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

Tracing the human family tree

0:00Hi, 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 Haile Selassie, 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 ramidus. You know Artie. We all know Artie. And fossils of the 5.8-million-year-old Ardipithecus cadaba.

0:57His list of finds also includes specimens from Australopithecus afarensis 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.

1:28Pleasure to be with you, Laura. We're delighted to have you. You've made these many spectacular finds.

From history student to fossil hunter

1:36I 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 written history. But there were a couple of 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.

2:14But 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 paleoanthropologists 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.

2:49Some 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.

What a fossil dig actually looks like

3:03Okay, 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, eat breakfast at 7, okay, and go out to the field looking for fossils.

3:39Now, 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 beachcombing? Like, you're just walking around with your head down?

4:12Exactly, 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 Warren Zemele is, like, the discovery of the partial skeleton of Australopicasapherensis, about 3.6-meter 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.

4:54That's a good sign that there might be more, and you end up excavating and definitely will 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, you 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.

5:25But other times, you don't mind staying until, like, 7 o'clock, even 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, you know, throughout the day,

6:02especially 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, you know, doing surveys with your eyes on the ground, you've got to tell your brain, like, exactly what you're looking for.

6:35Because 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, you know, little ones. Because, unfortunately, the fossil record is filled with isolated 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?

7:13So that was not my, you know, my expertise at the beginning. So whatever I found, which were really good, were, you know, identified by others, you know, 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.

7:45Otherwise, 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 fossil studies 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, you know, 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 part of the luck that you need to actually find good fossils.

8:24There's some serendipity there. Yes, correct.

Why rift systems hold ancient bones

8:29Why 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 hominid ancestors that lived outside Africa, right?

9:01And in Africa specifically, if you look at all these 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 these 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.

9:36So 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 hominids?

10:09I 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 these early homins live, but we're not finding them because the geological context is not, you know, 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.

10:58And 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, right? 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.

11:32I 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 field has to operate moving forward.

12:04And 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've 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 up for that? No problem.

Shifting goals in paleoanthropology

12:34What 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 like an evolutionary history of about 6 to 7 million years. And documenting that evolutionary history is probably the hardest, particularly when you think about how do we reconstruct our past? As paleoanthropologists, we tackle this question with, by going out to, you know, some remote areas to find ancient fossils of our ancestors.

13:14Not 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 7 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.

13:48That's interesting. From the outside, I keep seeing these sort of 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.

14:23So 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 these early human ancestors. And yes, 30 years ago, we wouldn't have thought about a ghost species. 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.

15:00Right 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... You think so? 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, you know, 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.

15:34When 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 referred to missing link as the ancestor that, you know, 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.

16:05So 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 paleoanthropology of Western scientists parachuting in to Africa, the sort of colonial legacy. How do you think about that in your field?

16:28I 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 researchers. We go to their area, we collect fossils, we go out, we describe the fossils and publish. Do we really communicate like 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?

16:58Those 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 in universities. As a matter of fact, ASU has an MOU with one of the local universities in Ethiopia, Samara University. And right now we're trying to open a new paleoanthropology master's program.

17:30So 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.

18:01All these results that we're finding like in terms of how the ghost species that you mentioned is one. But we're also seeing like some of these 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 of decades is really exciting.

18:37And 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 out and 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 Ardipatikaskadaba, 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 towards Emile, my dream changed.

19:20Because 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-Plaesian sites in the world, which is called Ardipatikaskadaba 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 and 3.5, 3.4 million years ago, my research question was to actually understand how Lucy's species, Ardipatikaskadafrensis, was connected to its potential ancestor, Ardipatikaskadamensis.

20:03So 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, Ardipatikaskadamensis, 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.

20:37And now I have a dream. My dream is, can I find the earliest homo in these 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 K. Reed, 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?

21:07Because 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.

21:33That's perfect. Why should people care about this research? That's an interesting question. So, 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.

22:03How 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 these early human ancestors lived.

22:41And 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.

23:13You 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 their shape and form. So we're pushing that connection way back to, like, the millions of years by tracing, like, who was the ancestor of, like, the Neanderthals, who was the ancestor of, like, the Denisovans, and Homo erectus, and beyond, Homo habilis.

23:50You 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, like, how our brains became, like, larger, or how we became more efficient, bipedal walkers, but also what's the next step?

24:34Where are we going? This is, like, something that we can learn from by compiling what, you know, 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.

24:55Dr. Johannes 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. 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 at 877-4SCY-FRI. This episode was produced by Charles Berquist. I'm Flora Lichtman. Thank you for listening.