Video: Teeth and Human Evolution


Modern humans and our closest-living ape relatives differ in developmental and reproductive biology, as well as in lifespans, but evolutionary anthropologists do not know when these distinctive characteristics evolved. It might seem that our development is invisible in the fossil record, but much can be learned from the faithful records of birth and growth embedded in teeth. Tanya Smith will discuss how she studies fossil teeth with state-of-the-art technologies to gain virtual access to these records and share what this research reveals about differences between Neanderthals and Homo sapiens, and about our evolution over the past seven million years.

Tanya M. Smith, Associate Professor, Department of Human Evolutionary Biology, Harvard University

Presented by the Peabody Museum of Archaeology & Ethnology and the Harvard Museum of Natural History

Recorded May 5, 2015


[00:00:05.07] It's a huge pleasure, as I said, to welcome Dr. Tanya Smith, who is an associate professor in the Department of Human Evolutionary Biology here at Harvard, and specializes in living and fossil human and primate tooth development and structure. She arrived here at Harvard in 2008 after fellowships at the Radcliffe Institute for Advanced Study and the Max Planck Institute for Evolutionary Anthropology in Germany, where she co-founded the Dental Tissues Working Group in the Department of Human Evolution there. She received her BS in biology from the State University of New York and her doctorate in anthropological sciences from Stony Brook University in 2004.

[00:00:49.06] Her research has helped to identify the origins of a fundamental human adaptation-- the costly yet advantageous shift from a "live fast, die young" strategy to the "live slow and grow old" strategy-- that has helped to make us one of the most successful mammals on the planet. She explores the evolution and development of human dentition. Teeth preserve remarkably faithful records, as we will hear, of the daily growth and infant diet, as well as stress experienced during birth, for many millions of years.

[00:01:22.74] Tanya's research is funded by the National Science Foundation, the Leakey Foundation, and the Wenner-Gren Foundation for anthropological research. Her work has been published in Nature, Proceedings of the National Academy of Sciences, and highlighted in many public venues, including the New York Times, National Geographic, Nature, Science, Smithsonian, and Discovery magazines as well as through NPR, PBS, History Channel, Voice of America, and BBC broadcast media-- i.e., she's everywhere.

[00:01:54.62] [CHUCKLES]

[00:01:55.78] On a personal note, I would also say I really admire her personal dedication to the advancement of women in science, and mentoring her graduate students and others, which I know she takes very seriously, and which I really admire in the work you do. So let me get off this stand and welcome Dr. Tanya Smith.

[00:02:15.79] [APPLAUSE]

[00:02:23.02] Thank you, Jane. Thank you, everyone, for joining me tonight. It's an absolute pleasure to share my passion for human history, human evolution, and, believe it or not, teeth.

[00:02:32.49] Before I start, though, I want to give full permission to anybody who feels compelled to keep their personal device in their hand. Harvard has been encouraging their faculty lately to join the Twitterverse. And so I want to invite you all, if you are so inclined, to tweet tonight. And feel free to call out my brand new hashtag.

[00:02:52.01] I'm pleased to be sharing research that we're doing here at Harvard, as well as in the broader anthropological field, with as large of an audience as possible. I think it's very important that we communicate to the general public the relevance of our research. And so I'm delighted that you'll be joining me in that endeavor tonight.

[00:03:09.00] All right. Why teeth? Why are you here this evening to listen to a lecture about teeth in human evolution?

[00:03:16.67] Well, first of all, when you think about it for more than a minute, you realize that your teeth are critical to your survival. Your teeth are what allow you to bring energy into your body, to process food, to metabolize food-- to grow, to develop, to reproduce, to pass your genes on to the next generation.

[00:03:36.49] And that's not just true for us. It's true for our favorite domesticated animals. It's true for all living species on the planet that are dependent on having a way of processing food orally.

[00:03:50.49] So they're critical for survival. And I hope tonight to give you a little bit more information about how information about their growth can better help us understand our own human evolutionary history. And so we're really going to focus in on both growth processes as well as kind of better understanding the last seven million years.

[00:04:10.77] The human fossil record has grown substantially in the last few decades. Those of you who have been following along, may have once thought of it as a linear progression from something primitive, several million years ago, to our own species today. And often it is depicted as this linear arrangement.

[00:04:27.44] When you look deeply at the human fossil record, what you realize is that the majority of the evidence we have for our own history actually derives from teeth. We have thousands of teeth in the fossil record. We really only have a few hundred skulls in the human fossil record, and if you want to study a skeleton, you're out of luck. There are very few skeletons in the human fossil record. So teeth represent the best evidence we have for our early history.

