited that you're here. This program is called "Fossilized Footprints Reveal a Snapshot of Early Human Life." We're going to give it another minute or two as people are joining.
My name is Briana Pobiner, and I work at the Smithsonian's National Museum of Natural History where I'm a research scientist and educator in our Human Origins Program, and I will be hosting today.

Thanks, everyone. We see a lot of people are here, almost 150 participants already. We're excited for you to join our program today, called "Fossilized Footprints Reveal a Snapshot of Early Human Life." We're going to continue to give it a few minutes as we let folks come in.

All right. Hi, and welcome to our program, "Fossilized Footprints Reveal a Snapshot of Early Human Life." I think we're about ready to get started. As I mentioned before, my name is Briana Pobiner, and I work at the Smithsonian's National Museum of Natural History where I'm a research scientist and an educator in our Human Origins Program. I'll be the program host for today.
Today's program is part of an ongoing series called HOT Topics, which stands for Human Origins Today. Our next HOT Topic program will be on August 19th, same place, same time, and we'll be sending you a link after the program in case you'd like to register for that one as well.

Before we begin, I'd like to acknowledge current events and let you know that the National Museum of Natural History stands with the Secretary of the Smithsonian, Dr. Lonnie Bunch, in expressing our deepest sympathy to the families and communities impacted by discrimination and violence. We'll be sharing Secretary Bunch's response to the events as well as a resource developed by our colleagues at the Smithsonian's National Museum of African American History and Culture. Talking About Race is designed to provide tools and guidance to inspire conversation about race and empower your journey. We'll share those links with you in the Q&A.

Please note that the Q&A box, which you can see either along the bottom of your screen if you're using a computer or maybe at the top if you're using a phone or a tablet, is where you'll be able to ask questions to the speakers. We'll try to answer as many of these questions as possible. We'll be answering questions during the program via text. We will write the responses back to your question. Then once the program is over, I'll be reading some of your questions you've posted in the Q&A and directing those questions to one of the program [inaudible] answer.

Today, I'm excited to have three members of the research team to talk about new findings from Engare Sero, Tanzania, where the lar- [inaudible] human footprint site in Africa has been found. Say that three times fast. Actually, four members because I'm also a member of this research team. I've helped assemble the team. I participated in on-the-ground field work at the footprint site, and I also helped broaden the collaboration to include two key members of the Smithsonian's 3D digitization team, Vince Rossi and Adam Mettelo. Later, we'll post a link to where you can see a fly-through of the site in 3D and you can download 3D files of the footprints yourself.

I will introduce our three participants. Dr. William Harcourt-Smith is an associate professor of Anthropology at Lehman College and the Graduate Center, City University of New York, and a research associate and Richard Gilder Graduate School faculty member at the American Museum of Natural History in New York. Will's field of expertise is the paleobiology of fossil hominins with an emphasis on the evolution of upright walking. He has conducted paleontological and archaeological field work across Africa and Eurasia for over 20 years, including projects in South Africa, Kenya, Tanzania, and Sudan. He has also helped curate numerous exhibits including the current Hall of Human Origins at the American Museum of Natural History in New York.

Dr. Cynthia Liutkus-Pierce is a rift basin sedimentologist and paleoenvironmental scientist and is the current director of the environmental science program in the Department of Geological and Environmental Sciences at Appalachian State University, where she's a full professor. She's also a National Geographic explorer. Cindy's research focuses on the reconstruction of paleoenvironmental and paleoecological conditions of fossil sites in Africa and the U.S. using a variety of sedimentological, stratographic, and geochemical tools including lake sediments, paleosols, and root carbonates.

Dr. Kevin Hatala is an assistant professor of biology at Chatham University. He's a paleoanthropologist who studies the evolution of human locomotion through a blend of experimental lab research and paleontological field research. Kevin's work has included a number of projects focusing on excavating and analyzing Plio-Pleistocene hominin footprints in Kenya as well as Tanzania.

All right. To start the program, I will ask Kevin and Cindy to turn their videos off for now, and they'll come back on and join us later. I will turn my video and microphone off in a moment. But for now, I'm going to ask Will to share his screen to start the program by giving an overview of the research and findings at Engare Sero. Thank you, Will for joining us all the way from the U.K. to talk with everybody today.

William E.H. Harcourt-Smith:
Thank you very much, Briana and everybody else for inviting me to do this, really looking forward to it. I'm going to just give you a brief overview of everything to do with the site and put it in a little bit of context first because we're all interested in the past. We're paleontologists and archaeologists and geologists and we're interested in reconstructing past events and life history and the relationships between different animals and what they did and how they moved and all those interesting things.

Traditionally, when we think about things like that, we think of fossils. You think of iconic fossils that are out there. On the left, there's the famous Taung child or Taung baby from South Africa, which was really the fossil that sort of got people really interested in searching for human origins in Africa in the early 1920s. Of course, on the right there, there's an iconic theropod dinosaur. Fossils really make up the majority of the evidence that we have for past life and indeed the majority of focus that we have on past-life history events.

But there are other types of records from the past of living creatures and what they did that are equally as interesting and fascinating. That's really what we're going to focus on today.

I want to just show you this graphic here. This is a really cool specimen that's found in Wyoming and displayed there in one of the local museums. You can see here two trails going across the screen that are preserved in this flat plaque. In fact, what this is, is these are the last moments of an extremely ancient fossil crab. This is a relative of today's horseshoe crabs. You can see it's leaving these trails along this flat plaque, and it's a preserved moment in time. Experts think that this creature likely died in an environment that it wasn't conducive to breathing properly, probably in a very shallow lake. So, you can see this whole trail of many meters winding and meandering, and eventually the poor creature dies and is preserved in the sand.

