fashion nova exchange
- [announcer] this ucsd tv program ispresented by university of california television. like what you learn? visit ourwebsite, or follow us on facebook and twitter to keep up with the latestprograms. ♪ [music] ♪ - [narrator] we are the paradoxical ape.bipedal, naked large-brained. long the master of fire, tools and language, butstill trying to understand ourselves. aware that death is inevitable, yet filledwith optimism. we grow up slowly. we hand down knowledge. we empathize and deceive.we shape the future from our shared understanding of the past. carta bringstogether experts from diverse disciplines
to exchange insights on who we are and howwe got here. an exploration made possible by the generosity of humans like you. - [william] good afternoon everyone andthank you to the organizers here of carta for inviting me to participate and thankall of you, a great crowd, for coming out to see this fascinating topic explored. itend sometimes to be accused of nihilism with regard to the origin of homo becausemy view is we actually know nothing about the origin of homo, just saying. and thereason is simple in my view; is that while it is true that we have a pretty goodfossil record of the genus homo, the homo lineage as bernard just finishedexplaining, by around 2 million years ago
with some diversity and different adaptivepackages in different species: erectus, habilis, rudolfensis. on the assumptionthat these three forms shared a common ancestor at some point. that commonancestor lived older than 2 million years ago in a period of time in which we havenot a fender and a tire and a piece of gear shift, but in which we have afragment of tire thread, which we have a fragment of a headlight. and we are tryingto reconstruct an evolutionary history of a group for which we basically have a carwreck. and this is what we have to solve, this is the problem we have to solve andthis comes from field work and i'm going to illustrate for you today in my viewwhere i think the genus, the homo lineage
arose and where we have to re-double ourefforts for increasing the representation of this lineage older than 2 million yearsago. now, as bernard ably suggested, the modern history of the study of theevolution of the genus homo really begins with louis leakey and colleagues and therecognition of the species, homo habilis in 1964 based on material from bed i inolduvai gorge dated to between around 1.7 and 1.75 million years, they discerned inthe type specimen of the species older by hominid vii what they thought was ahuman-like dextrous ability in the hands, they discerned a notable increase inendocranial volume, brain size, in relation to then known australopithecusspecies, mostly from southern africa and a
reduction in tooth size which they saw asemblematic of an overall gracilization of the chewing apparatus in almost ahuman-like arrangement. and putting these three characteristics together with theplentiful stone tools that had been recovered for years in these sediments,they arrived at the conclusion that this species, habilis, belonged near the baseof the genus homo. so convinced were they of this conclusion that philip tobias, onethe co-authors of the species, was able to write in 1965 that homo habilisrepresented that last remaining major gap in the pleistocene evolution of the genushomo, “of the story of human evolution, †to quote him directly. and in thisphylogeny shown here from one of tobias'
papers, you can see the genus homo isrepresented as a single, gradually evolving line characterized by uniquelyhuman characteristics related to large brain size, reduced canine teeth, aperfection of bipedal locomotion as we now see it, a slowing down of the growthtrajectory, technology, language and so forth. this was a package ofcharacteristics seen in modern humans and thought to go back in time to at least 2million years as an integrated whole, along this slowly emerging lineageculminating in homo sapiens. the problem was of course is that older than 2millions years ago, there was virtually no fossil record that could be confidentlyassociated uniquely with our lineage. and
so whether these characteristics emergedpiecemeal, step-wise and therefore each demanding a separate explanation fororigin, or whether they emerged as a package together, where one explanationwould take care of them all, could not be discerned. now a lot has happened, asbernard has pointed out in the years since early 1960s. and beginning in the 1980s,in large part due to the work that he and others have done in those years, we nowsee the genus homo as a much more complicated array of species. in my view,there are at least three broadly contemporaneous forms present at around 2million years ago whereas in 1964, the leakeys would have said there's one in thegenus homo: homo rudolfensis, homo habilis
and homo erectus. and one of the lessonsthat we have learned from the appreciation of greater diversity in our own genus atthis period of time, is the idea that there is not one adaptive package that candescribe them all, but there are perhaps multiple ones. and the question is which,if any, are germane to the origin of the lineage itself? or, are they all, in oneform or another, subsequent developments to the establishment of the lineage?following on bernard's talking about toyotas and clades, my appreciation, myrendering of the information available from these three forms between around 1.7and 2 million years ago is that they do, in fact, constitute a monophyletic group.this is not the place to go into a
