University Place

Finding Ancient Minds in the Human Evolutionary Tree

– Greetings everyone, my name is Jassim Al-Oboudi. I am a PhD student here at UW-Madison and chair of our Darwin Day events. Welcome to all of our in-person and online attendees for tonight’s session of Wednesday Nite @ the Lab, which has partnered with Wisconsin Evolution for this special Darwin Day seminar, celebrating the enduring contributions of one of biology’s most prominent thinkers. And I, of course, by that, mean John Hawks. [attendees chuckling] Now, technically, Darwin’s birthday is this Sunday, February 12th, but we just couldn’t wait to start celebrating, and I hope that some of our events might prompt some of you to have your own little Darwin Day celebrations this weekend. I want to briefly note that we have another excellent speaker lined up tomorrow, earlier in the day. And on Friday, we have a community celebration with some free food and science outreach activities for kids, which will culminate in an art exhibition which features participants of our Darwin Day 2023 art competition. More details can be found at evolution. wisc.edu/darwin-day.

I would also like to acknowledge support for our Darwin Day events from the College of Agricultural and Life Sciences, the College of Letters and Sciences, the Nelson Institute for Environmental Studies, and the Graduate School of UW-Madison. Now for the event at hand. It is a real privilege for me today to introduce John Hawks. He is the Vilas-Borghesi Distinguished Achievement Professor in the Department of Anthropology here at UW-Madison, a department he joined as an assistant professor in 2002. John engages in diverse science outreach, including an active social media presence, a personal website detailing scientific advances, and contributions to popular books and lecture series on evolutionary biology. He’s also conducted extensive work on human evolution, participating in projects that range from the origin of early humans all the way up to the last 10,000 years of our history as a species. This includes extensive field work in Africa, Asia, and Europe, analyzing archeological specimens and conducting complex analyses on genomic data from humans. Notably, John has extensive collaborations with members of the University of Witwatersrand in South Africa, with which he has made important and compelling contributions to our understanding of human evolution. But you are not here to hear me. You are here to listen to the expert. So without much more ado, and I appreciate your patience with me, please join me in welcoming Professor John Hawks.

[attendees applauding]

– That’s great; thank you so much. All righty, thank you everybody. I am so excited to be here tonight. You know, it’s been a long time since I was here, and I’m gonna give you some updates on what we’ve been up to. We have exciting things going on in the field in South Africa. I’m gonna focus tonight on the bigger picture. How do discoveries about the behavior of ancient hominins, who are very diverse, how do they inform our knowledge of ourselves and our connections with those other ancient hominins that existed in the past? We are truly encountering minds that are different from our own, minds that may be shared from common ancestors with us, and finding evidence of them is telling us about what those ancestors were like, but also minds that have undergone their own adaptive and evolutionary processes, minds that have become different from ours in intricate and interesting ways. I’m gonna start, as I often do, by taking you into the field. Since I was here last, we’ve had a number of discoveries in the Rising Star cave system of South Africa.

This one was in the news about a year ago, the discovery of a skull of a child Homo naledi individual that we named Ledi. The Rising Star cave system is underneath this really wonderful, sort of lovely, natural area now, and if we go underground as I was this last summer, we find a complex cave system with more than three kilometers of mapped passageways. I wanna take you virtually underground. This is a part of the cave system. We’re now flying past the Dragon’s Back and into the Dragon’s Back Chamber. And you’re gonna hear a good amount about that in a little bit. The narrow passage that you see there is the Superman’s Crawl, and as you emerge from it into a larger chamber, you have to turn and climb, and that climb takes you through tortuous passageways that involve a couple of ladders and some pretty narrow squeezes until you reach the upper parts of the cave system and the main entrance of the cave that we’re flying past now. This is just a small part of the cave system. It is a beautiful underground space. It’s intricate in its passageways, and it is challenging for our team to go through in many places.

There are parts of the cave system that are incredibly difficult to pass through that I cannot enter, that I have incredible colleagues and collaborators who have the skills and are able to do it, and it’s an amazing experience to be able to be there and be part of the research. As I’ve, probably you’ve heard before, the Homo naledi discovery began in a part of the cave system that we today call the Dinaledi Chamber at the far left of this image. That chamber is connected to the rest of the cave system through a very narrow vertical passageway that we call the Chute. That vertical passageway is about a 40-foot climb down into the Dinaledi subsystem of the cave, as we call it now, and it has a minimum width of about seven and a half inches. It is a very challenging climb based on size, and so it takes very specialized skills to enter this. When Steve Tucker and Rick Hunter entered that space for the first time in 2013, they discovered bones on the floor of the chamber. Lee Berger, my collaborator at the University of Witwatersrand and National Geographic and I have described all of this in our book Almost Human. This is old news at this point, and so I’m gonna talk about new news. But let me set the context. After we discovered this enormous array of bones, more than 2,500 fossil fragments of Homo naledi to date that we’ve uncovered and excavated from the cave system, as we uncovered this, we faced a number of interesting discoveries and a number of challenges of interpretation.