Of course, we have the fossil, but we also have a record of its behavior, and in this case, its last moments. Of course, this creature doesn't have feet in the way that we do. But when we think about things like this, what we call trace fossils or sometimes ichnofossils, we think of the classic things like footprints. We've all made footprints in mud or in snow and in sand. Of, course they can only last a moment and be washed away by the sea again, or in some cases they can last a very long time indeed.

Briana Pobiner:
Will, can I interrupt for a moment?

William E.H. Harcourt-Smith:
...

Briana Pobiner:
Will, your audio's going in and out of it. Can I just ask you to hold the microphone part a little closer to your mouth?

William E.H. Harcourt-Smith:
Absolutely. Sorry. Is that better?

Briana Pobiner:
Yes. Thank you.

William E.H. Harcourt-Smith:
Okay.

Briana Pobiner:
I'll let you know.

William E.H. Harcourt-Smith:
Sorry about that.

Briana Pobiner:
Yeah. No problem.

William E.H. Harcourt-Smith:
I tend to move around when I talk. All right. Thanks Briana.

Briana Pobiner:
Thanks. Sure.

William E.H. Harcourt-Smith:
Yes. So, footprints are a wonderful record of something different. They're a record of a behavior. They're a record of a moment in time, and they can tell you something about what an animal was doing. They're not the animal itself, but they can provide information that actually the fossil remains cannot.

I'll show you just some nice examples here. These are two beautiful examples of dinosaur fossil footprints. On the top left there, these are actually footprints preserved in the ceiling of a cavern due to strange geological upheaval of the layers. These are large, long-neck dinosaurs called sauropods that left tracks and trails across a landscape many hundreds of millions of years ago.

On the bottom left there, there's a whole group of very famous fossil footprints from Colorado showing numerous different species of dinosaurs that moved across the landscape collectively. Indeed, from fossil footprints, we can actually determine aspects of dinosaur behavior that we can't from the individual fossils. Go see a movie like Jurassic Park and you see some animals moving in herds. How do we actually know they did that? Well, we have fossil footprints preserved that overlay each other that were made at the same time, were made by multiple individuals. On the left are famous fossil footprints of sauropod dinosaurs from [inaudible] in the American Museum of Natural History in New York, and on the right a graphic from a recent paper showing that actually several theropod dinosaurs, a T. rex is a theropod, were actually moving together and possibly even hunting together.

So group behavior, herding behavior, what dinosaurs were actually doing can be deduced from footprints as opposed to the individual fossils. We can get other aspects of their biology. So, these trace fossils are super, super important.

Of course, that leads us to footprints that we leave and where we might leave them and the type of record they might be. Of course, one of the things I always think about is the iconic image of Buzz Aldrin's footprint on the moon. That footprint may well still be there. I often like to think of what an alien paleontologist might think if they would come to the moon and find this footprint and what they might deduce from it. They're one of the only records that we have of our presence on the moon and as a result are of great importance, which of course brings us to ourselves and our own evolution, what footprints can actually tell us about our past.

We're very lucky that there are a number of iconic sites out there where we have preserved footprints of us and our hominin ancestors that can tell us something about how we lived and who we were. Without doubt, the most famous and iconic are the Laetoli footprint trail. These are 3.7-million-years-old footprints preserved in an ash layer from northern Tanzania. They are exquisitely preserved. They don't exactly know which creature made them, but the creatures that were living then were small-brained bipedal creatures. At the time that they were found in the '70s, there was a lot of debate about which creatures had which attributes and when bipedalism, walking on two legs, first became an important part of the life history of fossil hominins.

Here was unequivocable direct evidence of at least two, and maybe several more, individuals moving across a landscape on two legs and only on two legs, with no support from the arms at all. Moving quite slowly. Indeed, the way that they were walking and the nature of the prints are in many ways quite similar to our own prints, and yet this was 3.7 million years ago, made by creatures that are quite distinct from us, much smaller brains and as far as we know, very limited technology.

The time period here on this graphic of how long different species within the human family tree lived. In yellow there, you can see this is the rough age. This is between 3 and 4 million years ago. So the sorts of creatures that may have made this include the iconic Australopithecus afarensis, of course, famous [inaudible] prints were very carefully excavated. There's a beautiful image there on the right of one of them in close detail and you [inaudible] to a human footprint. The big toe is largely in line with the rest of the foot. In fact, complex analyses of the topography of the footprint tells us that these creatures struck the ground with their heel first and then pushed off under their big toe. They're very similar to our prints in many ways, not in all ways, but in many ways.

We can, by looking at the distance between the prints, work out how quickly they were moving; we can work out the direction they were going in and all sorts. This is basically a version one of the graphics simplified for educational reasons, showing the two hominin footprints with a bigger set and the smaller set, very obviously bipedal, walking from left to right there. There's a number of other animal tracks there as well. In fact, Laetoli is an incredible site packed full of prints of all sorts of animals, even through to the tiny preserved fossilized raindrops. Indeed, when we analyze them, as I mentioned, they are very similar in their weight distribution to modern humans. They strike the feet, hit at the heel first, and then they push off from under the big toe. We can use all forms of software to analyze this and in most cases they fall within a human range of variation. They're a little more splayed out; they're a little bit different, and part of that might be about how they're worn, but essentially they're very humanlike.