detailed rendition about the evidence forit, but i think it speaks fairly clearly to the idea that these three at 2 milliondid in fact share a single unified ancestry predating that time period,moving back towards the 3 million year mark. and the question is, where is it?and who was it? and here's where we run up against a roadblock. now, why is thisimportant more than just for the purposes of putting cladograms or phylogenies onthe page is because in the last decade or two, information from global climatechange, paleo-climatic change, has made it clear that the tim period in which manypeople suspect the homo lineage arose was one of a very widespread, impactful changein global climate, creating an expansion
of ice sheets, reduction in sea levels,drying out of the african interior. and that time period has been focused rightafter the 3 million year mark; 2.8, 2.7 and so forth. and that drying out ofafrica has been seen as motive in the origin of the robust australopithecines,the origin of the genus homo, even to stone tool manufacture. this has becomethe prevailing hypothesis that the complexification, if you will, of hominidsand the origin of technology is all associated with the local impacts of theseglobal changes. the problem is that there's no fossil evidence for the genushomo that is informative on exactly what those changes were at this particularpoint in time. we do have of course
oldowan tools at around 2.6 million. andas bernard and others have pointed out, perhaps that is a proxy for the genus homoor maybe it isn't. it's not outside the realm of possibility given what we knowabout how chimpanzees can make tools that some australopithecus is capable of makingthem, too. so, questions and an absence of evidence. and here is the sum total of thefossil record of the genus homo between 2- and 2.5 million years ago. it would fit ina shoe box and leave room for a decent pair of shoes. all of these fossils havebeen promoted by one person or another, one group or another as identifying thegenus homo older than 2.0 million years ago and all of them have been doubted. andi'm not going to go through them here to
point out the weaknesses and strengths ofthe various arguments, other than to say that the very fact that there's debate canbe traced to the fact that there's relatively little evidence. and this iswhy groups return to africa, go to the field to african sites, in east africa, insouth africa all the time focusing on this time period which, in my view, is one ofthe most intriguing of all the time periods in human evolution to increase ourunderstanding of the fossil record. one area where the group from the institute ofhuman origins which i direct at asu has been focusing on, of course, for years isethiopia. we've worked at the lucy site more or less continuously since 1990. andcolleagues of mine, dr. kaye reed at asu
and chris campisano and others, haveexpanded the work, the iho work in ethiopia, to a place called ledi-geraru asseen here as slightly north and east of the hadar area. what attracted them tothis area? two things, knowledge that the environments represented by the sedimentsin this area looked different from those that were very common and well-understoodin the lucy time period, older than 3 million, some 20, 30 kilometers away athadar. and second, the suspicion verified since then that the rocks may actuallyrepresent a slightly younger time period and that's important because at hadar, asyou will see, we have lucy species, australopithecus afarensis up to about 3million years and then we jump across
three quarters of a million years and wehave a jaw of homo with some stone tools at 2.3 million. lucy, homo; older,younger. gap in the middle, let's try to fill it. and that was their mission. now,in the lower awash valley, these areas around hadar and middle ledi and gona anddikika and woranso-mille, there are excellent sediments going backwards intime from around 3 million years ago. and we have an excellent set of sediments inplaces like gona and hadar that take us forward from around 2.5 million years ago.it is the time period in between that is critical and is germane to the questionsabout where the three forms of homo that we know of at 2 million perhaps emergedfrom? and these sediments are present
amply, now well-studied in the ledi-geraruarea spanning in time from around 2.8 million years to about 2.6 million years.and what's really important to understand about these sediments, and this is both anadvantage and a disadvantage is that they are not continuous across time, butinstead are exposed in fault blocks, adjacent fault blocks which means thateach block of sediment is a unified slice of time separated from another block nextto it which has itself a unified period of time with slight gaps in between them.disadvantaged because we can't trace evolutionary events continuously butadvantage because fossils that come, demonstrably come, from particular faultblocks can be narrowed to a very narrow