[00:04:53.53] Tonight I want to give you a little context about the research that my colleagues and I have done. And I'll start out first by just giving us a 101 perspective on human evolution. And then we'll move into a better understanding of how teeth grow and develop, and how we can use them to understand the evolution of our own development.

[00:05:10.53] First of all, I just want to remind everyone we are primates. We often think of ourselves as humans, as hominins-- apes, maybe, if we're being generous. But to be fair, we're primates as well as mammals.

[00:05:20.96] And so many of us use a comparative perspective to understand our own growth and development-- what makes us unique, as well as what patterns of growth and development do we share with other primates. So when we look comparatively, our human history is a seven million year record. But when we look even deeper back in time, our primate history is a 70 million year record. So, many of us spend parts of our career better understanding how we are situated relative to other living primates.

[00:05:50.66] And I want to make a point many of you already realize. We are odd primates. We not only dress our babies up as other mammals, but aspects--

[00:05:59.05] [CHUCKLES]

[00:05:59.32] --of our growth and development are quite unique in a comparative sense.

[00:06:03.61] So when you look across apes, when you look across primates, you realize that humans are unusual. We wean our offspring very early relative to other primates, specifically great apes.

[00:06:14.34] We have a short inter-birth interval. What that means is that we have one infant and then a second infant with a relatively short time in between. We stack our offspring. We have many offspring, compared to an orangutan or a chimpanzee, for example, because we don't have a long spacing between them.

[00:06:31.21] However, we then have a long childhood period. Our infants spend a considerable period of time growing and developing, and they don't even begin reproducing and creating their own offspring until relatively late, when compared with other apes and some primates.

[00:06:47.54] Add all that up. We then see, in human history, that we have a very long post-reproductive period, which makes us unique. Human females go through a period of menopause, and then continue to live for several decades after this point in time. This, again, is a unique pattern when we look at comparisons with other apes.

[00:07:06.07] And finally, this together means that we have a very long lifespan. Something is quite unusual about the way we're growing and developing, and the way that we're timing these different key life history events.

[00:07:20.11] One of the questions many of us have been trying to answer is when did these key events evolve? Where in our past do we see evidence for our long childhood, for example-- for our early age at weaning, for our post-reproductive period? Can we find these in our human past?

[00:07:37.65] Here you see more of a contemporary view of human evolution. You see a bushy tree. You see a number of different species of human, hominin, throughout time, through about seven million years here. You're seeing different groups of hominins.

[00:07:52.05] You're seeing an early group which lived roughly seven to four million years ago. And we'll have a look at one member of that early group, Ardipithecus. This is an enigmatic genus which has a few species in it. One skeleton we know from this early hominin group-- again, living about 4 and 1/2 million years ago.

[00:08:11.70] And in many ways, this earliest group of hominins shows a very primitive morphology or skeletal pattern. Small-brained, short-statured, long arms, curved fingers, weird toes that splayed out-- not what you would expect in terms of an early human ancestor.

[00:08:32.69] As we pull the record forward, and we go into a group of hominins that many of you are familiar with-- the Australopithecines-- we again still see relatively primitive characteristics in terms of the skeletal morphology of these Australopiths. Many of you are familiar with this skeleton here, the Lucy individual from Tanzania. And many of you are also aware of the footprints that were found in Laetoli, these fossilized footprints laid down in volcanic ash.

[00:08:59.00] We know that these hominins were bipedal. They walked upright. We know that they, again, though, had relatively small brains and small bodies. So though they walked like we did to some degree, it doesn't quite look like, morphologically, they really were exactly like the pattern we find in our own genus and species today.

[00:09:19.60] When we move into the fossil record of the genus Homo, one of the charismatic individuals that many of us think of is this strapping youth, this fossil from East Africa, known as the Nariokotome Boy. This skeleton is the most well-preserved skeleton we have in the earlier part of the human fossil record. This individual was over 5 feet tall, and in many ways has an anatomy below the head that looks very much like our own-- long-limbed, as you can see-- long lower limbs. And this individual, again, still had a relatively small brain compared to our own brain. But below the neck, more or less, could be exchanged for anyone in this room.

[00:10:00.23] Finally, one of the key groups of earlier hominins that existed, contemporary with us and before us, were the Neanderthals. And this is one of the most well-known fossil species. We have hundreds of parts of skeletons-- of teeth, of skulls. We have over a hundred juveniles in the fossil record. We finally have a large number of individuals to really understand something about their variation and how they compare to us.

[00:10:27.98] Many of you also know we now have ancient DNA from the Neanderthals as well. There is available information on the entire genome. It's given us new insight into the fact that there was interbreeding between contemporary humans and Neanderthals. And I'd be happy to talk more about that during the question and answer period.