So, they were very direct evidence of a behavior of a moment in time when two or more individuals walked across a landscape, and it's led to some very iconic, if probably wrong in terms of how they are actually interpreted, reconstructions of these hominins and what they were doing. On the left there, the famous Australopithecus couple from the Museum of Natural History in New York walking arm in arm as if they're on a Sunday walk. The implications of this, that they were a monogamous couple, isn't necessarily correct. Then on the right there, you've got again a sort a couple of adults walking across the landscape.

So, these play a very central and a very important part in our understanding of the timing of bipedal locomotion as an adaptation. They were fully engaged in bipedal locomotion at least 3.7 million years ago and as a result, probably quite a bit earlier than that. So there's very strong evidence that bipedalism is probably one of the first major adaptations within the hominin lineage. It's of course within primates and mammals unique. We're the only creature that moves in the way that we do within mammals today. Here, was really direct evidence, obviously a long way ago, well before complex tool use and well before brain enlargement, which brings us on, really, to the main attraction.

Footprints are important. Our team, led by Dr. Cynthia Liutkus-Pierce, had been working at this site in northern Tanzania since 2009. This is an extraordinary site on the shores of Lake Natron in northern Tanzania. This is a view, actually, from one of our tents looking south. We're in the middle of the Rift Valley and we're looking here towards the only active volcano in Tanzania, called Ol Doinyo Lengai. It's a heavily volcanic landscape. Dr. Liutkus-Pierce will talk, perhaps, a little bit about this later. She's a geologist. Indeed, across the landscape are many, many areas of volcanic deposits, both current ash today and preserved ash deposits from the past. So one of the things that's wonderful about ash deposits is that you can convey them really, really well. It's a very desolate, quite remote landscape.

It became apparent to scientists in the 2000s that there were these preserved footprints rather like the ones at Laetoli, which is not that far from this site, preserved there. The local Maasai had known about them for a little bit and slowly the information trickled through to scientists and eventually a team was constructed that went out to visit this site and decide how to interpret it and analyze it and try and work out what it could tell us.

Here's a map, just to give you an indication of where we are. In the top graphic and the bottom right, you can see East Africa there, but right near the border between Kenya and Tanzania and just below Lake Natron, which is a soda lake, a very inhospitable environment, very little can live there, but it's quite lush around there. Indeed, there's plenty of wild animals when you're there camping and the like.

This is what the site looks like and this is certainly what the site looked like when I arrived at it at 2010. Carefully stones have been put around the area. You can see it's quite a barren, open landscape. The flat preserved area there are basically preserved volcanic ashes. Really, there were two main goals with the work. Initially, was, A, to try and analyze the footprints and document them and work out what was going on. And B, absolutely critical to work out the geology of the area and to work out the dates of the area.
This is a graphic showing what the footprints look like. I'll just tell you, many of us have worked at tough sites all over the world and had to sit in pits and break rocks and do all sorts of things. This was a wonderfully easy form of excavation. One literally has to sweep away the sand, and there are these exposed footprints below us. You can see a number of footprints that leading into this sand dune. In several cases, these are very obviously trails made by one individual. In other cases, the footprints cross over each other. This is just a small section of what I'm showing you.

Here is an example of a beautifully preserved print, and you can see it's almost as if it was made yesterday in a bit of mud. The person has stepped in and then walked away, and as they've done so, they've trailed a little bit of mud. A little bit of mud has splashed over to the sides of the print to the softness of what they're stepping into. All of that is preserved here on this volcanic ash pavement. It's absolutely incredible. Many of the prints are preserved like this. It's very hard. In many ways, they're better preserved than at Laetoli, and as we'll talk about in the moment, partly because they're a lot younger.

Here's another graphic just to show you. This is a beautiful graphic taken by Rob Clark for National Geographic, just to show you a way of photographing and looking at them. In this one, you can actually see the individual toe impressions. So these are a beautifully preserved series of trails, some of the best I've ever seen, and in quite hard sediments. So that's incredible. Of course, it takes a team to work on this type of material. Over a number of years [inaudible] others, we worked on the prints both in terms of cleaning and excavating them, in terms of documenting them. There's part of the Smithsonian imaging team in the bottom left there, sweeping them and cleaning them away so that we can actually analyze them properly and measure them and say something about them.

Not most importantly, but certainly of great importance, is the date. After a lot of very careful geology and a lot of very complex work, it was worked out that the footprints are really very young in age, anything between 6 or 7,000 years ago, and maybe 19 or 20,000 years, maybe a little older. It's complicated because we got a number of volcanic ash layers pressed into each other. But after very careful work by Dr. Liutkus-Pierce and other geologists, they worked out that they were quite young. That really puts us in an interesting time period. Modern humans, Homo sapiens, have been very much around for quite a while by this point. They are, as far as we know, the only species that is living in Africa. There are perhaps several other species just hanging on in a couple of other parts of the world. But in Africa, as far as we know, there were only modern humans.

So it was absolutely reasonable to have seen that these were made by modern humans, and indeed in every way there those prints are very, very similar to us and they could have been made by any of us watching this. They're young in age in the sense of the whole course of human origins. But there were so many of them, in fact over 400 different prints, we were able to say a little bit about what was going on.

This here is a graphic of the majority of the prints mapped out and you can see several things here. First of all, you can see there's a ton of prints. It was so densely marked with prints in certain areas. In fact, we had one area in the middle that we nicknamed the dance floor because it was full of so many different prints crisscrossing over each other.

But one thing you can see is they're all going in the same direction, which is really interesting. The vast majority are going in one direction, and a few are going back in the other direction, opposite. The ones that are all going in one direction are all made around about the same time. We've done some very careful [inaudible] So that's really, really interesting. Nothing deviating from this line. Whether this was a track or what have you, people are moving in one direction, and then some of them, perhaps not the same ones, perhaps the same ones are coming back in the same directions. That's one of the first things you can deduce from it.

