Januari 05, 2017
it's a huge pleasure, as i said,to welcome dr. tanya smith, who is an associate professorin the department of human evolutionary biologyhere at harvard, and specializes in living andfossil human and primate tooth development and structure. she arrived hereat harvard in 2008 after fellowships atthe radcliffe institute for advanced studyand the max planck institute for evolutionaryanthropology in germany,
where she co-founded thedental tissues working group in the departmentof human evolution there. she received her bs in biologyfrom the state university of new york and her doctoratein anthropological sciences from stony brookuniversity in 2004. her research hashelped to identify the origins of a fundamentalhuman adaptation-- the costly yet advantageousshift from a "live fast, die young" strategy to the"live slow and grow old"
strategy-- thathas helped to make us one of the most successfulmammals on the planet. she explores the evolution anddevelopment of human dentition. teeth preserve remarkablyfaithful records, as we will hear, of the dailygrowth and infant diet, as well as stress experiencedduring birth, for many millions of years. tanya's research is fundedby the national science foundation, the leakeyfoundation, and the wenner-gren
foundation foranthropological research. her work has been publishedin nature, proceedings of the nationalacademy of sciences, and highlighted in many publicvenues, including the new york times, national geographic,nature, science, smithsonian, anddiscovery magazines as well as through npr,pbs, history channel, voice of america, and bbc broadcastmedia-- i.e., she's everywhere. [chuckles]
on a personal note, iwould also say i really admire her personal dedicationto the advancement of women in science, and mentoring hergraduate students and others, which i know shetakes very seriously, and which i really admirein the work you do. so let me get off this standand welcome dr. tanya smith. [applause] thank you, jane. thank you, everyone,for joining me tonight.
it's an absolute pleasureto share my passion for human history,human evolution, and, believe it or not, teeth. before i start, though, iwant to give full permission to anybody who feels compelledto keep their personal device in their hand. harvard has been encouragingtheir faculty lately to join the twitterverse. and so i want to inviteyou all, if you are
so inclined, to tweet tonight. and feel free to callout my brand new hashtag. i'm pleased to besharing research that we're doing hereat harvard, as well as in the broaderanthropological field, with as large of anaudience as possible. i think it's veryimportant that we communicate tothe general public the relevance of our research.
and so i'm delighted that you'llbe joining me in that endeavor tonight. all right. why teeth? why are you here this evening tolisten to a lecture about teeth in human evolution? 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 youto bring energy into your body, to process food, tometabolize food-- to grow, to develop, to reproduce,to pass your genes on to the next generation. and that's not just true for us. it's true for our favoritedomesticated animals. it's true for all livingspecies on the planet that are dependent on having a wayof processing food orally. so they're criticalfor survival.
and i hope tonight to give youa little bit more information about how informationabout their growth can better help us understandour own human evolutionary history. and so we're really goingto focus in on both growth processes as well as kindof better understanding the last seven million years. the human fossil recordhas grown substantially in the last few decades.
those of you who havebeen following along, may have once thought ofit as a linear progression from something primitive,several million years ago, to our own species today. and often it is depictedas this linear arrangement. when you look deeply at thehuman fossil record, what you realize is that themajority of the evidence we have for our own historyactually derives from teeth. we have thousands of teethin the fossil record.
we really only havea few hundred skulls in the human fossilrecord, and if you want to study a skeleton,you're out of luck. there are very few skeletonsin the human fossil record. so teeth representthe best evidence we have for our early history. tonight i want to giveyou a little context about the research that mycolleagues and i have done. and i'll start out firstby just giving us a 101
perspective on human evolution. and then we'll move intoa better understanding of how teeth growand develop, and how we can use them tounderstand the evolution of our own development. first of all, ijust want to remind everyone we are primates. we often think ofourselves as humans, as hominins-- apes, maybe,if we're being generous.