range of environments and associationswith other animal species, etc. so, a plus and a minus. and here is the ledi-geraruarea. kaye and her team have been working her for more than a decade before theyfound their first hominid. looking at the fauna, looking at the geology, trying tounderstand the environments. and by the way, this is an area called the lee adoytabasin and you can see here, here's one fault block, here's another fault block,and here's a third fault block. they're about three or four fault blocks justexposed in this one view, very clearly delineated. you can see one of the faultsrunning right through here. now, back in 2013, kaye and her group ofpaleontologists were surveying an area in
the lee adoyta basin called the gurumahablock just in the one fault block. and at the base of this one hillside, there's avolcanic ash that is now well-dated, very precisely dated to 2.8 to 2, plus or minusa handful of years, in the million year range. and on one winter's day, one of ourgraduate students at asu, chalachew seyoum, was surveying up on this hillsideand found this little jaw. that jaw eroded out of this hill, perhaps in a recent rainstorm and resides about 10, maybe 12 meters above that volcanic ash. and on thehillside, there are no sediments up above younger that the jaw could have floateddown from. it eroded out of that hillside and it's around 10 meters above the tough.so here's the jaw after it has been
cleaned up. and i'm here to tell you thatit answers some questions, answer some very specific questions. it doesn't answerall the questions. but there's a myth out here in paleoanthropology that unless youhave a complete skeleton, you're not prepared to answer any meaningfulquestions and i wish to dispel that myth. you know, since raymond dart namedaustralopithecus in 1925, there have been a plethora of hominid species named,recognized; australopithecus africanus, paranthropus robustus, paranthropusboisei, homo habilis, on and on. many of them, if not most of them on the basis ofmaterial that we here today would consider, at best, imperfect. a fragmentof a jaw, a bit of a brain case, some
teeth, and the fact of the matter is isthat in the intervening years, the vast majority of those species recognized onthe basis of imperfect material have been verified as to be meaningful evolutionaryunits. we are not at sea when we have small fragments. we are limited in thetype of questions we can ask. if complete skeletons were the answer to all of ourquestions, then lucy would have settled, once and for all, the debate about whenearly humans made a commitment to terrestrial bipedality. instead, shegenerated what is now going on to five decades of debate about that question. itdepends on the question and this question, the question that we address to this jaw,is it the same thing as australopithecus
at 2.8 million or is it somethingdifferent? and i engaged in that question with my former phd student, brian brianvillmoar now at university of nevada, las vegas, and chalachew seyoum our graduatestudent who found the jaw. and we came to the conclusion that in many respects, itdiffers from your standard issue generalized australopithecus jaw. seenhere on the left is a nice jaw of lucy's species, australopithecus afarensis and onthe right is a reconstructed from a scan of the specimen from ledi-geraru. wenoticed that the jaw differs rather...these two jaws differ ratherremarkably. the afarensis jaw is typically long and narrow with fat molar teeth,primitive pre-molars and so forth. and our
major comparison was to something likethis, one of the jaws from the dmanisi site dated to about 1.8 million yearswhich is attributed to homo erectus. and there's a much greater similarity in theshape of the dental arch, in the form of the teeth, the pre-molars beingsymmetrical and so forth, to this 1.8 million year old homo erectus jaw than tolucy's species. and it extends also to the architecture of the jaw and i'm not goingto go into the details here, but underneath the pre-molar, the afarensisjaw is characterized by a highly sculpted out, contour like a chimpanzee probablydo, to the very large canine teeth absent in the ledi-geraru jaw. the back part ofthe mandible where the vertical part
called the ascending ramus arises from thebody of the jaw is located in the ledi jaw well back of the third molar, not forwardas it is in lucy's species over the second molar. and the upper and lower boundariesof the mandibular borders beneath the teeth and at the base are more or lessparallel. and in australopithecus, they're not, it gets shallower to the rear. and bythe way, it's also true of australopithecus africanus which isslightly closer in age to the ledi jaw in south africa, the same kind of thing. so,when we made the comparison to jaws of the genus homo, later in time obviouslybecause we don't have much in the 2.5 to 3 million year period, the similarities werevery apparent to us. this is a jaw that
exhibits characteristics that forecastanatomy that is common, the most common anatomical patterns in jaws of the genushomo younger than 2 million. so we published it, in not quite a year ago, inthe journal of science as a 2.8 million year old jaw of the genus homo. now doesit answer questions about what were the adaptive packages present early on in thelineage leading to us? of course. but what it does do is that it puts one data pointin an area that is otherwise a void in the evolution of our own genus. question iswhat kind of environment did it live in? did it live in a dry environment? did itlive in an open one? germane to the questions about what drove early evolutionof homo. and data that's been put together