Then one of the main other things one can do is one can measure the prints to work out how slowly or how quickly they're going. Mainly, they're moving very slowly or relatively slowly. There are a couple of individuals that are moving at a slightly faster pace, no real evidence of running except perhaps in one print. Dr. Kevin Hatala, one of our experts can talk a little bit more about that afterwards.

Of course, you can look at the lengths. On this graph here, what we've done is we've measured the lengths of the footprints in all the different trails. All those letters along the bottom there, D, H, E, B, those are all the names, the labels we've given to the different trails. A trail is a series of prints made by one person. On the left there, with those bars in orange and blue, these are the ranges of variation we get and the size of footprints of modern humans from today.

These are actually a huge sample of Americans from all different backgrounds that we actually were lucky enough to get from the Army. When we compare the size of all these prints to modern human comparative sample, you can see that there are a few there on the left that overlap in the dark blue with basically adult males. But the vast majority of them are too small to be adult males and in fact are more akin to the sizes we see in adult females and juvenile males and females. Of course, with adult females and more with juvenile males and females there's quite a bit of overlap. But generally most of the sizes of the prints are quite small, but a few of them, just two or three of them, are a lot bigger.

What we were able to do is to take all these measurements, and using a very clever complex statistical procedure, basically test the probability of each trail of prints and test the probability of whether it was more or less likely to be from an adult male, an adult female, or juvenile males or juvenile females. On this graph here, you can see for each trail along the bottom there, D, H, E, O/Y and [inaudible] of it being a male or a female or a juvenile. You can see the vast majority of the measurements make it likely that they are females or juveniles, and just a few are more than likely to be males. So, the overall composition of this group moving across the landscape are just a few males and then a lot of adult females and a number of juveniles.

This is really interesting because this provides us with a snapshot of the composition of the group moving across an open landscape, treading through this volcanic ash from the recent volcanic eruptions in the region. This is really, really interesting because we know from looking at several traditional societies that exist today that sometimes there is a division of labor. In some cases, women and juveniles move across a landscape to do certain things, gather foods and collect certain essential foodstuffs for the group, and men might be doing something else. There is in some cases an interesting division of labor. What this really amazing moment in time shows us is that we may have a snapshot of that kind of hunter gatherer behavior here preserved many, many thousands of years ago. That, of course, is of great interest to archaeologists and anthropologists and the like.

Briana Pobiner:
Willl, before you move on to the next slide, somebody's asking which color stands for which in these bars? Can you walk through that?

William E.H. Harcourt-Smith:
Ah. It's... Absolutely. Yes. So the dark blue represents... Sorry that there isn't a key. I thought there was a key on here. My apologies to everybody. The dark blue is for the males. The orange is for the females, and the light blue and the yellowy-orange are for male and female juveniles. But that is clear. Yeah.

This is a really interesting analysis done by Dr. Adam Gordon at SUNY Albany and Kevin Hatala and the rest of us looking into it, mainly by them. It's a really neat way of thinking about this and provides us with this really cool snapshot.

I want to just sort sum up here that footprint sites, apart from being intrinsically wonderful things to find and very beautiful, and this image here taken by Rob Clark looking at some of the footprints with the volcano in the background at dusk, they're not just wonderful and fascinating sites to find. They can really tell you something about the behavior of the creatures of interest in the past, whether they're dinosaurs, whether they're fossil crabs, whether they're our distant relatives like Australopithecus or whether they're us, in the case of these footprints. In this case, they provide a fascinating snapshot into how creatures behaved, and they can tell us things that actual fossils cannot tell us. So, they provide a really important extra line of evidence and a extra line of information about past lives and can be very, very useful and important as a result.

I want to finish this brief presentation of the material just by acknowledging the local people. This is a image of two local Maasai from the local village who came on an official visit to the site when we were excavating it in 2010. These two, this couple are actually walking alongside the footprints and having fun sort of tracing out their steps with the footprints that are laid out there. Indeed, it was a local Maasai who first alerted scientists to these footprints. This is their land. To be there at both their invitation and also their permission as well as the permission of the Tanzania authorities, this is ultimately their property and we were very lucky and privileged to be able to work with them and then to be allowed to work on this material.

So that, to me is a very important part of this. Indeed currently, the Tanzanians are working with us to think of important ways to conserve and preserve these trails for future generations. At that point, I will thank all the various funding agencies: National Science Foundation, National Geographic [inaudible] Foundation and a host of other people and institutions really made this wonderful work possible. Thank you very much.

Briana Pobiner:
Wonderful. Thank you so much, Will. I'm also going to ask now Cindy and Kevin to turn their videos and their microphones back on. One of the first things I want to do though, I apologize, I know we were having some audio issues during the presentation. I think just the connection, Will, because you're across the ocean was going in and out a bit.

William E.H. Harcourt-Smith:
I'm sorry. I've been holding the microphone but our Internet's been a bit patchy here. My apologies.

Briana Pobiner:
Yeah. No, thank you. Hopefully, we'll be able to go over some of the findings during the question and answer session. We apologize for the audio issues.

The first question, it's kind of the same question that's been asked by a couple different people. I'm going to direct this to Cindy. Elandra Norat asked, "How are the footprints dated?" And then Felix Glow asked the same thing: "How are footprints dated? Is it by using stratigraphy and how much uncertainty is there?" Pascual Lefleur asked, "Do you use carbon dating for footprints?" So maybe you could just talk a little bit in general about how footprints are dated. But actually... Sorry. Before we get to that question, I wanted Cindy and Kevin to reintroduce themselves and let us know what their roles on this project were.

