Science Friday - Telling the tale of human origins, one fossil tooth at a time

Episode Date: September 16, 2026

Where did we come from? How did humanity, with our complicated civilizations and all our human quirks and foibles, come to be? Much of what we know about those questions traces back to important fossil finds in Ethiopia.  Paleoanthropologist Yohannes Haile-Selassie has been key in that hunt for answers. A skilled fossil-finder, he’s helped discover some of the most important known hominid fossils, including the bones used to describe the 2.5-million-year-old Australopithecus garhi, the first pieces of a 4.4-million-year-old partial skeleton of Ardipithecus ramidus, and fossils of the 5.8-million-year-old Ardipithecus kadabba, to name a few.He joins Host Flora Lichtman to talk about what fieldwork is really like, big questions in human origins, and making the field of paleoanthropology more equitable. Guest:Dr. Yohannes Haile-Selassie is the director of the Institute of Human Origins at Arizona State University, and a professor in the School of Human Evolution and Social Change.Transcripts for each episode are available within 1-3 days at sciencefriday.com. Subscribe to this podcast. Follow our show on Instagram, TikTok, Facebook, and Bluesky @scifri and sign up for our newsletters. Got a science question that’s keeping you up at night? Call us: 877-472-4374 Hosted by Simplecast, an AdsWizz company. See pcm.adswizz.com for information about our collection and use of personal data for advertising.

Hi, it's Flora and you're listening to Science Friday. Where did we come from? How did humanity with our complicated civilizations and all of our human quirks and foibles come to be? This is one of science's most basic questions, and much of what we know about the answer traces back to important fossil finds in Ethiopia. The paleoanthropologist Dr. Johannes Heila-Salasi has been key, in that hunt for answers. He's helped discover some of the most important known hominid fossils, like the bones used to describe the 2.5 million-year-old Australopithecus Garhi, and the first

pieces of a 4.4 million-year-old partial skeleton of Ardipithecus ramedus. You know Arty. We all know Arty. And fossils of the 5.8 million-year-old Artypithicus Kadaba. His list of finds also include specimens from Australopithecus apharynxus and more modern species of Homo, including Homo erectus. So many branches of the human family tree. Today, he's the director of the Institute of Human Origins at Arizona State University and a professor in the School of Human Evolution and Social Change. And he's here to talk to us today about some of the big questions in human origins research and his own origin story.

Welcome to Science Friday. Pleasure to be with you, Laura. We're delighted to have you. You've made these many spectacular finds. I read that originally you went to school for history, that you didn't really know that paleoanthropology was a thing. Do I have that right? Yeah. So my first degree, which is from Addis Abba University in Ethiopia, was in history.

And history as a subject relates to, like, you know, written history. But there were a couple courses on archaeology. one of them being early man. And I really loved that course because that was the first course that kind of introduced me to fossils and also introduced me to the name Australopithecus, which I had a lot of problems saying. But clearly, that interest kind of built up when I started to work for the Ministry of Culture in Ethiopia where I was assigned to work in a paleoanthropology laboratory where there were. so many fossils. And when I started going out to the field with pale anthropologists as antiquities

officer, seeing the fossils on the ground was another excitement, which kind of increased my interest in them. And luckily, I ended up being like a good fossil finder. Some of them, I didn't know what I was finding at that time, but they were really great fossils. Some of them, fossils of human ancestors. What are these digs like? Are you camping? Is it hot? Paint me a picture. Okay, well, when I take you to the field, first of all, what I can tell you is you're going to a place where the temperatures go as high as 110 degrees. It's very hot.

And you're going to be living in a tent, in a small tent, for four to five weeks. Okay. And you're going to have to think about everything that you need while you're out there and all of your stuff. You have to pack and get ready. But once you're in the field, you get up at 6 o'clock in the morning. It breakfast at 7, okay, and go out to the field, looking for fossils. Now, what do we do once we're out in the field? A lot of people, when I come back from the field, they usually ask me, like, how was the dig?

And I'm like, I was not digging, you know. I was just walking around with my eyes on the ground looking for fossils and identifying whatever fossils we can find and catalog them at the end of the day. Okay, is this like beach combing? Like you're just walking around with your head down. Exactly. That's what you do for 99% of the time.

