Ardipithecus ramidus

[Palmela]

In a special issue of Science, an international team of scientists has for the first time thoroughly described Ardipithecus ramidus, a hominid species that lived 4.4 million years ago in what is now Ethiopia. This research, in the form of 11 detailed papers and more general summaries, was published today in the journal's 2 October 2009 issue. Science is published by AAAS, the nonprofit science society.

The package of research offers the first comprehensive, peer-reviewed description of the Ardipithecus fossils, which include a partial skeleton of a female, nicknamed “Ardi.” Publication of the new research was the subject of simultaneous news conferences today in the Ethiopian capital of Addis Ababa, and at AAAS/Science headquarters in Washington, D.C., with major international news media quickly conveying the story to a worldwide audience.

“What we celebrate here today are the results of a scientific mission to the very deep past,” said Tim White of the University of California Berkeley, one of the lead authors of the research, at the AAAS news conference.

The discovery and publication of the research is “an extraordinary event,” Samuel Assefa, Ethiopian ambassador to the United States, said at AAAS. “The deeper point for all of us is a deeper sense of our interconnectedness.”

Alan I. Leshner, chief executive officer of AAAS and executive publisher of Science, called the publication “truly a landmark event in our understanding of human origins.”

The last common ancestor shared by humans and chimpanzees is thought to have lived six million or more years ago. Though Ardipithecus is not itself this last common ancestor, it likely shared many of this ancestor's characteristics. For comparison, Ardipithecus is more than a million years older than “Lucy,” the partial female skeleton of Australopithecus afarensis. Until the discovery of the new Ardipithecus remains, the fossil record contained scant evidence of other hominids older than Australopithecus.

Through an analysis of the skull, teeth, pelvis, hands, feet and other bones, the researchers have determined that Ardipithecus had a mix of “primitive” traits, shared with its predecessors, the primates of the Miocene epoch, and “derived” traits, which it shares exclusively with later hominids.

Because of its antiquity, Ardipithecus takes us closer to the still-elusive last common ancestor. However, many of its traits do not appear in modern-day African apes. One surprising conclusion, therefore, is that it is likely that the African apes have evolved extensively since we shared that last common ancestor, which thus makes living chimpanzees and gorillas poor models for the last common ancestor and for understanding our own evolution since that time.

“In Ardipithecus we have an unspecialized form that hasn't evolved very far in the direction of Australopithecus,” said White, a professor at the Human Evolution Research Center and the Department of Integrative Biology at the

University of California at Berkeley. “So when you go from head to toe, you're seeing a mosaic creature that is neither chimpanzee, nor is it human. It is Ardipithecus.”

“With such a complete skeleton, and with so many other individuals of the same species at the same time horizon, we can really understand the biology of this hominid,” Gen Suwa of the University of Tokyo, the project paleoanthropologist and also a lead Science author, said in an interview.

C. Owen Lovejoy, a professor of anthropology at Kent State University in Ohio, reconstructed the “Lucy” skeleton and was a lead researcher and author on the “Ardi” project. He called the “Ardi” skeleton “a treasure-trove of surprises” and “one of the most revealing hominid fossils I ever could have imagined.”


Brooks Hanson, deputy editor for physical sciences at Science, hailed the importance of the work, calling it “one of those special and wonderful moments in science.”

“These articles contain an enormous amount of data collected and analyzed through a major international research effort,” Hanson said. “They throw open a window into a period of human evolution we have known little about, when early hominids were establishing themselves in Africa, soon after diverging from the last ancestor they shared with the African apes.

“Science is delighted to be publishing this wealth of new information, which gives us important new insights into the roots of hominid evolution and into what makes humans unique among primates,” said Hanson.

The special collection of Science articles is published 150 years after the publication of Charles Darwin's “On the Origin of Species.” The special issue begins with an overview paper that summarizes the main findings of this research effort. In this article, White and his coauthors introduce their discovery of over 110 Ardipithecus specimens, including a partial skeleton with much of the skull, hands, feet, limbs and pelvis. This individual, “Ardi,” was a female who weighed about 50 kilograms (110 pounds) and stood about 120 centimeters (just under 4 feet) tall.