by kaye reed and given to me for thispurpose shows that this jaw is found in a context of animal species that lived inessentially grassland environments, very different in terms of how open or closedthe habitats were compared to time periods in which lucy's species lived. and this isjust a couple hundred thousand years later. now, i hasten to add here, i am notasserting that the origin of the genus homo is due to a drying out of theenvironment. but one thing we can say, because of the very confined time periodof the gurumaha fault block in which the mandible and the fauna on which thisinference is made, suggest that the modal environmental signal at 2.8 in this areais one essentially of a grassland
environment. and we can see that bylooking at some of the other animal fossils that have been found associatedwith the horizon from which that mandible has come. this is the gurumaha block,these are el salafin bovid frequencies and the horse frequencies, both of which ofcourse are well-known grazers. and together, in the gurumaha block, theyconstitute nearly 40% of the macrofauna, excludes elephants and hippos and stuff.i'm not saying it's dry, we're saying it's open. so it's 40% of the macrofauna andthat is very impressive compared to the frequencies back in lucy's time startingjust 200,000 years earlier. opens up areas for inquiry. and finally, some new datacoming out of kenya from sonia harmand's
group suggests that stone tool use, infact, began not with the genus homo, maybe. but perhaps as long ago as 3.5million years when we have australopithecus. and if these finds areverified, it opens up a whole new range of possibilities looking at the adaptivepackages that constitute the ancestral platform from which the genus homoemerged. and so to finish up, here we have ledi-geraru, here we have our formerlyfirst appearance, a former first appearance of stone tools now pushed backhere perhaps and does that imply that the genus homo itself has even an earlierorigin than we think of at 2.8, perhaps back as far as lucy? or could lucy herselfhave been the first stone tool maker?
thank you. - [philip] we've heard a lot at this pointabout the evolution of hominids in africa. it's a complicated history for sure. letme at this point just cut to the chase and say that humans moved out of africaprobably just after 2 million years ago and it will be that part of the recordthat i want to emphasize this afternoon. the site of dmanisi in the georgiancaucasus is very important, records the oldest known, at this point, the oldestknown occupations of eurasia beginning before 1.85 million years ago. we don'tactually have human remains that are that old, but certainly there are stone toolsapproaching that date. the good part about
dmanisi is that in fact we have not justscraps of headlamp and bumper and so forth, but virtually whole skeletons and anumber of them. now we have five skulls in various states of repair or disrepair.along with them, there are post cranial bones associated with one juvenileindividual particularly perhaps but not clearly associated with one of the adults.also the material is extremely well-preserved. we're very fortunate inthat respect. along with the humans, of course there are animal bones and many,many of them and there is a very complete lithic record to go along with thismaterial which is well-preserved, as i say, in a very carefully studiedstratigraphic context. several of the
specimens from dmanisi have been in thepast likened to african homo erectus but the skeletons are quite primitive. one ofthem in particular is strikingly so, skull 5 which i will talk about. at the moment,i think it's fair to say that the taxidermic identity and thepaleobiological significance of the dmanisi materials remain controversial.certainly there have been plenty of suggestions and i'm afraid i've beenresponsible for some of them but we'll see where things go. dmanisi is situated ingeorgia. georgia is stuck there between the black sea and the caspian withazerbaijan off to the east. from tbilisi, the capital, it's about an hour and ahalf, an hour and 45 minutes ride. roads
are pretty good these days. roads wereterrible 15 years ago. things have improved. down to the site, dmanisi isjust a few kilometers from the armenian border. this is the obligatory excavationsin progress slide. there are about four meters of sedimentary deposits at dmanisi.much of this stuff is volcanic in origin, it's very ashy. there are some othersediments and silts, but ash is always a primary component which is a good thing.the stratigraphy is complex, all of the sediments are piled atop the mashaverabasalt which is about 1.85 million years old, that's the bottom of the site, theearliest record, 1.85 million years is the date obtained from radiometricmethodology. it is secure. the
stratigraphy at the site is complexpartially because there are a number of piping features. water was present nearthe site during periods of heavy rain and so on. pipes formed underground and thenprogressed through breaching to collapse towards the surface filled with sedimentsthen got buried again. so it's been a mess, it's been very hard to sort it out.our geologist, reid ferring, has done a huge amount of work in this respect, hugein the trumpian since. but there have been problems. the first traces of humanmaterial were found at dmanisi in 1991. excavations in fact have been underway atthe site for quite a period of time before that. the site is underneath an oldmedieval town that was on the silk road.