Now, sometimes you might end up having an excavation. And one good example from my site at Guaranzumila is like the discovery of the partial skeleton of Astral Casarenses, about 3.6 million partial skeleton. And we excavated that locality for like five years in a row. Okay. Now, when do you decide when to do excavation? Now, a lot of times you might end up finding like a few specimens just coming out of the ground and you find different. elements of the body coming out. That's a good sign that there might be more, and you end up excavating and definitely we'll find more, right? Otherwise, for most of the time, you just keep

walking and walking for like hours every single day. And in the end up collecting, like, you know, probably hundreds of fossils or even sometimes three or four, depending on, you know, how rich the locality that you're exploring is. Some of the days you may not find anything. and you get disappointed going back to your camp, but other times you don't mind staying until like 7 o'clock, even though if it's dark, you know, you just stay because you're finding so many exciting things, right? And once you're done and you come back home,

everything that you think about is like, when am I going to go back to do the same thing? So that's how addictive fieldwork is, right? So if you were to go to the field, that's the kind of feeling you would have. But again, you might feel tired at the end of the day, but there's always that excitement because of, you know, what you have found throughout the day, especially if you found something good. It's like a treasure hunt. You mentioned at the beginning that you had a knack for this.

You happen to be good at finding fossils. Is there a trick to it? Do you have a secret? Well, there's no trick to it. It's just about how you train your eyes. and what you focus on. So when you're exploring and doing surveys with your eyes on the ground,

you've got to tell your brain exactly what you're looking for because your eyes are going to probably focus on the bigger items on the ground than the smaller ones, but you can train your eyes to blur the bigger ones and focus on the little ones. Because unfortunately, the fossil record

is filled with isosos, teeth more than anything else, right? And teeth are like, you know, small. So most of the discoveries that I've made started with, like, isolated teeth or isolated fragments of other parts of the body. So it's about how you train yourself in addition to eventually learning to know what you're finding. Okay. So that was not my, you know, my expertise at the beginning. So whatever I found, which were really good, were.

were identified by others, my seniors, who had the experience doing that kind of work. But eventually, I developed the knowledge as well, and now every fossil that we find in the field, I have to identify not only in terms of what element it is, but also what animal it belongs to. So that's how you develop the experience. Otherwise, there is a lot of luck associated with finding

fossils. Sometimes it's just being at the right place at the right moment because the fossils study you see this year on the surface. If you leave them there next year, they may not be there, right? Because there is like erosion. They could be displaced. There are animals who trample the bones and they can, you know, kick them and remove them and rebury them sometimes. So when I say you have to be like at the right place at the right time, that's also possible. part of the luck that you need to actually find good fossils. There's some serendipity there. Correct.

Why are there places with just enormous numbers of human remains? Like, why are there certain fossil hotspots? So when you think about, like, where are the hotspots located? Of course, most of our evolutionary history happened in Africa, right? So when you think about all the discoveries that we have up until, I would say, 2 million years, 1.9 million years, there were no hominant ancestors that lived outside Africa. And in Africa specifically, if you look at all this localities that we go explore every single year, they're all located in the Rift system. Right. So the Rift system is an area where the, you know, the Earth has been opening up because of plate tectonics. It's been opening up and exposing this deep trenches with sediments that have been buried for millions of years. And as the rift gets more wider and wider, what you will see is that the sediments start getting eroded. So whatever life lived in those ancient sediments is what gets exposed. So there is a geological component to why these areas are really significant.

Now, you got to also think about, like, primates are tropical animals. We've always lived in the tropics, and it's less likely that you would find, like, the earliest human ancestors, like, further to the north in the colder regions. And if we had, like, rift systems like the East African Rift System somewhere else in Africa, could there be a possibility to find fossil hominins? I wouldn't say no, we could. But again, our target has always been this region. But I wouldn't be surprised if there are other areas where this early homes live,

but we're not finding them because the geological context is not convenient for us to look for these fossils. I've heard this field is very competitive and fierce and secretive. Do you find that to be true? and if so, why do you think that is? I would have to admit that it is true because I think in the late 1900s, when there were so many discoveries

coming out of sites in Africa, people were actually studying whatever they were finding by themselves and publishing it, and there has been a lack of data sharing. That, in fact, might have made individual researchers famous, but it was not really good for the science

because that thing is changing right now, and we can see the difference. People have started communicating, collaborating, and sharing data. And some of it is because of, like, you know, rules from funding agencies where you have to really share data, otherwise you're not going to get money to do your research. But now, not only fossils, but when you look at the genetic research that is being done, it's highly collaborative. I think not only do we think about sharing the data, but also the timing of when we share the data, right? Because in our field, things change really quickly.