but to be fair, we'reprimates as well as mammals. and so many of us use acomparative perspective to understand our owngrowth and development-- what makes us unique,as well as what patterns of growthand 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 evendeeper back in time,
our primate history is a70 million year record. so, many of us spend parts ofour career better understanding how we are situated relativeto other living primates. and i want to make a pointmany of you already realize. we are odd primates. we not only dress our babies upas other mammals, but aspects-- --of our growth anddevelopment are quite unique in a comparative sense. so when you look across apes,when you look across primates,
you realize thathumans are unusual. we wean our offspring very earlyrelative to other primates, specifically great apes. we have a shortinter-birth interval. what that means is that wehave one infant and then a second infantwith 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. however, we then have along childhood period. our infants spend aconsiderable period of time growing anddeveloping, and they don't even begin reproducing andcreating their own offspring until relativelylate, when compared with other apesand some primates. add all that up.
we then see, in humanhistory, that we have a very longpost-reproductive period, which makes us unique. human females go througha period of menopause, and then continue tolive for several decades after this point in time. this, again, is a uniquepattern when we look at comparisons with other apes. and finally, thistogether means that we
have a very long lifespan. something is quite unusualabout the way we're growing and developing,and the way that we're timing these differentkey life history events. one of the questions many ofus have been trying to answer is when did thesekey events evolve? where in our pastdo we see evidence for our long childhood,for example-- for our early ageat weaning, for
our post-reproductive period? can we find thesein our human past? here you see more ofa contemporary view of human evolution. you see a bushy tree. you see a number of differentspecies of human, hominin, throughout time, through aboutseven million years here. you're seeing differentgroups of hominins. you're seeing an early groupwhich lived roughly seven
to four million years ago. and we'll have a look at onemember of that early group, ardipithecus. this is an enigmatic genuswhich has a few species in it. one skeleton we know fromthis early hominin group-- again, living about 4 and1/2 million years ago. and in many ways, thisearliest group of hominins shows a very primitivemorphology or skeletal pattern. small-brained,short-statured, long arms,
curved fingers, weird toesthat splayed out-- not what you would expect in termsof an early human ancestor. as we pull the recordforward, and we go into a group ofhominins that many of you are familiar with-- theaustralopithecines-- we again still see relativelyprimitive characteristics in terms of theskeletal morphology of these australopiths. many of you are familiarwith this skeleton
here, the lucyindividual from tanzania. and many of you are alsoaware of the footprints that were found in laetoli, thesefossilized footprints laid down in volcanic ash. we know that thesehominins were bipedal. they walked upright. we know that they,again, though, had relatively smallbrains and small bodies. so though they walked likewe did to some degree,
it doesn't quite look like,morphologically, they really were exactly like the patternwe find in our own genus and species today. when we move into the fossilrecord of the genus homo, one of the charismaticindividuals that many of us think of is thisstrapping youth, this fossil from east africa,known as the nariokotome boy. this skeleton is the mostwell-preserved skeleton we have in the earlier partof the human fossil record.
this individual wasover 5 feet tall, and in many ways has ananatomy below the head that looks very much likeour own-- long-limbed, as you can see--long lower limbs. and this individual,again, still had a relatively small braincompared to our own brain. but below theneck, more or less, could be exchanged foranyone in this room. finally, one of the key groupsof earlier hominins that
existed, contemporarywith us and before us, were the neanderthals. and this is one of the mostwell-known fossil species. we have hundreds ofparts of skeletons-- of teeth, of skulls. we have over a hundredjuveniles in the fossil record. we finally have a largenumber of individuals to really understand somethingabout their variation and how they compare to us.
many of you also knowwe now have ancient dna from the neanderthals as well. there is available informationon the entire genome. it's given us newinsight into the fact that there was interbreedingbetween contemporary humans and neanderthals. and i'd be happy to talk moreabout that during the question and answer period. tonight, though,i want to tell you
a little bit abouthow neanderthals grew and developed. and i'll come back to thatpoint in just a few moments. finally, many of ushave speculated perhaps it was the origin of ourown genus and species, homo sapiens, that wasthe place that we found the first evidence forour own life cycle, for our characteristic patternof growth and development, that makes us really uniqueamong all living primates.
was it, perhaps,our use of tools? was it our big brains? was it the very complicatedsocial structure that we're partof today that may have driven a period ofgrowth and development that is so long andunusual, uncharacteristic? well, how do we use teethto better understand these questions? why would someone spent 20years studying fossil teeth?