the archeologists were busy at dmanisi forsome time poking around the foundations of the old buildings and eventually theybegan to dig up stuff that didn't seem to belong there, not just the goats and fishbones from medieval suppers, but things that looked quite antique indeed. thepaleontologists came in and ascertain that yes, the material was ancient, deeperexcavations got underway and in 1991, the folks at dmanisis were rewarded with thisjaw, the d211 mandible. it's remarkably complete, not all of it is there. but whatthere is remarkably well-preserved. it's a small jaw and in a number of respects, itdoes look like homo erectus. you've seen one reference to this specimen already.the teeth are about right for homo erectus
as are the proportions of the mandibleitself. the cranium which turns out to be the match to the little mandible was foundlater in 1999, d2282 is a small cranium, very small capacity, surprisingly so forhomo erectus. only a bit more than 650 ccs in this case. despite the small brain, thething does share a number of characteristics with particularly earlyafrican erectus. this hulk turned up at the site in 2005. it's way down at thebottom of the site and within a few days after the fossil had been uncovered andcleaning was underway prior to trying to lift it out, there was a very heavy rain.things were very nearly washed out. of course we have a cover over the site,there is protection, but it rained so much
and so long that water began to trickle inaround the sides of the excavations and things were dicey for a while.fortunately, d4500 survived. there it is all cleaned up. it turns out that thecranium found in 2005 is a perfect match, once again, to a mandible, d2600, whichhad been found earlier in the year 2000. the upper and the lower, the cranium andthe mandible simply clicked together once the stuff have been cleaned off, there wasno doubt at all. there is some pathology on the mandible that matches, comparablepathology in the region of the ear of the cranium so there is no doubt about thematch. more than other dmanisi hominins, skull 5, as this one is known, exhibitsvery robust morphology. it's pretty
clearly a male individual. determining sexin the case of these fossil hominins is often tricky, often can't be done veryaccurately. but in this case, we think we have a match, the skull says male allover. such a pattern given the fact that it has the smallest brain of all thedmanisi hominins because it is unexpected. normally for other primates, humans too ofcourse, but for primates, higher primates generally, males tend to exceed thefemales in brain size by something like 8 to 10 to 15%. so, having the tiniest brainattached to the most robust cranium and jaw is a bit surprising. you can see thatthere is a good deal of variation within the dmanisi assemblage. the little jawgoes with a smallish brain case which is
quite gracile in its construction. we pegthat one skull 2 as likely a female. skull 5, on the other hand is much more robust,clearly distinctive in a number of respects. number 1 is like homo erectus,number 4 is a small individual. that one seems to have lived to a ripe old agesince it had lost almost its entire dentition, maybe one tooth was still inplace at the time the individual died. that one may or may not be male, we're notsure. anyway, there is a great deal of diversity at dmanisi. the crania do lookdifferent. this raises the question of "how many species might be documented atthe site?" this is a question that's been plaguing us for some time. i think myselfon the basis of the shared anatomy among
the dmanisi individuals, they have acommon bow plan extending not just to the cranial vault but to the face insofar aswe have it represented. and also to the details of the cranial base suggest, thecommon bow plan suggests that all of the individuals are drawn from just one group.we've done extensive resampling analysis as well which cause us to come to the sameconclusion that really, in fact, the skulls, the post-cranial remains that gowith them, are drawn from just one population. now there is stratigraphicevidence relating to this question. it doesn't solve the question of course, butit's important information. it's good to know that the material was all washed intothese deposits or arrived in the site by
one means or another at about the sametime that is the duration here can't be more than a few hundred or perhaps athousand years or so according to the best analyses conducted by the geological sideof the team. we'll say, then, that it's very likely that the dmanisi assemblagesamples a population belonging to a single species. i know there may be objections tothis. i'm sure there will be, there have been in the past. if it's true, then sucha situation is quite rare. of course, at most localities where hominins arediscovered, you've heard a lot about east africa at this point, koobi fora, olduvai.also at sangiran in java where there are a number of fossils. the material isscattered through a very long sequence of
deposits covering a long period in time.time as a contributor variation just cannot be discounted. if the dmanisifossils document what we can call a population in the past extending overquite a number of years of course, then the next question, the next importantquestion is how the dmanisi sample may relate to the hominin taxa that havepreviously been recognized. skill 5 of course has a very small brain case, a verylarge projecting face in the vault. also in the basal cranium, there are someresemblances, not a lot, but some resemblances to homo erectus. skull 3which i have not shown you a picture of before is the sub-adult from dmanisi.skull 3 is pictured here down below. skull
