David Reich: Ancient DNA and the New Science of the Human Past | Town Hall Seattle

okay now on to the actual introduction david reich is professor of genetics at harvard medical school and a howard hughes medical institute investigator with a reputation as one of the world's leading pioneers in analyzing ancient human dna in a 2015 article in nature he was named one of 10 people who matter in all of the sciences for his contribution to transforming ancient dna data from niche pursuit to industrial process that was a quote awards he has received include the newcomb cleveland prize for the american association for the advancement of science as well as the dan david prize in the archaeological and natural science sciences for his computational discovery of intermixing between neanderthals and homo sapiens his book that he's here to discuss tonight is who we are and how we got here ancient dna and the new science of the human past please join me in giving a big town hall welcome to dr david wright um thank you it's a pleasure to be here and um to be visiting your beautiful city for these two days i had a beautiful flight over from the east coast and beautiful clear weather and it's really magnificent here as you know um so i wanted to talk about uh uh the ancient dna revolution which has unfolded over really the last eight years and maybe even really within the last four years um which is really profoundly changing our understanding of who we are how we're related to different people in the world today and how each population in the world that exists got to where it is today and i think if there's a single take-home message of this talk it's that profound mixture of people who are very different from each other it's not just a phenomenon in the last 500 years in the americas but is in everybody's history and no population is or could really ever be pure in any sense so i'm going to begin by talking about the disruptive power of ancient dna um and by someone who for me is really a mentor or not a mentor exactly but like an inspiration luca cavalli sports who died last month and in 1960 he made a bet that it might be possible to reconstruct the history of human populations around the world how they got to where they are today by comparing the genetic variation in groups living today to each other and and the idea was that if people moved to each place in the world where they are today and just stayed there after that point without mixing very much then the people who live in each part of the world today should be direct representatives of those ancestral population and by measuring how closely related people are to each other we can build a tree of relationships and determine the branching pattern that brought people to each part of the world today so on the basis of this type of idea he analyzed genetic variation beginning in 1978 from uh diverse european people and looked at the type of genetic data that was available at the time which was protein polymorphism data so this was blood group variation so for example your blood group system or your rhesus blood group system that's measured in your blood and that people look for determining antibody matches and by looking at the frequencies and how they differ in different populations around the world he saw that the primary gradient of variation in europe today moves in a southeast to northwest direction like you see in this plot and he interpreted this as the genetic imprint of the movement of farmers into europe which we know from archaeology occurred sometime after 9 000 years ago from turkey or anatolia and moved eventually to the british isles within a few thousand years and so what he thought he was seeing is the dilution of ancestry from those first farmers from turkey by mixing with local hunter gatherers as they moved through the region now with ancient dna when we can actually measure the proportion of farmer ancestry you see the proportion of farmer ancestry is actually highest in southern or even southwestern europe and decreases in the opposite perpendicular direction and that's because people have moved that's because there was a later major event or multiple events that have changed that pattern and so it's actually not possible to reconstruct human history based on patterns of variation today because people have moved and removed and mixed and migrated again and again and again i'm going to tell you some evidence for that so i'm going to begin by talking about this new scientific instrument of ancient dna and it begins with an ancient human skeletal remain for example a bone or a tooth in a clean room where the goal is to protect this tooth from the people who are handling it the archaeologists and the lab people who are handling it who have much more dna than is still in the bone and a little bit of dna from you can overwhelm the dna that's left in the sample so we take a number of measures like gloves and face masks and clean suits and purified air and positive pressure and ultraviolet light to protect the sample we drill beneath the surface of the bone with a dental drill or another type of implement we extract powder we release dna from the powder in a watery mix that removes the protein and mineral content and we convert the dna fragments which are short and degraded but still long enough in some cases to sequence into a form that can be sequenced in one of these modern sequencers in the late 2000s there was a jump in dna sequencing technology which made it possible to sequence dna for literally a million times less expensively than it was the case two decades ago and this has made it possible to whole genome sequence modern humans but also ancient humans so here's a not related to genetics but moore's law which many people here might be familiar with which is the doubling of density of uh transistors or or or other operational elements on electric circuits over the last century or so which has doubled every one or two years and has driven a lot of the computer revolution so there's an even more dramatic increase in ancient dna data the first ancient dna was published in 2010 it reached 10 in 2012 it passed a hundred in 2015 and only this year it reached a thousand and there's a lot of unpublished data as well and so this orders of magnitude increase in the amount of data makes it possible to ask and answer questions about the past that simply were not possible to answer before so i'm now going to sort of briefly give a little bit of information about how the genome reveals the past and can be used to reveal the past and so the data that we're looking at to look at human history can be thought of as follows so you see on the left of this picture there's a picture of a human cell in stylized form inside of which is a nucleus which contains your 23 pairs of chromosomes so you get one chromosome from each of your parents your mother and your father and um for example and and inside a cell is a nucleus most cells have 23 pairs so here's a blow up of those 23 pairs and if you blow it up even further you'll see the famous double helical strand and this will be composed of a strings of adenine cytosines thymines and guanines the four letters of dna a c t and g and so there's three billion of these in all approximately and what we actually do when we look at human history is we line up two sequences for example the sequence of dna you get from your mother and the sequence of the dna you get from your father and humans are extremely similar to each other so if you look at the dna c copies you have in you one from your mother and one from your father they're almost identical 99.9 of the positions in the dna that you can line up the letters are identical but 0.1 percent are different due to mutations that occurred randomly in the past since your mother and father share a common ancestor and 1.1 percent of 3 billion is 3 million differences and that's a lot of information so what we do for example is you can look in positions in the genome where there are a few differences between your mother and father and that tells you there's not been much time since your mother and father share a common ancestor then because mutations occur at a pretty constant rate over time if you see a stretch with few differences it means that there's a relatively recent ancestor my brother and i have large sections of the genome where we have no differences at all from each other because we share immediate common ancestor on the other hand there are places where there's a high density of mutations reflecting a typical time to the common ancestor that's very far back in the past typical time to the common ancestor between two human sequences for example your mothers and fathers sequences at a random place in your genome is about one to two million years so how can the genome reveal the past so if you take your dna and you say how many ancestors is your dna coming from well it's coming from two parents so that doesn't seem like a lot of ancestors but then that's coming from four grandparents and eight great-grandparents and sixteen great-grandparents so there's this exponential increase in the number of ancestors and then the dna itself is getting fragmented into those parents it breaks about about uh 70 times per generation due to the splicing togethers of mothers and fathers dna that occurs when you produce an agarose sperm and so it starts in 46 or 47 chunks your chromosomes