Until now, researchers have generally assumed that chimpanzees, gorillas, and other modern African apes have retained many of the traits of the last ancestor they shared with humans--in other words, this presumed ancestor was thought to be much more chimpanzee-like than human-like. For example, it would have been adapted for swinging and hanging from tree branches, and perhaps walked on its knuckles while on the ground.

Ardipithecus challenges these assumptions, however. These hominids appear to have lived in a woodland environment, where they climbed on all fours along tree branches--as some of the Miocene primates did--and walked, upright, on two legs, while on the ground. They do not appear to have been knuckle-walkers, or to have spent much time swinging and hanging from tree-branches, especially as chimps do. Overall, the findings suggest that hominids and African apes have each followed different evolutionary pathways, and we can no longer consider chimps as “proxies” for our last common ancestor.

“Darwin was very wise on this matter,” said White.

“Darwin said we have to be really careful,” he added. “The only way we're really going to know what this last common ancestor looked like is to go and find it. Well, at 4.4 million years ago we found something pretty close to it. And, just like Darwin appreciated, evolution of the ape lineages and the human lineage has been going on independently since the time those lines split, since that last common ancestor we shared.”

The special issue of Science includes an overview article, three articles that describe the environment Ardipithecus inhabited, five that analyze specific parts of Ardipithecus' anatomy, and two that discuss what this new body of scientific information may imply for human evolution.

Altogether, 47 different authors from around the world contributed to the total study of Ardipithecus and its environment. The primary authors are Tim White of the University of California, Berkeley; Berhane Asfaw of Rift Valley Research Service in Addis Ababa; Giday WoldeGabriel of Los Alamos National Laboratory; Gen Suwa of the University of Tokyo; and C. Owen Lovejoy of Kent State University.

“These are the results of a mission to our deep African past,” WoldeGabriel, project co-director and geologist, said before his appearance at AAAS.

“Ardi's” bones will be quartered at the Ethiopian National Museum, where there's already a significant “Lucy” exhibit. WoldeGabriel reminded reporters at AAAS of the extensive collections of hominid fossils discovered in his nation, some dated to 6 million years ago. “Ethiopia can rightly claim to be the cradle of mankind,” he said.

The research was funded by the National Science Foundation; the Institute of Geophysics and Planetary Physics of the University of California at Los Alamos National Laboratory (LANL); the Japan Society for the Promotion of Science; and others.

http://www.aaas.org/news/releases/2009/1001sp_ardi.shtml


http://www.aaas.org/news/releases/2009/1001sp_ardi.shtml

 

[Phronesis]

Interesting, when did they find the skeleton?

 

[Palmela]

In Ethiopia 15 years ago.

Interview with Dr. Tim D. White,
Professor, Human Evolution Research Center, and
Department of Integrative Biology,
University of California at Berkeley


EWL: Hello, I'm Ed Lempinen of the AAAS [American Association for the Advancement of Science] Office of Public Programs, and I'm speaking today with Dr. Tim White of the University of California at Berkeley. Dr. White is one of the lead authors of a collection of articles that will appear in Science this week [publication date 2 October, 2009]. The articles present the first major fossil analysis of Ardipithecus ramidus, including the oldest known hominid skeleton, from 4.4 million years ago, in what is now Ethiopia.

Dr. White, could you begin by telling me briefly the story of how you discovered and excavated these fossils, and especially how you dealt with the condition of the “Ardi” skeleton?

TDW: Well, that story starts a long time ago. What you're seeing now is a publication by 47 total authors. I'm number 47. [Numbers] 46, 45 and so forth aren't here at the table with me table so I'm going to kind of summarize our work there, and the work has been going on since 1981 in this region of Ethiopia.
The team is a very large team and our basic overall goals are to find as much evidence as we can for what happened in that part of the world over the last 6 million years.

So back in 1992, we were at the Aramis locality in Ethiopia. It's out in the desert in Ethiopia in the Afar Triangle, and we were looking for hominid remains. My colleague from the University of Tokyo, Gen Suwa, found a molar tooth. It was very old. We knew the beds we were working were much older than the Lucy discovery, which is around 3.2 million [years ago]. We knew we were older than 4 million [years ago].