3 is similar to homo habilis. this is truefor the bow ridge, the extended brow ridge development. it's true particularly forthe shape of the vault, the rounding at the back and for the mid-facial profile.skull 3, i must point out, is sub-adult so we must allow for some extra growth tohave occurred if the individual had grown up. it might have looked, had it grown up,a bit more robust and a bit like the skull to the left there, homo habilis knm-er1813. skulls 2 and 4 also have their peculiar aspects, of course. they have anumber of primitive characters that they also share some features with homohabilis. so, which species? there is, as i've pointed out, much variation withinthe dmanisi paleodeme. this is not an easy
question, the question as to which speciesmay be represented. skull 5, the very small brained and very robust and veryprimitive looking individual does indeed share some characters withaustralopithecus as well as homo. so perhaps in this case, the line, thedivision between australopithecus on the one hand and earlier homo on the other isnot so clear cut after all. many of these shared similarities are primitivecharacters and unfortunately they don't help us much in answering key questionsabout phylogenetic affinities. other characters expressed in the dmanisimaterials are homo erectus-like and pretty clearly they are specialized characters,characters that have changed during the
course of evolution, characters that aresaid to be derived. these characters include the form of the brow ridge forexample which is larger and bar-like, a little bit of midline, keeling on thevault, details of temporal bone construction, things of that sort on theunderside of the vault. indeed, when skulls 1 and 2 were first described backin the year 2000, they were grouped with early homo erectus from the turkana basin.if the fossils are included with homo erectus, clearly that's one way to dealwith the material is simply to lump it with homo erectus. if that is the coursewe take, then it must be recognized that the boundaries between homo erectus on theone hand and other early homo taxa will
become less distinct. it will beparticularly difficult to distinguish early homo erectus, african homo erectusfrom specimens attributed to earlier homo to homo habilis in particular. homohabilis is considered apart from homo rudolfensis. so, to sum up at this point,here is again a speciose view of hominin phylogeny done by bernard wood with meaveleakey several years ago. you can read the caution sign. to sum up then, there isapparently no simple answer to the question as to which species may berepresented at our site. indeed this question is often a tricky one. it's beena hard one for paleoanthropologists to deal with for a long time. in one view,this view expressed on the slide, dr. wood
showed you another version of this veryspaciose hominin phylogeny. in that view hominin evolution has produced a veritableflowering of lineages over more than 6 million years. such bushiness, as it were,is particular evident for the 2.5 to 1 million year ago interval. this intervalin which paranthropus on the one hand, australopithecus and homo are representedby multiple species for each group. at dmanisi the fossils seem clearly to behomo. now there are some points of overlap with australopithecus as i pointed out,but i would say unbalanced the evidence favors grouping all of our fossils withthe genus homo, very little doubt about that. at the same time, the assemblage atdmanisi does not fall neatly into one of
the taxonomic packages that have beenproposed: homo habilis, homo rudolfensis, homo ergaster, homo erectus and so on. ifi were pressed and i do feel pressed at this point, given the morphologicalresemblances of dmanisi to both homo habilis in a very strict sense, just thosefossils allocated to homo habilis, not to homo rudolfensis. given the resemblancesof our material to homo habilis, and to early homo erectus, particularly africanhomo erectus, i would probably argue, i will argue, that it is most reasonable toplace all of these fossils within a single evolutionary species. i would say that thedmanisi fossils constitute just one population within this unbranched lineage.now, this is not to raise the specter of
just one species at a time, or to suggestthat there isn't a great deal of diversity in the hominin record, clearly there wasparticularly in that interval after, about 2.5 million years. but as far as ourevidence is concerned, it seems to me the best way to go, simply to place all thefossils within one evolutionary species. then of course we'd have to argue aboutwhat to call it, but this is not the place for that. so, with that, thanks forlistening. thanks very much. - [pascal] thanks very much for this greatafternoon, to all the speakers. i'm especially grateful to dan lieberman forhaving set the stage with a lot of carnage and meat-eating, because the molecules italk about today have to do with
vertebrates and what we eat and whathappens to it in our bodies. so this afternoon i'd like to share an idea abouta way that a molecule could be driving speciation and share some evidence for itin vigo [sp], not in primates but in mice. i like to start with by acknowledging thepeople who've done some of the heavy lifting, including fang ma who's back inchengdu, a former lab member and darius ghaderi and my team here especially stevanspringer and miriam cohen, as well as my collaborators, ajit varki and his team.the kelp there in the background is actually an analogy i use when i talkabout the glycocalyx. every living life, every living cell has a sugar coat calledglycocalyx which consist of glycolipids