and your mitochondrial dna and it adds about 70 every generation so what's happening is that your genome is fragmenting into more and more ancestors adding 70 fragments per generation and by about uh 10 10 or um generations or so it's fragmented into more fragments than there are ancestors or or um and so you're actually there are ancestors who didn't even contribute any dna to you but what this is telling you is that if you go back far enough into the past you have thousands of independent ancestors and your genome is allowing you to find information from all of those ancestors so you're not one person you're actually a multitude of people and by sequencing your genome you're averaging together many ancestors and you could find quite powerful information so my talk is about four structured around four examples about how the genome revolution in the last few years is showing us that we're wrong again and again and again about our understanding of who we are so the first example i'm going to tell you about is our wrong assumption about that we all came from africa in the last 50 or 100 000 years so it's mostly true uh 98 of the ancestry of non-africans is from africa but actually there was mixture with local humans and i came into this work for example in 2007 quite convinced that the genetic data had shown that all of the ancestry of non-africans today was entirely from africa within 50 to 100 000 years in fact i was part of the scientific lineage of people who had demonstrated that although i didn't play a particular wrong role in that so the context for this is that the first skeletons of anatomically modern humans people who look like us appear in the african fossil record about 300 000 years ago and then after a hundred thousand years or especially 50 000 years ago explode out of africa into different parts of eurasia but those parts of eurasia were not empty of people the first neanderthals who are archaic humans whose brains were as large as ours who made tools as complex as our own ancestors did at the same time were spread across europe in western asia at the same time so a question was always when we found the evidence archaeologically of the spread of modern humans out of africa after 50 000 years ago was their interbreeding when they encountered each other and if there was interpreting where did it happen so what we did in collaboration with our colleagues in germany who were sequencing the genome was we developed a test for whether interbreeding had occurred and the basic test that we developed was extremely simple so i'll explain it now so if you line up a genome sequence this is supposed to represent one of your 23 chromosomes from a european person and a nigerian person and you look at a place where they differ in their dna letter at thiamine or guanine and then you lift the veil you look at the neanderthal sequence does it want match one more than the other what we see is that neanderthals match europeans more than they do nigerians it's not just europeans it's also east asians it's not just nigerians it's also any of any sub-saharan african population so this is evidence of gene flow which we established in various ways specifically of neanderthal contribution to the ancestors of all non-africans in about the same proportion so we replaced uh in this analysis we compared a nigerian to a french person um but in uh and then we could then we replace the the the french person with a second neanderthal and we asked how much of the skew that you see in a french person is compared to what you see if you replace it with a second neanderthal and that allows you to say how far of the way is a french person or another non-african person to a complete neanderthal and when you do this it's about two percent of the way and so this allows us to obtain an accurate and correct we think measurement of the proportion of ancestry of non-africans that's from neanderthals it's about two percent but significantly higher in east asians than it is in europeans which is a real effect and we know what explains it i could tell you afterward what explains that so once we had this observation we wanted to know how long it's been since the mixture occurred between neanderthals and modern humans and the way we did this is again by looking at the segments of dna so here in population two it might be neanderthals is green population one is red there's two stripes here which are the chromosomes in which your dna is packaged for example two copies of your chromosome three if you have a kid with someone who's a neanderthal you'll have that kid will have one entirely neanderthal one entirely modern human chromosome and when you produce an egg or a sperm you produce spliced together chunks of your parents dna and you get these mosaic patterns and many generations later you just chop it up and by looking at the dice size of the chunk size of these fragments you can learn how long it's been since the mixture started between these two parents it actually is a very accurate way of estimating the age since mixture so when we analyzed this and looked for the sizes of segments of shared ancestry between nigerians and neanderthals they were very small measured on this plot of the length versus the size of the ancestry but in french people it was much larger reflecting the recent mixture and then we also had data that we later gathered from a 40 5 000 year old modern human who lived close in time to the mixture event and for this individual that individual lived within five to ten thousand years of the mixture so by looking at the sizes of these segments of sharing with neanderthals especially of ancient individuals who were close in time to the event we now have a rather precise date of when the neanderthal mixture occurred into modern humans which is between about 54 to 49 000 years ago that's when it happened so the conclusions is that at that time 54 to 49 000 years ago there was interbreeding from neanderthals into the ancestors of all non-africans and that population spread out across eurasia bringing the neanderthal admixture with it to spread throughout the rest of eurasia so in 2010 just as we were publishing these findings about neanderthal we also our colleagues in germany obtained dna from this place in siberia southern siberia denise of a cave a finger bone of a little girl we could tell it was a little girl because the growth plates were not fully formed in the finger bone so it was a child and you can tell it's a girl because of the dna it has no y chromosome and two x chromosomes and there was also a big tooth from an adult that had dna similar to the finger bone when we looked genetically at how closely related it is to the other neanderthals and to the modern humans it's more closely related to neanderthals than to modern humans but not particularly close to neanderthals this denisova specimen it's actually separated by many hundreds of thousands of years from neanderthals so what we had identified was a new population neither neanderthal nor modern human but a third population altogether so this is very exciting and amazing and of course we did the same test we took different pairs of modern human populations like people from new guinea people from china and other pairs and we said does denisova match some more than others and when we did this analysis we saw that denisova matched specifically new guineans more than any other group and it was a highly significant effect and when you further did more analysis we could estimate that this denisovan population or one distantly related to it contributed four to six percent of the ancestry of new guineans and nearby populations and very little ancestry in the mainland so this is the proportion of denisovan ancestry we subsequently estimated in diverse populations around the world it's five percent in new guinea and this is so a full pie represents five percent and maybe a 20th of that in east asians and almost nothing in europe and in africa so the conclusion is that neanderthals mixed into the ancestors of non-africans as they spread 50 to 100 000 years ago out of africa then there was a further mixture on the way to people in new guinea of four to six percent just published not by us but by another group a month or two ago by this group in germany is another sample from denise of a cave who's uh of an individual who has a neanderthal mother and a denisovan father and so this is an inter individual who's in the result of interbreeding between group two groups separated by many hundreds of thousands of years we estimate four or five hundred thousand years and produce this child who happen to have leave left a bone that has been sequenced and we have their full genome sequence so what's actually happened with this ancient genetic data is it's opened a whole pandora's boxes of mixtures that we didn't know about it identified the denisovans that we didn't even know about before there's no clearfully documented fossil record or archaeology of them the way there is with neanderthals but we now know about them and we know that they interbred in ways that we had not anticipated before we've identified multiple and mixture events between different modern human denisovin and neanderthal and lineages altogether different like this one that's contributed to the denisovans and so it's quite clear that the past 50 or 60 000 years ago was one full of archaic humans and modern humans