[00:10:44.19] Tonight, though, I want to tell you a little bit about how Neanderthals grew and developed. And I'll come back to that point in just a few moments.

[00:10:51.03] Finally, many of us have speculated perhaps it was the origin of our own genus and species, Homo sapiens, that was the place that we found the first evidence for our own life cycle, for our characteristic pattern of growth and development, that makes us really unique among all living primates.

[00:11:08.51] Was it, perhaps, our use of tools? Was it our big brains? Was it the very complicated social structure that we're part of today that may have driven a period of growth and development that is so long and unusual, uncharacteristic?

[00:11:24.17] Well, how do we use teeth to better understand these questions? Why would someone spent 20 years studying fossil teeth? What kind of information can we extract that can better help us really get at where these points of time show these key transitions?

[00:11:39.98] I'll give you a clue. This was one of the most exciting areas of research from my perspective, because there is an intimate record of growth and development. And the clue here-- you can see my thumbprint. The clue is a tiny clue.

[00:11:52.22] On this cast of a Neanderthal canine to the left-- this black cast of a tooth-- you can see some tiny ridges. It turns out that teeth have tiny timelines inside them. This is a tongue twister-- teeth have tiny timelines.

[00:12:07.15] Locked inside all of the teeth in your mouth are biological rhythms, which are reflective of your growth and development throughout your entire childhood. These tiny timelines are not unique to teeth. In fact, many of you know about them in trees. When you cut a cross-section of a tree trunk, you recognize rings that represent time.

[00:12:27.30] As it turns out, many hard tissues show timelines in them. Bones, teeth, mollusk shells, otoliths-- which are these ear bones in fish-- basically anything that's calcifying shows a record of its development through time. We may know about trees, but very few of us know about our own teeth-- the fact that our childhoods are recorded in our teeth. It's a remarkable biological system. It's highly faithful.

[00:12:53.17] For those of you who haven't thought too hard and long about teeth, let me just give you a little bit of an overview of how a tooth is built. A tooth is composed of a crown and a root. The crown is made up of enamel, which overlays the dentine. This is a hard tissue that then surrounds the pulp.

[00:13:09.42] Many of you are familiar with your pulp. You're not friends with your pulp if you've had a root canal.

[00:13:14.07] That is how that tooth fruit is held in the bone-- by cementum. It's the glue. These are the hard tissues of teeth. These are the hard tissues that are built to last.

[00:13:24.61] They don't just last throughout your lifetime. They actually can last for millions of years. These tiny records of growth and development, that are created while you're growing and developing during your childhood, are permanently recorded in tissues that can last for millions of years.

[00:13:43.11] Let me orient you to the type of approach that we use when we want to understand this record of growth and development in teeth. What you're seeing here is a chimpanzee molar on the top. It hadn't finished forming its root before this individual died.

[00:13:56.20] In the middle, you're seeing a cross-section of this tooth. You can see the enamel, the lighter color tissue, overlaying the dentine and some of the root. And then in the lower corner, you can see microscopic growth lines.

[00:14:07.81] How do we recognize these tiny timelines? Well, what we do in my lab is we prepare thin sections of teeth very carefully. It takes many hours to prepare a very well-polished, very thin, 1/10 of a millimeter section of a tooth. When we put that section of a tooth, that microscope slide, underneath polarized light, we can see these very fine biological rhythms inside the teeth.

[00:14:34.52] This is a cross-section of a molar tooth of a chimpanzee that hadn't finished forming before the individual died. What I want you to notice here are a number of these stacked lines that you can see. These lines are like these trees, these ridges in trees.

[00:14:49.09] What we're seeing here are the successive progression of the development of this tooth, laid down and registered through time. So you're seeing here the way this tooth grew, both outward and downward simultaneously. So you can reconstruct its formation from the very beginning of its growth to a later point in time by simply recognizing these lines as they were being produced.

[00:15:12.58] Not only do teeth mineralize their development on a very fine scale, they also show daily rhythms as they're forming. The cells that are secreting the enamel on the dentine secrete on a 24-hour rhythm, and they lock that process in.

[00:15:28.28] The reason that we're confident that there are these 24-hour rhythms is because of experimental studies that have used bio-marking techniques to give an individual-- in this case, monkey-- a marker, and then a few days later, another marker. And so we're able to pick up these markers in the individual's tooth that was growing and developing, and relate that to the number of these little growth lines.

[00:15:50.80] So in this particular individual, there were two markers given-- fluoresced under a certain kind of illumination. The first marker, the XO marker, was followed eight days later by a minocycline marker. And what you can see here, and particularly if you squint, is light and dark bands. There are actually eight of these light and dark bands between these two biomarkers, and these two biomarkers were administered eight days apart.