Homo naledi was very similar to us in some important ways. They were upright walking. They walked bipedally, like we do. Their legs were relatively long for their body size, and so we think that they were efficient long-distance walkers. They were relatively skinny. They stood about the height of very small-bodied human populations today, something like 4′ 6″ to 5′ 2″ in females and males. Their body mass was approximately 40 to 50 kilograms, so something like 80 to 120 pounds, so they’re small-bodied human in size. They’re walking like us, but they have adaptations in their shoulders and their hands that indicate to us that they were probably much more efficient climbers than humans are today. Their hands, however, have morphologies that are specialized in the wrist, in the fingertips, and in the sort of flexibility of the fingers, the length of the thumb in particular, that are associated in humans and Neanderthals with stone tool manufacture. And so we think that they were competent tool makers in addition to being very good climbers.

There were many differences between their skeleton and ours that set them closer to some earlier relatives of ours, relatives like Australopithecus. In particular, the size of their brains was about a third the size of the average human brains today. Their range about 450 milliliters to about 600 milliliters. That’s the size of very early hominins and very different from today’s humans. So this species was really different from us in really interesting ways. We discovered over the course of a couple of years of work that the time that these hominins were deposited in this cave system was between 241,000 and 335,000 years ago. So this species existed at around the same time as our own species was first coming into existence in the range of about 200,000 to 300,000 years. That made them much more recent than the very early hominins that they resemble in some details. Australopithecus lived in this part of South Africa before about two million years ago. So this species is here at the same time as our immediate ancestors, and that posed a number of challenges. What were they doing there and how did they potentially coexist with other species, including our own?

In 2017, we announced the discovery of a second chamber in the cave system, the Lesedi Chamber, that included this amazing skeleton, the Neo skeleton, as we named it, and a number of other fossil discoveries. This second chamber highlighted to us that this species was not just an accidental occurrence in one remote chamber. This was a species that was actually using substantial parts of this underground space, space that has not changed in its depth or in its difficulty to reach in the most distant parts of the cave since Homo naledi was using the cave system. Since I talked with you last, we had some of the Homo naledi fossils here in the United States. In Dallas at the Perot Museum of Nature and Science, our team helped assemble an exhibition of Homo naledi material that included the reconstruction of this beautiful skeleton reconstruction by an artist of Neo, so you get an idea of what Homo naledi looked like from a distance. And I think that in skeletal terms, this is a pretty good reconstruction. Since I talked with you last here at Wednesday Nite @ the Lab, we also have done a really amazing amount of work in visualization and understanding the scope of the cave system. I wanna take you on a virtual reality tour of the Dinaledi subsystem of the cave. You’re now entering via the Chute into the part of the cave that we call the Hill Antechamber. You can see on the floor here right in front of us the excavation area that I’m gonna talk about in a moment, where we uncovered a block of fossil material of Homo naledi that is really astounding.

We’re flying over the excavation area and now down a steep slope further into the subsystem, where we arrive at a choice of two passageways. And we’re about to enter into the left-hand one and squeeze along another about five meters through this very narrow passageway into the Dinaledi chamber itself. That chamber is a special space. It’s a place where Homo naledi existed at one time, where we’ve found more bones of them than we’ve found of any other hominin except for Neanderthals in an assemblage this large. It’s a remarkable space, and the floor area of this still contains abundant fossil material. You can see here an area that we had been excavating in 2018, and that remains there on the floor of the chamber as we study it and try to understand its arrangement and what it represents. The Letimela discovery came from further yet. If you go past the Dinaledi Chamber, you enter into an intricate series of fissures that, as you’ll see Steve Tucker here moving through them, are incredibly narrow. You’re gonna hear the word incredibly narrow from me a lot tonight, because that’s a theme in this cave system. Here you’ll see Steve taking off his helmet so that he can squeeze through this place that’s too small for his helmet to pass. And continue on. You’ll see the light there past him. It’s about three meters distant.

