Culture-Gene Interactions in Human Origins

this ucsd-tv program is a presentation of university of california television your support makes you see tv's programming possible contribute online at UC TV TV slash support check out the youtube original channel you see TV prime at youtube.com slash you see TV prime subscribe today to get new programs every week you among the oldest questions of humans are the following who are we what are we doing here where did we come from how did we get here and where are we going two of these questions are now tractable for scientific enquiry and they are where did we come from and how did we get here and this subsumes the subject of anthropogenic explaining the origin of humans our mission statement is as follows we use all rational ethical approaches to seek all verifiable facts from all relevant disciplines to explore explain the origins of the human phenomenon I'd like to go back more than 50 years when CP snow pointed out that there were two domains of academia humanities and Sciences and they seemed to have a difficult time getting together well it's not very different from the ongoing nature nature debates and for that matter it really comes down to culture and genes now if you want to push you anthropogenic me to avoid such false dichotomies so we need to look at the humanities and the sciences study no chill with nature and the theme of today is culture gene coevolution well I'm I'm very grateful to be here and for the opportunity to engage in this interdisciplinary conversation I'm very humbled to be here and hope that I have something useful to add to the discussion I'm not a geneticist so dealing primarily with with the origins of culture in a sense so we all toss this word around culture and in general it's being used to apply beyond humans to learn behaviors that differ among different populations of a species so great apes have culture now orcas have culture Birds have culture lots of things that we never imagined would be included in this term have culture but lots of my anthropological colleagues would reserve their word culture for humans they're very uncomfortable with extending this to great apes and they it involves a much more cerebral version of culture that includes shared beliefs shared values and symbols so it includes in a sense their definition and implies language so one of the things we're looking at is the transition from an 1/8 version of culture to a human version of culture and we also have to as the introducer said to realize that humans not only are dependent on culture but we're also shaped by culture that be our behavior and our morphology have evolved together in a feedback relationship the fact that we chose to consume tubers wenton when things were bad we're going to hear a little bit more about this later created a set up a selective situation which selected for more genes that could that help the digestion of starches and the fact that we kill and process animals that were bigger than we are at least we did for 99% of human evolution also shaped other aspects of our morphology to be able to do it better our weapons evolved and so did our shoulders our stone tools evolved and so did our wrists to make them better and we became better communicators with each other and the speech apparatus and the brain both evolved to facilitate that so we've come to the point where we can't survive without culture you leave a human on a desert island and give them no possibility of making anything to help them survive they wouldn't survive very long and we have in order to acquire that culture we have an extraordinarily long period of dependency relative to the rest of our lifecycle not just infancy which many animals have but a juvenile period which is normally a very dangerous period for a species and I would in moving towards the end of human evolution and the appearance of our own species Homo sapiens I would underline three important aspects of our own culture one is our capacity for technological and economic innovation our ability to build on past innovations to ratchet up what what our forebears have done to build something new and to accumulate culture is sometimes called cumulative culture we also have extraordinarily large social groups that constitute imagined communities because if you think about who your family is there are people in your family I'm sure it is there are in mind who you've never met there are people who have not even alive while you've been alive that you think of as your family there may even be in some cases members of families who are spiritual beings or who are going to be born in the future that and the same goes for communities or Nations all of these things have an imagined a men aspect to them not just a physical on-the-ground aspect and these large social groups are essential for faithful long-term cultural transmission of complex behavior they also increase survivorship because there are more individuals that you can go to for help if things are bad where you are or in your life and the third aspect is the world of symbols which reifies these social groupings and increases the potential for information sharing and also helps survivorship and there are lots of ways of trying to understand the past the way I try to understand the past is by the direct evidence that it leaves behind which as you see includes at the bottom genetic inheritance which i think is a direct proxy of what happened in the past but so are the stones and the bones of a archaeologist the landforms and sediments where we find the stones in the bones and the biotic and chemical environmental proxies that are encapsulated in those sites so the question is for me when does modern human culture began and was it a gradual or a rapid transformation if we look at the end of the Pleistocene we find that the first Homo sapiens in Europe had many of these or if not all of these characteristics material and economic innovation