Cynthia Liutkus-Pierce:
Okay. I guess I'll go first. I unmuted already. I'm Cynthia Liutkus-Pierce, as Dr. Pobiner said earlier. I'm a professor in the Department of Geological and Environmental Sciences at Appalachian State University. We're here in western North Carolina. I've been there for 15 years and I am the, I guess, scientific director of the Engare Sero footprint project. I've been working in Tanzania for since like 1999 and so on and so forth. But I have had the opportunity for the past 10 years to work with these amazing people. So, I'm pretty thankful for that. Kevin.

Kevin Hatala:
I'm Kevin Hatala, as Dr. Pobiner mentioned. I'm an assistant professor of biology at Chatham University. I started working on the project back in 2010, initially because I was very interested in the evolution of human locomotion and sort of trying to figure out what these footprints might be able to tell us about that. But obviously, sort of the trajectory change of the analysis that we ended up doing and the ways in which we started to look at these footprints... But yeah, have also been very grateful to be a part of this project for these past 10 years.

Briana Pobiner:
Wonderful. Thank you both. I also want to mention to people that part of the reason why we're doing this webinar now is that on May 14th, the team published a paper in the journal Scientific Reports entitled "Snapshots of Human Anatomy, Locomotion, and Behavior from Late Pleistocene Footprints in Engare Sero, Tanzania." So we were very excited when that paper came out. Now, I will turn it back over to Cindy to talk about the dating techniques.

Cynthia Liutkus-Pierce:
Great question about the dating. I was kind of following along here in the Q&A. Sometimes it's easy to date footprints. In fact, when they are pressed into certain geologic materials, specifically volcanic ash, life is easy. What we do is then we go to the ash layer that the footprints are pressed into and we take minerals out of the ash layer. Certain minerals can be radiometrically dated because those minerals have radioactive elements in them that decay at a known rate.

So, if you have a certain mineral, certain feldspars will do this; certain micas will do this; it's wonderful, and the idea there is that if you can date the minerals in the ash layer, you know that a footprint that then gets pressed into the ash layer must be either as old or younger than the ash because the idea here is that if you can press something into the ash layer, the ash layer had to already be there.

If you are very lucky, like totally in other sites, you have a pristine volcanic eruption that blankets the area with ash and then you can date the minerals in [inaudible] layer that we have at in Engare Sero is a bit more challenging because the ash was not a volcanic eruption. The volcanic ash that the Engare Sero prints were pressed into was a remobilized, kind of like a debris flow, almost like a mud flow. So, it had been a volcanic ash that erupted previously, but we don't know how much previously it had been erupted. Then sometime after that, rainfall and whatever else reworked that ash, moved it down towards the edge of the lake, and the ash mixed with water kind of made this muddy slurry. The humans walked across it and then the mud dried out and when volcanic ash gets wet and then dries out, it hardens like concrete. So, we had a little bit of a challenge in dating the footprints because by just dating the minerals in that ash, you could be dating minerals from a whole slew of eruptions that had been mobilized on the flanks of the volcano.

So, what we tried to do, and I'm very thankful for the multiple universities that helped us do these [inaudible] tried to date as many as possible in that volcanic ash and find the youngest one. The idea here is that all that stuff that has been kind of mixed up and remobilized, the youngest one would give us ... It has to be younger than that date. The youngest mineral that we found in that ash was on the order of 19-20,000 years.

What we then did is we dated, and I saw somebody asked about carbon dating. While I can't use carbon dating to date the footprints, what I did use is I carbon dated a snail shell in the unit on top of the footprints, and that snail shell came back at about 5,000 years. So we know that if the footprints are essentially younger than the volcanic ash but older than the snail, that gives us a bracket of about 5,000-6,000, anywhere up to potentially 19,000 years ago. Long answer, but believe it or not, it took us 10 years to figure out all of that stuff. Great question and I hope I answered it. If I need to clarify, please let me know.

Briana Pobiner:
That's awesome. Thank you. I think this question might also be to you. Cecilia asked, "How can the footprints stay so clear for years?" And Elandra asked, "How's it possible that the footprints lasted so many years without being destroyed?"

Cynthia Liutkus-Pierce:
Great, great questions. One of the things is that, as Dr. Harcourt-Smith was telling us, they are a lot younger than some of the other footprint sites, so they have not been exposed as long as Laetoli. However, one thing that we are really thankful for is that the type of ash that these footprints are pressed into, once it hardens, it literally hardens like concrete. In order for us to take a sample of that rock unit that has the footprints in it, we were using chisels and sledge hammers. So you can imagine. We also did modern experiments, too, where we took ash, we wet it, and then we let it dry. You could actually see how this stuff hardened over time. So, you can imagine it's very similar to where you lay concrete, you scratch your name in it, and then once it hardens now it is solid.
Once it solidified, it was pretty well indurated and was able to then weather all of the natural erosive processes. The other part about it, too, is that it was also covered by some sand dunes. One of the pictures that Will showed, you noticed that the trails kind of go up to and go underneath a sand dune. That sand dune was probably very, very important in making sure that those footprints didn't get eroded.

Briana Pobiner:
Wonderful. Thank you. I'll do one more follow up on this line of questioning and then I'm going to take a few other questions. Kevin Takashita Binum asked, "Do carbonatite ashes possess properties that preserve footprints or imprints better than your standard silica-based flavor of ashes?"

Cynthia Liutkus-Pierce:
First of all, hi, Kevin. Kevin is actually one of my former research students, so that's super exciting to see Kevin here amongst all of the other folks that are watching. Great question.

