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Practice English Speaking&Listening with: Origins of Genus Homo–Australopiths and Early Homo; Variation of Early Homo; Speciation of Homo

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- [Narrator] We are the paradoxical ape. Bipedal, naked large-brained. Long the

master of fire, tools and language, but still trying to understand ourselves.

Aware that death is inevitable, yet filled with 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 brings together experts from diverse disciplines

to exchange insights on who we are and how we got here. An exploration made possible

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- [William] Good afternoon everyone and thank you to the organizers here of CARTA

for inviting me to participate and thank all of you, a great crowd, for coming out

to see this fascinating topic explored. I tend sometimes to be accused of nihilism

with regard to the origin of Homo because my view is we actually know nothing about

the origin of Homo, just saying. And the reason is simple in my view; is that while

it is true that we have a pretty good fossil record of the genus Homo, the Homo

lineage as Bernard just finished explaining, by around 2 million years ago

with some diversity and different adaptive packages in different species: erectus,

habilis, rudolfensis. On the assumption that these three forms shared a common

ancestor at some point. That common ancestor lived older than 2 million years

ago in a period of time in which we have not a fender and a tire and a piece of

gear shift, but in which we have a fragment of tire thread, which we have a

fragment of a headlight. And we are trying to reconstruct an evolutionary history of

a group for which we basically have a car wreck. And this is what we have to solve,

this is the problem we have to solve and this comes from field work and I'm going

to illustrate for you today in my view where I think the genus, the Homo lineage

arose and where we have to re-double our efforts for increasing the representation

of this lineage older than 2 million years ago. Now, as Bernard ably suggested, the

modern history of the study of the evolution of the genus Homo really begins

with Louis Leakey and colleagues and the recognition of the species, Homo habilis

in 1964 based on material from Bed I in Olduvai Gorge dated to between around 1.7

and 1.75 million years, they discerned in the type specimen of the species older by

Hominid VII what they thought was a human-like dextrous ability in the hands,

they discerned a notable increase in endocranial volume, brain size, in

relation to then known Australopithecus species, mostly from southern Africa and a

reduction in tooth size which they saw as emblematic of an overall gracilization of

the chewing apparatus in almost a human-like arrangement. And putting these

three characteristics together with the plentiful stone tools that had been

recovered for years in these sediments, they arrived at the conclusion that this

species, habilis, belonged near the base of the genus Homo. So convinced were they

of this conclusion that Philip Tobias, one the co-authors of the species, was able to

write in 1965 that Homo habilis represented that last remaining major gap

in the pleistocene evolution of the genus Homo, “of the story of human evolution, ”

to quote him directly. And in this phylogeny shown here from one of Tobias'

papers, you can see the genus Homo is represented as a single, gradually

evolving line characterized by uniquely human characteristics related to large

brain size, reduced canine teeth, a perfection of bipedal locomotion as we now

see it, a slowing down of the growth trajectory, technology, language and so

forth. This was a package of characteristics seen in modern humans and

thought to go back in time to at least 2 million years as an integrated whole,

along this slowly emerging lineage culminating in Homo sapiens. The problem

was of course is that older than 2 millions years ago, there was virtually no

fossil record that could be confidently associated uniquely with our lineage. And

so whether these characteristics emerged piecemeal, step-wise and therefore each

demanding a separate explanation for origin, or whether they emerged as a

package together, where one explanation would take care of them all, could not be

discerned. Now a lot has happened, as Bernard 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 now see 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 2 million years ago whereas in 1964, the

Leakeys would have said there's one in the genus Homo: Homo rudolfensis, Homo habilis

and Homo erectus. And one of the lessons that we have learned from the appreciation

of greater diversity in our own genus at this period of time, is the idea that

there is not one adaptive package that can describe 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 one form or another, subsequent developments

to the establishment of the lineage? Following on Bernard's talking about

Toyotas and Clades, my appreciation, my rendering of the information available

from these three forms between around 1.7 and 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 for it, but I think it speaks fairly clearly

to the idea that these three at 2 million did 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 this important more than just for the purposes

of putting cladograms or phylogenies on the page is because in the last decade or

two, information from global climate change, paleo-climatic change, has made it

clear that the tim period in which many people suspect the Homo lineage arose was

one of a very widespread, impactful change in 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 right after the 3 million year mark; 2.8, 2.7

and so forth. And that drying out of Africa 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 become the prevailing hypothesis that the