So we were happy to get this and we worked for a couple years and found more teeth, an arm from another individual; basically, you could have put all of those fossils into a baseball cap at that point, and I figured, we're not going to get too much more because this site had a lot of hyena activity at it and at 4.4 million years ago, the bones are highly fragmentary.

But we returned to the field after publishing those first discoveries and naming this new creature, Ardipithecus, and you know, you - as a paleontologist, you always have a hope of finding something more complete. Skeletons are really key to this because if you can find all the parts of a single individual skeleton, then you can really start to understand the biology, and here in this place, we had a chance to understand more than just the skeleton biology and the individual biology. But we had found lots of fossil wood, fossil monkeys, lots of other animals in these deposits, so we were out there collecting these things, and during one of these operations to clear the surface of all the bones that eroded to the surface, Yohannes Haile-Selassie, who was a graduate student here at Berkeley at the time, found a piece of a bone from the palm of a hand.

And we started to do the recovery operation there by screening the loose sediment on the surface and [we] started to find other bones. And then we thought, “Well, maybe we have something more than what you usually get.” What you usually get is a tooth or a jaw, or if you're lucky, a piece of a face. And we found more and more of this individual and by the end of that field season in 1994, we had found over 100 pieces of this single individual skeleton.

The problem was that these pieces had been very broken at the time that they were deposited. They were in very bad shape. They were at that point in time cemented into a very hard silky matrix so those had to be removed from the field, encased in plaster jackets and hardened with a consolidant. Very, very fragile.

And it took us many, many years to clean the bones in the National Museum of Ethiopia and then set about to restore this skeleton to its original dimensions and form and then study it and compare it with all the other fossils that are known from Africa and elsewhere as well as with modern age.

So that's sort of the story - we excavated back at that skeleton site for three separate years before we ran out of pieces. We don't have the entire skeleton, but we have some of the most important parts of it.

EWL:: Was there ever a moment where you were in the field or working in the lab and you knew you had made a significant new discovery?

TDW: Well, anytime that you're in a previously unknown time period as we were there, we knew that anything we got was going to be important. We hoped of course that we would have much of the skeleton. But this took weeks to excavate. So there was never really a point at which we said, “Wow,” you know, “we feel pretty happy here. We've got a partial skeleton.” It sort of evolved as we dug along.

The high points were finding the mandible, the lower jaw, with a complete dentition. That was great. We had found foot bones, hand bones. We found the base of the skull, and when we took it out from the field, I remember digging a pedestal around this thing to lift it out in a plaster jacket, hoping that we had the whole skull, the face and the back of the head. Turned out, years later as we cleaned this from the jacket, in the laboratory, only the back of the head was there, and not only that, it had been completely crushed so that the dimension from the top to the bottom of the cranium was only a couple of centimeters and completely flattened.

And then we went back and dug more, and we found isolated teeth and then one side of the face, and so we had lots of pieces that had been smashed a long time ago and we had to put these back together again to understand what it looked like.

EWL:: I wonder if you could expand the discussion of the bones a little bit? You found these bones in an ecological, environmental context. What other things did you find there, and not just the hominid bones, but plants, other animals?

TDW: This is a locality at a place call Aramis in the Middle Awash study area of Ethiopia. And if you were to go out to that site today, you'd stand on a volcanic ash horizon that was deposited 4.4 million years ago. Just under that horizon, there are reddish brown sediments that are silts from an ancient river system. As you walk along the surface, these soft sediments are eroding today.

So their content, all the fossils that were entombed there 4.5 million years ago have eroded to the surface. You're seeing more of them come out every year. So we've been back to this place many times, and we crawl across it and we pick up everything: Every seed, every piece of fossil wood, every scrap of bone.

So we're in a very good position, at the end of many years of crawling these outcrops and especially locality six - I think we have over 2,000 identified fossils from that site - and on an average crawl, if you were just to go out there and crawl along and pick up every tooth of a mammal, the ones you'd find most often are spiral-horned antelope, that we call “kudus.” We find lots of Colobus monkeys. We find lots of very small mammals, things like shrews and bats. You find occasional birds. You find lots and lots of seeds and fossil wood.

So we're able to then understand what the place was like - not just what the hominids were like, but the entire flora and fauna that they were embedded with as this ancient floodplain was accumulating sediments at around 4.4 million years ago.