symbolic culture language large social networks they had flutes they had strange sculptures that looked like half human half animal and so they lived in a rich symbolic world before 36 thousand years ago Chauvet Cave is another example and a beautiful example of some of these imagery and symbolism that we see but it's not just in Europe I've colleagues who think that you had to have cold weather and to grow the symbolic brain to do these things but we also have finds from Africa this is a little bit less well preserved but you can see that it's got human legs on the right and it's turning into an animal on the left so we can be sure that this capacity for making these images did not develop in Europe it came into Europe with the modern humans and if we go back further into the past we see that in South Africa Namibia Tanzania Ethiopia Senegal and the countries of North Africa as well we find these this kind of rich symbolism that implies fully human language we have beads we have geometric designs in two different forms or three different forms we have them on bone I don't have a picture this is on this little slab of ochre there are two of these and then we have a series of geometric designs on ostrich egg shell that look very much like the eggshell in the center that was collected in the Kalahari Desert in the 1960s there around the hole that was made to use the egg shell as a canteen and there are 16 or 17 different motifs that repeat themselves through the collection of the egg shells from deep cliff so these may be individual styles or group styles or family styles in some way but they're very distinctive from each other and these beads are not just at a couple of sites but this is a map that we made for a paper on African climate in the and the development of early human modern human culture you can see that there's this great density of dated sites in South Africa there's a quite a density in North Africa but less and East Africa is being filled in so there's still a big concentration of the record in South Africa that doesn't mean that that's the place where most of the people lived it's the place where most of the archaeologists live but about 10 percent of these sites have beads so beads are not a rare phenomenon they're lots of the sites they go back to about a hundred and five thousand years ago so already we're back into a much earlier time period we also have between a hundred and sixty thousand years ago point styles that differ as you go around Africa and this could be argued to be a regional tradition that is shared among a large group of people and they preferentially use everywhere exotic raw materials materials they had to go somewhere to get often 150 to 200 miles away which implies they're not necessarily going there themselves there may be some kind of exchange networks going on but if you think about the problems of exchanging things with people that you never met if you were a chimpanzee they would kill you as soon as you came over the horizon so you have to come up with a way of telling them that you are a friend as you walk into their territory to get some of their stone and we also have sophisticated hunting technology between a hundred and sixty thousand years ago bone harpoons in the lower left for fishing from some sites that we excavated in the Congo stone points from blombo's cave and upper left which were hafted as spear heads and then Mike Willis from a number of places these are from Tanzania there are a lot of them from South Africa as well that were hafted on to what we are pretty sure were arrowheads so these people had at least by sixty and probably much earlier bows and arrow technology so that would have allowed them to wipe out the game or the enemies at a considerable distance rather than having to walk up to something and stab it with a spear which necessarily is a little dangerous for yourself we also have burials I showed this one from cough so because there's a better picture of it but there's another one from South Africa with a shell pendant that's about ninety thousand years old so these are burials the distinctive thing about is that they have great goods they were giving these dead people something some kind of special treatment to take with them into another life to provision them sometimes these things are food or beads or ochre to make them look as if they're alive there's some kind of ceremonial elaborate symbolism going on in these burials Eurasia is very different Neanderthals sixty to a hundred thousand years ago had no personal ornaments they had very comp lit incomparable artifact diversity to what we see in Africa they had no engraved geometric designs and I'm my colleague Francesco derecho cites some things but I think he himself would recognize that the objects the engraved ostrich egg shells where the geometric designs are far more complex than anything we see and then the internal sites which just could be interpreted as scratches there's little evidence of large social networks most of the stone is coming from local sources and there are no burials with grave Goods there is a personal ornament which I show here from Cueva antonin Spain dated to 50,000 years ago when the modern humans are already on the periphery or in Europe and there are several sites with bone and tooth beads that overlap in age with the edges of the earliest European modern humans so whether the Neanderthals are actually making these things or whether they're developed developing them in imitation or whether they're so stressed out that they're finally getting with it and inventing symbolism is an open question but in any case it's a very late phenomenon the Neanderthals as you see on this map lived in Europe and into Central Asia and we've recently become aware of another group of archaic humans who apparently lived in East and Southeast Asia although the only remain we have of them is from a place in Western Siberia now Western Siberia might