[00:16:15.93] This is the kind of evidence that people have marshaled, both in primates as well as in rodents, to be able to demonstrate that there are these faithful 24-hour rhythms-- again, in teeth as well as in bone. So you have a clock recording your life as you're growing and developing in your mouths.

[00:16:32.80] Not only does each day record, but your birth is recorded. That stress, the physiological stress of being born, actually leaves behind an accentuated growth line. You can see that here. This is an orangutan that started forming its first molar just a couple weeks before it was born, and the stress of birth registered as a permanent line in this individual's mouth.

[00:16:56.38] And this is true for all of us, as well. We start forming our first molars a few weeks before birth, as well as all of our baby teeth, and that event is permanently recorded. So you all have a birth certificate--

[00:17:06.91] [CHUCKLES]

[00:17:07.82] --in your mouths, as do all the fossils that we are interested in studying.

[00:17:12.57] So this is really a very precise way of not just understanding the amount of time a tooth took to grow, but actually being able to get at an age of an individual.

[00:17:23.53] To walk you through how this works, I'll show you-- this is the chimpanzee from before, in cross-section with magnification using the microscope. And what I was able to do in this individual was I was able to find its birth line with very high-resolution imaging. I assigned that day 0 there. And then I was able to count each one of these 24-hour rhythms, up through developmental time, and assign ages to stress lines.

[00:17:48.41] This was an individual that lived in West Africa, in the wild, and in its early life it went through some periods of developmental disruption. And I was able to assign ages to these disruptions. And then I was able to continue counting time through the crown, into the root, and I was able to pick up a few more stress lines in the route before this tooth stopped forming.

[00:18:08.10] And I estimated, based on my counts and measurements, that this individual was 1,396 days of age. And I didn't actually know how old it was. But when the field notes were presented, it became clear that this individual was 1,372 days of age. So I was 24 days off from its actual age at death. So that's a 2% error in this case.

[00:18:32.53] This can be a very precise and faithful way to assign an age to an individual when we don't have any other information. So it can be a very powerful technique. It's also very time-consuming, in full disclosure, and perhaps you have to be a little obsessive-compulsive to count 1,396 days.

[00:18:50.37] So this type of approach allows us to better situate human growth and development, not just by comparing ourselves to other fossil hominins, like Neanderthals, but to be able to compare ourselves to our closest living relative, the chimpanzee, or to other primates-- for example, to macaques.

[00:19:10.13] So my colleagues and I have spent a good deal of time characterizing how teeth grow in these different primates, again, to better understand whether our development is unique or whether it's a primitive pattern shared with other primates.

[00:19:22.94] We've also started to understand how these closest living relatives, the chimpanzees, erupt their teeth into their mouths. And I want to just highlight some work that I've done with a colleague who's here tonight, Zarin Machanda. We've spent several years collaborating with Richard Wrangham and colleagues at the Kanyawara field site to conduct a photographic study of tooth eruption in wild chimpanzees.

[00:19:47.34] And we worked with a couple semi-professional photographers who gave up a few years of their lives to take photographs of chimpanzees. And what they did was they captured tooth eruption in 25 sub-adults from this community, whose birth was known. And they followed some of these individuals for up to three years.

[00:20:07.57] And we were also able to understand how their dental development related to their maternal behavior, as well as their feeding history. So for example, we were able to establish there was a relationship between when their teeth were erupting and when they were transitioning onto an adult diet. Let me just show you the kind of data we were able to take. And first I'll show you a video, because these are just amazing to watch. This is a very special field site, and we were able to get quite close to the individuals until we were able to really see their dental development through video as well as through high-resolution images.

[00:20:41.14] So this is a video just to show you a couple juveniles, who are main focal animals. And you can see they actually open their mouths quite frequently. They will yawn. They will open their mouths during play. They're play fighting here-- they're biting each other.

[00:20:56.54] And our photographers would work with these individuals and just very patiently wait until the right moment and then capture images such as this. This is, I think, our favorite image from the entire study.

[00:21:08.96] This is Azania. This is Azania's first molars erupting into her oral cavity. This is the first documentation of this particular developmental event in our closest living relative in the wild.

[00:21:21.37] This is happening at three years of age in these individuals. In most of us, this happens at about six years of age. So chimpanzees are showing a more rapid pattern of growth and development for their first molars.

[00:21:33.83] It's not just their first molars. It's their baby teeth as well. We followed another female infant, Buke, from her second month of life-- here she is, lovely individual, where you can see her baby teeth are just cutting the gums here, incisors and her premolars there-- up through 2.8 years of age, when her first molars are erupting as well.