Believe it or not, so Kevin's question is asking about the composition of the lava, so composition of the ash that is preserving the footprints. While Ol Doinyo Lengai, the volcano that produced the ash that these footprints are pressed into, while Ol Doinyo Lengai does tend to have a bit of a carbonatite history, the ash that it produced at this time for the Engare Sero prints was actually nephelinite. It was an ash made of a mineral called nepheline. It wasn't carbonatite. Still silica-depleted, but we're just kind of finding that different types of ashes don't necessarily have a whole lot of difference in what they can do in terms of their preservation.
A lot of it also has to do with the type of water that it interacts with and also, too, the type of water that comes through the subsurface before it comes up because likely what we're seeing at the Engare Sero footprints is that there was a lot of calcium carbonate precipitated and that probably was not in the ash itself; it was the water that was going through the local bedrock, then got up into the ash and when it evaporated, that's what kind of helps to solidify the ash layer. But thanks, Kevin. Great question.

Briana Pobiner:
Thank you. This next question goes to Kevin, the Kevin that's here on the screen. I apologize if I pronounce your name wrong. But Nagas asks, "Does the stride length give information about the height of the hominin?"

Kevin Hatala:
Yeah. Stride length is going to be very much related to the length of the lower limbs of the individuals who made those footprints. Stride length also varies with speed, though. So as you walk faster, you start to take longer and longer strides in order to do so. It's also very closely related with the speed at which you're going to be moving. So really, when we look at something like stride length, we need to account for both of those variables. We know that it's going to be related to speed, but that relationship is dependent upon how large the person is, too.

Briana Pobiner:
Great. Thank you. A follow-up from the same person: "Why the comparison with American footprints? Would size profiles differ in African populations now?"

Kevin Hatala:
Yeah. That's definitely the case, that you would get size variation between different groups. The way we actually ran this analysis, though, the analysis is dependent upon the distribution of sizes, and so the size variation that you see in this group. As long as you have basically the same shape of the curve of your distribution, then you can apply that analysis across different groups that might differ in their absolute size. So, we have reason to believe that this group of individuals 10, 12,000 years probably had ranges of anatomical variation that would've been pretty similar to what we would see within a modern human population.

Briana Pobiner:
Thank you. Speaking of sizes, we have another question from Neils Nielsen, who asks, "Is there any way to use this data to make a guess about the size of the population at that time?"

Kevin Hatala:
That's a tough question. Probably, we're touching such a limited snapshot. I think when you think about the context in which these footprints were made, if you set up a motion-activated camera on the side of a lake in the town you live in or something like that, and it's activated once, and you capture one small snapshot of some individuals who happen to walk through that area at that time, it's going to then be really, really difficult to say too much. I guess you have a sort of minimum bound on what the population size would be. You know it's at least as many individuals that you're capturing in that small snapshot. But there are probably going to be a whole bunch of other individuals that you're not able to sample.

Briana Pobiner:
Yeah. That's fair. Another question for you, Kevin. This is from Sarah. "Is it difficult to interpret fossil footprints because the preservation of samples is so limited compared to all the footprints people or animals ever left?" You sort of answered that, but in case you wanted to add anything else.

Kevin Hatala:
Yeah. There are always going to be difficulties, especially if we're looking at something like trying to reconstruct how these individuals were moving. We get one tiny little glimpse at how they were moving at one particular instant in time. Obviously, there might be different ways that people do things. We might happen to capture an odd event. You might capture a footprint where someone slipped and fell. Obviously, that's going to tell you something very different about the sort of stereotypical pattern by which that individual may have walked. There's always going to be variation. We kind of have to do our best to sort out, using all of these different lines of evidence, looking at how modern people walk, using experimental data to see what kinds of variation we actually see among these kinds of data sets to really do our best to say are we looking at something that we should believe to be more of the norm or something that might be outliers.

Briana Pobiner:
Thank you. I'll direct this one to Will. "Do you know of other situations in which footprints would be preserved other than in ash layers?"

William E.H. Harcourt-Smith:
Absolutely. Cindy may want to add to this, too, because she's worked on some of the other sites. Can you hear me, by the way?

Briana Pobiner:
Yes.

William E.H. Harcourt-Smith:
Great. Preserved mud of all types and the right conditions, the preserved floors of caves. We find footprints in really any type of muddy sediment that dries and then for some reason gets covered or preserved in some ways. I live in New York. Normally, you sometimes see the odd errant footprint in concrete, people or a pigeon or something that preserves. So you have all types of what we call substrate, that's the thing the footprint is made in, that can preserve prints.

This volcanic ash wouldn't necessarily always preserve a footprint. You've got to have the right conditions and you've got to have the ash and then you got to have the right water content, and then you've got to have it drying and you've got to have it preserved. You need a number of things to happen for prints to be preserved, but anything, any sort of fine or even less fine particle-based sediment can preserve a footprint. But I'm actually going to pass over to Dr. Liutkus-Pierce here because she has actually done some experimental work on this.

Briana Pobiner:
I will interrupt and add a question onto this. Steven Rockhauer asked, basically, "Why didn't the prints get washed away by occasional rainstorms?"

Cynthia Liutkus-Pierce:
Great question. To build off what Will said, there have been a lot of modern studies about what are the ideal conditions for footprints. That is one of the things that, as a geologist, we're certainly curious about. The finer the sediment, definitely. Muds and silts great; sand, a little bit more difficult. You can imagine when you're at the beach, too, right? Sand footprints. The water content, certainly important. You can certainly make impressions in wet sand versus dry sand. So that is definitely one thing. But we do have lots of sites, especially in Africa, where there's footprints preserved even in coastal sand dunes. If you're able to kind of glue the sand together because of sea spray that has the right minerals into it. So, all of that works out really well. Briana, remind me again, the question was about how did they not get destroyed before?