EWL:: And what kind of environment was it? I understand maybe it was a forest and that wasn't necessarily expected?

TDW: Well, for years, people have thought that the earliest stages of human evolution happened as a response to living in grassland savannas, and even early in the analysis, starting back in the 1990s when we started to collect these things, we became very suspicious of this because all of the things found with Ardipithecus were indicating a more closed woodland habitat. We didn't have the grassland, grazing antelope. We didn't have the rodents that live in dry savanna regions. We had lots of Colobus monkeys. We had lots of animals that are adapted to woodland conditions.

So in the last 15 years, as we've been analyzing these hominid fossils and extracting them from the matrix, we've returned to these sites. We've crawled them. We've accumulated an avalanche of information and we have specialists. These other authors I've talked about - specialists in things ranging from pollen to seeds to bats, giraffids, rhinos, monkeys, you name it, right down to the person at the University of Illinois who studies the isotopes of the ancient soils and the isotopic composition of the enamel in the teeth of these various creatures, including Ardipithecus.

So what we have are multiple lines of evidence from geology, from paleontology, from geochemistry, from all across the spectrum and they all point to the same thing: These creatures were living and dying in a woodland habitat, not an open savanna.

continued

 

[Palmela]

EWL:: One of the exciting things about Ardipithecus is that, because it's so old, it may give researchers some ideas about the last common ancestor shared by humans and chimpanzees. I wonder if you could explain a bit more about the connection between Ardipithecus and this last common ancestor.

TDW: Well, ever since the days of [Charles] Darwin and [Thomas] Huxley, it's been appreciated, first based on anatomy and later of course on genetics, independently on genetics, that our closest living relatives are chimpanzees - African apes that live today in west Africa.

The issue for Darwin was the lack of a fossil record, and in fact, he devoted an entire chapter in his book, On the Origin of Species, to imperfections in the geological record in general. And the human fossil record in those days was extremely imperfect. There were a few Neanderthals known. Well, over the hundred-some [150] years since the Origin of Species was published, people have been collecting human fossils from all over the world, Europe and Asia and Africa, and we had a fairly good fossil record that went back to the time of Lucy and beyond, to between 3 and 4 million years ago. Now, that doesn't get us far enough back to reach that last common ancestor that we shared with chimpanzees.

So, it's been a kind of an elusive organism, as it was for Darwin. We just had a lack of fossils. What we found in Ethiopia at 4.4 million years ago is the closest we've ever come to that ancestor along our own line - unfortunately, the lines leading to modern chimps and gorillas are not even represented in the fossil record. These animals live in places that don't produce a good fossil record. So we have virtually no fossil record for these modern apes. But we have a really good one for fossil humans. Well, really good except it doesn't get us back far enough.

That's really one of the things that lead us in the Middle Awash to want to explore these older deposits, to see what we would find because ever since Darwin, people have sort of assumed that modern chimpanzees haven't evolved very much, that the last common ancestor was more or less like a chimpanzee and that it's been the hominid branch of the family tree, the human lineage, if you want, that's done all of the evolving. But without a fossil record, it's very hard to test that.

So this new evidence, coming as close as we've ever come to that last common ancestor really allows us to infer what that creature was like.

A lot of people predicted that when you found something at 4.4 [million years ago], it would look like a chimp. And in fact, this creature is a very interesting mosaic.

It is not chimpanzee-like - in its hand, for example. It's even more primitive than a chimpanzee. What this indicates is that the chimpanzee hand has evolved radically from the last common ancestor. We can't assume that it was the primitive condition, in fact, it's highly evolved [condition]. Our hand was more human-like from almost the beginning in terms of the length of the palm or the length of the digits, for example.

When we look to the foot of Ardipithecus, from 4.4 [million years ago], we see that it maintains the ability to grasp as does the foot of a chimpanzee. But the chimpanzee, again, has specialized its foot to be compliant to substrate because it's a climber, a suspender, and it walks on its knuckles.

So we see in modern living apes, a lot of evolution has happened since the last common ancestor. By approaching it in Ardipithecus, we now know, for example, in the teeth, the chimpanzees are highly specialized. They're frugivores [fruit-eaters]. They have very broad front teeth. These are the incisors [he shows a model to the interviewer]. Ardipithecus doesn't [have the same type of teeth]. It's much more of an omnivorous creature. It turns out that frugivory evolved along chimpanzee lineages.