[00:21:53.50] Chimpanzees are erupting their primary dentition, or their baby dentition, at about half the age that we are. By about a year and a half, they've erupted their entire deciduous dentition, or their baby teeth, whereas modern humans tend to require about three years before our full complement of baby teeth are erupted.

[00:22:11.86] So this is relevant because we want to better understand, again, what our closest living relatives' growth and development was like. We often infer that the earliest members of the human fossil record were like chimpanzees. But until this work, we didn't actually have a good handle on what wild chimpanzee development was like. So we're able now to characterize tooth eruption across the life course of the juveniles that we were able to study-- and these are 25 juveniles.

[00:22:37.35] And there's a lot of data on this slide. I just want to show you, for example, the first molars here, in both wild and living captive primates, are erupting between about two and four years of age here. Human first molars, again, tend to erupt at about six years of age.

[00:22:52.63] So these chimpanzees-- again, growing and developing their first molars much more rapidly. Similarly, their second molars here-- again, forming and developing about half the age that our second molars are erupting out into our oral cavity.

[00:23:06.73] And their third molars show an even greater offset. Humans-- we fall off the chart here. On average, modern humans, when you're lucky enough to erupt your third molars-- when they're not impacted-- we tend to erupt them between about 18 to 22 years of age, when it's a normal process. So these chimpanzees, again, are showing much more rapid development, much more rapid eruption of their teeth. And this, again, is the pattern that we often infer was more of an ancestral or primitive-like condition.

[00:23:34.82] So this brings me to one of the key questions that we've been trying to address, which is, where do we find this transition from this more primitive pattern of growth and development to what we see today, that characterizes humans? It's fairly well-established that our tooth development is correlated with our overall growth and development. Our period of childhood tends to be related to our period of erupting our final molars. And by the time those come out, we tend to call ourselves adults.

[00:24:01.35] This seems to be true as well from a chimpanzee perspective. Chimpanzees begin reproducing at about the age their third molars erupt. So this is a marker of the duration of childhood. If we know something about how long it takes to grow your teeth, you can say something about, again, the length of your childhood.

[00:24:16.88] So in order to try to address this question, I was fortunate enough to team up with a colleague in France. His name is Paul Tafforeau. He's an incredible scientist who uses synchrotron x-rays to be able to virtually go inside mineralized tissue on a microscopic scale. We've been working together in France for a decade.

[00:24:37.38] And the facility-- you can see here, it's in the foothills of the Alps. The synchrotron is this ring in the photograph, which was taken from a nearby mountain. It's 700 meters across, and it's a special facility that accelerates electrons to a very, very high velocity, and then bends them. And as they bend, they give off synchrotron x-rays.

[00:24:56.96] These are much more powerful than medical x-rays, or conventional facilities that we use in many universities here in the US. And they allow us to do things that you can't do in other facilities. They allow us to go inside very highly mineralized tissues on a microscopic scale. So we've taken to calling this virtual histology. This is very different than what I do in my histology lab across the street.

[00:25:20.88] These are three-dimensional cubes of tooth enamel that were never physically cut. These are high-resolution scans that were taken that allow us to access the three-dimensional information locked inside these teeth. I'm going to show you a video now to give you some context about how this technology works. And I have to show this on a different piece of software.

[00:25:42.14] Here you can see an individual. This is a fossil from Israel-- 90,000 year old individual. And we took this individual to Grenoble, and we were able to scan it using this facility and visualize how its teeth were growing.

[00:25:55.82] What we first started with was-- I'll get this to play. What we first started with was an overview of its lower jaw here. And what you can see are the teeth virtually rendered here, using this three-dimensional software that allows us to visualize the inner aspects of this individual's jaw.

[00:26:16.64] And you can see this is a child. Its permanent dentition is still inside the bone. These are its baby teeth that have erupted outside, as well as one first molar tooth that had erupted. And so here's that first molar tooth coming out to us.

[00:26:30.00] And now you can see, with higher resolution, some of those tiny ridges on the outside of the tooth crown. Again, this is virtual technology. We never cut this tooth to be able to go inside.

[00:26:39.73] And now you can see some stress, as well as-- with an even higher-resolution approach, we can see the fundamental tissue of the enamel. You're seeing what are called enamel prisms. These are these building blocks-- they're five microns across.

[00:26:53.15] And we can peel through virtually in three dimensions and follow the course of growth of this tooth, as well as see these tiny daily lines, which you're seeing-- these light and dark bands here in the video. So this approach allows us to be able to assign ages to individuals without having to cut or break any teeth.