be a lovely place today in a big interglacial it was not a great place to live and the high glaciation so it's very unlikely that this is the homeland of the DNI's events that they've become to be called Denis of itself has stone tools similar to those of Neanderthals and it also has the onion turtle romaine these mysterion tools are replaced by tools of early modern humans and again there are colleagues who think these early modern humans were made by tools were made by Denis events seems unlikely because they look like Near Eastern tools and Denis events don't go that far West but this is more likely the north west edge of the denis oven/range not the center if we go to South China and Southeast Asia between a hundred thousand and thirty thousand years ago the tools look like the tools of Homo erectus a much earlier ancestor they're very simple Oldowan kinds of artifacts so Africa before Homo sapiens so we've already realized that by a hundred hundred and ten hundred and thirty thousand years ago humans were pretty modern and the oldest human fossils attributed to Homo sapiens are between 200 and 160 what about before that in the Middle Pleistocene this is marks a period of major climate variability which differs by region within Africa it's more severe at the top and bottom of Africa and less in the middle but before Homo sapiens we now have evidence for hafted weapons for large quantities of ochre which worth being processed into powder presumably for painting something your face your body your clothes whatever we have long distance networks of where raw material is moving almost all African fossil brain sizes after five hundred thousand years ago are 1,200 cubic centimeters or larger that is they're in our range we used to think they didn't do anything special but that's because we were looking at the Europeans almost in the head laval WA and blade technologies which many cognitive and archaeological scientists have argued imply sophisticated sequential action conceptualization teaching and very likely language it's a very difficult technique to learn and it involves conceptualizing the final form of a piece on a core that you specially a pair to produce that final form okay so there are five places in Africa where these long posters surely in sequences have been explored and I've worked in two of them the middle I wash and larga silently and talk for three minutes about a Lorca Sally this is a site we finished excavating last summer called the Largo sily be okay too it has five separate horizons of occupation and it's under a tough that you can see on the left is dated to three hundred and forty thousand years ago more or less the date is not fixed totally might we might get a surprise but repeated samples have confirmed it and every black spot you see on the floor in this picture is a piece of obsidian there is no obsidian in the Allura Ghazali basin and the obsidian here this is a scale at the bottom left of 28 or 30 kilometers the source is from B okay to come primarily from the south about 30 40 kilometers away and from the north about 60 kilometers away and by the time you get to the north you're getting uphill into better water the kinds of environments but if we go to a slightly younger site that's still probably older than 300,000 across a little valley we find obsidian from the Ibero complex which would have been more than a hundred kilometers away on foot so long distance transport of this material what are they doing with the material they're making small and medium sized points that were hafted they're making platelets and little bay liqueurs and small n scrapers and Laval ma technology is the Darwin technology it's also more than five hundred thousand years old at both the Loire gazali and wonder where it came in South Africa how do you maintain a large social network you use pigment and we have this and other pieces of stone that were used to grind ochre this is of the chart I published in 2000 with Sally mccurdy and all the red lines show where we've gone gone since 2000 and pushing back some of these distinctions of behavioral innovations that we noted in that paper and just to give you a line here's the Homo sapiens appearance first appearance data and you can see the lines go well back before anything we're going they're going to call Homo sapiens and just to show you we also have discontinuous preservation which could be showing us that we have these things in perhaps small packages that appear and disappear perhaps the people go somewhere else for a while and then they come back but we have periods of time when we don't have a lot of evidence and periods when we do but you can see that the record is not enormous for all of Africa so in conclusion I have just a few points which I'll read very quickly most technological innovations are older in Africa and they employ at least four to five out of Africa events not just two and Africa had probably consistently fairly large populations compared to Eurasia because it's a wonderful place particularly East Africa to be a terrestrial herbivore and as a friend in this audience once pointed out to me it's wonderful to have two rainy seasons a year because then if one of them fails you have a second one to hope for it there's the Neanderthal denisa van split from our ancestral lineage maybe one such event the onset of accelerated behavioral evolution innovation considerably predates the Homo sapiens in Africa and the Neanderthal split so Neanderthals eventually share pigment use of alwa technology and probably cognitive capacities for language but later Africans with projectile weapons and large-scale networks would have held a major competitive advantage over Neanderthals and eventually as we know what up them out the discontinuous nature of the African record implies small or fluctuating skill groups with potential for extinction especially in subtropical ends of the continent and it also argues that ancestral