Briana Pobiner:
Destroyed. Yes. Basically. Yeah.

Cynthia Liutkus-Pierce:
Yeah. We actually did some modern experiments with ash from Ol Doinyo Lengai. We made impressions into it and then we kind of rewet it. One of the cool things that we found was that if you rewet the sediment from Ol Doinyo Lengai, that ash, before it has really kind of hardened and whatever, you can actually wipe away all the footprints. We knew that the footprints had been made, the ash had dried out, and then it had to have been very quickly covered by some other unit, some other deposit. That is a very typical thing. Paleo friends, help me out here. One of the best ways to make any type of fossil, whether it's a body fossil or trace fossil, is quickly bury it. So, the less time between the fossils being deposited and burial, the better the fossil is. So, we kind of knew that the next ash layer that came on top had to have come in pretty quickly after that ash layer had been receptive to the footprints, and that's why we have a really exquisite preservation.

Briana Pobiner:
Thank you. I'm going to pivot to Kevin for this question. This is from Sonya Price. "About how many human footprint sites are known?"

Kevin Hatala:
Oh, wow. Good question. I can't say for sure off the top of my head. I would estimate. Once you get into the younger end of the geological time scale, the more recent, there are many, many, many, many more sites at that end. I don't have in my head sort of a firm number, but I would say it would have to be more than 100, if you started including everything. Yeah. My research has more focus on the older end of the scale where you can count on two hands or now that more sites are being discovered, maybe three hands, you can count the number of sites that are known. But, yeah. There are many, many more from the more recent end of the geological time scale.

Briana Pobiner:
Great. No. That makes total sense. Also, two more questions to you that are similar. Mason Hintermeister asked, "Are there any species other than hominins represented at the site?" And Dia Kash Varden asked, "Did you find anything else other than footprints near the site?"

Kevin Hatala:
There are some other footprints. There are some animal footprints, footprints of other animals, things including zebra, buffalo, some other bovids. There are different animals that seem to be walking across this landscape. But it is surprising that we don't see more of them given how many human footprints that we have in this particular location. But, yeah. We do have other things that are represented there. To the best of my knowledge, and maybe Cindy would have more input on this as well, but I don't believe that there have been any real fossils aside from trace fossils that have been found so far on this immediate surface where the footprints are, but also in the surrounding areas where there are some other extensions of the same surface.

Cynthia Liutkus-Pierce:
Just shells, the only real true fossil that we've actually found. We have two different species of snails. Like Kevin was saying, we do have footprints of other critters. But other than that, yeah, there are stone tools found in the area, but they are not at the same layer and I think they're probably considerably younger.

Briana Pobiner:
Thank you. Kevin, I'm going to point this question to you also, in part to help do a little review because I know some of the audio was going in and out for Will. How did the footprints help us understand about the species; life and behavioral practices?
I'm going to tack on one question that I know we've discussed a little bit from... Oh. Where... There's lots of questions that came in. Yeah. From Lori DeMarco. "Could you tell from any of the footprints if any supposed female was carrying a child, like perched on her hip?"

Kevin Hatala:
Yeah. The way that we go about interpreting evidence like this is largely we're tied to looking only at the feet of these individuals and the feet of these individuals as they moved across the landscape. When you think about broader biological questions, there are some things that you're going to be able to make a very informed guess on, and there are other things that you're just never going to know. We're never going to know what the entire rest of the body of this individual looked like. We only have this glimpse of the bottom of their foot. So, we can do things, like we can estimate based on the size of their foot, we can estimate how large these individuals were, or we can do the sort of analysis that we'll touch on before in the presentation. We can estimate, assuming body size patterns looked the same in this group as they do in modern people, we can estimate how many of them might have been adult males or adult females or juveniles and so on.

There are certain limits to the level of detail of what we can infer. Turning to Lori's question and whether ... This was the carrying question, is that right? About-

Briana Pobiner:
Yeah.

Kevin Hatala:
... carrying infants potentially? We don't see very many footprints of infants. But if we're going with this interpretation, which we do want to emphasize this is just one possibility that this is a group of females. If this is a large group of adult females, you would expect, as Lori, you probably expected that many of them would probably have children, and they might have children in tow as they're going out on this forging expedition. They aren't necessarily going to leave them behind. It's certainly very plausible. It's very possible that is the type of scenario that we're looking at. But there's really not a good way to say for sure because we don't really have any really clear lines of evidence to suggest that.

Briana Pobiner:
Yeah. That's fair. This pair of questions will go to Cindy because they're similar. Sophia G. Asks, "Is the environment of the footprint site at the time that they were made known?" And Rick Potts says, "Hi, fine colleagues. Are there traces made by the roots of grass or trees preserved in the ash layer from the same time as the footprints? Or do you think vegetation was destroyed when the ash covered the ancient landscape?"

Cynthia Liutkus-Pierce:
Great questions, guys. We definitely have been working to reconstruct the environment. I saw somebody else asked a question, too, about were there any other vegetation indicators? We do actually see impressions of leaves. We do have those two different species of snails. So, all of those things can help us figure out what the environment might have looked like. The other thing, too, that we can do... Without getting into too much of the details, you can look at the geochemistry of some of the minerals that have been deposited and you can tell whether those minerals were deposited in fresh water or saline water. So, we've done all of that and you can even actually speculate maybe what temperature those minerals had been deposited. So, we can use all of that to figure out that this was probably a time when the Lake Natron was at a higher extent. It probably had areas around it that had freshwater springs. The snails that we were looking at actually do have freshwater affinity. Then those plants and whatever can tell us a little bit about what was going on in the area.