[00:27:12.11] And this has then given us access to many more fossils than we ever would have been able to study in the past. As you can imagine, for some of these fossil species there's only one juvenile, maybe two juveniles. It's absolutely impossible to convince a curator to allow me to cut up a couple of their teeth to count tiny lines inside. And now we don't need to. We have a new approach.

[00:27:32.70] So by applying this technology, we first studied this interesting fossil from North Africa, from a site called Jebel Irhoud. And we first started just with a tiny chip of enamel. We took this tiny chip of enamel in the middle here to the synchrotron in Grenoble.

[00:27:48.14] And we were able to find this key piece of developmental information that we could only see internally. And we used that to reconstruct how old this individual was when it died.

[00:27:58.01] You can see from the jawbone here of this individual that its first molar had erupted, its second molar had not erupted yet, its canine tooth also was locked inside the bone, but it's incisor tooth had erupted. So by modern human standards, this individual should have been about seven to eight years of age. By using this faithful record of growth and development locked inside, we were able to assign the age of 7.8 years to this individual.

[00:28:24.22] So this fossil-- 160,000 year old fossil-- actually grew and developed like we did. And it turned out this was the first fossil we've studied that showed a modern human pattern of growth and development.

[00:28:35.30] We then thought, all right, let's look more deeply at the Neanderthal. So let's look at our cousins. Everyone wants to know, were Neanderthals growing and developing like us? They certainly had large brains like we did. They used technology. Did they grow and develop like we do?

[00:28:48.50] Well, our first individual that we studied was an individual from Belgium. And I'm going to come back to this individual later in the talk. This is its lower jawbone here. In this case, I actually did physically section one of the teeth, and I was able to work out its age. And this individual, we estimated, was eight years of age. And this is give or take maybe a month or two.

[00:29:07.42] And this individual showed a very different pattern then the Homo sapiens individual from Morocco, which you can see here. The Moroccan Homo sapiens has its first molar erupted, but again, that second molar-- deep inside the bone, no root formed, absolutely nowhere near erupting. The Neanderthal, however, showed a first molar erupted and worn down, and a second molar is well erupted and coming into place-- basically at the stage you'd expect of a modern human 12 year old or so to be. So this shows a very different pattern of growth and development than the fossil from Morocco. We would argue this is an advanced pattern of growth and development-- more similar, in fact, to something like a chimpanzee than to a modern human.

[00:29:49.27] What about other Neanderthals? Maybe this was just a weird Belgian fossil. Well, when we added additional individuals, we found a similarly rapid pattern of growth and development.

[00:29:59.48] This is an exquisite maxilla of a baby-- you can see here on this specimen holder. Tiny little baby maxilla-- rendered in the middle with x-rays so you can see the stage of growth of all of its teeth, as well as a very high-resolution image of its lower first molar. Originally, people thought this individual must have been about four to five years of age, just given its overall development.

[00:30:22.12] And it was a remarkable fossil. First of all, this is the first fossil ever found, a hominin fossil ever found-- in the winter of 1829 to 1830. And at the time, people didn't really have much of an appreciation for human evolution. So they thought it was weird, stuck it in a box, set it aside for a few decades.

[00:30:41.37] They stumbled back over this a few decades later, and they realized this shared affinities with some material coming out of Germany from the Neander valley, which gave its name to the Homo Neanderthalensis. And they recognized that this, as it turned out, was the very first hominin ever discovered.

[00:30:55.51] This baby maxilla has an associated neurocranium with it. That neurocranium has a capacity of about 1,400 cubic centimeters. That's larger than most of us in this room today. So this little baby maxilla goes with this major brain, and if it was the case that this died at four or five years of age, it's a pretty rapid period of growth and development.

[00:31:16.28] As it turned out, counting these tiny timelines, we estimated this was a three-year-old. So by three years of age, this Neanderthal had grown a brain bigger than most of us. Again, a very rapid pattern of growth and development-- not like the modern condition. It takes several more years to reach a cranial capacity that size, if some of us ever get there.

[00:31:37.17] [CHUCKLES]

[00:31:39.11] We compared other Neanderthals. We created a comparison, which I'll show you in a moment, looking at individuals from three years of age up to 12 years of age. So here is that Engis Neanderthal. And what you're seeing here is our histological estimate of its age plotted against how old it would be, conservatively, by modern human standard.

[00:31:58.76] So this individual is actually younger than you'd expect it to be. The eight-year-old individual, again, also younger than you would expect it to be-- this was the other Belgian Neanderthal. So all these Neanderthals are basically showing a more rapid pattern of growth and development then living humans-- in green circles-- or fossil Homo sapiens here.