what kind of informationcan we extract that can betterhelp us really get at where these points of timeshow these key transitions? i'll give you a clue. this was one of the mostexciting areas of research from my perspective, becausethere is an intimate record of growth and development. and the clue here-- youcan see my thumbprint. the clue is a tiny clue.
on this cast of aneanderthal canine to the left-- thisblack cast of a tooth-- you can see some tiny ridges. it turns out that teeth havetiny timelines inside them. this is a tongue twister--teeth have tiny timelines. locked inside all ofthe teeth in your mouth are biological rhythms, whichare reflective of your growth and development throughoutyour entire childhood. these tiny timelinesare not unique to teeth.
in fact, many of youknow about them in trees. when you cut a cross-sectionof a tree trunk, you recognize ringsthat represent time. as it turns out, many hardtissues show timelines in them. bones, teeth, molluskshells, otoliths-- which are these earbones in fish-- basically anything that's calcifying showsa record of its development through time. we may know about trees,but very few of us
know about our own teeth-- thefact that our childhoods are recorded in our teeth. it's a remarkablebiological system. it's highly faithful. for those of you who haven'tthought too hard and long about teeth, letme just give you a little bit of an overviewof how a tooth is built. a tooth is composedof a crown and a root. the crown is made up of enamel,which overlays the dentine.
this is a hard tissue thatthen surrounds the pulp. many of you arefamiliar with your pulp. you're not friendswith your pulp if you've had a root canal. that is how that tooth fruit isheld in the bone-- by cementum. it's the glue. these are the hardtissues of teeth. these are the hard tissuesthat are built to last. they don't just lastthroughout your lifetime.
they actually can lastfor millions of years. these tiny records ofgrowth and development, that are created whileyou're growing and developing during your childhood,are permanently recorded in tissues that canlast for millions of years. let me orient you tothe type of approach that we use when wewant to understand this record of growthand development in teeth. what you're seeing here is achimpanzee molar on the top.
it hadn't finishedforming its root before this individual died. in the middle, you're seeinga cross-section of this tooth. you can see the enamel,the lighter color tissue, overlaying the dentineand some of the root. and then in thelower corner, you can see microscopicgrowth lines. how do we recognizethese tiny timelines? well, what we do in my lab is weprepare thin sections of teeth
very carefully. it takes many hours to preparea very well-polished, very thin, 1/10 of a millimetersection of a tooth. when we put that section of atooth, that microscope slide, underneath polarizedlight, we can see these very fine biologicalrhythms inside the teeth. this is a cross-sectionof a molar tooth of a chimpanzee thathadn't finished forming before the individual died.
what i want youto notice here are a number of these stackedlines that you can see. these lines are like thesetrees, these ridges in trees. what we're seeing here arethe successive progression of the development of thistooth, laid down and registered so you're seeing herethe way this tooth grew, both outward anddownward simultaneously. so you can reconstructits formation from the very beginning of itsgrowth to a later point in time
by simply recognizingthese lines as they were being produced. not only do teethmineralize their development on a very fine scale, theyalso show daily rhythms as they're forming. the cells that are secretingthe enamel on the dentine secrete on a 24-hour rhythm,and they lock that process in. the reason that we'reconfident that there are these 24-hour rhythms isbecause of experimental studies
that have used bio-markingtechniques to give an individual-- inthis case, monkey-- a marker, and then a fewdays later, another marker. and so we're able topick up these markers in the individual's tooth thatwas growing and developing, and relate that to the numberof these little growth lines. so in this particularindividual, 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, andparticularly if you squint, is light and dark bands. there are actually eight ofthese light and dark bands between these two biomarkers,and these two biomarkers were administeredeight days apart. this is the kind of evidencethat people have marshaled, both in primates aswell as in rodents,
to be able todemonstrate that there are these faithful24-hour rhythms-- again, in teeth as well as in bone. so you have a clockrecording your life as you're growing anddeveloping in your mouths. not only does each day record,but your birth is recorded. that stress, the physiologicalstress of being born, actually leaves behind anaccentuated growth line. you can see that here.