Now, Dr. Potts, you asked too about whether we could tell a little bit about the vegetation and whether it was kind of syndepositional. We have so few plant material, so little plant material that I would really hesitate, especially knowing that this is kind of debris flow deposits, a lot of that could have been translocated. The other thing, too, that I would say about that is we found what we thought at the time was preserved plant material kind of smooshed between the footprint layer and the layer above it, and we tried to carbon date it and it came back as no carbon. So, I think a lot of what we originally thought might have been syndepositional plant material was probably modern plants that was kind of getting into the cracks and crevices and was actually just kind of modern contamination.

But we do know a little bit about the environment, and we do know that this was probably an area where the lake was somewhere else, a little bit salty, but the margins of it would have been fresh. So, there would've been plenty of opportunities for the humans as well as the animals to go to those little marshes and springs to get roots and tubers and fish and water to drink and all sorts of stuff like that.

Briana Pobiner:
All right. Thank you. We have time for just one or two more questions. I'm going to ask a question to Will from Chris Andrade. "Do any footprints show evidence of lower limb injuries that may have impaired walking?" And Dee Akosh Varden asked a similar question: "Can a footprint tell that somebody's happy or sad or even suffering from some disease?"

William E.H. Harcourt-Smith:
Great questions. They're sort of linked, so I'll hopefully be able to answer them both. Obviously, it's hard if there's a direct injury to the foot and the foot is in some way deformed as a result of that injury, then of course you'd be able to directly tell that. In terms of a very badly injured leg or hip, one way you would be able to tell would be to look at the weight distribution of prints in a trail. Perhaps, if somebody was putting a lot of weight on one leg and much less weight on the other, you would look at the pressure maps of the footprints and you may be able to work that out. Of course, if they were with a stick, you would see impressions of that stick.

As far as we can tell, there is nothing to indicate any particular injury here, that people are putting more weight on one foot rather than the other or any particularly sort of vagaries of the gait cycles of these individuals to indicate any injuries. As far as we can tell, there isn't anything there. I think in some cases you would be able to tell about injuries.

But in terms of mood and happy and sad, sadly, unless they're skipping around for joy, I think it's probably hard to tell, fun as it would be. But certainly, anomalies in gait can be picked up, and those anomalies, if you were to talk to modern clinicians, can sometimes be linked to injuries, but nothing at this site.

Briana Pobiner:
Yeah. That's fair. Okay. So, last question, I think, and I'll direct this to Cindy. Well, it's a pair of questions again that are similar. Lori DeMarco asks, "Is it possible these tracks will be removed from the site for preservation purposes or will a permanent covering be constructed on site?" And then Mason Hintermeister asks, "How are these footprints to be preserved going into the future?"

Cynthia Liutkus-Pierce:
Great questions. Right now I feel like the answer to that is stay tuned. We have lots of really great ideas, and thankfully we've been in touch with the curator of the museum in Arusha, Tanzania and have been trying to develop the best way to move forward. One of the biggest concerns that I know this group has is to make sure that nothing damages these footprints going forward. So while I don't think they're going to be removed, that is one thing that we certainly want to remember is that this is the heritage of Tanzania and it should stay in Tanzania.

Instead of removing that, what we have done is, as a team, we've photographed them down to sub-millimeter resolution. I think in a few minutes, Briana's going to show you guys the link to where you can literally do a fly through of the entire site down to sub-millimeter resolution. You can zoom in and see the actual duct tape that we put next to a footprint to label its code. So that is one way that we have preserved it. We can literally 3D print, if we wanted to, the entire site, we can duplicate it.

Whether we're going to put a structure over it or not, I think that is something we would certainly like to do. We are currently discussing options with the Tanzanian government to do that. One of the big hiccups, of course, is funding. So finding enough funding and getting kind of a ground swell behind the idea of what do we do that would be a best practice, right? [inaudible 00:59:05] something and then have it actually damage the footprints. But also then having the funding to make sure that it is actually not only done right but then also has an education and an outreach component as well to really kind of highlight how special this place is for the local region, but also for anybody who wants to come visit it. Fingers crossed. Coming soon.

Briana Pobiner:
Wonderful. Thank you. I just posted an answer to Ashley Peery noted resources where you can learn more about today's webinar. So, I posted a link to 3d.si.edu. There's a collection called Engare Sero Human Footprints, and so you can check out that link in order to download 3D files to print them yourselves or visit this site on a fly through. So, I hope you have a really good time exploring further that way.

All right. This concludes today's virtual program. Thank you to everybody for joining us. Please stay tuned for future programs from the National Museum of Natural History. We hope you'll join us next month.

A few people had asked, where can you see the recording for this? We have put a link in the Q&A to see our upcoming programs as well as a link to see the archives of all of our video programs. So, the archive of this program will be available in the next few weeks.
After this webinar ends, you'll see a survey pop up asking for some feedback about the program. Please take a moment to respond. We're really interested in hearing what topics you might be interested in seeing for future programs.

I want to give a big thanks to Will, who's joining us from the U.K., and to Cindy and Kevin for also tuning in and answering all the questions. I'm sorry we couldn't get to all of them, but we really appreciate all your wonderful questions. Thank you to our participants and thank you to you, the audience, and we'll see you next time. Bye.

Kevin Hatala:
Bye.