complexification, if you will, of hominids and the origin of technology is all

associated with the local impacts of these global changes. The problem is that

there's no fossil evidence for the genus Homo that is informative on exactly what

those changes were at this particular point in time. We do have of course

Oldowan tools at around 2.6 million. And as Bernard and others have pointed out,

perhaps that is a proxy for the genus Homo or maybe it isn't. It's not outside the

realm of possibility given what we know about how chimpanzees can make tools that

some Australopithecus is capable of making them, too. So, questions and an absence of

evidence. And here is the sum total of the fossil record of the genus Homo between 2-

and 2.5 million years ago. It would fit in a shoe box and leave room for a decent

pair of shoes. All of these fossils have been promoted by one person or another,

one group or another as identifying the genus Homo older than 2.0 million years

ago and all of them have been doubted. And I'm not going to go through them here to

point out the weaknesses and strengths of the various arguments, other than to say

that the very fact that there's debate can be traced to the fact that there's

relatively little evidence. And this is why groups return to Africa, go to the

field to African sites, in East Africa, in South Africa all the time focusing on this

time period which, in my view, is one of the most intriguing of all the time

periods in human evolution to increase our understanding of the fossil record. One

area where the group from The Institute of Human Origins which I direct at ASU has

been focusing on, of course, for years is Ethiopia. We've worked at the Lucy site

more or less continuously since 1990. And colleagues of mine, Dr. Kaye Reed at ASU

and Chris Campisano and others, have expanded the work, the IHO work in

Ethiopia, to a place called Ledi-Geraru as seen here as slightly north and east of

the Hadar area. What attracted them to this area? Two things, knowledge that the

environments represented by the sediments in this area looked different from those

that were very common and well-understood in the Lucy time period, older than 3

million, some 20, 30 kilometers away at Hadar. And second, the suspicion verified

since then that the rocks may actually represent a slightly younger time period

and that's important because at Hadar, as you will see, we have Lucy species,

Australopithecus afarensis up to about 3 million years and then we jump across

three quarters of a million years and we have 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 and Dikika and Woranso-Mille, there are

excellent sediments going backwards in time from around 3 million years ago. And

we have an excellent set of sediments in places 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 questions about where the three forms of Homo that

we know of at 2 million perhaps emerged from? And these sediments are present

amply, now well-studied in the Ledi-Geraru area 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 an advantage and a disadvantage is that they

are not continuous across time, but instead are exposed in fault blocks,

adjacent fault blocks which means that each block of sediment is a unified slice

of time separated from another block next to it which has itself a unified period of

time with slight gaps in between them. Disadvantaged because we can't trace

evolutionary events continuously but advantage because fossils that come,

demonstrably come, from particular fault blocks can be narrowed to a very narrow

range of environments and associations with other animal species, etc. So, a plus

and a minus. And here is the Ledi-Geraru area. Kaye and her team have been working

her for more than a decade before they found their first hominid. Looking at the

fauna, looking at the geology, trying to understand the environments. And by the

way, this is an area called the Lee Adoyta basin 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 just exposed in this one view, very clearly

delineated. You can see one of the faults running right through here. Now, back in

2013, Kaye and her group of paleontologists were surveying an area in

the Lee Adoyta basin called the Gurumaha block just in the one fault block. And at

the base of this one hillside, there's a volcanic ash that is now well-dated, very

precisely dated to 2.8 to 2, plus or minus a handful of years, in the million year

range. And on one winter's day, one of our graduate students at ASU, Chalachew

Seyoum, was surveying up on this hillside and found this little jaw. That jaw eroded

out of this hill, perhaps in a recent rain storm and resides about 10, maybe 12

meters above that volcanic ash. And on the hillside, there are no sediments up above

younger that the jaw could have floated down 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 that it answers some questions, answer some

very specific questions. It doesn't answer all the questions. But there's a myth out

here in Paleoanthropology that unless you have a complete skeleton, you're not

prepared to answer any meaningful questions and I wish to dispel that myth.