[00:32:19.34] So here's our individual from Morocco, as well as our individual from Israel. Those two individuals fall right on the modern human condition. But the Neanderthals are showing something more rapid. They're offset from us.

[00:32:33.15] What about other hominins? What about Lucy's child? What about some of these enigmatic juveniles that have been turning up lately? Perhaps you've heard about Australopithecus sediba-- very interesting South African hominin-- multiple skeletons for this individual as well as a juvenile skull.

[00:32:51.21] Well, we're in the process of analyzing it, and I can't give away its age quite yet. But I can tell you that is actually similar to other Australopithecines. When we did a large analysis of over 16 individuals from east and south Africa, we found an even more rapid pattern of growth and development.

[00:33:08.54] You can see that here-- this is a similar plot. It's a little busy. These are ages for fossils that we estimated using tooth histology, counting these tiny growth lines, and comparisons against modern human growth standards. And they're plotted here against humans, in green circles-- chimpanzees, in red circles.

[00:33:27.02] And what you can see is that a number of these Australopiths actually show even more rapid growth and development than chimpanzees, whereas others overlap to some degree with chimpanzees. But none of them show modern patterns of tooth growth and development. And so we would argue none of them show our characteristically long childhood, our late age at first reproduction.

[00:33:49.46] All right. I just want to turn, in my last few minutes, to a related area of research that I'm also very excited about, that I undertook with a colleague who's in the audience tonight. And this is a study of early life diets. And this is a key aspect of our growth and development. Again, we talked about we have an early age at weaning. It's earlier than one would expect, given how large we are. When you compare us to chimpanzees, or orangutans, or gorillas, for example, we have roughly half the time devoted to providing nutrition to our offspring.

[00:34:21.75] And so one of the questions has been, well, when did that pattern evolve? And as it turns out, teeth can help us get at this as well. And so the work that I'm going to show you is collaboration with a colleague in New York, who formally was at Harvard, at the School of Public Health.

[00:34:35.79] And he's developed a method of actually mapping the elements inside teeth. And so this is an image showing you a cross-section of a human baby tooth with basically a 30 micron raster, or laser, going across his tooth, and giving you a map of the elements that are present in that tooth. And in this case, you're seeing trace elements, things that are found in low levels.

[00:35:00.50] This is particularly interesting for people in public health who want to look at lead exposure. They want to know, for example, were children subjected to high levels of lead during development, because that has, obviously, serious consequences for later life.

[00:35:14.00] This is also interesting for evolutionary anthropologists, because we can track the incorporation of key elements that are found exclusively, or to particularly high levels, in mother's milk. So many ask, can we use teeth to get at diet? And in fact, yes, we can. We can use the record of time, coupled with the record of chemistry, to be able to look at transitions in our early life diets.

[00:35:39.15] This is a map showing you the integration of these two ideas here. So the work I do looking at the birth line, of the neonatal line in a tooth crown, is integrated here with chemistry. And so I'm showing you a model for the incorporation of one particular trace element called barium.

[00:35:56.52] Barium is something that's found in the environment in very low levels. It's not particularly good for us in high levels throughout our life course, but it's not something that we tend to take on in our diets. Before we're born, barium is found in very low levels in the fetus, in the embryo, because the placenta blocks the transfer of this trace element. After birth, however, barium levels increase because of the input in mother's milk. Barium is transported across the mammary gland, much like calcium is transported. It's enriched in mother's milk.

[00:36:32.52] And so we have a marker of maternal input that's recorded in your teeth while you're growing and developing. And so just after birth here-- the colors here relate to the intensity, where low levels of barium are in the cool colors, and higher levels, again, are in the warmer colors. And what you can see here is for a short period of time after birth, you have an enrichment. You have a high amount of barium, which is due to exclusive maternal milk input. And as you then transition onto a mixed feeding regime, or you have supplemental foods introduced, your barium levels go down, until eventually there is no more milk being provided to an individual, and you go back to this low level here.

[00:37:13.13] This is the model we can use to basically interpret the record of early life diet in an individual's tooth. So we did this very thing using, first, monkeys that were from a captive colony. And this is a busy slide-- I just want to take you through the basic pattern in here. These are first molars of monkey teeth, which were cross-sectioned. And I was able to work out the time of formation, again, using this temporal approach, using all these tiny lines. And my colleague was able to map out their chemistry. And my other colleague was able to give us records of their growth and their behavior-- when their mothers, for example, were exclusively feeding them, and when they started to transition onto monkey chow, in this case.

[00:37:55.29] And what you can see here is that, again, you have these warm colors just after birth, representing an exclusive period of mother's milk. And that barium, by a quantitative standard, ramps up for the first few months of life as the mother is providing more and more milk. And then again, as the individual transitions onto an adult diet, the barium starts to come down. When milk is removed, the barium again goes back to that low level, or that flat line here, you can see, which is representative of that prenatal level.