interacting with each other and when they were in close proximity producing children so that was the first example of how we were wrong about our understanding of the past the simple understanding being disproven by the genetic data once we look at it now a second lesson is uh given by the history of west eurasians or whites so what we uh which is that the population that people think of as whites simply didn't exist 10 000 years ago um so if you look in western eurasia uh uh europe um the near east central asia today genetically people are pretty similar to each other the level of differentiation between people from iran and people from england for example is maybe a tenth of that between europeans and east asians and that's given rise to the idea that these people might be broadly part of the same group so we collected and published in 2016 data from diverse people from the last 10 000 years across this region and if you measure the average differentiation 10 000 years ago on this top row between these people across this region on this scale which is just the average frequency difference of these genetically variable positions between two populations it's about 0.1 which is about the level between europeans and east asians so 10 000 years ago across west eurasia if you take random populations the level of differentiation was quite large similar to europeans and east asians today it's a tenth of that and it collapsed by a factor of three already by six thousand years ago and then it collapsed yet again to approximately present day levels by about 4 000 years ago and so how did that happen one possibility you might imagine is that of the ancient populations that exist one expanded and displaced by the others that's not what happened what happened is this so we now have pretty accurate models for the history of the ancestry of west eurasians based on all the ancient dna data we have and we know that present-day west eurasians are comprised of mixtures of four ancestral populations you can think of them some of them as iranian early farmers that we have genetic data from from about nine or ten thousand years ago uh levantine farmers people from present-day jordan in israel for example from about the same time western european hunter-gatherers so people who were been hunter-gatherers in spain or britain or central europe and eastern european hunter-gatherers from the eastern part of europe and those populations did not disappear instead they mixed with each other and the reason that there's low levels of differentiation across west eurasia today was this profound mixing that occurred so the third lesson in humility is that large-scale migration was common in our past and so there's been a prevailing view in uh archaeology uh by in many people that movement that when you see changes in material culture and the types of pots people make and the types of artifacts people left behind people have argued for 100 years about what that reflects does it reflect movements of new people in or does it reflect communications of ideas does it reflect new fads that are spreading and recently in the last 50 years there's been a lot of favoring of ideas where the changes are thought to be often due to spreading of ideas not to movements of people but of course with ancient dna you can actually look to test whether that occurred that's exactly what ancient dna likes you do you excavate an ancient grave it's associated with an archaeological culture it has some pots that are characteristic that are grave guards inside the cave inside the grave and you compare it to other ancient dna samples and to people living today and with great precision because of the large number of ancestors each person has you can get a precise measurement of how that individual is related to under their individuals that's what it allows us to do so i'll show you how ancient dna data is showing that large-scale migration was common in our past so now i'm going to give you a little bit of background on the history of europe this is not because europe is a more important place than the rest of the world it's not it's just because it's where we happen to have the most data right now three years into the ancient dna revolution um and so i'm going to tell you a story of this not as uh as an exemplar of what's possible so agriculture arrives in europe sometime after 9 000 years ago from anatolia present-day turkey arrives in greece and the balkans and then spreads to britain within a few thousand years to spain more rapidly and to sweden and so spreading farmers were seen as the most likely vector for the spread of indo-european languages another cultural phenomenon that is known from europe is not from the archaeology but rather from the languages that people speak so in europe almost everybody with some interesting exceptions like basque and hungarian and finnish and sami and estonian speak these tightly related languages that are also related to armenian and to iranian languages and to northern indian languages this has been known for 250 years it's an amazing observation and it still hasn't been explained up to the last few years genetics has made important strides in contributing to explaining why this is so so it was thought that why are the languages all the same well maybe this is because the languages were brought by the first farmers this was the prevailing idea up till three years ago but in fact there was a later made large-scale migration that's almost certainly responsible for spreading these languages and i'm going to show you the evidence for that so with ancient dna we and others had collected begin in by 2015 data from a fair number of individuals who were 5 000 years ago and earlier and all of these individuals had one of two types of ancestries primarily this blue type of ancestry which is consistent with that of the hunter-gatherers of europe or this orange type of ancestry which was characteristic of the first turkish farmers and farmers so people were a mixture of farmer and hunter-gatherer ancestry so today though when you apply the same analysis there's a third ancestry type shown in green here that had shown up and so it must have shown up somewhere between 5000 years ago and the present so how did it get there so this was a great mystery we knew about this in 2014 we knew that today europeans are comprised of three ancestral populations but five years ago they were comprised of only two so what happened so this was very exciting and we of course looked to try to see what happened by collecting samples that covered the period where we thought these transitions might have occurred so three years before this we had gotten our first clue about the existence and possible origin of the third source population for europeans and we got it in the following way so we developed a statistical test i'm a statistical geneticist by background where we was a formal test for whether a population today is mixed and the way it works is as follows so typically we look at data from a population like french people today and we look at them at 600 000 places in the in in our dna that are variable across people and we can measure the frequency of dna letter like cytosine thymine adenine so cytosine it might be 30 in a large group of french people and you can ask the question on average are the french people intermediate between any other pair of populations in your data set and if they are you can prove mathematically that the french people are the result of a mixture between at least two groups that are differently related to the two groups you're comparing them to we call this the three population test and we then with this toy this is tool that we had we applied it to all the populations we could so we had a wonderful data set of 50 present-day populations analyzed at about 600 000 positions and for each population the data set we looked at all other possible of the 50 populations and we saw do they give a signal of mixture toward this target population and when we took northern european populations like french people we got a huge signal of mixture when one population was southern europeans like sardinians and the other population of all people was native americans it was a crazy result very strange result it was very strong result and it was definitely native americans it was not siberians it was not east asians it was not south asians and we of course didn't think that it was migration across the atlantic from the americas into europe instead we argued that sometime before 15 000 years there was a population we called the ancient north eurasians we made this up so this is a population called the ancient north eurasians which we argued existed sometime more than 15 000 years ago and that population contributed to the population that moved across the bering land bridge into the americas and some people from that population also later contributed to europeans so this was the first articulation of the idea of a ghost population a theme that i return to again and again in the book which is a population you statistically reconstruct from the mixed remains of it in people living later but that you don't have access to an unmixed form so it's a statistical prediction so a year and a half later a group working working in copenhagen in denmark found this ghost population they call sequence dna from a 24 000 year old little boy uh who was buried near the shores