this is an orangutan thatstarted forming its first molar just a couple weeksbefore it was born, and the stress of birthregistered as a permanent line in this individual's mouth. and this is true forall of us, as well. we start forming ourfirst molars a few weeks before birth, as well asall of our baby teeth, and that event ispermanently recorded. so you all have abirth certificate--
--in your mouths, asdo all the fossils that we are interestedin studying. so this is really a very preciseway of not just understanding the amount of time atooth took to grow, but actually being able to getat an age of an individual. to walk you throughhow this works, i'll show you-- this isthe chimpanzee from before, in cross-sectionwith magnification using the microscope.
and what i was able todo in this individual was i was able to findits birth line with very high-resolution imaging. i assigned that day 0 there. and then i was ableto count each one of these 24-hour rhythms, upthrough developmental time, and assign ages to stress lines. this was an individualthat lived in west africa, in the wild, andin its early life
it went through some periodsof developmental disruption. and i was able to assignages to these disruptions. and then i was able tocontinue counting time through the crown,into the root, and i was able to pick up a fewmore stress lines in the route before this toothstopped forming. and i estimated, based onmy counts and measurements, that this individualwas 1,396 days of age. and i didn't actuallyknow how old it was.
but when the fieldnotes were presented, it became clearthat this individual was 1,372 days of age. so i was 24 days off fromits actual age at death. so that's a 2%error in this case. this can be a veryprecise and faithful way to assign an ageto an individual when we don't haveany other information. so it can be a verypowerful technique.
it's also very time-consuming,in full disclosure, and perhaps you have to be alittle obsessive-compulsive to count 1,396 days. so this type of approachallows us to better situate human growth and development,not just by comparing ourselves to other fossil hominins,like neanderthals, but to be able tocompare ourselves to our closest livingrelative, the chimpanzee, or to other primates--for example, to macaques.
so my colleagues and i havespent a good deal of time characterizing how teeth growin these different primates, again, to better understandwhether our development is unique or whether it's aprimitive pattern shared with other primates. we've also started to understandhow these closest living relatives, thechimpanzees, erupt their teeth into their mouths. and i want to justhighlight some work
that i've done witha colleague who's here tonight, zarin machanda. we've spent severalyears collaborating with richard wrangham andcolleagues at the kanyawara field site to conduct aphotographic study of tooth eruption in wild chimpanzees. and we worked with a couplesemi-professional photographers who gave up a fewyears of their lives to take photographsof chimpanzees.
and what they did was theycaptured tooth eruption in 25 sub-adultsfrom this community, whose birth was known. and they followed someof these individuals for up to three years. and we were alsoable to understand how their dentaldevelopment related to their maternal behavior, aswell as their feeding history. so for example, wewere able to establish
there was a relationship betweenwhen their teeth were erupting and when they were transitioningonto an adult diet. let me just show you the kindof data we were able to take. and first i'll show you avideo, because these are just amazing to watch. this is a veryspecial field site, and we were able to get quiteclose to the individuals until we were able to reallysee their dental development through video as well asthrough high-resolution images.
so this is a video just to showyou a couple juveniles, who are main focal animals. and you can see theyactually open their mouths quite frequently. they will yawn. they will open theirmouths during play. they're play fighting here--they're biting each other. and our photographers wouldwork with these individuals and just very patientlywait until the right moment
and then captureimages such as this. this is, i think, our favoriteimage from the entire study. this is azania. this is azania's first molarserupting into her oral cavity. this is the firstdocumentation of this particular developmentalevent in our closest living relative in the wild. this is happening at three yearsof age in these individuals. in most of us, this happensat about six years of age.
so chimpanzees are showinga more rapid pattern of growth and developmentfor their first molars. it's not justtheir first molars. it's their baby teeth as well. we followed anotherfemale infant, buke, from her second monthof life-- here she is, lovely individual, where youcan see her baby teeth are just cutting the gums here, incisorsand her premolars there-- up through 2.8 years of age,when her first molars
are erupting as well. chimpanzees are eruptingtheir primary dentition, or their baby dentition, atabout half the age that we are. by about a yearand a half, they've erupted their entire deciduousdentition, or their baby teeth, whereas modern humanstend to require about three years before ourfull complement of baby teeth are erupted. so this is relevantbecause we want to better
understand, again, what ourclosest living relatives' growth and development was like. we often infer thatthe earliest members of the human fossil recordwere like chimpanzees. but until this work,we didn't actually have a good handle on whatwild chimpanzee development was like. so we're able now tocharacterize tooth eruption across the life courseof the juveniles
that we were able to study--and these are 25 juveniles. and there's a lot ofdata on this slide. i just want to show you, forexample, the first molars here, in both wild and livingcaptive primates, are erupting between about twoand four years of age here. human first molars,again, tend to erupt at about six years of age. so these chimpanzees--again, growing and developing their firstmolars much more rapidly.