morphs and artefact styles could well have survived and did survive the the second early homo early sapiens in Ethiopia is associated with the Australian with her Nexus and so we have we also have a new archaic human that was described this past year from Nigeria that stated to 11,000 years ago so again there are mysteries in Africa always something new from Africa and I just like to thank Carta and the organizers of this conference many of my closer collaborators on several of these projects and the funding organizations thank you so my talk here is to give you a little insight into how at least I think culture influenced the evolution of human genes in the recent and perhaps not quite so recent past so what kinds of evolutionary relationships between genes and culture might we imagine there are several modern proposals on the table that reflect the old dichotomy between nature and and nurtures that propose for example that culture is really largely under the control of genes a natural selection on genes the most prominent spokesman for this point of view I think is Edward Wilson who has this proposed with Charles lump stand clear back in 1981 that culture was on no kind of a genetic leash that that natural selection acting on genes would give us a psychology that then shaped culture and ways that were dictated ultimately by the survival value for our genes as that would all boil down to a selection on genes the opposite kind of hypothesis is that cultural evolution at some point took over from genes and in human evolutionary history and genes no longer have anything interesting going on in terms of of evolution and this is a common idea and it's it's perhaps tacit throughout most of the social sciences that social scientists don't have to worry about genes of genetic evolution because it's all the interesting stuff is going on in culture and the position that I defend and I'm going to talk about here is that the relationship is much more intimate between genes and culture and in particular that culture often leads the gene-culture co-evolution process so cultural evolution creates novel environments and then those novel environments exert selection pressures on genes so that genes play a excuse me culture has played a very active role in the formation of our human nature if you want to think of it that way so the old nature-nurture dichotomy really has to be broken down into three kinds of elements that are important in human behavior first genes second culture and and third individual learning so we that we get the direct impress of the environment on ourselves and then we get the the evolution of these two inheritance systems part of this depends upon the idea that culture is a form of inheritance so it's a lot like genes and in some important respects you get your your culture from other people from your parents and from other people that influence you your peers a large number of other people may influence you but it has in common with genes that something is is transmitted that behavioral variants are acquired by imitation and teaching and and spread among people much like genes spread but of course there are these dramatic differences and one of the most important differences is that culture is a form of inheritance of acquired variation I mean a meeting like this is all about the inheritance acquired variation those of us who speak think we've learned something in our scientific work and we're trying to teach it to you to convince you that we've learned something interesting so we're an example right here a laboratory example of of how that system works we can also use the much larger community of people in just our parents to acquire our culture from so if dad is a poor hunter or mom can't weave a basket to save her soul and we can learn from aunts and uncles and and other people and of course we dramatically amplify that kind of thing in the modern world one when formal teaching comes along and we learn a large number of skills in formal settings by professional teachers but nevertheless the pattern that so struck Darwin of descent with modification is something that obtains in the cultural system as well as in the the genetic system and so culture links learning and inheritance through this inheritance of acquired variation and the important part of that is that it means that cultures can evolve much more rapidly than than gene pool so it adds to natural selection natural selection may still act on cultural variation but we add to it the ability to use learning and to use selectively borrowing from other people to cause evolution to happen in a much more rapid pace so Darwin himself was already a pretty sophisticated cultural evolutionist in you can extract passages from the descent of man like the one I have here in which he remarks that or it's really an opinion that in the highly civilized nations natural selection is no longer so important and the continuing evolution depends more upon the teaching of children and good education he says and high standards of excellence depending upon the best and the brightest to acquiring culture from the best and the brightest and and these are then embodied in laws and customs that are taught directly to children so this he just barely uses the word culture and in the modern sense in one place but it's full of ideas like tradition and customs that are related to the modern concept of of culture unfortunately that the Senate man was an unduly neglected book particularly after about nineteen hundred and and Darwin's influence was lost and for three-quarters of a century or so so how can we imagine that culture could have a major impact on human evolution that we could be substantially made by a cultural evolution even to the point of of our genes and following adapting to culture so that we invent so the first place culture has been important for at least a quarter of a million years partly be more like two-and-a-half million years so