And if we go further on, Steve has the video now and he’s recording Marina Elliott, who is excavating in this situation in a remote fissure passage that’s about 10 meters further than the Dinaledi Chamber, where on the floor surface were bones of Homo naledi. We are finding bones of naledi in incredibly difficult to reach places, in places that are rather difficult for us to understand in terms of how the bones reached this place, what the behavioral interactions were that were leading naledi to be in some of these spaces. They’re remarkable, and they’re a big challenge for us to understand. Here, where Marina was working, was where the Letimela skull was. We now have discoveries of naledi bones from more than seven localities throughout the cave system. This is an amazing array of evidence of the behavior of this fossil species. We are working to understand that now.

One hypothesis that we’re working with is the hypothesis that naledi was using these spaces for mortuary practices, that there was something special about their interaction with these deepest parts of the cave system. For naledi to be so familiar with some of these very remote spaces indicates to us that they must have been using the cave system in its more easy to reach surface areas really quite a lot. And so you know, they must have been living there and they must have been carrying on ordinary activities in some parts of the cave system for them to be familiar enough with it to move into these very deep parts of the cave system where they apparently carried out different activities, special activities. Why do I say special? Because ordinarily, when we find hominin remains inside of caves where the hominins are hanging out, we find abundant evidence of their lives. We find the stone tools and the refuse of tool manufacture. We find the bones of animals that they were interacting with. We find evidence of their behaviors other than their bones. In this context so far, we’re finding evidence of naledi remains, including articulated parts of skeletons. So we have set out during the past several years to investigate more broadly the utilization of the cave system to try to understand the behavior of this ancient species. We are working to understand a cultural pattern of activity, and we’re doing it by probing, exploring, and excavating in spaces where we have no evidence previously.

Here you’ll see Maropeng Ramalepa, one of our exploration team working in the Hill Antechamber. He’s digging right here on a shelf of flow stone right at the base of the Chute as you enter into the chamber. This is Becca Peixotto working very near where you just saw Maropeng, and she’s working on a ladder beneath her on the Hill Antechamber floor. I told you that there was an excavation area. This is what she’s working on. And in 2017, our team exposed a massive assemblage of naledi fossil material that we documented and put inside a plaster jacket, like a dinosaur bone, and brought it out of the Chute. This was a major accomplishment, and the team had to have enormous teamwork to do this. We have, since that time in the spring of 2018, been studying the contents of this block. And the contents of the block are nothing short of amazing. You can see abundant skeletal and dental material in this. There’s evidence from many parts of the body of at least two individuals. It’s a remarkable array of bone.

We’re studying it with techniques that have never been applied before, including right now, this block is in Grenoble, France, where it is undergoing synchrotron-aided micro CT scanning, which gives us a scan of the inside of this thing at nano, at micrometer resolution. So it is going to be A, an enormous amount of data, I gotta tell you, but B, it’s going to be one of the most detailed documentations of a fossil assemblage of hominins that’s ever been uncovered. Unfortunately, that kind of work takes time, and so as that develops, we’re continuing to work by exploring and excavating in other parts of the cave system. The Dinaledi Chamber, which you reach through the Chute, is adjacent to a massive block called the Dragon’s Back, which is a narrow ridge of rock that extends to a maximum height of about 12 meters or about 40 feet, which you climb to get to the area where you can access the Dinaledi subsystem of the cave. That Dragon’s Back is our access point to the Dinaledi subsystem. We think that it was also Homo naledi’s access point, and our work in geology and in excavation is targeted toward understanding what the interaction of naledi with this area might have been. This last year in 2022, we began an excavation in the Dragon’s Back Chamber. It’s logical that if Homo naledi was accessing the deeper parts of the cave through the Dragon’s Back Chamber, that indeed we might find evidence of Homo naledi’s activity in the Dragon’s Back Chamber also. But we had never excavated there. It was a passageway for us, but we hadn’t dug to see what was underneath the surface.

I will say, we had some pretty good reasons to think that there might be something interesting under the surface. There are brecciated fossils that are in the ceilings and some low overhangs within this chamber that include bones that look like Homo naledi bones. And so this is a pretty promising place to look for fossils. And in 2022, our team assembled in July and August and began working in the Dragon’s Back Chamber. You see here the team getting going, including on the right here, Sarah Johnson, who’s here in the crowd, our graduate, one of our graduate students here at the University of Wisconsin. Yeah, give her a round of applause please. [attendees applauding] You gotta understand, she is sitting here listening to me say, “This is an incredibly challenging climb, this is so difficult.” And when Sarah came out of the cave having done this climb, she says, “I don’t see what’s so hard about that.” [audience chuckling] Now, there’s some difference in dimensions [audience chuckling] between me and her that make this somehow a different kind of physical challenge. And I will say that it’s not only about body size. It’s also about climbing skill; it’s also about body awareness. And Sarah is a climber and she’s really good at it. So having her join this expedition to work in Dragon’s Back was really amazing.