New ideas come up as new fossils are coming out of the ground, right? And if we can't really communicate that on a timely manner, we're delaying the progress in the science. And it looks like every pale anthropologist now understands how this, this, this, field has to operate moving forward, and if we have to make it relevant to the society, we are the ones who should now change that trend and make sure that data sharing is the norm, not like the exception. I got to take a break, but when we come back, I want to zoom out from the tiny teeth in the field to the very big picture.

You open for that? No problem. What does it mean to be human? Well, that's the $64 million question. What is it to be human? We have a very long evolutionary history. We're talking about an evolutionary history of about six to seven million years. And documenting that evolutionary history is probably the hardest,

particularly when you think about how do we reconstruct our past. As paleontropologists, we tackle this question with by going out to some remote areas to find ancient fossils of our ancestors. not only our ancestors, but also fossils of other animals who lived with them. Because to understand really how we became human involves really understanding the circumstances in which we kept evolving for the last seven million years. So compiling all that data to actually answer the question of how do we came about to be who we are today and where we're going in the future is going to be the most challenging

question for all of us. And we're still trying to piece together evidence to actually answer that question. That's interesting. From the outside, I keep seeing these sort tantalizing discoveries. We covered one pushing back the timeline of when modern humans appeared, possibly. Just recently I saw a science paper suggesting we have ghost ancestors, not Neanderthals, not Denisovans, some other hominid population that early humans bred with. Do you feel like this is an exciting time for your field?

Very exciting because now in the 21st century, we have so many tools that we didn't have in the 20th century. So now we're talking about like at the genetic level, not just, you know, shape and form of fossils, but also deep diving into the genetic components of this early human ancestors. And yes, 30 years ago, we wouldn't have thought about a ghost speech. But now, thanks to the developments in genetics, we can actually see that there was a lineage that we haven't sampled in the fossil record yet, which could be like, you know, part of our evolutionary history. So hopefully we will keep pushing the ability of genetic analysis further back in time.

Right now, it might be limited to like a few hundred thousand years. But eventually we're going to probably push it back in time to the millions. so that we can have. Well, we hope because, you know, it's not just the DNA. There's also some analysis on ancient proteins now that is kind of pushing the time back to like millions of years. So there is a lot of hope that there's going to be another component to our analysis that we can further understand our evolutionary history. When I first started covering this field, there was a lot of talk about a missing link. Do people still think about it that way?

I would say no, because missing link has always been a misnomer. In the old days, people refer to missing link as the ancestor that we shared with the chimpanzees and gorillas. And that's what used to be called like the missing link. But there is no missing link. There are so many links that we still haven't recovered, but they're not missing links. So that idea has completely changed now. we don't really think that there is something called missing link. I know that there's this issue in paleo anthropology of Western scientists parachuting in to Africa,

the sort of colonial legacy. How do you think about that in your field? I think a lot of us agree that it has to change, and there has been a lot of movement. Communities have not been involved as much in terms of what we do as, you know, researchers We go to their area, we collect fossils, we go out, we describe the fossils, and publish. Do we really communicate with the local communities in terms of what we're doing in their area, what the significance of the discoveries we're making, what it means to them? Those were like the lacking points that we're trying to change by making sure that communities are engaged

and also make sure that they benefit from the research that we're doing. And there are so many ways to do that. and some of it is by collaborating with local institutions, opening programs in paleoanthropology and universities. As a matter of fact, ASU has an MOU with one of the local universities in Ethiopia, some are our university. And right now we're trying to open a new paleoanthropology master's program. So this is a huge agenda that paleoanthropologists, paleoanthropologists all over the world are

discussing right now, and I can see that in a lot of ways some changes are happening, and I can see that a lot of our researchers are actually expanding community engagement in so many ways and making sure that they benefit from the research that we're doing. What's the most exciting discovery of the last couple of years for you? Most of the discoveries in the last couple of years have been related to genetics. All these results that we're finding, like in terms of how the ghost species that mentioned is one, but we also seen like some of this early human fossils that we've had for like decades that we couldn't figure out what they were. Like the Denisovans, for example, we've had

fossils from China for many, many decades that we didn't realize what they could be. And now genetics is kind of linking all these fossils to either Denisovans. So all of the research, genetic research that's happening that has been happening in the last couple decades is really exciting and it's really changing how we understand our origins. That's so fascinating. Is there something you dream of finding, like just where you're just, you're going on, you're like, you know what, I really want to find is this? When I started in this field, my dream was to find the earliest human ancestor, right?