But that's not the only thing. The fascinating thing we see in Ardipithecus is that it had a chimp-sized brain, and yet its front face is very much smaller than a chimpanzee's. Its incisors were not only smaller, its canines were really small, especially the male individuals.

Now the fossil we found, the skeleton, is a female. But we have lots of other individuals from the same horizon of Ardipithecus. And we have not a single large chimpanzee-like canine. So even the males by that time in hominid evolution had feminized the canine tooth. And this tells us about the social behavior that would have been practiced.

And again, this very aggressive behavior you see in chimpanzees where males fight with one another all the time over estrous females, this wasn't happening early in our lineage because that canine tooth is very much reduced. So you could never predict what the last common ancestor looked like from just looking at humans and chimps.

That was all that Darwin had. Darwin was very wise on this matter. Darwin said, “We have to be really careful. The only way we're really going to know what this last common ancestor looked like is to go and find it.”

Well, we haven't found it [haven't found the last common ancestor]. But we've come closer than we've ever come, at 4.4 million years ago. And just like Darwin, [we have] appreciated that the evolution of the ape lineages and the human lineage has been ongoing since the time that these lineages split, since that last common ancestor.


EWL:: You've talked a little bit about some of these traits that distinguish the apes from Ardipithecus. I wonder if you could talk a little bit more. Give me some other traits that researchers found compelling in this analysis.

TDW: Well, the most important thing about the discovery of the Ardipithecus skeleton is that in one individual, all the parts are sort of put together for you. We have from head to toe, as it were.

So we can start with the head. What does the head show us? Well, the head shows that the brain is very small, like a chimpanzee. The head shows the canine tooth is very small but it's a female individual. Fortunately, we have many other canine teeth from the same layer of the males. So that allows us to say, “This is a female individual.”

One of the important things about this compared to, say, a gorilla, or even an early Australopithecus is that it doesn't have very much body size difference between male and female. That tells us a dimension of the species that we didn't have prior to the time we got the skeleton and had all the pieces.

So you start to unravel the biology in the head. You take a look at the isotopes in the teeth - these isotopes are telling us that the diet didn't include grassland, savanna plants.

We find now, moving down to the arm and the hand, we find an organism that was not adapted to knuckle-walking, to walking on its knuckles like living chimpanzees. It was in fact very capable as a climber. Its hand was not a human hand, but it lacked the specializations that we see in great apes today in Africa for knuckle-walking. So it could climb.

What else? Well, you go down to its pelvis. Here we have the key part of the skeleton that allows us to walk on two legs. We see substantial evolution in Ardipithecus compared to any other quadruped. We have a pelvis shape that indicates some early form of walking on two legs - very important.

Then we go down and we look at the leg bones and compare them to the length of the arm bones. We go down to the foot and we find a foot that, unlike a human foot, it lacks an arch. The large toe in this foot diverges from the rest of the foot, so it's a grasping foot, much more like the foot of a chimpanzee. It's not a chimpanzee foot. But it can grasp. What does this tell us about the capabilities of this organism?

It can climb in that woodland environment. It has not abandoned the ability to climb even though it's an early biped. It hasn't forsaken a lot of the ape characteristics that we see both in earlier apes and in living apes which are pretty much common to all primates. They are subsisting on an arboreal substrate. Their diet is fruits and leaves and things like that. Here we have, in Ardipithecus, an unspecialized form that hasn't evolved very far in the direction of Australopithecus. It's not expanded its molar teeth, for example. Its canines are still big although not as big as the thing that came before it and certainly not as big as chimpanzee canines.

So when you go from head to toe, you're seeing a mosaic creature that is neither chimpanzee, nor is it human. It is Ardipithecus. That's why we gave it this different name. The only way that we could have figured out that it ever lived here was not by triangulating from living humans and living chimps and figuring on some halfway house. This is actually hard data from the skeleton biology and from the context that allows you to look back in time and see something that was and is very unique.