The Dragon’s Back excavation season this year was supervised by Dr. Keneiloe Molopyane, who is our team member and collaborator at the University of Witwatersrand and included two of our graduate students here from the University of Wisconsin, Sarah and Erica Noble. So this was an amazing get back into the field and get back into Rising Star kind of field season, and it was rewarded with some amazing discoveries. You see the team here working in the Dragon’s Back Chamber, Keneiloe here in the front, behind her Erica Noble, then Ginika Ramsawak, and then Sarah there furthest in the back, and we were maxing out the cave, right? This is about as much excavation as we could possibly do in this area, sampling different parts of this area of the cave system. And in this area of the cave system, beneath the surface, we found abundant evidence of charcoal and fire.

This is a first for us in the cave system. And Lee Berger, my friend, announced it to the world last fall. As we’re exploring in this cave system and beginning to understand that this is not just a fossil space, it’s actually a cultural space, it becomes really important for us to begin to draw the bigger picture of how fossil hominins are interacting with each other and interacting with the spaces. We’ve begun to understand that this cave system is not only a repository for Homo naledi fossils, it’s in part created by Homo naledi. And that interaction, the interaction of a species and its landscape, in this case a subterranean landscape, the interaction of a species using technology potentially to find ways into deeper portions, that’s something that’s very special. Now, it takes a while for us to understand the full context of something like discoveries here, charred animal bones, in the previous slide you saw a hearth. It takes a while for us to do the chemistry and to really understand, okay, how much can we really say about this? What I can say is that we’ve found this in an area that previously we have only evidence of Homo naledi’s entry, so from that standpoint, we’re working with the hypothesis that here we have some kind of interaction. What we’ll continue to do is excavate and contextualize this kind of evidence, and that will take us some time. But immediately, as we discovered that there was fire evidence, we realized that we had to reevaluate larger parts of the cave system to try to discover what other evidence in the cave system there might be besides the fossil bones that we’ve found. Here you see Lee and Dirk van Rooyen, one of our exploration team members going out the south entrance of the cave system, and I’m taking the picture.

As we explored the cave system looking for potential cultural evidence, the cave system, as I said, was three kilometers underground, and it’s complicated. I’m gonna take us, it’s an extensive system underground, and I’m gonna take us in closer to the central areas where we have naledi evidence that includes the Dinaledi subsystem here and the Lesedi Chamber here in two different passageways areas of the cave system. Our entrance to the cave that we use is here on the right. As we explored the cave system, we found in another chamber, the Rising Star Chamber, which is one of the most remote from surface entrances that we’ve entered so far, we’ve found quite abundant evidence of charcoal, including really distinctive chunks of charcoal, scatters of ash, and animal bones in association with charcoal. We cannot yet say if these animal bones are left there by naledi, right? There’s a lot of dating and a lot of work that we have to do to understand what the context is. But we’re happening upon a context which is very unusual. Most human-used caves have scatters of lithics. They have stone tools everywhere. They have evidence of large mammal bones. This is something that’s looking kind of different.

And as we study it and probably in the upcoming year we’ll be excavating in this chamber, we’ll be working to understand what the interactions in the spaces were. Now, let me say a few words about Charles Darwin. It’s Darwin Day at our celebration of this, and the study of evolution in a biological sense really began with Darwin. A lot of people read Darwin. A lot of people think about Darwin and his contributions. Fewer people read his work on human origins, The Descent of Man, than read The Origin of Species, his more famous work. Darwin thought really hard about where humans came from, and he was famously reticent to share his thoughts in public about this after the publication of The Origin of Species. A lot of people know that in The Origin, he sort of concludes with the line that light will be shed on man and his origins, but he doesn’t shed any light. It takes him another 13 years of work to get to where he’s ready to share his ideas about human origins and human evolution with the world. In the meantime, other people are working on this, people like Thomas Huxley, one of his longtime collaborators, who writes Man’s Place in Nature.