You know, since Raymond Dart named Australopithecus in 1925, there have been

a plethora of hominid species named, recognized; Australopithecus africanus,

Paranthropus robustus, Paranthropus boisei, Homo Habilis, on and on. Many of

them, if not most of them on the basis of material that we here today would

consider, at best, imperfect. A fragment of a jaw, a bit of a brain case, some

teeth, and the fact of the matter is is that in the intervening years, the vast

majority of those species recognized on the basis of imperfect material have been

verified as to be meaningful evolutionary units. We are not at sea when we have

small fragments. We are limited in the type of questions we can ask. If complete

skeletons were the answer to all of our questions, then Lucy would have settled,

once and for all, the debate about when early humans made a commitment to

terrestrial bipedality. Instead, she generated what is now going on to five

decades of debate about that question. It depends 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 something different? And I engaged in that question

with my former PhD student, Brian Brian Villmoar now at University of Nevada, Las

Vegas, and Chalachew Seyoum our graduate student who found the jaw. And we came to

the conclusion that in many respects, it differs from your standard issue

generalized Australopithecus jaw. Seen here on the left is a nice jaw of Lucy's

species, Australopithecus afarensis and on the right is a reconstructed from a scan

of the specimen from Ledi-Geraru. We noticed that the jaw differs

rather...these two jaws differ rather remarkably. 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 like this, one of the jaws from the Dmanisi

site dated to about 1.8 million years which is attributed to Homo erectus. And

there's a much greater similarity in the shape of the dental arch, in the form of

the teeth, the pre-molars being symmetrical and so forth, to this 1.8

million year old Homo erectus jaw than to Lucy's species. And it extends also to the

architecture of the jaw and I'm not going to go into the details here, but

underneath the pre-molar, the afarensis jaw is characterized by a highly sculpted

out, contour like a chimpanzee probably do, to the very large canine teeth absent

in the Ledi-Geraru jaw. The back part of the mandible where the vertical part

called the ascending ramus arises from the body of the jaw is located in the Ledi jaw

well back of the third molar, not forward as it is in Lucy's species over the second

molar. And the upper and lower boundaries of the mandibular borders beneath the

teeth and at the base are more or less parallel. And in Australopithecus, they're

not, it gets shallower to the rear. And by the way, it's also true of

Australopithecus africanus which is slightly 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 obviously because we don't have much in the 2.5 to 3

million year period, the similarities were very apparent to us. This is a jaw that

exhibits characteristics that forecast anatomy that is common, the most common

anatomical patterns in jaws of the genus Homo younger than 2 million. So we

published it, in not quite a year ago, in the Journal of Science as a 2.8 million

year old jaw of the genus Homo. Now does it answer questions about what were the

adaptive packages present early on in the lineage leading to us? Of course. But what

it does do is that it puts one data point in an area that is otherwise a void in the

evolution of our own genus. Question is what kind of environment did it live in?

Did it live in a dry environment? Did it live in an open one? Germane to the

questions about what drove early evolution of Homo. And data that's been put together

by Kaye Reed and given to me for this purpose shows that this jaw is found in a

context of animal species that lived in essentially grassland environments, very

different in terms of how open or closed the habitats were compared to time periods

in which Lucy's species lived. And this is just a couple hundred thousand years

later. Now, I hasten to add here, I am not asserting that the origin of the genus

Homo is due to a drying out of the environment. But one thing we can say,

because of the very confined time period of the Gurumaha fault block in which the

mandible and the fauna on which this inference is made, suggest that the modal

environmental signal at 2.8 in this area is one essentially of a grassland

environment. And we can see that by looking at some of the other animal

fossils that have been found associated with 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 of course are well-known grazers. And

together, in the Gurumaha block, they constitute 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 and that is very impressive compared to the

frequencies back in Lucy's time starting just 200,000 years earlier. Opens up areas

for inquiry. And finally, some new data coming out of Kenya from Sonia Harmand's

group suggests that stone tool use, in fact, began not with the genus Homo,

maybe. But perhaps as long ago as 3.5 million years when we have

Australopithecus. And if these finds are verified, it opens up a whole new range of

possibilities looking at the adaptive packages that constitute the ancestral

platform from which the genus Homo emerged. And so to finish up, here we have

Ledi-Geraru, here we have our formerly first appearance, a former first

appearance of stone tools now pushed back here perhaps and does that imply that the

genus Homo itself has even an earlier origin than we think of at 2.8, perhaps

back as far as Lucy? Or could Lucy herself have been the first stone tool maker?

Thank you.

♪ [music] ♪

- [Philip] We've heard a lot at this point about the evolution of hominids in Africa.

It's a complicated history for sure. Let me at this point just cut to the chase and

say that humans moved out of Africa probably just after 2 million years ago

and it will be that part of the record that I want to emphasize this afternoon.