and glycoprotiens that completely coverthe cell and give it its molecular identity. these molecules swing aroundvery much like the kelp in the ocean right here. and if we make ourselves hundreds ofmillions of times smaller and land on a cell, one of my favorite cells, amammalian sperm cell, we would see something that reminds us of one of thesekelp forests. these are the glycolipids and glycoproteins. all of them share shortsugar chains on them, each little sugar is about one nanometer big. and what theyshare is that most of these chains terminate in a sugar coat sialic acid thathelps define the identity of the cell type and it tells the body that this is a selfcell. so sialic acid can be thought of as
a very potent self signal. you canvisualize them here with an antibody and you see the entire surface of the sperm,it's payload so to speak is the haploid genome in blue. and the surface of thesperm is completely covered in sialic acid. to go back to the kelp analogy,macrocystis kelp has these big terminal bulbs that help it float and it definesthe outer edge, the molecular frontier of every cell, that's what you can think ofas sialic acids. they're very important because they're telling the body that thisis self and they play a role in fertilization, in gestation, indevelopment including during pregnancy. peter medawar many years ago coined thephrase "immunological paradox of mammalian
pregnancy" where a female, a mother, isgestating an individual that is genetically not identical to her withinher body, in a very intimate contact. the interface between the fetal cells of thetrophoblast and the mother includes these sialic acids, these sugar molecules foundon cell surfaces of all vertebrates. this matters, it can be quite dramatic. thisyoung fellow here was born in 1963 and he had a problem; his mother was type o, hada lot of antibodies against type a blood. and they had a former son and his fatherwas type a. so what happened during the pregnancy is that the antibodies that hismother were making passed through the placenta and almost killed him. he wasborn with massive hemolytic disease of the
newborn that had only been described acouple of years earlier. and it's only because he was given two complete bloodtransfusions that he can stand here today and give a talk. so sugars and mismatchesof sugars are really important. but i would like to propose that there isanother immunological paradox and it's the one about mammalian fertilization. how dosperm manage to survive this journey from insemination to the place of fertilizationway up in the ampulla of the oviduct? if you think back, when you were a sperm, thereproductive tract of your mother was about the equivalent of six kilometers,we're back to running. of course, hundreds of millions of potential you's wereinseminated, but only one made it up all
the way to the ampulla. and that is partlydue to a massive influx of immune cells of the mother upon insemination that take outand actively kill, potentially select, most of the sperm. only a few hundredsmake it to the oviduct where they capacitate, they start sprinting,galloping and one of them meets the egg and fertilizes it. so, females may bescrutinizing and even selecting sperm. why would they do that? well, one thing theymight have to look out for is, is it the correct species? male mammals are quitefamous for trying to mate with anything that is shaped roughly like that. but thefemale might be interested in some read out of the fitness of the male who madethe sperm or in the fact that the male is
genetically compatible also moreimportantly that the sperm is still functional. wasting one precious egg on asperm that already lost its acrosome and is not fit to make a surviving embryowould be a terrible waste. so, i come back to the sialic acid on the surface of cellsand in most mammals, the two most common sialic acids are called n-acetylneuraminicacid and n-glycolylneuraminic acid, ac and gc for short. what is interesting is thatthere's an enzyme the modifies ac to gc and humans are natural knock outs. this iswork by ajit varki's laboratory that over the last 15 years has found the mechanism.we are knock out for a gene that was an insertion of a selfish piece of dna thatdestroyed the gene. we cannot make the gc
anymore. all of us in this room are pureac on our cell surfaces. so the cell surface of a human differs dramaticallyfrom that of most other mammals. so with all our close living ape relatives, weshare ac. but because of a mutation that is quite well-timed with three differentmethods using coalescence, molecular clock and the type of element that is present,we know that this mutation happened between 2 and 3 million years ago. and weknow that it causes us to only have one type of sugar on our cell surface, ofsialic acid. now you'd think it's a tiny, tiny change in dna, why could this be abig deal? i haven't mentioned that your average cell has tens to hundreds ofmillions of this molecule. so a tiny
change in your dna changes the flavor, themolecular flavor of your cells in a big way. a human cell would appear in oneflavor and a non-human cell in a completely different flavor. what couldhave driven this? you're looking at a model of a cell surface of a red bloodcell which make up over 80% of all your cells. and they're targeted by some of themost important pathogens that we know for human kind such as falciparum, malaria.this molecule encircled is glycophorin a that carries a lot of sialic acid whichmalaria uses to get into the host. so a couple of years back with ajit, wecommented on the fact that it is known that the apes that still make most of ourgreat ape cousins sick, they use sialic