People like George Mivart, who’s working on primate evolution and trying to understand the connections of primates. And Darwin is corresponding with all of these people and trying to come into an understanding of where humans fit in the natural scheme. Darwin made many contributions to biology. Natural selection, the idea of biological evolution, the mechanism for evolution. To me, the most central and important one is the idea that species are connected by a tree. This was really an original idea of Darwin. People had thought about evolution before. People had not thought about the tree of relationships of species. And the innovation of the tree gives us the power to understand ancestors by studying their many descendants and to understand different stages of the evolutionary process by looking at the branches that connect at those stages. Darwin famously in his notebook made this diagram of a tree, the one that says, “I think,” at the top.

This is the most popular biology tattoo, by the way. I recommend it. It’s actually, it’s an iconic image. This changed the nature of science, this image. Darwin did not publish very many trees, and in his entire work on human origins, he published no pictures of the tree of relationships of primates or humans to them. But he did think about this a lot and put in his notebooks his ideas of the relationships of species. You see my transliteration of this on the right and Darwin’s original notebook on the left. And you see that he puts humans on the top left next to the great apes, gorillas, chimpanzees, orangutans, and gibbons. They’re close relatives, and a branch yet further from them are the other catarrhine primates, Cercopithecus, Macaca, baboons, the Cercopithecoid monkeys as we consider them today. Another branch further out, the new world monkeys, who we call the Platyrrhine primates today. And then further, the lemurs.

This is fundamentally the tree of relationships of primates as we understand it today, with the exception that Darwin didn’t quite get the place of humans correct. Darwin, like most people at the time, thought that humans have a long branch that separates us from other great apes, other closely related primates. Today, we understand that we are among the great apes. Our branch is most closely related to a branch that includes chimpanzees and bonobos, Pan paniscus and troglodytes on this tree. The human chimpanzee branch is related to a gorilla branch, Gorilla gorilla, Gorilla beringei, the western and eastern gorillas. That branch is connected to an orangutan branch, which today is occupied by three living species and many fossil species. Here you see Pongo pygmaeus, Bornean orangutans, Pongo abeliis, Sumatran orangutans, and Pongo tapanuliensis, a recently discovered species in a small area of Aceh in Sumatra. Their ancestry goes back as long as three million years ago. The ancestors of today’s great apes go back in time into a time when our own lineage was also diverse.

This isn’t as widely appreciated as it should be. These primates were diversifying in the same timeframe as our ancestors were diversifying. We have an abundant record of human origins. This is at the Smithsonian, a wall of skulls, where you see, I think, more than 150 different skulls of fossil hominins from the beginnings of our lineage more than six million years ago up until the recent past. It’s an enormous array and it’s full of skulls. There are relatively few or complete or partial skeletons of things, but there are some impressive ones. And as we go closer and closer to the present, we find partial skeletons of Neanderthals and more recent modern forms of humans. We have abundant skeletal evidence of our evolutionary history. If I look at some of that evidence, I’ve put up here sort of my own wall of skulls, this is not super representative except insofar as I’ve tried to put a skull up here for a number of species, and you’ll see 17 of them. And if we look at these skulls and their arrangement, we can make a tree very much along the lines that Darwin might’ve made, where we have species like our own at the far right and species that are close on the tree to us that lived in the past that are now extinct, and as we go further to the left, species that are progressively more and more distant from us in terms of their common ancestry on the tree.

If we look at that leftmost part of the tree and consider the kinds of hominins that were there, we see hominins that belong to genera called Australopithecus and Paranthropus, genera that are today extinct and have been extinct in the case of Paranthropus for more than 800,000 years; in the case of Australopithecus, for nearly two million years. These were diverse and they lived for millions of years. Today they’re gone, but one branch of them survived, and that branch, after two million years ago, also diversified. That branch includes all of the extinct species that we consider to be members of our own genus, the genus Homo. You’ll see here a wide array of them, humans again on the farthest right, surrounded by other humans that are kind of like us in some really important ways, humans that we today call the Neanderthals or archaic humans. We see extinct species like Homo erectus. An extinct species, Homo naledi, has its place on this tree. Earlier extinct species like Homo rudolfensis, Homo habilis. These represent ancient diversity that once existed that our own branch evolved from and that we have survived. Our origin was an origin that was not alone. It was an origin that was among many others.