of lake baikal in siberia which is the deepest freshwater lake in the world and this individual was a better surrogate for french people than was the native americans you can see that they're genetically related both to native americans and to europeans but not particularly closely related to people in that part of siberia today and that's the result of post-ice age movements from the south into that part of siberia that displaced that population so i'm now going to show you what we've we and others have succeeded in reconstructing in terms of a time series of population turnovers in western eurasia so here is a principal component analysis where we're taking data from about 800 present-day europeans so all of most all of these individual dots are from present-day europeans or west eurasian people and you should think of the data going into this analysis as a rectangular grid like an excel spreadsheet where there's 600 000 rows corresponding to all the polymorphic that the variable genetic positions we're analyzing where some people have an adenine and some people have a cytosine um a or a c and the columns correspond to all 800 people in our data set so it's an 800 by 600 000 grid and in each cell of the grid is a zero one or two depending on whether you have zero one or two copies of the adenine so you multiply the matrix by itself to get an 800 by 800 matrix which shows how closely related every sample is to every other sample and you perform the mathematical operation of principal component analysis which finds combinations of these variable positions that best separate these samples from each other and then you plot the x-axis is the there is the variable that most separates the samples and the y-axis is the second best variable and when you do this to present-day europeans you see there's two parallel gradients that contain most of the samples there's the one that contains europeans and there's the one that contains the near easterners and a gap in between with relatively few populations the populations that do exist in between are groups that are have known or plausible more recent contacts between europe and the near east such as island mediterranean populations jewish populations and greek populations at this end of the european gradient on the top is northern europeans on the other end is southern europeans especially sardinians in the near east this is our army our armenians at the top northern people from the caucasus and down here is uh levantine is levantine people such as people from jordan and israel and arabia so how does this arise well i'm going to now gray out these points so here's the two parallel gradients of present-day people in the near east and in europe and i'm going to see where the ancient samples fall relative to these so the hunter-gatherers of europe we have data from fall beyond europe in the direction of european differentiation from the near east and that's because europeans are a mixture of these hunter-gatherers and near-eastern nurse but the hunter-gatherers no longer exist in unmixed form then the first farmers of europe you see there's this rightward shift move and fall on top of where present-day sardinians are that's because sardinians today are a relatively isolated population that is relatively directly descended without much subsequent mixture from the first farmers who moved into that region but you still don't see people like europeans in most european countries today who are in this cloud over here there's some mixture between the farmers and hunter-gatherers who haven't completely disappeared meanwhile in far eastern europe five thousand years ago there's this formation of this yamnaya population and this population is a mixture of the far eastern european hunter-gatherers and samples that we now have data from who are here who are iranian farmers and yet you still haven't seen people like europeans today that only happens after 5000 years ago when suddenly you see people like europeans today and they come in an association with a culture called the corded ware culture which is named after the pots that people made which had decorations made by imprinted corded twine twi cords of twine and ever after in europe you see people like those living today so a summary is that europe was massively transformed by two migrations one after nine thousand years ago which brought in these first farmers and almost all the ancestry of the first people was from farmers there was some subsequent mixture between farmers and hunter-gatherers and then bang after 5000 years ago this new ancestry arrives in association with this culture called the corded wear complex that is coming from the yamnaya in some populations it replaces at least 70 percent of the ancestry of the populations who were these yamnaya so the yamnaya were a population that spread over the step north of the black of the caspian seas shown in this blue shading here beginning about 5300 years ago so they were the first people who brought the newly invented wheel out into the open steps this was an important invention and also the newly domesticated horse and they hitched their horses to wagons and brought brought supplies out to the open step lands which were economically unexploitable before because there was no water sources and they used it to graze their cattle and their sheep out in the open steplands and were extremely successful and so prior to the yamnaya there were many smaller cultures that left their settlements and different pottery types across the region that the yamuna has spread and then with the yamnaya they almost all disappeared and were replaced by this homogeneous culture the settlements too disappeared and all that's left for the most part are these big graves that they left big rich graves and it's uh the archaeologists interpret this as people moving in in mobile homes out into the open step and just moving from place to place these people spread all the way from hungary to europe to siberia to the altai mountains and left a massive impact and genetically the data is showing that they left a massive impact too so the implication also is that they likely spread languages because of this massive population replacement it's thought that major movements of people also spread languages and there is a long debate about how these languages indo-european languages spread and most people the predominant view had recently been that must have been spread by farming because how could there have been a major movement after the spread of farming into a place like europe once there were densely settled people how could step pastoralists for example make an impact i don't know how they made an impact but they did because we see it in the genetic data so it's a very profound thing indo-european languages are interesting because there's shared languages shared words all the way from india and iran all the way to all of the spoken indo-european languages for a whole set of words related to carts horses and wheels and that suggests that the languages shared a common ancestral language after the development of these techniques and really is another reason why the yamaya are a plausible vector for the spread of these languages and why it would be very hard to imagine that it was spread by farming which spread three or four thousand years before so i'm now going to tell you a little bit about the further spread of indo-european languages and in particular in britain where we have our best data so at the same time that the corded wear complex was over spreading eastern europe an equally impressive phenomenon as reflected in the pots people made and the tools people made and the buttons people buried in their burials and the whole series of ritual artifacts that were left behind in graves was spreading in western europe and it was these bell beakers were at one of the pot was named after the bell beakers these bell-shaped beautiful pots that they made that first appear in iberia in spain about 4 700 years ago and then spread within 200 years to central europe and once they get to central europe they go everywhere so they go to britain to ireland they go to central europe and they go all the way to poland and to hungary to southern into northern italy and a question that we wanted to ask is what was the ancestry that was spread in 2015 we had some samples from germany where they had spread 200 years after they originated and they were like the corded ware lots of step ancestry so we collected at the beginning of this year 400 new ancient dna samples um and uh um and many of them were from bell beaker associated burials here in red is where the bell beaker associated burials were and we looked at their ancestry and we got a surprise and the surprise is that in principle component analysis the same type of analysis i showed you before they were not all in the same place not like the yamana so different from our previous supposition it was tempting to think based on the previous work that always a genetic culture people making pots in a certain way would correspond to people who all look genetically a certain way but this is not the case with the bell beakers they fall into two clusters one over here one over here the cluster at the bottom is almost all iberian people from spain where it first developed and the cluster over here is almost all from central europe and what we were able to show is that there's almost no