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

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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 by the generosity of humans like you. ♪ [music] ♪ – [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] ♪


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