And those others had their own ways of living, their own ways of thinking, their own ways of being. We’ve recently gotten a lot more evidence about these relationships from genetics. And it would be wrong for me to talk about the tree without showing you a genetic tree. DNA evidence goes back only in the maximum case in humans so far, around 430,000 years. We have DNA from an early population of Neanderthals from Sima de los Huesos, Spain that is that age. So we know something about Neanderthals, we know something about other kinds of hominins, including a fossil population called the Denisovans discovered initially from this tiny piece of a finger bone from Denisova Cave in Siberia. Today, we understand that this was a diverse branch of ancient hominins that lived alongside Neanderthals and our own species probably in the eastern and southeastern parts of Asia. And we understand that today’s people and these ancient people had interactions with others, others that we have echoes of in the DNA that survived. So those others we cannot yet identify with fossil evidence.

We have genetic fossils in this sense. The Denisovans are a great example of that. Initially found from a finger bone, today we still do not have a skull that I can show you the face of that we know to be associated genetically with this population. We do have a jaw, and so there’s something, and we will have more. There’s almost certain we’re gonna have more. But as we go further back in the past, we discover that these lineages interacted with each other. They mixed with each other and they mixed with others. Our tree was a tree in which our species originated among others, and those others did not just form part of the scenery; those others were interacting with and interbreeding with, in some cases, each other. We come from a tree with hybridization. That means that these different species interacted and they interacted through cultures.

Now, as we look at this tree and see the diversity of hominins that are part of it and understand that they each had their way of living, they each had their time, they each had their place, and they mixed with each other, we can start to assemble a timeline to try to understand their interactions. Here we see humans at the far right again, that’s the present day, and there’s a density of evidence of these other kinds of species coexisting with our species at various places. We see that there’s Neanderthals at the top and archaic humans from East Asia next to them. There’s modern humans and archaic humans from Africa. There’s Homo naledi that coexists with early modern humans, and there’s the very small-bodied species from Indonesia, Homo floresiensis, that also persists until 65,000 years ago. As we trace our branch earlier in time, there are other times that we see many species coexisting. These interactions among these species must have mattered to our ancestors. They formed a part of what made us human. Now, I’m very concerned now with understanding their behavior. How did these ancient hominins interact? What was their way of living? What were their minds like? When the title of this talk went out on Twitter, people sort of kicked back at me. They’re like, “Minds, what are you talking about, minds? This is fossils, this is archeology. We’re not looking at minds.” Minds are our way of experiencing the world. They’re also our only way of connecting that experience to the world itself. Minds are our interface with the world around us. That means that they’re also our interface to the other individuals in the world around us and the interface to the other species that existed in the world in the past. When we look at the brains of ancient hominins, we’re not looking at minds. It’s a challenge for us scientifically to try to connect those things with each other with evidence. Here you see exceptional endocasts, natural casts formed inside of skulls. We can study the insides of skulls and we can look at them and get a lot of them.

An obvious one that I pointed out earlier is that Homo naledi’s brain size is about a third the size of the other hominins that coexisted in Africa at the same time. That includes our own species, Homo sapiens, and archaic forms of humans, sometimes called Homo rhodesiensis. These crania all date to the same time. They’re very different from each other in size. An important question is, what difference does that make to behavior? We know a lot about the evolution of the size of the brain. This is endocranial volume, how big the inside of the skull is over time, over the last four million years in hominins. I’m gonna parse this out a bit ’cause there’s a lot of data here, and that data actually is chunky when you look at the details. If we look within Africa, here’s the trajectory. Earlier hominins before two million years ago all have brains around 500 cubic centimeters or so. After that time, some hominins get bigger brains, but not all.

In Africa, you can see that there’s later hominins, including Homo naledi, that retain that ancestral, smaller brain size or evolve convergently with earlier hominins toward a smaller brain size. If I stretch out the last two million years and look at that timeframe, you’ll see that there are individuals of Homo erectus throughout this time that approach Homo naledi’s brain size. Small brain size is something that exists across a long span of time. We’re used to thinking of our species as having been the end of a trajectory, where that trajectory is toward larger and larger brains. It’s actually not the case. Here, you see archaic forms of humans like Homo rhodesiensis compared to living, modern humans and our immediate fossil ancestors. They’re a little different from each other. If we look at evidence from Southeast Asia and East Asia, you can see that erectus. Then there’s these archaic humans, and they’re big in brain size, and then modern humans are a little smaller. Neanderthals pick up in Europe after about 430,000 years ago, and modern humans immediately follow them.