shared ancestry between the spanish practitioners of this culture and the central european one what that means is that the bell beaker culture spread between these two regions through communication of ideas not through movement of people kind of like cell phones spread across people of different ancestries a lot of archaeologists interpret the beaker burial tradition as a kind of ancient religion and it's one that was clearly copied in france and in hungary we even have graves where we have men and women buried side by side both with beakers on their heads next bury next to their heads but some of them have step ancestry and some of who didn't so it was clearly a kind of proselytizing belief system which allowed new people to come in so but so what we see is that there's not a lot of beak of step ancestry in spain but there is a lot in europe in central europe but once this belief system gets to europe it spreads further through migration and brings the ancestry with it we know this best in the case of britain where we have 150 people that we reported at the beginning this year between 6 000 and 3000 years ago here's a beautiful beaker burial in britain you see here's the beaker pot there's a second burial with another beaker pot in this grave and what you can see is that the people who brought this tradition replaced the population that had just built stonehenge so the last big stone built stones at stonehenge went up about 4 500 years ago and at that time the ancestry of the people was all blue it was all first farmer type ancestry but bang at 4 400 years ago every single sample has typically 90 or more step ancestry so there's a mass movement into britain that essentially almost completely replaces the local population and people in britain today are largely descended from this new wave of people and share very little ancestry from the people who built stonehenge in iberia and spain meanwhile at almost exactly the same time we see something very similar this is a different type of plot but this y-axis is measuring the proportion of ancestry from the far east ultimately from the yamnaya and what you see is that there's no change between six thousand and two thousand uh six thousand and four thousand years ago in terms of the proportion of step ancestry um but then beginning 4500 years ago a new group the step ancestry people begin to come in niberia there's a 500 or 300 year period of overlap and then the two groups mixed together the mixed population is not as dramatic as what you see in britain it's 60 from the indigenous local farmer population 40 percent from the step people but the coloring of these dots is the males in this group and we're coloring them by their y chromosome type so there are typical farmer y chromosome types which are colored here in different shades of blue and different step y chromosomes types which are colored here in red and what you see is that after 4 000 years ago the farmer y chromosomes completely disappeared it was 100 y chromosome replacement and what this is telling you is that only 40 percent of the ancestry in this population is from the step people but 100 of the y chromosomes are so what that means is that the people coming in completely displaced the local males that is they had preferential access to local females and their kids had preferend their the newcomers kept having preferential to access to local females generation after generation so it's telling you about a complex interaction about these call between these groups which could have hardly been a simple interaction so the fourth lesson and the final lesson in humility pertains to south asia and it's uh re pertains to something that has been the longest standing interest that i've had in human population history and in 2009 we analyzed modern data from 130 people from 25 groups in india and we came up with a model of south asian population history where most groups in india today are mixtures in different proportions between two highly divergent ancestral population one we call the ancestral north indian related deeply to europeans and central asians and near easterners and the other we call the ancestral south indian related deeply to east asians and indigenous other peoples of east asians and people in india today are mixtures in different proportions of these two groups we were able to quantify the proportions of ancestry from each of these two groups and they ranged from about 20 related to europeans and near easterners to 80 percent and they were correlated people with relatively more western ancestry tended to speak more into european languages and tended to be even controlling for that have traditionally higher status in the traditional caste system in india so last year we tried to look at this much harder with 250 or so groups with shown here in the dots on the map color coded by the languages people speak language groups so in india today there are many language groups but they can categorize in a few of language families so the maroon color is indo-european languages related to the languages i told you about before in europe the blue is dravidian languages which are very distantly related and we're not obvious it's not clear what the relationship is all the the triangles here are tibeto-burman languages related to chinese and there's also these austroasiatic languages in red but i'm going to talk about the indo-european and dravidian here so the hypothesis we entered into at the beginning of our work in 2016 was that the ancestral north indians the ones with the west eurasian relatedness were a mixture of step related and iranian related ancestry we already knew from work that we had done in the meantime that both of those components contributed to india and that the ancestral south indians had no west eurasian related ancestry so we were wrong about this and it was really interesting how this happened and i'm going to tell you how we were wrong so we now have this model of three sources that works as a model for india one of them is step one of them is asian related to east asians and indigenous people from south asia and the other is iranian and we were able to quantify in these diverse groups the proportions of ancestry from diverse indian groups today shown with these plus signs here and if you extend this great you can see there's a gradient of different proportions of ancestry and if you extend this gradient as far to the top of the plot as possible you get to what's expected if there's no step ancestry at all where you intersect this green line and what you notice is it doesn't intersect at this vertex of all asian ancestry like we thought it actually intersects in the middle of this line and what that means is that the ancestral south indians had a lot of iranian-related ancestry they were not a non-westerasian population they already had about 30 percent iranian ancestry and that suggests that the iranian ancestry which plausibly brought was related to the spread of farming technology was already in the ancestral south indians when the mixture occurred so this was extremely interesting and we wanted to look at it harder in light of ancient dna so this is a bad map but this is a map of central asia in northern south asia and we now have ancient dna that's not yet published but we're beginning we're working it toward publication of 550 new samples 200 from kazakhstan in russia uh 200 from iran and uh central asia in parts of the that historically have had has had iranian-related ancestry and sometimes languages so from turkish menastan in uzbekistan and tajlikestan and about 140 samples from the swat valley of northern pakistan where we a little bit of data from northern south asia so with this huge ancient dna sample size which is possible with this new technological revolution for generating data we actually can do something we couldn't do before this is one of the many things you can do by pumping up the sample size you can look not just at the main cluster of people but you can look at the outliers so in this room if we were analyzing and sampling just four people in this room we might all look like we're from the same group but if we analyzed 80 people from the group we'll see different people and those different people will be informative about what's happening in the city of seattle there's a primary group of people and there are secondary groups and we'll be able to see that so this is exactly what we see at towns like this one this is gonor tepe this is a one of the four towns we have data from from what something called the battery of margiana archaeological complex was one of the great civilizations of the ancient world 4 000 years ago and the first one of the first and it was only discovered by archaeologists in the 1990s they made these great walled towns and genetically most of the individuals are similar to contemporary iranians from around the same time but there are outliers and we were able to study the outliers and the outliers are almost the most interesting part prior to 4 000 years ago the outliers look like other samples we have from western siberia and from kazakhstan who have hunter-gatherer ancestry so the iranian-related primary group who were burying their dead in the cemeteries outside this town were mixing and interacting with a little bit with these local hunter-gatherer-related populations after four thousand years ago people descended from yamnaya