similarly, their secondmolars here-- again, forming and developingabout half the age that our second molarsare erupting out into our oral cavity. and their third molars showan even greater offset. humans-- we falloff 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 betweenabout 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 thepattern that we often infer was more of an ancestralor primitive-like condition. so this brings me toone of the key questions that we've been trying toaddress, which is, where do we find this transition fromthis more primitive pattern
of growth and developmentto what we see today, that characterizes humans? it's fairly well-establishedthat our tooth development is correlated with our overallgrowth and development. our period ofchildhood tends to be related to our period oferupting our final molars. and by the timethose come out, we tend to call ourselves adults. this seems to be true as wellfrom a chimpanzee perspective.
chimpanzees beginreproducing at about the age their third molars erupt. so this is a marker ofthe duration of childhood. if we know somethingabout how long it takes to grow your teeth, youcan say something about, again, the length of your childhood. so in order to try toaddress this question, i was fortunate enough to teamup with a colleague in france. his name is paul tafforeau.
he's an incredible scientist whouses synchrotron x-rays to be able to virtually goinside mineralized tissue on a microscopic scale. we've been working togetherin france for a decade. and the facility--you can see here, it's in the foothillsof the alps. the synchrotron is this ringin the photograph, which was taken from a nearby mountain. it's 700 meters across, andit's a special facility that
accelerates electrons to avery, very high velocity, and then bends them. and as they bend, they giveoff synchrotron x-rays. these are much more powerfulthan medical x-rays, or conventional facilities thatwe use in many universities here in the us. and they allow us todo things that you can't do in other facilities. they allow us to go insidevery highly mineralized tissues
so we've taken to callingthis virtual histology. this is verydifferent than what i do in my histologylab across the street. these are three-dimensionalcubes of tooth enamel that were never physically cut. these are high-resolutionscans that were taken thatallow us to access the three-dimensionalinformation locked inside these teeth.
i'm going to showyou a video now to give you some context abouthow this technology works. and i have to show this on adifferent piece of software. here you can see an individual. this is a fossil from israel--90,000 year old individual. and we took thisindividual to grenoble, and we were able to scanit using this facility and visualize how itsteeth were growing. what we first started withwas-- i'll get this to play.
what we first started with wasan overview of its lower jaw here. and what you can see are theteeth virtually rendered here, using this three-dimensionalsoftware that allows us to visualize the inneraspects of this individual's jaw. and you can see this is a child. its permanent dentitionis still inside the bone. these are its baby teeththat have erupted outside,
as well as one first molartooth that had erupted. and so here's that firstmolar tooth coming out to us. and now you can see,with higher resolution, some of those tiny ridges onthe outside of the tooth crown. again, this isvirtual technology. we never cut this toothto be able to go inside. and now you can see somestress, as well as-- with an evenhigher-resolution approach, we can see the fundamentaltissue of the enamel.
you're seeing what arecalled enamel prisms. these are thesebuilding blocks-- they're five microns across. and we can peel throughvirtually in three dimensions and follow the courseof growth of this tooth, as well as see thesetiny daily lines, which you're seeing-- these light anddark bands here in the video. so this approach allows usto be able to assign ages to individuals without havingto cut or break any teeth.
and this has then given usaccess to many more fossils than we ever would have beenable to study in the past. as you can imagine, forsome of these fossil species there's only one juvenile,maybe two juveniles. it's absolutely impossibleto convince a curator to allow me to cut upa couple of their teeth to count tiny lines inside. and now we don't need to. we have a new approach.