This starts to look kind of chunky. It starts to look like groups. Homo floresiensis persists in Southeast Asia. So as we look at the overall tree of the last two million years of evolution, we see that there’s a pattern of diversification. Hominins are becoming different. Those hominins are all classified within our genus. They all share common ancestors with us around two to two and a half million years ago. They’re part of the same tree, but this is a tree that’s becoming diverse in its brain size and as you’ve seen a hint of with Homo naledi, it’s becoming diverse in strategies for interacting with the landscape. So going back to this tree, we’re now gonna try to understand the whole organism. How are these species interacting? Well, there’s other things to the brain besides its size.

This is evidence of the insides of the skulls of Homo naledi brains. We’ve studied these, and in a paper in 2018, our team showed that the frontal lobe of naledi’s brain actually is very human-like in its organization. It’s different from earlier hominins that had smaller brains. It’s actually human in this respect. There’s a structural aspect to our brains that is shared with a species like naledi, despite the fact that there’s a big size difference in the brains. That may mean something very important. This part of the brain in humans, the left frontal lobe, is essential to forming language, syntactic language, making sentences that make sense. We don’t know whether that would’ve been true in ancestral hominins. We cannot see their forms of communication directly. We have only a little bit of evidence about the vocal tract in some forms of hominins, and it does suggest that Homo shares a vocalization capacity with humans. Whether it shared a linguistic capacity or not is unclear.

When we think of what big brains are for, there’s been a lot of scenarios, a lot of hypotheses about this, and technology is maybe one of the most widely known. These stone points, which were hafted onto spears, come from Kathu Pan in southern Africa about 500,000 to 300,000 years ago. This is the archeological evidence that occurs in the area where we find Homo naledi. We don’t know whether this evidence was produced by naledi or whether it’s produced by other forms of hominins that may have also existed at the same time in the same place. We know that our immediate ancestors in this timeframe existed and they could well have been here. There’s also chunks of ochre, a mineral pigment that you can see the engraved lines on. These are made from rubbing the ochre onto hard surfaces. We have other chunks of ochre that looked like they were rubbed on soft surfaces. In southern Africa, we don’t have fossil remains in direct association with archeology like this.

We haven’t yet found archeology like this in the Rising Star Cave, for example. In North Africa, however, there are very similar archeological forms, and they are found in cave layers with early members of our own species, Homo sapiens. In East Africa at Olorgesailie, this is a famous site that is just littered with hand axes that you can go as a tourist and walk on the walkway and see the hand axe floor there. Those hand axes are around a million years old, and they are found in the same landscape as this fossil skull, which is very similar to a naledi skull. This was once classified as Homo erectus. Today, I would say we don’t know for sure. But there are also pigments at Olorgesailie at a younger timeframe, around 300,000 to 400,000 years old. Pigment use, incision of lines becomes characteristic on pigments after around 100,000 years ago. Those are the things that people look at and say, maybe brains are doing this, maybe this is what communication takes a form, technology takes a form that requires innovation from big brains.

People used to say that Neanderthals would’ve been incapable of this kind of thing. Neanderthals have very big brains. Their brain sizes are human in their size. Over the last two decades, a revolution in understanding of the behavioral evidence of Neanderthals has happened. Neanderthals are primarily a Western Eurasian fossil population. Here’s a skeleton of a Neanderthal that has a hyoid bone, that has a, shows a human-like vocal tract. The evidence of Neanderthals as we’ve pored over it over the years, these fossil fragments from Krapina in Croatia where I’m gonna be in a few weeks. This is my friend, the late Jakov Radovcic who used to curate the collection. Those fossil fragments, as we’ve studied them, we’ve uncovered evidence of incised lines on the frontal bone of one of these. My friend Davorka Radovcic, who is Jakov’s daughter and presently the curator of this collection, identified eagle talons that have evidence of where they were strung together. This engraving on the floor of Gorham’s Cave in Gibraltar, more than 60,000 years old, made by Neanderthals.

In addition, pigment spots in various caves in Spain and southern France appear to be Neanderthal in manufacture. And Neanderthals abundantly interacted with pigments. And in a deep cave in France called Bruniquel, they constructed a circle out of stalagmites. Hominins are experiencing their worlds and altering them. They’re going into spaces that other species don’t access and they’re making things that are different. They’re marking their environments and they’re marking themselves. Now, many people look at this kind of evidence and think, “Wow, you know, this is what humans do, so therefore they must be human. ” That’s kind of a chauvinistic point of view. And there’s nowhere that’s better evidence for this than Southeast Asia where, on the island of Flores, separated from the Asian mainland always by a permanent water crossing, even at lowest levels of sea level, there are hominins that are very different from us, brains that are around 420 cubic centimeters in size, smaller than naledi’s, skeletons that are smaller than naledi’s. This species, Homo floresiensis, evolved on an island or reached this island by crossing over permanent water straits.