pastoralists or the outliers or the primary outliers in kazakhstan north of this region a little bit before that is when these people come in so we're actually seeing the wave of advance of these step pastoralist descendants hitting these towns they don't take over these towns the towns are still primarily occupied by these iranian-related people but archaeologically we actually see their encampments outside the towns there's people who make different types of buildings and different types of pots they're trading and sometimes they get integrated into the cemeteries in this town in the pakistani samples we have from 3000 years ago we have the step ancestry is already in pakistan uh 3 000 years ago and we can see through the size of the chunks it's already been there for 500 years so that allows us to bracket the arrival of step ancestry into south asia where today it can tr it can comprises up to 30 percent of the ancestry of some populations between 4 000 and 3 500 years ago so in this way in the talk that i've given here today we've now established chains of transmission of this yamnaya step ancestry into europe and south asia between 5000 and 3500 years ago that tracks very well the spread of indus spoken into european languages and it even tracks features of the way those languages are related like funny features such as baltoslavic languages have a particular relatedness to indo-iranian languages so what's particularly exciting about the south asian work is that we actually also have now data that probably comes from a civilization that was to the south of the bacteria-margiana complex which was in turkmenistan and uzbekistan this one from south asia itself so around the same time as the bactria-margiana civilization around the indus valley and other regions of present-day south asia northwestern south asia especially there are these great towns that are formed like mohenjo-daro and harappa and we don't have data from those towns however we have data from outlier individuals living in their periphery so we have from eastern iran and from ghonor the same town i showed you before we have outlier individuals who don't look anything like the other people there these towns both have close cultural contacts with the indus valley civilization they're seals from the indus valley civilization found in their artifacts and many a lot of evidence of trade of contact and exchange between these places these people all 14 of these individuals are a mixture of typical south indian related ancestry which is not seen in the rest of the samples and a different type of iranian ancestry so we think we're seeing migrants from a cosmopolitan population to the south east and so here is our updated model of indian population history so to now we think of india as a mixture of late bronze age pastoralist send it from the yamnaya and an ancient iranian population and gradients and different proportions in this indus periphery indus valley civilization samples of these two poles asian and iranian so this gradient existed more than four thousand years ago and we have multiple samples along it now from ancient dna from iran and turkmenistan kind of like if you sampled here from seattle you would see immigrants from other countries and that you would be get a picture of what the ancestry was like then the step pastoralists come in between 4 000 and 3 500 years ago and we see multiple samples in pakistan that are in different proportions on this line from the step to a point that we haven't directly sampled but we've close to sampled from this existing pre-existing gradient there's an unsampled point on this gradient but we think it must have existed where we think the ancestral north indians must have lived and people in india today are mixtures of these two mixed populations so three gradients the one four thousand years ago been before the one between three and four thousand years ago and then this mixture of mixed populations pulling outward toward eurasia one way to think of eurasia is as there's a core region with two peripheral peninsulas one of them being europe one of them being india with two parallel histories agriculture develops 12 to 11 000 years ago in the near east that's where agriculture first develops it spreads about the same time to each of these regions into europe from turkey about 9 000 years ago into south asia from iran about 9 000 years ago takes a few thousand years to spread across each of these regions because of the climatic challenges that need to be overcome for west asian crops to spread into the complicated climatic zones in each of the regions meanwhile in the step after five thousand years ago the yamana has spread to the peripheries of each of these regions and its mixtures of these mixed populations that form the primary gradient of each of these groups today producing a mixture of three ancestral populations hunter-gatherers farmers and yamnaya step pastoralists so a summary and this is a picture of the yamnaya individual that we got data from ancient dna is teaching us again and again that much of what we thought we knew is wrong that we're all mixed and that no one is pure and if this is an unusual field where scratching the surface is guaranteed to surprise i was prior to working to writing this book which i decided to do five years ago i just wanted to write papers all the time and i people would say you should write a book or you know i want to find out about what this is going on what's going on but i just wanted to write papers more and more papers because it was so exciting to work on this but it became increasingly clear that it was impossible to understand what was going on in this field through the journalistic articles about it and people even people super interested like members of the public but also archaeologists and linguists and historians whose work was being impacted by these findings just couldn't interact with our cryptic papers that were written in jargon and so i decided to write this book to try to explain it to anybody who is interested without jargon it's sort of serious books so it's actually you know not not uh but it's it's hopefully readable um because i think it's really important to try to understand what this is finding and we're of course clearly only just at the beginning there are so many things to do uh you know europe is just a tiny corner part of the world we've there's many times in and not just european history but all parts of the world which we can access with this technology we can learn you know how often migration occurs how often mixture occurs what the tempo of it is we can learn about frequency changes of mutations that affect biology and it's so exciting to work on this thank you very much thank you so much we have about 15 minutes for questions we already have a bit of a line so you might want to run up if you do want to try to get a question in so um let's just move through as many as we as we can and then um there are books for sale and we'll have a signing in the back of the room thank you very much for writing the book thank you very much for coming to talk about the book to a broad public audience um some members of the alt-right movement have misused your book in your new york times op-ed to suggest that genetics can now be used to distinguish human races some academics have suggested that a sociologist as a co-author might have been a way to prevent such hijacking of your ideas and your thoughts going forward what is your what is your view on the role of geneticists such as yourself and their role in the discussion of such questions about race and social in society sure thank you thank you for the question which is an important question um i think that uh geneticists are the experts on the nature of human variation and i think that the message that's been coming from the genetics community for the last 50 years has really been a very simple message which is that the differences among human populations are very small relative to the differences across individuals that's true insofar as it goes but yet the differences across populations on average are not trivial and so i think it's very important to be able to communicate that because as science advances whether we like it or not medical geneticists are going to be able to be measuring differences in genetic traits that will differ on average across populations and it will be impossible to get away from these findings so i think we geneticists need to provide guidance for how to think about the nature of population differences so race itself is a social construct it changes over time for example definitions of what it is to be an african-american were what are different uh in 100 years ago than they are today definitions of what it means to be black is in brazil is different from what it is in this country some definitions of race have nothing to do with ancestry at all especially in latinos for example it doesn't correspond to a homogeneous group in any way at all but we need as geneticists to provide guidance because if we don't that vacuum will be filled by people just making up stories about the nature of differences amongst groups and that the genetics is suggesting that it corresponds to long-held stereotypes but what the genetics is showing again and again is that what we thought is wrong and so i think that this is the sort of thing we need to be able to be discussing thank you