and I found Artifica Skadapa, which was great. And you did? Mission accomplished, yeah. But then after I graduated and then wanted to have my own project and I started this project, Tora Zemile, my dream changed because this time I wanted to have a site of my own that I can work at instead of working, collaborating with others at other sites. and that was also another mission accomplished because I now have one of the best mid-plies insights in the world,

which is called Rwanda Mili site. And when I got there, of course, what's the next dream? The next dream was because these sediments are somewhere between 3.8, 3.5, 3.4 million years ago, my research question was to actually understand how Lucy's species are of the Casafranes, was connected to its potential ancestor, Altaxana Menses. So my question was like,

can I find anything that would inform me about this transition from the sediments that are between 3.6 and 3.8 million years ago? And that's another dream. And we found so many fossils from this time period, including finding the most complete cranium of Lucy's ancestor of Sopicaaenae animinsis from 3.8 million years ago.

So now I'm also working in the sediments that are between 3.2 and 2.9 million years ago. And now I have a dream. My dream is, can I find the earliest homo in this deposits? Because we have sediments at Ledi Gararu project that are about 2.8 to 2.9. And this project is actually led by one of our faculty here, Professor Kay Reid, who found this earliest Homo at 2.8 million years ago. So now my dream is, can we push that record a little bit older because we have the sediments? So what that tells you is that I never had a single dream. My dreams have always changed with the circumstances. And so far, my dreams have, you know,

been realized, and now I have a new dream that I hope will be realized as well. So that's how, you know, my dreams have been evolving through time. That's perfect. Why should people care about this research? That's an interesting question. Positive discoveries have always been interesting to the general public. Now, as generations change, what we're hearing now is beyond the excitement of these discoveries. People have started asking, so what? What do they mean to me?

How are they relevant to me? And that's the big challenge for scientists. How can they answer that question? So what does it mean to me is a question that you really need to address by reframing how you tell the story, the human story based on what you're finding and what you're publishing, right?

But also, how do we connect it to our future? Okay. Now, every discovery comes with interpretations that include the animal communities, the environment, and everything that was like part of the ecosystem when this early human ancestors lived. And today we talk about climate change, right? Now, climate change is not something new. How did our ancestors cope up with the climate change that have happened so many times in their evolutionary history? So there is a connection between what we can learn from the past to actually understand what we might expect in the future.

So that's the kind of connection that we need to create for people to really understand and appreciate the value of studying our past. The past is not just the past. The past is also about understanding our future. You know, when we say all humans are connected, we're lucky that we have now so many ways to explain that connection to our ancestors. As I said, the genetic work is really amazing, and it's connecting so many dots that we were not able to connect using just fossils in the shape and form. We're pushing that connection way back to like the millions of years by tracing, like, who was, who was the ancestor of like the Neanderthals, who was the ancestor of like the Denisovans and Homo erectus and beyond Homo habilis.

You can go all the way to like about six million years ago. So we know where we came from, where we started. But where are we going, right? So why did Homo erectus go extinct? Or why did, you know, homo habilis go extinct? Is Homo sapiens our species going to go extinct? How can we reconstruct the sequence of events? that have happened in the past to make it relevant to us today.

So that's why we investigate our past in so many ways, not only in terms of how we changed, in terms of how our race became larger or how we became more efficient bipedal walkers, but also what's the next step? Where are we going? This is like something that we can learn from by compiling what we've learned from the past. I mean, what could be more relevant and existential than whether we make it?

Well, that's the big question. Yes. That's the big question. Dr. Johannes-Hila Salasi is the director of the Institute of Human Origins at Arizona State University and a professor in the School of Human Evolution and Social Change. Thank you so much for talking to me today. Well, thanks for having me, Flora. It was really a pleasure. And if you have questions about where we came from or where we're going that you think Science Friday could answer, give us a call 877 for SciFry. This episode was produced by Charles Berkwist. I'm Flora Lichten.

Thank you for listening.