acid that we don't have anymore. theycannot infect us. so, one possible driving force for the loss of this sugar initiallymight have been to escape a pathogen such as an ancestral malaria. we got a nicebreak. unfortunately, much, much later in the neolithic with agriculture and theexpansion of anopheles mosquito species, malaria caught up with us with a vengeanceand is now highly specific for the sugar we have on human cells. now,interestingly, as we've heard somewhere along the line to homo, we became toppredators and we regularly engage in hunting or scavenging or a combination ofboth. and when you eat this non-human sugar, you actually incorporate it and youstart making an immune reaction to it. so
after having lost the sugar, we startedgetting regularly immunized to it and making antibodies. and there is ongoingwork showing that this is incorporated, is relevant for modern homo sapiens, all ofus who eat red meat which is the biggest source of this non-human sugar, continueto immunize ourselves and to incorporate it. so several years back, we asked,"well, could it be that if sperm differs so dramatically that a change in cellsurface sugars could actually have been involved in this reproductiveincompatibility that i was unlucky enough to suffer from when i was born? that wasabo blood groups, very unusual, a rare case. but this would have been a verypowerful point, a way to immunize the
mother against the sugar she doesn't have.could this have been involved, this mutation? the fixation of it, could thathave been driven? could that have driven the speciation along the lineage to modernhomo? so one thing we could do at the time is obtain chimpanzee sperm in anon-invasive fashion. i shall not go into details. and expose those chimpanzee spermto human serum with antibodies and show that they die. but the reverse is nottrue. we could also show that compliment gets deposited on sperm. it seems to be anantibody-driven killing mechanism in human serum. luckily by then, there was a modelmouse that carried the same mutation that we humans carry. you can see that thewild-type mouse has the non-human sialic
acid on its sperm, the knock out mousedoesn't. and so we said, "well, let's use these mice to prove whether this mechanismcould function." and we could show that yes, the immunized mice would makeantibodies that stick to the sperm. the two groups of females, we mated withdifferent males, had similar comparable levels. and there were antibodies veryimportantly in the female reproductive tract of these mice. so, that would be away to model female immunity being hostile to the ancestral molecule. and afterhundreds of mating experiments, effectively the only group of pairingswhere there was a 30% reduction in fertility was with females lacking asugar, making antibodies against a sugar
that was on sperm that was mismatched.now, 30% reduction in fertility, is that important? could that drive speciation?and this is where stevan springer came in and came up with an instantaneous model ofselection with pay-off matrices of all possible combination between the genotypesof males and females. interestingly, this process could not start by sexualselection. this is a type of sexual selection, female immunity, punishingsperm for carrying the wrong sugar. but if a pathogen were to introduce and favor themutation, very quickly as the frequency of the mutation rises in the females, malesget rewarded by also losing the sugar because they gain compatibility. and aftera certain moment, you can cross a
threshold from negative selection infemales, in pink, to a net positive selection in black. in males, as soon asthere is enough females that lack the sugar, it is worth losing the sugar. sothat should gain compatibility. we modeled the effect of both the degree ofincompatibility and promiscuity and it showed that with a promiscuity of aboutthree matings per ovulation. so a female would have to mate between two and fivemales for each egg. and a frequency of only about 0.4% which is 1 in 25 femaleswill be homozygous. this process would become a directional selection fixing theallele. so in a cartoon version, the idea is that pathogens are prominently drivingthe sugars on your cell surfaces, that's
why we have blood groups. but usuallythese changes go back and forth. and they generate selection that oscillates andresults in polymorphisms that do not fix. but if this process comes on the sexualselection via female antibodies against molecules on the sperm, you havedirectional selection that rapidly fixes the loss of functional mutation. so whatwe propose might have happened somewhere in the past and possibly at the beginningof the genus homo is a very important pathogen driving changes in the glycocalyxof the host. some of the hosts would have been homozygous. they would have lookedvery different. the females by virtue of their new mode of life with much huntingand contact with other animal products
would have been immunized against thissugar. that immunity also protected them from infection from these bugs, but itwould preclude optimal compatibility with males that still had the ancestralmolecule, eventually driving apart two populations even within the same sympatricenvironment. so as a model for sympatric speciation of ancestral hominids. the hopenow is to get fossil material to actually look for incorporated monosaccharides asfar back as 3 or 4 million years and find out which lineages still have both sugarsas opposed to which lineages already had just one sugar and would make a bettercandidate for our ancestors. thank you very much.
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