awesome thank you for again to come to for coming to seattle so finding actually human or hominid fossils is really difficult but presumably it's much easier to find animals that traveled with um were domesticated or or they were forced by ice age migration they were forced to move um could uh like applying the same techniques on on other species help or provide extra data set or augment the models yeah so looking at animal ancient dna that's an incredibly interesting area as well and there's been really amazing work especially in dogs and in cattle and in horses which have shown big transformations of these domesticated animals with the movements of people but also of wild animals such as elephants or passenger pigeons and in in some ways the opportunities there are even richer than for humans and so i think that uh there's all sorts of skeletal remains it's not true that there are not a lot of human skeletons so if you dig a highway here for example you will run into many many human skeletons or almost anywhere in the world and people are excavating them all the time i mean here it's a complicated issue because it's native american skeletons and i think special attention needs to be placed in care and analyzing them but there's there are in the last 10 000 years literally millions of skeletons that we're uncovering all the time and so there's not a limitation uh in terms of being able to analyze data from this period what are your current thoughts on the origin of the native americans um and the challenges of trying to figure out the details of that so can you explain more about your question there well where where did the native america the first americans come from so i don't think that genetics sheds much specific light on that because we don't have ancient dna from the exact time and place when native americans first arise genetic data doesn't give you any information about where exactly people were unless you actually have data from the individual we don't have individuals who are on the bearing land bridge we have dna from however what the genetic data says is that all present-day native americans with small exceptions the great majority of the ancestry descends from a single homogeneous ancestral population which was already in place in the americas at the time of the oldest excavated skeletons that we have now which are almost 13 000 years old for which we have data and so what's very clear is that most native american ancestry descends from a common stem population at that time and there's some additional interesting events that happened since then and the origin being siberia native americans uh in terms of their relationships to eurasian groups are a mixture of about two-thirds ancestry related to deeply to east asians and one-third ancestry related deeply to this ancient north eurasian population i talked about at the beginning of my talk thank you hi um the traditional story of man is that like homo sapiens are 150 000 or 200 000 years old and then we left africa 50 000 years ago but with since we interbred with with neanderthals and we had healthy offspring and we interbred with denisovians had a healthy offspring neanderthals interbred with denisovians and had healthy offspring is that an incorrect way that we're really seeing species because aren't we really then a 1.2 or 1.3 million year old species that has gone in and out of africa probably on multiple times because we have these other type of groups and that it's kind of a misnomer that where a species that's 200 000 years old and separate from denisovans and neanderthals yeah that's a great question and actually we struggled with it a lot in our collaboration i actually described this extensively in my book our conversations about this so species are defined in the biological literature as groups that do not in practice interbreed with each other and what you see in the ancient dna data is that when ancient humans or archaic humans were in contact with each other they were interbreeding all the time and producing offspring and in fact many people in the world descend today descend from mixtures between archaic and modern humans in different parts of the world in different proportions and so what does this mean about whether these different groups of humans were distinct species or not so when we wrote our papers we had discussions with some of our archaeologists and anthropologists co-authors who were very strongly arguing that we should name neanderthals homo neanderthalensis or call the newly discovered denisovans homo altiensis or something like this and we had a principal decision that this was not our business as geneticists in fact the genetic data there was already some debate about whether these should be called distinct species or not the separation time between neanderthals and modern humans and denisovans is about half a million years different groups of chimpanzees separated by half a million years are called subspecies not different species and the anthropologists argue that morphologically these were neanderthals and modern humans were very distinct group and maybe should be considered species but we just punt it and what we said is we're just going to call them by colloquial names neanderthals and denisovans after the first places they were discovered and we're not going to make a statement and leave it to other people but for me the boundary between what a species is and not is not so important what's very clear is that the mixture between neanderthals and modern humans and denisovans and modern humans produced a lot of natural selection so if you look at the genomes of non-africans today there have been segments of neanderthal dna that have been removed through the action of natural selection so for example there's a deficiency of neanderthal ancestry near genes compared to far away from genes and that can only be due to natural selection pushing away the neanderthal ancestry which is a toxic on a modern human background a genetic background and so this is clearly groups that were far enough away that the genomes had to kind of react to the insult of of mixture between two such different populations which we know from other species when they mix produces even greater natural selection but i don't know what to think about the nature of these groups thank you for your talk i have two questions uh the first is kind of about the molecular property of the dna so i worked with i've worked with bacterial genome and you know in lab you're very careful not to expose it to cycles of time freeze just because the dna is very prone to fragmentation but i'm very curious how has this ancient dna survived over such extreme environmental conditions and while being exposed to many bacterial strains many enzymes that could easily degrade the dna um i was just simply naively thinking maybe by after so many years we just end up with different nucleotides you know rather than even just short segments and the second question is as human species have we improved with regard to us being resistant to diseases over time in other words have we increased the diversity in our dna or um have it decreased that because on one side the lifestyle has diverged has converged actually but on the other side like you were presenting you know based on the segmentation of the dna you're actually being able to calculate how diverse the dna over generations have become so yeah thank you two great and very different questions so the first question is the preservation of dna and it's a complete miracle dna is a very stable molecule rna is very unstable molecule but dna is a very stable molecule and in bones it seems to bind to hydroxyapatite after the cells this is a which is a basic mineral material in bone after the cells break apart and is a relatively stably bound for a long period of time it doesn't survive very well in soft tissue so for example if you look at an egyptian mummy or a or a individual like the iceman preserved in cold for a long time the soft tissue doesn't preserve the dna nearly as well as the bones which happens to be a good preservation condition so it's like an incredible piece of luck that this is a stable molecule that lasts for so long the oldest dna that's been successfully obtained from a mammal is from about 700 000 years ago for a complete genome of a horse from alaska but there's even dna from temperate humans from 400 000 years ago from spain that's substantially preserved um your other question um was uh about could you just remind me for a second what your other question was about disease disease right so so disease the that's an interesting question whether we're more or less resistant to disease than we were before um i don't think the uh we've really answered that in a meaningful way with ancient dna um so sort of a general comment is that um uh you know we're living in more dense conditions than we used to be uh before to infectious disease so we're exposed to many more diseases before than people perhaps were a long time ago on the other hand you know in the last couple of hundred last hundred years i think that um we've been propped up by a lot of medical care and it's possible that uh the natural selection that has always been pressured on us to preserve uh resistance has been weakened and you know maybe maybe we're becoming more susceptible on average to a number of things i have two questions a very uh technical question