Allison's talk just before mine gave you a sketch of what we know about about that and if cultural evolution is faster than genetic evolution and culture is going to often play the leading role the evolution of this gene-culture co-evolution system not the not a lagging rule but so we have this long period of time in which culture and genes interacted and in which the cultural system could have played a leading role and so cultural processes create novel environments and then such environments lead to selection on the gene so just to reiterate the point of the previous speaker we have this succession of fairly sophisticated and increasingly sophisticated strong tools over that long two and a half million year longer period of human evolution now so what direct evidence do we have that change cultural environments have imposed themselves substantially on genes quite recently we have made a major transformation and our environments particularly in the elaboration of agricultural subsistence systems which have replaced meat largely in our diet by plant materials and and we done things like invent dairying and and generated an entirely new nutritional system for most human populations that no longer depend upon hunted and gathered resources and this seems to have led to when we survey the genome looking for events of strong selection in the genes something we can do looking for selective sweeps as a jargon goes we can estimate the age of these sweeps by the amount of material on either side of the allele under selection that have been dragged to near fixation by the selective sweep of the target gene because of linkage so we could take it and then this linkage breaks down gradually over time so it provides a kind of a molecular clock to date these selective sweeps and at least if the preliminary data that's in the hawksin happening at all paper in 2007 is indicative that there was a huge wave of of change associated with with agriculture so that's a recent very dramatic change some quite convincing cases were actually known before modern molecular genetics so in human populations with daring most adults secrete lactase throughout their life and could can digest the sugars in milk throughout their life and most other mammals and most human populations this gene has shut down about the time of weaning because it's no longer useful only humans with daring get much milk to drink after their weed there are many hemoglobin polymorphisms that turn out to be adaptations to exposure to malarial parasites and and these polymorphisms are all seem to be quite recent because human populations seem not to have been dense enough to sustain malaria as a specialist parasite on humans until the advent of Agriculture so this is another case of the imposition of an agricultural environment in this case making humans dense enough to support diseases many other epidemic diseases have a similar history that came to afflict humans after the development of Agriculture and more dense population so there's a presumably going to turn out to be a large raft of these some of these things are much more speculative so there's some idea that genes that affect behavior might have been selected by the advent of of societies that are hierarchical so there's a proposal at certain serotonin transporter genes that make people more tolerant an inequality have arisen in the last few thousand years as you know gala teri and state-level societies have replaced the egalitarian societies of our hunting and gathering days and then the human dispersed a lot of Africa was presumably led by cultural innovation so when people penetrated from tropical Africa subtropical Africa into the mammoth steppe into Europe and and these other cold even periglacial environments we we built fires and and made shelters and and in order to make highly tailored clothing but in addition a considerable number of genetic adaptations occurred as we drag our culture dragged us into these novel environments skin pigmentation is a well worked out one in which exposure to low UV environments and particularly in North Western Europe North Western Europe is a cloudiest the environment in the world and and so it's notes and white colored skin allows people to to synthesize more vitamin D adaptations to high altitudes there's a interesting adaptation and Tibet in which the the way in which the humans respond to low a low oxygen by producing fewer red blood cells and they otherwise than the rest of us turns out that that confers health advantages on on people living in high altitudes and it's President Tibet but not in the Andes and the Tibetans Tibet was settled all 30,000 years ago or so in the Andes not until till a few thousand years ago so the Andean people have not yet acquired that adaptation if they ever would a conjecture here then is that culture may have played a big role in in all of the attributes of humans and and one particularly interesting one is is human social behavior so humans are are docile they live in these large groups Allison already mentioned that and so might human se have acquired these proclivities through some form of group selection it's a curd old people going clear back to Darwin that people look like they've been selected as tribes that we have loyalty to tribes we've recognized a boundary of tribes we act altruistically towards other tribal members we behave honestly with other tribal members not necessarily so honestly with regard to others and and Darwin thought and again in the dissent a man has this little passage that suggests that that humans indeed are our group selected so this is just a quote you've had time to glance at it from the again from the descent of man so one of my students a couple of years ago Adrienne Bell got the idea that maybe we could look at the variation between human groups the genetic variation in the cultural variation and put it on the same scale so the the problem with selection among groups