The site of Dmanisi in the Georgian Caucasus is very important, records the

oldest known, at this point, the oldest known occupations of Eurasia beginning

before 1.85 million years ago. We don't actually have human remains that are that

old, but certainly there are stone tools approaching that date. The good part about

Dmanisi is that in fact we have not just scraps of headlamp and bumper and so

forth, but virtually whole skeletons and a number 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 juvenile individual particularly perhaps but not

clearly associated with one of the adults. Also the material is extremely

well-preserved. We're very fortunate in that 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 this material which is well-preserved, as I

say, in a very carefully studied stratigraphic context. Several of the

specimens from Dmanisi have been in the past likened to African Homo erectus but

the skeletons are quite primitive. One of them 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 the paleobiological significance of the

Dmanisi materials remain controversial. Certainly there have been plenty of

suggestions and I'm afraid I've been responsible for some of them but we'll see

where things go. Dmanisi is situated in Georgia. Georgia is stuck there between

the Black Sea and the Caspian with Azerbaijan off to the east. From Tbilisi,

the capital, it's about an hour and a half, an hour and 45 minutes ride. Roads

are pretty good these days. Roads were terrible 15 years ago. Things have

improved. Down to the site, Dmanisi is just a few kilometers from the Armenian

border. This is the obligatory excavations in 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 other sediments 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 Mashavera basalt which is about 1.85 million years

old, that's the bottom of the site, the earliest record, 1.85 million years is the

date obtained from radiometric methodology. It is secure. The

stratigraphy at the site is complex partially because there are a number of

piping features. Water was present near the site during periods of heavy rain and

so on. Pipes formed underground and then progressed through breaching to collapse

towards the surface filled with sediments then 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, huge in the Trumpian since. But there have been

problems. The first traces of human material were found at Dmanisi in 1991.

Excavations in fact have been underway at the site for quite a period of time before

that. The site is underneath an old medieval town that was on the Silk Road.

The archeologists were busy at Dmanisi for some time poking around the foundations of

the old buildings and eventually they began to dig up stuff that didn't seem to

belong there, not just the goats and fish bones from medieval suppers, but things

that looked quite antique indeed. The paleontologists came in and ascertain that

yes, the material was ancient, deeper excavations got underway and in 1991, the

folks at Dmanisis were rewarded with this jaw, the D211 mandible. It's remarkably

complete, not all of it is there. But what there is remarkably well-preserved. It's a

small jaw and in a number of respects, it does 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 mandible itself. The cranium which turns out to be

the match to the little mandible was found later in 1999, D2282 is a small cranium,

very small capacity, surprisingly so for Homo erectus. Only a bit more than 650 CCs

in this case. Despite the small brain, the thing does share a number of

characteristics with particularly early African erectus. This hulk turned up at

the site in 2005. It's way down at the bottom of the site and within a few days

after the fossil had been uncovered and cleaning 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 in around 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 the cranium found in 2005 is a perfect match,

once again, to a mandible, D2600, which had been found earlier in the year 2000.

The upper and the lower, the cranium and the mandible simply clicked together once

the stuff have been cleaned off, there was no doubt at all. There is some pathology

on the mandible that matches, comparable pathology in the region of the ear of the

cranium so there is no doubt about the match. More than other Dmanisi hominins,

Skull 5, as this one is known, exhibits very robust morphology. It's pretty

clearly a male individual. Determining sex in the case of these fossil hominins is

often tricky, often can't be done very accurately. But in this case, we think we

have a match, the skull says male all over. Such a pattern given the fact that

it has the smallest brain of all the Dmanisi hominins because it is unexpected.

Normally for other primates, humans too of course, but for primates, higher primates

generally, males tend to exceed the females in brain size by something like 8

to 10 to 15%. So, having the tiniest brain attached to the most robust cranium and

jaw is a bit surprising. You can see that there is a good deal of variation within

the Dmanisi assemblage. The little jaw goes with a smallish brain case which is

quite gracile in its construction. We peg that 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 age since it had lost almost its entire

dentition, maybe one tooth was still in place at the time the individual died.