and a more broad question the technical one you confidently called dates is that from radiocarbon dating uh which dates are you talking about well when you say this happened you know this is from a a a a sample from 3 500 years ago the sample okay correct radiocarbon dating is a very precise technology and is pretty accurate in terms of the dating within a small coefficient of variation great my other question i mean this is i mean you've in just a few years you've discovered so many things that are big surprises that i know it's hard to project from that to the future but some things you can project to to the future moore's law and the number of samples and that sort of thing so my my more general question is where do you see this going in the next you know 10 years 25 years 50 years so i think what's going to happen in the next 10 years it's really communal as a community we ancient dna specialists are going to build kind of an ancient dna atlas of of humanity of all of the ancient skeletons that's possible to get dna from over the last 50 000 years which is where there are a lot of skeletons available from all different cultures all around the world and so we're going to be able to build an atlas a map of how different ancient people are related to each other and to people today we're going to be able to draw uh figure out which groups replaced which groups which groups mixed with rich groups how the rate of migration between all of these groups and their relative contributions to people today and so that will be an atlas building or a map building communal effort and interpreting that will be a rich source of data for the future i think that there's another access to this that's going to be if anything even more rewarding which is that the lang we geneticists speak a language which is very different from that that sociologists and historians and anthropologists and linguists speak but we're talking about the same thing so we need to learn to speak across disciplines and need to adjust and translate our our what we find and also learn from people in those fields how to contextualize our results better and so what you're going to be see is an integration of genetics into these other fields where they properly belong so i'm pretty excited about that too great thank you so i was just curious when you talk about analyzing variation between genomes it sounds like you're talking on the genomic level is there any research you've done or analysis into what parts of the genome might be changing more or less than others like are they the non-coding part are they like you know physical attributes digestive things like i'm just curious if that's been something that which parts of the genome are mutating and changing more or more conserved under natural selection yeah so uh we know a lot about that um and we know a lot about the relative rates of mutation and so there's um for example certain dna letters and combinations of dna letters like cytosines and guanines together are much more mutation prone than others uh due to the chemical processes in the way that replication works and in genes and other important biological segments the sequence is conserved due to the action of natural selection removing bad mutations which affect function there and so if you look at pairs of species that are diverged there'll be more sequence similarity on those regions because of the selection to to remove those errors cool thank you i'm gonna have to preface this with i promise it's a serious question uh it is an argument that's been going on in my department at my school for about eight months now from a genetic and archaeological standpoint is there a hard line between archaeology and grave robbing and grave robbing yeah so the question is from a genetic and archaeological standpoint is there a hard line but so so i think that's a very serious question and what we're doing actually in ancient dna is we're opening up ancient skeletons perhaps from people who would not have liked their skeletons to be um sampled and i think that when we do this work uh we have to in every case keep in mind that these are the bones of real people and who may or may not wish to have had their skeleton sampled and we need to think about what we're doing and weigh the costs and benefits of doing that analysis so some of the arguments that are put that are suggested often when people talk about this is you should ask the question are these connected to a present-day people who might have some cultural connection to these bones so that's often the litmus test used to determine whether you should be able to analyze the skeletal remains for example a muslim burial we muslims are still all over the place and have very strong feelings about opening up of graves maybe we shouldn't open up a muslim grave because there are present-day muslims that would violate their feelings but i think that why should we actually think just that it's important in ancient egyptian tomb it's very clear those people didn't want their graves being violated their belief system was that their bodies should be intact they protected their tombs but there's no people who believe in that particular religious system anymore but should we respect those people by not opening up their tombs so i think that uh we science i think that there is an argument to be made that obtaining data from ancient skeletons breaks down barriers between people shows that people are related in ways different from what they thought and actually shows that people are more similar to each other than a lot of people would have thought before in a way that draws connections between people and i think that that's a mitigating factor but i think you have to in each case and do it in a case-by-case basis and it varies between each part of the world and often we shouldn't do this all right thank you thank you any other questions question back there the origin of the term yamnaya and does it have anything to do with the yamato and i am not a linguist and i do not know the origins of these terms but i doubt that there's a connection between those two terms but that's an uninformed opinion yamnaya i do not know the the the etymology of that term i think that's what you're asking what does it mean to you it's a name of an ancient of an archaeological uh culture associated with a particular style of burial and set of material artifacts that are associated with these people other questions i have a question this is the last question right here okay um so when when a group like the yamnaya do come into an area can you tell if there's some sort of genetic superiority or or or benefit rather than just to sheer numbers you know if there's thousands of people moving it seems like that would present itself in the in the gene record without necessarily being a better quote better set of genes or is it the other way around or can you tell the difference i would be extremely um skeptical of a claim that these changes were driven by genetic um traits that some people had more than others i think we see again and again in our modern society of group examples of groups colliding with each other that you know just have different cultures or different technologies you know that runover or diseases they're carrying that run over each other in different ways i think this is reflecting cultural differences and luck or you know ways of being and ways of moving in ways of organizing life and i think that that's almost certainly what we're seeing i think establish there might be small genetic contributions to some of these changes but i would guess they would be overwhelmed by cultural ones that's my bias unless like something like disease you know for there could be diseases europeans coming into north america yeah so so one interesting one interesting observation i talk about this in detail in my book um is that these step pastoralists uh the grave this group in in denmark discovered after um sequencing dna from these early bronze age people including yamnaya that many of them maybe ten percent of the ones in the first study they studied or maybe five to ten percent carried the black death pathogen so an early form of the black death um and this was you know well before the 1347 you know great black death and the fact that they had it in their teeth when they died almost certainly means they died of it because you don't have bacteria in your bloodstream normally and so a one possibility that they speculated in their paper and is quite an interesting possibility but is far from proven is that what actually happened is that a disease like this one spread was was was endemic in a group like the yamnaya and wasn't uh present in a naive group like central european farmers and was introduced from the step and resulted in a kind of decimation of the local population which made room for the people coming in from the east demographically even though they weren't as densely spread on the landscape they were pastoralists rather than farmers it made it possible for them to expand that would be ironic if true because in the same way much long after as you said uh native americans sort of encountered european diseases from these very people uh who perhaps were impacted by similar phenomena themselves earlier great thank you thank you so much thank you all so much for coming

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Published: Thu Oct 18 2018