You can't therefore straight-jacket the world of the past just by looking at the modern world. You've got to go and seek the evidence from the deep past to find out what the deep past was really like.

continued

 

[Palmela]

EWL:: Given the traits that you're discovering, given what you're learning about Ardipithecus, can you infer a lifestyle for this creature, or what can you infer?

TDW: Maybe the best way to think about that is what it would have been like 4.4 million years ago, had you been able to get into a time machine. Let's say we took our time machine to Aramis today, and before we step into the time machine, we look around, and we see a desert. It's a very harsh place. There are pastures there, no agriculture is possible, very little water, very little rain, very little vegetation. It's a desert, and these ancient sediments are eroding.

Now imagine we step into the time machine, set the dial to 4.4 million years ago and go back in time, and we get out of the time machine and the place has completely changed. We're in the same place. But 4.4 million years ago, this was a really different world. All of a sudden, we're not even on a grassland, we're no longer on a desert, we are in a woodland environment.

And we look up in the trees, and we see that they're full of monkeys. We look around on the ground and we see that there are a lot of kudus. And we see an occasional hyena. And we see elephants and we see lots of small mammals. And we know what all these mammals and birds and plants - we know what they are - because we've found evidence of them. And then we look through and if we're lucky on that particular day, in that particular place, we might come across an Ardipithecus.

When you observed it from a distance, from several hundred meters away, through the vegetation, you would have been struck that it's not walking on its knuckles, it's not a knuckle-walker, it's not a chimpanzee, and yet as it gets closer, it's not a human either, it's a very small head atop this body but the body is erect. It's walking on two feet. And then we'd probably see it move quickly away from us and find that it's able to move around in the trees in a way that we're not [able to do] because its foot can still grasp. It has very, very long arms, very long fingers, very effective at moving through this arboreal habitat.

Now if we were lucky enough to watch it eat, we'd be able to see what kinds of foods it was after, and it was an omnivore. It would have been gathering small game and perhaps invertebrates along the woodland floor. It also would have been able to go effectively up into the trees to gather ripe fruit, figs, and we know these things because, for instance, we've found the pollen that indicates the figs were there. We've found the seeds from the hackberry tree. We've found the bird fossils showing us that peacocks were present 4.4 million years ago.

So we're able to put this ecological snapshot together from hard evidence and so in a sense, the fossils and the sediments they're in become our “time machine.” They allow us to look back and to see this. So what we have done is to reassemble the most complete Ardipithecus that we have managed to find. This partial skeleton with hands and feet and a head and arms and legs and a pelvis.

The natural history artist, Jay Matternes, has sketched out what the skeleton looked like. It's a female individual. It would have been about 4 feet tall and would have had of course muscles and ultimately hair. Those are gone now, we only have the bones. So Jay, through his great knowledge of comparing primate anatomy and going over these bones, and how they fit together and how they might have moved, he's recreated what Ardipithecus would have looked like, had we been able to go back into that environment at 4.4 million years ago. So we are dealing with a creature that is no longer here. You can't find them anywhere in Africa, except through the fossil record.

EWL:: The research talks, the papers in Science talk to some degree about the lack of long, sharp canines in Ardipithecus. What does that tell you about their social behavior?

TDW: When you look at other primates, whether you look at baboons or gorillas or chimpanzees, the males of the species have very big, very sharp, very projecting canine teeth, and they mostly use these teeth for conflicts among themselves. And the conflicts are often over access to female individuals ready for mating - estrous females in the populations, and again, this is based on living primates.

The real exception to that rule in living primates is modern people. If you look at modern people, we don't have big sharp canines and we don't go around threatening each other with big sharp canines. Of course, Darwin and many other scholars of his day saw this and wanted to come up with an explanation for why is it that humans have lost these weapons?

For many years, it's been thought that stone tools were invented at some point and the canine would have been reduced as hand-held weapons supplanted its function. This was the general explanation.

But what we've found, starting in the 1970s with the discovery of the Lucy species in Ethiopia and in Tanzania, early hominids had small canines, from even three and a half to four million years ago - even the males, small non-projecting canines, and they were fully bipedal. And so we were interested of course in finding older fossils to see if they would be more chimpanzee-like.

And now we've found Ardipithecus. We have a good many individuals. We know that their canine teeth are not only smaller than chimpanzees, but they are also shaped very differently. They have a diamond shape. They're blunt. Even the largest male of Ardipithecus would not have had a canine that functioned in that way. So what does this mean? And what does it have to do with bipedality?