That one may or may not be male, we're not sure. Anyway, there is a great deal of

diversity at Dmanisi. The crania do look different. This raises the question of

"How many species might be documented at the site?" This is a question that's been

plaguing us for some time. I think myself on the basis of the shared anatomy among

the Dmanisi individuals, they have a common bow plan extending not just to the

cranial vault but to the face insofar as we have it represented. And also to the

details of the cranial base suggest, the common 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 same conclusion that really, in fact, the

skulls, the post-cranial remains that go with them, are drawn from just one

population. Now there is stratigraphic evidence relating to this question. It

doesn't solve the question of course, but it's important information. It's good to

know that the material was all washed into these deposits or arrived in the site by

one means or another at about the same time that is the duration here can't be

more than a few hundred or perhaps a thousand years or so according to the best

analyses conducted by the geological side of the team. We'll say, then, that it's

very likely that the Dmanisi assemblage samples a population belonging to a single

species. I know there may be objections to this. I'm sure there will be, there have

been in the past. If it's true, then such a situation is quite rare. Of course, at

most localities where hominins are discovered, 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 is scattered through a very long sequence of

deposits covering a long period in time. Time as a contributor variation just

cannot be discounted. If the Dmanisi fossils document what we can call a

population in the past extending over quite a number of years of course, then

the next question, the next important question is how the Dmanisi sample may

relate to the hominin taxa that have previously been recognized. Skill 5 of

course has a very small brain case, a very large projecting face in the vault. Also

in the basal cranium, there are some resemblances, not a lot, but some

resemblances to Homo erectus. Skull 3 which 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 true for the bow ridge, the extended brow ridge

development. It's true particularly for the 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 to have 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-ER 1813. Skulls 2 and 4 also have their

peculiar aspects, of course. They have a number of primitive characters that they

also share some features with Homo habilis. So, which species? There is, as

I've pointed out, much variation within the Dmanisi paleodeme. This is not an easy

question, the question as to which species may be represented. Skull 5, the very

small brained and very robust and very primitive looking individual does indeed

share some characters with Australopithecus as well as Homo. So

perhaps in this case, the line, the division between Australopithecus on the

one hand and earlier Homo on the other is not so clear cut after all. Many of these

shared similarities are primitive characters and unfortunately they don't

help us much in answering key questions about phylogenetic affinities. Other

characters expressed in the Dmanisi materials are Homo erectus-like and pretty

clearly they are specialized characters, characters that have changed during the

course of evolution, characters that are said to be derived. These characters

include the form of the brow ridge for example which is larger and bar-like, a

little bit of midline, keeling on the vault, details of temporal bone

construction, things of that sort on the underside of the vault. Indeed, when

Skulls 1 and 2 were first described back in 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 deal with the material is simply to lump it

with Homo erectus. If that is the course we take, then it must be recognized that

the boundaries between Homo erectus on the one hand and other early Homo taxa will

become less distinct. It will be particularly difficult to distinguish

early Homo erectus, African Homo erectus from specimens attributed to earlier Homo

to Homo habilis in particular. Homo habilis 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 Meave Leakey several years ago. You can read the

caution sign. To sum up then, there is apparently no simple answer to the

question as to which species may be represented at our site. Indeed this

question is often a tricky one. It's been a 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 very spaciose hominin phylogeny. In that view

hominin evolution has produced a veritable flowering 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 interval in which Paranthropus on the one hand,

Australopithecus and Homo are represented by multiple species for each group. At

Dmanisi the fossils seem clearly to be Homo. 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 with the genus Homo, very little doubt about

that. At the same time, the assemblage at Dmanisi does not fall neatly into one of

the taxonomic packages that have been proposed: Homo habilis, Homo rudolfensis,

Homo ergaster, Homo erectus and so on. If I were pressed and I do feel pressed at

this point, given the morphological resemblances of Dmanisi to both Homo

habilis in a very strict sense, just those fossils allocated to Homo habilis, not to

Homo rudolfensis. Given the resemblances of our material to Homo habilis, and to

early Homo erectus, particularly African Homo erectus, I would probably argue, I

will argue, that it is most reasonable to place all of these fossils within a single

evolutionary species. I would say that the Dmanisi 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 suggest that there isn't a great deal of diversity

in the hominin record, clearly there was particularly in that interval after, about

2.5 million years. But as far as our evidence is concerned, it seems to me the

best way to go, simply to place all the fossils within one evolutionary species.

Then of course we'd have to argue about what to call it, but this is not the place

for that. So, with that, thanks for listening. Thanks very much.