You may say, “Well, okay, that could happen. That seems like an amazing thing.” They make stone artifacts that are very interesting, not necessarily super complex, although these blades are beautifully manufactured from Liang Bua Cave, where the skeleton that I just showed you was found. From Mata Menge, these artifacts more than 800,000 years old indicate floresiensis’ presence on the island at that date. But floresiensis is not alone. On the island of Luzon in the Philippines is another small-bodied and small-toothed species that existed up until the last 100,000 years. This species, discovered in 2018, named Homo luzonensis, we don’t have a skeleton of with a skull that we can say the brain size of. So we can’t say for sure whether this was like floresiensis, but it looks like floresiensis. These are species that are crossing water. They’re capable of colonizing new places, living in new ecologies, and doing so with very different biological equipment than we’re used to having.

They’re doing it with small bodies and small brains. And they’re doing it not 100,000 years ago, not 300,000 years ago. They’re doing it 800 to a million, 800,000 to a million years ago. That kind of behavioral complexity is evidenced in the material record. At Wonderwerk Cave in South Africa, some of the earliest evidence of charcoal and fire, vastly older than the evidence that we found in the Rising Star cave system, indicates that hominins are interacting with fire going way back. Gesher Benot Ya’aqov in Israel is an open-air site. It’s not a cave site; it’s by the Jordan River. And in this site, there are hearths where ancient people, 800,000 years ago, roasted pistachios. Another thing people say brains are for is social behavior, and there’s a deep reality to this. Hanuman langurs are well-known for their coalitions that they form between males to compete to displace dominant males within a group.

That kind of ability to ramp on and ramp off aggression in very intricate social contexts, something called Machiavellian intelligence, is something that many scientists think brain size is evolved for. Grooming, getting along with other individuals, affiliating with them is something that people think, this is what brain size is really for. Mountain gorillas. This is a male mountain gorilla who’s affiliating with and taking care of this young juvenile. That kind of social behavior is something that we use our minds for, right? These are primate minds. I put this slide up here to remind everyone that I’m gonna show a disturbing slide next that includes a dead gorilla. So if that’s something that you don’t wanna look at, I wanted to alert you to it. Because the other side of affiliation, the other side of Machiavellian intelligence, the other side of aggression is grief and mourning. Here’s a gorilla mourning his mother. We find this in the fossil record also.

At Sima de los Huesos in Spain, the single largest accumulation of fossil evidence of any hominin, more than 30 individuals of an early Neanderthal population, representing a group of some kind in a pit with their bones tossed in, probably originally as bodies, many of them the victims of aggressive or violent deaths as evidenced by perimortem wounds on their crania and other parts of their bodies. An intentional act of some kind, deep in a cave. Homo antecessor, also in the Atapuerca Mountain around 780,000 years ago, cannibalized. Groups are interacting with each other in violent ways, in aggressive ways, and also in many cases in caring ways, in ways that manifest the emotion of grief. When I look at the remains of Homo naledi that we’re working on in the Rising Star cave system, this is what I want to try to understand. What do these remains tell us about the interactions of the hominins, of the individuals, and we are learning that. So as we look across the tree, we are seeing that there are different experiences. And those experiences, we have fragments of evidence of. Our field encounters more evidence every year, and we find new ways to analyze it. What I’m telling you about tonight includes evidence from around the world that didn’t exist five years ago. Very little of this stuff existed 15 years ago. We’re making rapid advances in understanding the lives and cultures of ancient hominins. And as we make those advances, we experience joy, but we think about the emotions and the joy that the ancient people also experienced. This is where our emotions came from. This is where our minds came from.

So I’ll leave you with Homo naledi. I think about it a lot. And you saw the reconstruction earlier that was very human-like in its look. And John Gurche, who’s a good friend of mine, a paleo artist, has a reconstruction that is a little bit edgier. We have our ways of looking at the past and bringing to it something different from our own experience. It’s a challenge for us as scientists to try to go beyond our own minds and understand the minds of others, but it is so essential because our minds came from them. The tree that connects us is the tree that will tell us how our minds originated. With that, I’ll end. And thank you all for coming. I’ll be happy to take questions afterwards. But before I do that, I just wanna say I really appreciate you all coming out tonight. It is amazing to be back in person here in Madison and giving talks. And I’m really happy that it’s happening again. So thank you all, and I’ll be looking forward to coming back again and giving you the next installment.

[audience applauding]