from a genetic point of view is it that it's a little bit of migration tends to homogenize the the genetics of the two groups and and human groups are really not very different genetically which means it's hard to imagine that the the group selection process that operated on on humans operated directly on genes so they're in the left of that diagram or the FS Jesus is a proportion of variants that's between groups relative to the total genetic variants in those bars on the narrow bars on the left and you can see they're quite small and the the gray bars are the cultural variation based upon the World Values Survey and you can see that it runs about an order of magnitude or maybe a little bit more higher than than the genetic variation so there's lots more cultural variation to work with the genetic data is from Cavalli sorts and colleagues famous compendium of genetic variation in human populations so the the picture then that we have is that humans could have evolved by gene-culture co-evolution and humans that are essentially then became a domesticated animal that that we were domesticated by this cultural group selection process so we still have these ancient social instincts we tend to dominate other people people are not are often selfish and and they're often nepotistic but on the other hand we also have tribal scale institutions and attitudes that that allow us to have cooperation and much larger groups so if if there was first cultural group selection and then primitive norms and institutions and the evolution of emotions like sympathy and patriotism and and more docile behavior and the evolution of symbolic group boundaries that certainly are very important in human groups then we could exert social selection on people who deviate from from the rules of society this is something we certainly do today if people don't obey the rules they go to prison and people in prisons don't have the same reproductive success as people out of prison so there's social selection that that way can be can be very important so this is a picture of a chimpanzee brain and a chimpanzee testicles the brains are on the right the chimpanzee testicles are on on the left and chimpanzee males are really rough customers and and part of the reason is that they're you know I mean people think of human males particularly juvenile males as testosterone poisoned but we got nothing on on chimpanzees so there's I can't if any of you here pathologists or know a pathologist or do with deal with human anatomy I'd love to have a picture that of human testicles and brains that correspond to the to the the chimpanzee once there I haven't yet been able to persuade anybody that I know to do this so the best I could come up with is this picture here my picture here with there's a human testicles about the size of a walnut I guess and the human brain about three times the size of a chimpanzee brain and so you can imagine what that picture would would look like if I could put my hands on on one okay that's the end of my remarks thank you very much for your attention as we've heard from a number of talks today we know that humans originated somewhere in sub-saharan Africa and from there they expanded to occupy essentially every corner of the earth landmass and in doing so they have encountered a tremendous diversity of habitats and environments environments that differ in terms of climate which is a major focus of our research but also in terms of nutrient availability resource availability degree of solar radiation and so forth and these environments are illustrated here in this map of the eco regions of the continents and so this different aspects of human environments have exerted strong selective pressures on human metabolic processes and physiological processes and certainly they adapt Asians have a reason in response to these selective pressures so environmental change is truly a defining feature of human evolution where impart is due to movement of populations therefore experience in new environments but there is also variation of habitats and environments over time illustrated here by in this diagram that has population size on the vertical axis and time on the original axis and as you can see there are a number of changes marked by different transitions during human evolution among these transitions we recognize - as perhaps the most important ones meaning the ones that have probably been associated with most selective pressures the first one is the Out of Africa expansion which occurred sometime earlier than forty thousand years ago during this transition humans were exposed to colder climates lower degrees of UV radiation different nutrients and also lower pathogen loads and then much more recently sometime more recently than 9,000 years ago in the Neolithic Revolution humans shifted away from a subsistence based foraging which characterized much of human existence up to that point and the adoption of different subsistence strategies based on horticultural pastoralism and intensive agriculture this changes in turn induced a number of other changes for example at the level of the diet which became much more rich in carbohydrates and milk became a major staple of adult diet but also there were massive increases in population densities which led to an increase in pathogen loads and increase in trans transmission rates of infectious diseases so really as I said environmental change has been a major defining feature of human evolution and humans have encountered this tremendous diversity of environments responding with cultural behavioral and genetic adaptations that led ultimately to the wonderful diversity of phenotypes and cultures that we see in human populations today some of the phenotypes that vary across human populations are actually diseases in fact many common diseases have significant inter-ethnic differences and these include diseases of the immune response like asthma multiple sclerosis but also metabolic