so by applying thistechnology, we first studied this interesting fossilfrom north africa, from a site called jebel irhoud. and we first started justwith a tiny chip of enamel. we took this tiny chipof enamel in the middle here to the synchrotronin grenoble. and we were able tofind this key piece of developmental informationthat we could only see internally.
and we used that to reconstructhow old this individual was when it died. you can see from the jawbonehere of this individual that its firstmolar had erupted, its second molarhad not erupted yet, its canine tooth also waslocked inside the bone, but it's incisortooth had erupted. so by modern humanstandards, this individual should have been aboutseven to eight years of age.
by using this faithful recordof growth and development locked inside, we were able toassign the age of 7.8 years to this individual. so this fossil-- 160,000year old fossil-- actually grew and developed like we did. and it turned out thiswas the first fossil we've studied that showed amodern human pattern of growth and development. we then thought, all right,let's look more deeply
at the neanderthal. so let's look at our cousins. everyone wants toknow, were neanderthals growing and developing like us? they certainly had largebrains like we did. they used technology. did they grow anddevelop like we do? well, our firstindividual that we studied was an individual from belgium.
and i'm going to come backto this individual later in the talk. this is its lower jawbone here. in this case, i actuallydid physically section one of the teeth, and i wasable to work out its age. and this individual,we estimated, was eight years of age. and this is give or takemaybe a month or two. and this individual showed avery different pattern then
the homo sapiensindividual from morocco, which you can see here. the moroccan homo sapienshas its first molar erupted, but again,that second molar-- deep inside the bone, noroot formed, absolutely nowhere near erupting. the neanderthal, however, showeda first molar erupted and worn down, and a second molar is wellerupted and coming into place-- basically at thestage you'd expect
of a modern human 12year old or so to be. so this shows a verydifferent pattern of growth and developmentthan the fossil from morocco. we would argue this is anadvanced pattern of growth and development-- moresimilar, in fact, to something like a chimpanzee thanto a modern human. what about other neanderthals? maybe this was just aweird belgian fossil. well, when we addedadditional individuals,
we found a similarlyrapid pattern this is an exquisite maxillaof a baby-- you can see here on this specimen holder. tiny little babymaxilla-- rendered in the middle withx-rays so you can see the stage of growthof all of its teeth, as well as a veryhigh-resolution image of its lower first molar. originally, peoplethought this individual
must have been about fourto five years of age, just given itsoverall development. and it was a remarkable fossil. first of all, this is thefirst 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 appreciationfor human evolution. so they thought it wasweird, stuck it in a box, set it aside for a few decades.
they stumbled back overthis a few decades later, and they realizedthis shared affinities with some materialcoming out of germany from the neander valley, whichgave its name to the homo neanderthalensis. and they recognized thatthis, as it turned out, was the very firsthominin ever discovered. this baby maxilla has anassociated neurocranium with it.
that neurocranium has acapacity of about 1,400 cubic centimeters. that's larger than mostof us in this room today. so this little baby maxillagoes with this major brain, and if it was the case thatthis died at four or five years of age, it's apretty rapid period as it turned out, countingthese tiny timelines, we estimated thiswas a three-year-old. so by three years ofage, this neanderthal
had grown a brainbigger than most of us. again, a very rapid patternof growth and development-- not like the modern condition. it takes several more yearsto reach a cranial capacity that size, if someof us ever get there. we compared other neanderthals. we created a comparison, whichi'll show you in a moment, looking at individualsfrom three years of age up to 12 years of age.
so here is thatengis neanderthal. and what you're seeing hereis our histological estimate of its age plotted againsthow old it would be, conservatively, bymodern human standard. so this individualis actually younger than you'd expect it to be. the eight-year-old individual,again, also younger than you would expectit to be-- this was the otherbelgian neanderthal.
so all these neanderthalsare basically showing a more rapid patternof growth and development then living humans-- in greencircles-- or fossil homo sapiens here. so here's ourindividual from morocco, as well as ourindividual from israel. those two individuals fall righton the modern human condition. but the neanderthals areshowing something more rapid. they're offset from us.