So my colleague Owen Lovejoy at Kent State University has come up, in one of the papers in this package published in Science, with the expansion and elaboration of ideas that he started to have when Lucy was found. Now we see in Ardipithecus a very early biped with a lack of these big, projecting male canines.

In fact, we've tracked this. We have even older remains of Ardipithecus in our area, and remains from Chad, and remains from Kenya, and none of these, all the way back to 6 million years ago, have big, projecting male canines. And they all seem to be early bipeds.

So Lovejoy thinks that this goes together, that the reduction of the male canine is telling us something very important about social organization, that the male individual has been brought into the parenting process and that the carrying of food is deeply involved in this and really set apart our family, our branch of the family tree, from what we see in all these other primates.

He links walking on two legs with the reduction of the canine tooth. That's the hypothesis. Now as we go back beyond Ardipithecus, into that zone at perhaps 7 million years ago of the last common ancestor, we'll be very interested to see what the last common ancestor had in the way of canine dimorphism. It looks like in chimpanzees, at least common chimpanzee, that canine dimorphism has become highly evolved and we know these today to be the most aggressive primates when it comes to male-male competition over estrous females.

So what we see is, very early in the hominid line, this is not going on. Natural selection has led to the reduction of this male canine tooth very, very early in time, right at the base of our branch of the family tree.

EWL:: Does that tell us then that there is less conflict of the type you might see among chimpanzees and [a greater] tendency to cooperation?

TDW: Yes, and Lovejoy's model is involving the cooperation between the male individuals who are no longer fighting over estrous females but rather, putting time and energy into the provisioning of dependent young. So Lovejoy's model links social structure in the form of monogamy in these early hominids that we don't see in chimps or gorillas or anything else. What he sees is a pair-bonding between males and females, very early in time, and that's basically the only way to capture the male energy, which in all these other primates goes into the weaponry display, aggression and mating based on that.

So Lovejoy points to things that are not commonly thought of in paleontology, things like the fact that at the end of the human lineage, which is where we are today, ovulation is actually fairly concealed, even from modern married people. So that, we don't think of that as making us very different from other modern primates but it really does. We don't have an estrous like all these other primates have. The permanently enlarged female breast could be another emblematic biological signal that has very deep evolutionary roots. The male no longer is able to say, to tell, to discern whether the female is ovulating or breast-feeding.

So he places, Lovejoy, my colleague at Kent State, places a lot of attention on aspects that are often unavailable to us through the fossil record - that is the soft-tissue evidence from modern people. He cites physiological evidence and the biology of modern primates: both chimps and humans. He cites the fossil evidence, the ecological evidence. So what his paper does is to integrate all of this information and to come up with a hypothesis about how it was that our lineage diverged from lineages leading on to chimpanzees and became, through evolution, the very different primate that we are today.


continued

 

[Palmela]

EWL:: The research finds that there is relative similarity between male and female body sizes. What does this tell us about their behavior?

TDW: This goes along with the same principles involved in the competition between males over estrous females that we see in other primates. The low level of sexual dimorphism seen in body size is coupled with the low level of dimorphism in canine size. What that suggests is that the males are not competing with one another and they're not being chosen by females for being the strongest with the biggest canines, the largest, or whatever, but this is signaling a new social structure in which males are cooperating, at first not a lot, but later, more and more, cooperating in the provisioning of offspring.


EWL:: I wonder if you could talk at all about the possible evolutionary relationships between Ardipithecus and Australopithecus?

TDW: One of the first things that we look at, as paleontologists is, what does the anatomy suggest about different species and their relationships? So as paleontologists, we're able to do that, not just in a comparative sense, by comparing living humans and living chimps, but we're actually able to put things in temporal order. As we go back through the fossil record, from Homo sapiens, we get to a point about 160,000 years ago where people don't look like people anymore, as we go back in time. As we go [further] back in time, they come into a creature known as Homo erectus, at a million years ago.

We have that creature. We found it, in the Middle Awash, higher up in the stack of sediments. And we're able to take these snapshots in time and work our way back into the past in this unique area of Ethiopia.