diseases like type 2 diabetes or different types of cancers like prostate cancer there are some phenotypes that in addition to varying greatly across human populations they also have striking Geographic patterns this is the case for example for body size and proportions which are known based on classical work to be correlated with climate variables as shown here on the vertical axis we have the body mass index and under example axis the mean annual temperature in worldwide human populations so that populations leading at opposite ends of the climatic range like this Messiah in Eastern Africa and is Inuit from eastern Siberia have rather different body sizes and proportions which suggests that humans just like most other mammals conform to general ecological rules that suggests that body size and proportions are adaptations to different types of climates another phenotype that has striking differences across ethnic groups and also striking Geographic patterns is pigmentation we know that the skin reflectance which is a measure of pigmentation is strongly correlated in worldwide populations with latitude or distance from the equator which as we heard before suggests that variation in pigmentation is adaptive with regard to different degrees of shortwave radiation so these are two cases in which phenotypic variation appears to be a function of a particular environmental variable so we thought that based on this information that one could look for genetic variants that have similar Geographic patterns with the idea that these genetic variants might be adaptive with regard to different aspects of the environment and we do that by using this approach we refer to as environmental correlations where essentially we look for correlations between allele frequencies and specific aspects of the environment so we first start by classifying populations based on the environment they live in and then we search for genetic variants whose frequency is correlated with the environmental variable of interest in the populations that we studied and then if we find a correlation shown in this diagram here between the environmental variable and allele frequency we infer that this is an adaptive variant with regard to environmental variation however if the allele frequency is not predicted by the environmental variable we assume that this variant does not confer a selective advantage in different environments as the by the particular environmental factors that we've considered so we use this approach on a particular data set which includes more than 640,000 autosomal markers which were genotyped in more than 1300 individuals from 61 indigenous populations worldwide the distribution of which is shown here on this map we regard to the environment we use a set of environmental variable that we that can be broadly classified into three categories climate variables which are all continuous and then eco region and subsistence variables which are categorical and analyzed as dichotomous variable the climate variable the climate variables were chosen among those that are most likely to reflect the impact of cold stress and heat stress on human physiology as well as different degrees of UV radiation ecoregion is as I said a categorical variable that mainly contains information about climate but as I said it's a it uses the information in a dichotomous manner and then the subsistence variables can be further subdivided into mode of subsistence and main dietary component where the subsistence mode includes foraging horticulture pastoralism and advanced agriculture and the main dietary components are cereals roots and tubers fat milk and meat so we've applied this approach to this genome-wide data set and I'm just going to show you a couple of examples so in these two diagrams which reflect two strong signals of correlation between a snip and in this case a foraging subsistence while in this case it's a correlation at this knee between allele frequency and relative humidity what we find is that on your example axes we had the individual populations but grouped based on the major geographic area and then on the vertical axis we have allele frequencies now if we focus on the left portion of this slide I color-coded the populations or at least the categories of populations in that we have the foragers in light blue the known foragers and then this horizontal line indicates the mean allele frequency for populations within a certain category and within a certain geographic area and so what you can see is that the signal that we find is that not all ferreters have the same allele frequencies and all non foragers have a different an identical allele frequency rather what we find is that there is a consistent shift in allele frequencies between foragers and non foragers that is observed in multiple geographic regions as if selection acted on subdivided populations in parallel to increase the frequency of a particular variant that is advantageous with regard to that particular aspect of the environment and this snip cause this snip is in a gene that codes for interleukin 22 which is a important mediator of the innate immune response and so it may make sense with regard to the changes in pathogens associated with a forage in tune on foraging subsistence on the right we have a correlation between a little frequencies and a climate variable which is continuous and again the pattern that we see is that there are clients of a little frequencies that repeat themselves in multiple geographic locations and in this case this is a non synonymous substitution in the keratin 77 gene which is expressed in the ducts of actin sweat glands and therefore it makes sense that this snip might reflect adaptations to heat stress in influence in the way the body can cool down its temperature by sweating another example in this case where the dietary component was observed at this PL RP 2 gene this is this gene codes for an enzyme that metabolizes