what about other hominins? what about lucy's child? what about someof these enigmatic juveniles that havebeen turning up lately? perhaps you've heard aboutaustralopithecus sediba-- very interesting south africanhominin-- multiple skeletons for this individual aswell as a juvenile skull. well, we're in theprocess of analyzing it, and i can't give awayits age quite yet.
but i can tell youthat is actually similar to otheraustralopithecines. when we did a large analysisof over 16 individuals from east and southafrica, we found an even more rapid patternof growth and development. you can see that here--this is a similar plot. it's a little busy. these are ages for fossilsthat we estimated using tooth histology, countingthese tiny growth lines,
and comparisons againstmodern human growth standards. and they're plotted here againsthumans, in green circles-- chimpanzees, in red circles. and what you can see is that anumber of these australopiths actually show even morerapid growth and development than chimpanzees,whereas others overlap to some degree with chimpanzees. but none of them show modernpatterns of tooth growth and so we wouldargue none of them
show our characteristicallylong childhood, our late age at first reproduction. i just want to turn,in my last few minutes, to a related area ofresearch that i'm also very excited about, that iundertook with a colleague who's in the audience tonight. and this is a studyof early life diets. and this is a key aspect ofour growth and development. again, we talked about wehave an early age at weaning.
it's earlier than one wouldexpect, given how large we are. when you compare us tochimpanzees, or orangutans, or gorillas, for example, wehave roughly half the time devoted to providingnutrition to our offspring. and so one of thequestions has been, well, when did that pattern evolve? and as it turns out, teeth canhelp us get at this as well. and so the work thati'm going to show you is collaborationwith a colleague
in new york, whoformally was at harvard, at the school of public health. and he's developed amethod of actually mapping the elements inside teeth. and so this is animage showing you a cross-section ofa human baby tooth with basically a 30micron raster, or laser, going across histooth, and giving you a map of the elements thatare present in that tooth.
and in this case, you'reseeing trace elements, things that are found in low levels. this is particularly interestingfor people in public health who want to lookat lead exposure. they want to know,for example, were children subjected tohigh levels of lead during development, becausethat has, obviously, serious consequencesfor later life. this is also interesting forevolutionary anthropologists,
because we can track theincorporation of key elements that are found exclusively, orto particularly high levels, in mother's milk. so many ask, can we useteeth to get at diet? and in fact, yes, we can. we can use the recordof time, coupled with the record ofchemistry, to be able to look at transitionsin our early life diets. this is a map showing you theintegration of these two ideas
so the work i do lookingat the birth line, of the neonatal linein a tooth crown, is integrated herewith chemistry. and so i'm showing you amodel for the incorporation of one particular traceelement called barium. barium is something that'sfound in the environment in very low levels. it's not particularlygood for us in high levels throughoutour life course,
but it's not something that wetend to take on in our diets. before we're born, bariumis found in very low levels in the fetus, in the embryo,because the placenta blocks the transfer ofthis trace element. after birth, however,barium levels increase because of theinput in mother's milk. barium is transportedacross the mammary gland, much like calciumis transported. it's enriched in mother's milk.
and so we have a markerof maternal input that's recorded inyour teeth while you're growing and developing. and so just after birthhere-- 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 isfor a short period of time after birth, youhave an enrichment.
you have a highamount of barium, which is due to exclusivematernal milk input. and as you then transitiononto a mixed feeding regime, or you have supplementalfoods introduced, your barium levels go down,until eventually there is no more milk beingprovided to an individual, and you go back tothis low level here. this is the model we canuse to basically interpret the record of early life dietin an individual's tooth.
so we did this verything using, first, monkeys that werefrom a captive colony. and this is a busyslide-- i just want to take you throughthe basic pattern in here. these are first molarsof monkey teeth, which were cross-sectioned. and i was able to workout the time of formation, again, using thistemporal approach, using all these tiny lines.
and my colleague was ableto map out their chemistry. and my other colleaguewas able to give us records of their growthand their behavior-- when their mothers, for example,were exclusively feeding them, and when they started totransition onto monkey chow, in this case. and what you can seehere is that, again, you have these warm colorsjust after birth, representing an exclusiveperiod of mother's milk.