♪ [music] ♪

- [Pascal] Thanks very much for this great afternoon, to all the speakers. I'm

especially grateful to Dan Lieberman for having set the stage with a lot of carnage

and meat-eating, because the molecules I talk about today have to do with

vertebrates and what we eat and what happens to it in our bodies. So this

afternoon I'd like to share an idea about a way that a molecule could be driving

speciation and share some evidence for it in Vigo [sp], not in primates but in mice.

I like to start with by acknowledging the people who've done some of the heavy

lifting, including Fang Ma who's back in Chengdu, a former lab member and Darius

Ghaderi and my team here especially Stevan Springer 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 talk about the glycocalyx. Every living life,

every living cell has a sugar coat called glycocalyx which consist of glycolipids

and glycoprotiens that completely cover the cell and give it its molecular

identity. These molecules swing around very much like the kelp in the ocean right

here. And if we make ourselves hundreds of millions of times smaller and land on a

cell, one of my favorite cells, a mammalian sperm cell, we would see

something that reminds us of one of these kelp forests. These are the glycolipids

and glycoproteins. All of them share short sugar chains on them, each little sugar is

about one nanometer big. And what they share is that most of these chains

terminate in a sugar coat sialic acid that helps define the identity of the cell type

and it tells the body that this is a self cell. So sialic acid can be thought of as

a very potent self signal. You can visualize 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 the sperm 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 defines the outer edge, the molecular frontier of

every cell, that's what you can think of as sialic acids. They're very important

because they're telling the body that this is self and they play a role in

fertilization, in gestation, in development including during pregnancy.

Peter Medawar many years ago coined the phrase "immunological paradox of mammalian

pregnancy" where a female, a mother, is gestating an individual that is

genetically not identical to her within her body, in a very intimate contact. The

interface between the fetal cells of the trophoblast and the mother includes these

sialic acids, these sugar molecules found on cell surfaces of all vertebrates. This

matters, it can be quite dramatic. This young fellow here was born in 1963 and he

had a problem; his mother was type O, had a lot of antibodies against type A blood.

And they had a former son and his father was type A. So what happened during the

pregnancy is that the antibodies that his mother were making passed through the

placenta and almost killed him. He was born with massive hemolytic disease of the

newborn that had only been described a couple of years earlier. And it's only

because he was given two complete blood transfusions that he can stand here today

and give a talk. So sugars and mismatches of sugars are really important. But I

would like to propose that there is another immunological paradox and it's the

one about mammalian fertilization. How do sperm manage to survive this journey from

insemination to the place of fertilization way up in the ampulla of the oviduct? If

you think back, when you were a sperm, the reproductive 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 were inseminated, but only one made it up all

the way to the ampulla. And that is partly due to a massive influx of immune cells of

the mother upon insemination that take out and actively kill, potentially select,

most of the sperm. Only a few hundreds make it to the oviduct where they

capacitate, they start sprinting, galloping and one of them meets the egg

and fertilizes it. So, females may be scrutinizing and even selecting sperm. Why

would they do that? Well, one thing they might have to look out for is, is it the

correct species? Male mammals are quite famous for trying to mate with anything

that is shaped roughly like that. But the female might be interested in some read

out of the fitness of the male who made the sperm or in the fact that the male is

genetically compatible also more importantly that the sperm is still

functional. Wasting one precious egg on a sperm that already lost its acrosome and

is not fit to make a surviving embryo would be a terrible waste. So, I come back

to the sialic acid on the surface of cells and in most mammals, the two most common

sialic acids are called N-acetylneuraminic acid and N-glycolylneuraminic acid, AC and

GC for short. What is interesting is that there's an enzyme the modifies AC to GC

and humans are natural knock outs. This is work 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 that destroyed the gene. We cannot make the GC

anymore. All of us in this room are pure AC on our cell surfaces. So the cell

surface of a human differs dramatically from that of most other mammals. So with

all our close living ape relatives, we share AC. But because of a mutation that

is quite well-timed with three different methods 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 we know that it causes us to only have one

type of sugar on our cell surface, of sialic acid. Now you'd think it's a tiny,

tiny change in DNA, why could this be a big deal? I haven't mentioned that your

average cell has tens to hundreds of millions of this molecule. So a tiny

change in your DNA changes the flavor, the molecular flavor of your cells in a big