galacto lipids which are a main component of plants and interestingly what we found is that there is a polymorphic stop codon so in other words this is a shorter protein that occurs at higher frequency in populations that specialize in serial compared to populations that do not as shown here where the red indicates the frequency of the variant in populations that specialize on cereals versus the blue populations so again this consistent shifts in allele frequencies that are observed in multiple independent locations and the interesting thing is that we predict based on the biochemical evidence that the stop codon even though it results in a truncated protein this protein is more active and therefore it makes a lot of sense that it's found in populations that have a plant-based diet which is rich in cereal another type of analysis that one can do to learn more about the biology of these signals but also to sort of increase our confidence that we're finding something real is to look at whether these signals are enriched in genes that have specific biological functions and so we looked at dietary specializations in roots and tubers which is one of the classes where we have some of the strongest signals and what we find is that the two biological pathways that are most enriched in the signals of environmental correlations are starch and sucrose metabolism and followed biosynthesis and that's actually what you would expect for roots and tubers which are rich in starch and poor in folate and so it's very comforting to see this particular result we regard to adaptations to the polar ecoregion we find several metabolic pathways that are involved involved in energy metabolism and therefore in the production of heat and the maintenance of body temperature in the face of a cold climate another type of approach that we can use to learn about the biology of these environmental adaptations is by comparing our results with those from genome-wide Association studies so as I just explained this a power approach connects particular polymorphisms with specific aspects of the environments that are considered proxies for the selective pressures that underlie the signals that we observe genome-wide Association studies in contrast connect polymorphism with specific phenotypes and so now if we look at the strongest signal from our analysis as well as the strongest signal from genome-wide Association studies and we asked which polymorphisms have strong environmental correlations as well as strong association with phenotypes we can start to make a connection along this side of this triangle and ask questions such as which phenotypes were acted on by natural selection and which specific aspects of the environment shaped these phenotypes which selective pressures shaped these phenotypes when we do this exercise we find that there are many snips that have strong signals with climate and are associated with pigmentation and tanning phenotypes in genome-wide Association studies this is exactly what we would expect it based on what I told you in the beginning that the pigmentation phenotypes have this Cline of distribution with a gradual change as a distance as a function of distance from the equator as a function of solar radiation the other result that we have that perhaps is less expected but very interesting is that many of the overlap between the two analyses identified the phenotypes of the immune response and in particular autoimmune diseases like multiple sclerosis systemic lupus erythematosus celiac disease Crohn's disease psoriasis and so forth this is also not entirely unexpected because we know that the a great deal of diversity in the way in there is a very striking pattern of the geographic distribution of pathogens which is very established in the literature that pathogen diversity decreases as a function of distance from the equator and it was shown a few years ago that climate and in particularly humidity are very important factors in shaping this gradient this latitudinal gradient of diversity so it makes sense that we find these particular observations which suggests that pathogens are probably the true selective pressure acting on these polymorphisms and that climate is essentially acting on pathogens and which in turn act on on on polymorphism so we're looking basically at the indirect effect of climate on selected alleles now by no means this pathogens represent the only selective pressure shaping this kind of signals but it's certainly as based on this analysis it's it's certainly an important component of it so to conclude I showed you that there are that is strong genome-wide signals of adaptations to climate to ecoregion to dietary components and subsistence in worldwide population samples many of these signals are found in gene whose function can be easily connected with the biology of the adaptations and the specific environmental factor that was used to identify the signals the signals are due to relatively subtle and I didn't talk much about this but relatively subtle but consistent shifts in allele frequencies that occur in multiple geographic regions across populations that experience different environments and we found that adaptation to different climates make an important contribution to pigmentation phenotypes as we expected as well as to diseases of the immune response I'd like to put a plug for this decline this is database that we have generated in my lab you can find it at this URL and you can search the database for signals of environmental correlations within 21 environmental variables and we have links to other selection browsers and then finally I want to thank the people in my lab and my collaborators who contributed to this particular research

Channel: University of California Television (UCTV)

Views: 57,681

Keywords: Brooks, Richerson, Di Rienzo, evolution, human origins, genes, culture

Id: Pj7mWE1R5xw

Length: 58min 59sec (3539 seconds)

Published: Fri Jun 01 2012