[00:38:25.71] So we have a marker in teeth, again, of our early life diet, and we have a marker of when that diet is disrupted.

[00:38:32.60] This is a summary of the points I've made about these key regions. But I want to draw your attention to the lower panel here, because this individual unfortunately was quite ill during its early life. And I was able to map a number of stresses this individual experienced, as well as estimate its age. In this case, I was only one day off from its true age at death, so I was able to come up with a really faithful record of its stress, as well as some of the key periods where it had to be hospitalized because it was just so sick.

[00:39:01.12] And it turned out that at 166 days of age, this individual had to be separated from its mother for a few weeks, treated in the hospital, and then released, and at which point the mother was no longer producing milk. And so this individual experienced an abrupt weaning event. And you can actually see that here, because the color goes from green-- this transition diet-- back down to blue. So we're basically seeing an artificial weaning event in this monkey because of its illness.

[00:39:28.00] This is pretty exciting, because this gives us more access to an individual's early life. It was also exciting to us because we then extended this approach to look at that Neanderthal-- the one that I had cut that first molar from. Well, I was able to look at this Belgian Neanderthal-- and this was the eight-year-old.

[00:39:43.79] And I was able to look at the map I created of its first molar, starting at birth, and registering stress throughout its first 2.4 years of age. And then we were able to map the chemistry of this tooth, and integrate the time of its formation with its chemistry. Looking here at the enamel, what you can pick up are a few key regions here.

[00:40:03.40] First, in blue, we've got a 13 day window-- I'm going to give you the time, here-- 13 day window of prenatal formation. And then you get a ramping up here, a very high level of barium for the next seven months. This is what we think was a period of exclusive mother's milk in this Neanderthal.

[00:40:20.47] And then at seven months, we see a transition. We see a decrease to what looks like a mixed feeding regime. That's the green part here on this tooth crown. And that also lasted for about seven months.

[00:40:32.71] But in this individual, we see a very abrupt falling off here, which corresponds to the blue in this elemental map of this individual. And this looks just like that macaque that was separated from its mother at a 166 days of age. So we think what we're picking up here is a very abrupt weaning event in a Neanderthal. And that correlates with a stress line at 435 days of age in this tooth.

[00:40:57.28] So we see here what we think the first evidence for early life diet transitions in the human fossil record. But what we think we see here is something quite unusual. We don't necessarily think that all Neanderthals stopped nursing at 1.2 years of age. We think that this individual experienced something-- led to maternal separation or the cessation of suckling. And that individual continued living until eight years of age.

[00:41:22.90] So I just want to leave you with a few points before we have a few minutes for questions. I hope that, if nothing else, I've really convinced you that teeth have this intimate record of our growth and development locked inside them.

[00:41:34.06] We can recognize our own birth. We can recognize stress during development. We can recognize periods of illness. We can also assign age at death to individuals that died while they were still forming their teeth. And we can do this with very high fidelity when we're able to access this information.

[00:41:50.79] When we look comparatively-- we look at our closest living relative as well as other primates-- we really can hone in on what is particularly unique about our own growth and development, and what do we think might be a more primitive shared condition with other primates.

[00:42:06.16] When we look using this new technology at Neanderthals, as well as earlier Australopiths, we don't see this clear transition from this small-brained, short-bodied, early hominin to this tall, large-brained, late-developing hominin in one fell swoop. This is a mosaic period of growth and of change through the fossil record. It looks like our lengthening childhoods were one of the last key features to come into play, probably with the origin of our own species, but certainly not in the first members of the genus Homo.

[00:42:43.56] And finally, we're really excited about this new approach to be able to put information together, from elemental chemistry along with temporal mapping, to be able to better understand early life exposures to various elements in the environment, as well as to key elements within mother's milk, such as barium and strontium.

[00:43:01.94] So I want to just stop by saying that no animals were harmed in the production--

[00:43:06.28] [CHUCKLES]

[00:43:06.62] --of this research, although you may wonder how I got all these dead monkeys and apes. All of the individuals died naturally of natural causes. I'm very grateful to them for giving up their teeth, as well as to my colleagues who have been amazing collaborators along the way.

[00:43:23.02] And I want to thank the members of my lab group as well as a number of curators and the funding agencies that have made this work possible. It's a team effort. Everything that I've shown you tonight is really a team effort. And I want to thank Jane, and Diana, and the museums for having me. And I look forward to some robust discussion. Thank you.

[00:43:40.29] [APPLAUSE]