way. A human cell would appear in one flavor and a non-human cell in a

completely different flavor. What could have driven this? You're looking at a

model of a cell surface of a red blood cell which make up over 80% of all your

cells. And they're targeted by some of the most 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 which malaria uses to get into the host. So a

couple of years back with Ajit, we commented on the fact that it is known

that the apes that still make most of our great ape cousins sick, they use sialic

acid that we don't have anymore. They cannot infect us. So, one possible driving

force for the loss of this sugar initially might have been to escape a pathogen such

as an ancestral malaria. We got a nice break. Unfortunately, much, much later in

the Neolithic with agriculture and the expansion of Anopheles mosquito species,

malaria caught up with us with a vengeance and 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 top predators and we regularly engage in

hunting or scavenging or a combination of both. And when you eat this non-human

sugar, you actually incorporate it and you start making an immune reaction to it. So

after having lost the sugar, we started getting regularly immunized to it and

making antibodies. And there is ongoing work showing that this is incorporated, is

relevant for modern Homo sapiens, all of us who eat red meat which is the biggest

source of this non-human sugar, continue to 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 cell surface sugars could actually have been

involved in this reproductive incompatibility that I was unlucky enough

to suffer from when I was born? That was ABO blood groups, very unusual, a rare

case. But this would have been a very powerful 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 that have been driven? Could that have driven

the speciation along the lineage to modern Homo? So one thing we could do at the time

is obtain chimpanzee sperm in a non-invasive fashion. I shall not go into

details. And expose those chimpanzee sperm to human serum with antibodies and show

that they die. But the reverse is not true. We could also show that compliment

gets deposited on sperm. It seems to be an antibody-driven killing mechanism in human

serum. Luckily by then, there was a model mouse that carried the same mutation that

we humans carry. You can see that the wild-type mouse has the non-human sialic

acid on its sperm, the knock out mouse doesn't. And so we said, "Well, let's use

these mice to prove whether this mechanism could function." And we could show that

yes, the immunized mice would make antibodies that stick to the sperm. The

two groups of females, we mated with different males, had similar comparable

levels. And there were antibodies very importantly in the female reproductive

tract of these mice. So, that would be a way to model female immunity being hostile

to the ancestral molecule. And after hundreds of mating experiments,

effectively the only group of pairings where there was a 30% reduction in

fertility was with females lacking a sugar, 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 of selection with pay-off matrices of all

possible combination between the genotypes of males and females. Interestingly, this

process could not start by sexual selection. This is a type of sexual

selection, female immunity, punishing sperm for carrying the wrong sugar. But if

a pathogen were to introduce and favor the mutation, very quickly as the frequency of

the mutation rises in the females, males get rewarded by also losing the sugar

because they gain compatibility. And after a certain moment, you can cross a

threshold from negative selection in females, in pink, to a net positive

selection in black. In males, as soon as there is enough females that lack the

sugar, it is worth losing the sugar. So that should gain compatibility. We modeled

the effect of both the degree of incompatibility and promiscuity and it

showed that with a promiscuity of about three matings per ovulation. So a female

would have to mate between two and five males for each egg. And a frequency of

only about 0.4% which is 1 in 25 females will be homozygous. This process would

become a directional selection fixing the allele. So in a cartoon version, the idea

is that pathogens are prominently driving the sugars on your cell surfaces, that's

why we have blood groups. But usually these changes go back and forth. And they

generate selection that oscillates and results in polymorphisms that do not fix.

But if this process comes on the sexual selection via female antibodies against

molecules on the sperm, you have directional selection that rapidly fixes

the loss of functional mutation. So what we propose might have happened somewhere

in the past and possibly at the beginning of the genus Homo is a very important

pathogen driving changes in the glycocalyx of the host. Some of the hosts would have

been homozygous. They would have looked very different. The females by virtue of

their new mode of life with much hunting and contact with other animal products

would have been immunized against this sugar. That immunity also protected them

from infection from these bugs, but it would preclude optimal compatibility with

males that still had the ancestral molecule, eventually driving apart two

populations even within the same sympatric environment. So as a model for sympatric

speciation of ancestral hominids. The hope now is to get fossil material to actually

look for incorporated monosaccharides as far back as 3 or 4 million years and find

out which lineages still have both sugars as opposed to which lineages already had

just one sugar and would make a better candidate for our ancestors.

Thank you very much.

.♪ [music] ♪

The Description of Origins of Genus Homo–Australopiths and Early Homo; Variation of Early Homo; Speciation of Homo