and that barium, by aquantitative standard, ramps up for the firstfew months of life as the mother is providingmore 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 representativeof that prenatal level. so we have a marker in teeth,again, of our early life diet,
and we have a marker ofwhen that diet is disrupted. this is a summaryof the points i've made about these key regions. but i want to drawyour attention to the lower panel here, becausethis individual unfortunately was quite ill duringits early life. and i was able to mapa number of stresses this individual experienced,as well as estimate its age. in this case, i was only one dayoff from its true age at death,
so i was able to come upwith a really faithful record of its stress, as well assome of the key periods where it had to be hospitalizedbecause it was just so sick. and it turned out that at 166days of age, this individual had to be separated fromits mother for a few weeks, treated in the hospital,and then released, and at which point the motherwas no longer producing milk. and so this individualexperienced an abrupt weaning event.
and you can actuallysee that here, because the color goes fromgreen-- this transition diet-- back down to blue. so we're basically seeingan artificial weaning event in this monkey becauseof its illness. this is pretty exciting, becausethis gives us more access to an individual's early life. it was also excitingto us because we then extended this approach tolook at that neanderthal--
the one that i had cutthat first molar from. well, i was able to look atthis belgian neanderthal-- and this was the eight-year-old. and i was able to lookat the map i created of its first molar,starting at birth, and registering stressthroughout its first 2.4 years of age. and then we were able to mapthe chemistry of this tooth, and integrate the time of itsformation with its chemistry.
looking here at the enamel,what you can pick up are a few key regions here. first, in blue, we'vegot a 13 day window-- i'm going to give youthe time, here-- 13 day window of prenatal formation. and then you geta ramping up here, a very high level of bariumfor the next seven months. this is what we think was aperiod of exclusive mother's milk in this neanderthal.
and then at seven months,we see a transition. we see a decrease to what lookslike a mixed feeding regime. that's the green parthere on this tooth crown. and that also lastedfor about seven months. but in this individual, we seea very abrupt falling off here, which corresponds to theblue in this elemental map of this individual. and this looks justlike that macaque that was separated from itsmother at a 166 days of age.
so we think whatwe're picking up here is a very abrupt weaningevent in a neanderthal. and that correlateswith a stress line at 435 days ofage in this tooth. so we see here what wethink the first evidence for early life diet transitionsin the human fossil record. but what we think we see hereis something quite unusual. we don't necessarily thinkthat all neanderthals stopped nursing at 1.2 years of age.
we think that this individualexperienced something-- led to maternal separationor the cessation of suckling. and that individualcontinued living until eight years of age. so i just want to leaveyou with a few points before we have a fewminutes for questions. i hope that, if nothing else,i've really convinced you that teeth have thisintimate record of our growth and developmentlocked inside them.
we can recognize our own birth. we can recognize stressduring development. we can recognizeperiods of illness. we can also assign ageat death to individuals that died while they werestill forming their teeth. and we can do thiswith very high fidelity when we're able toaccess this information. when we look comparatively--we look at our closest living relative as wellas other primates--
we really can hone inon what is particularly unique about our owngrowth and development, and what do we think might be amore primitive shared condition when we look using this newtechnology at neanderthals, as well as earlieraustralopiths, we don't see thisclear transition from this small-brained,short-bodied, early hominin to this tall, large-brained,late-developing hominin in one fell swoop.
this is a mosaicperiod of growth and of change throughthe fossil record. it looks like ourlengthening childhoods were one of thelast key features to come into play, probably withthe origin of our own species, but certainly not in the firstmembers of the genus homo. and finally, we're reallyexcited about this new approach to be able to putinformation together, from elemental chemistryalong with temporal mapping,
to be able to betterunderstand early life exposures to variouselements in the environment, as well as to key elementswithin mother's milk, such as barium and strontium. so i want to just stop bysaying that no animals were harmed in the production-- --of this research,although you may wonder how i got all these deadmonkeys and apes. all of the individuals diednaturally of natural causes.
i'm very grateful to themfor giving up their teeth, as well as to my colleagues whohave been amazing collaborators along the way. and i want to thank themembers of my lab group as well as a number of curatorsand the funding agencies that have madethis work possible. it's a team effort. everything that i'veshown 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 tosome robust discussion. thank you.
Tidak ada komentar
Posting Komentar