What we find is that around 3.2 million years ago we hit a very well-known species, Australopithecus afarensis, the Lucy species. The reason it's so well-known is because of that partial skeleton of Lucy. We know a lot about the biology of these creatures. We know males were substantially bigger than females. We know they had small brains. We know they were basically committed to terrestrial bipedality. They lacked the grasping foot of Ardipithecus.

And until now, we didn't know much about the biology of Ardipithecus. But what we're able to do now is to step back again in time, back to 4.4 million years ago, and when we get there and we meet this creature that is very different from the Lucy species, it's got a grasping foot. It lacks the specializations of the head, of the teeth, of the face, to a heavily chewed diet that characterizes Australopithecus - not there, in Ardipithecus. We know it's not a frugivore like a chimpanzee. Neither is it an Australopithecus. It's something new.

So by looking at the relationship of Ardipithecus and Australopithecus, we are able to say, “Maybe one is older than the other and maybe what we're looking at is evolution.”

In fact, in this area of the Middle Awash, right in the very geographic location that the new skeleton was found, if you go up in the rock column, about 80 meters higher, about 200,000 to 300,000 years younger, you hit the first Australopithecus. And so it's possible that we are seeing the local evolution from Ardipithecus into Australopithecus.

Even locally, when in the Afar, our big problem as paleontologists is, this is the only place in the world so far where we've been able to go back to 4.4 [million years ago]. There's a place just north of us where they've got 4.5 million year old fossils, not as complete as this skeleton. They tell the same story - no Australopithecus at 4.4 [million years ago], lots of Australopithecus later, only Ardipithecus earlier. Well, does that characterize the entire African continent? Not a question we can answer right now.

So we don't know exactly when Australopithecus emerged or what kind of a speciation event it was that lead to the abandonment of arboreality, that lead to the expansion of the back teeth, that lead to the arched foot. All we know is that at 3.7 [million years ago] in the Lucy species, it's there, in the Ardi species, it was not there.

What we're doing is trying to use these little tiny windows that we have in time and link them together. And from my point of view, it's kind of premature to do that with such a limited sample, which is why we go back to the field every year to try to find more evidence of this.

But it is possible now, because of Ardipithecus at 4.4 [million years ago] to really have a concept of the basic way that we became human, to think of it perhaps in stages.

The earliest stage would be represented by Ardipithecus— whether the species ramidus directly gave rise to Australopithecus, we can't tell yet. But certainly, it represents an adaptation, a grade of hominid evolution in which the brain was very small, the foot could grasp, it wasn't really a biped like Lucy was a biped.

And then we were able - because of all discoveries in South Africa and the Lucy discovery and so forth - to characterize the Australopithecus grade very, very well. There are a number of different species. Some of them became so specialized that their back teeth expanded into great big chewing platforms and their front teeth became very diminished and their faces became really flat and then about a million years ago, they went extinct.

And if we go back in that record, in this area of Ethiopia, we find that stone tools appear at 2.6 million years ago. Shortly after that, we find organisms, hominids that have bigger brains. And then shortly after that, at around 2 million years ago, 1.8, the first hominids outside of Africa.

So human evolution is an African phenomenon until around 1.8 million years ago and then there's a rapid spread all the way from Portugal, Spain, all the way to Indonesia. And we can track this now because we have these records, these fossil records from various parts of the world. The Middle Awash is one of these records, but because of the geological setting, because of the ability to take the rocks, because of the great thickness, we're able to track it better than anywhere else, and so we're able to see our own genus, Homo, as sort of a third stage: This technological primate that's fully bipedal that has an expanding brain and a shrinking face and that evolves in Africa eventually to become Homo sapiens.

So it's a really unique record because of the unique geological circumstances there in the Horn of Africa.

EWL:: What significant new research questions might be raised by these discoveries and the new analysis that you and other members of your team have done?

TDW: One of the interesting questions now that we have moved back to 4.4 million [years ago] is, “What happened before that? What comes just before that?” Here we are seeking that last common ancestor. No one has found it yet. It must be older than 6 million years but there are very few deposits with the appropriate fossils. But one, as a paleontologist, always has to be hopeful.

If we remember back in 1992, there were no fossils older than the Lucy species. We broke through that 4 million-year barrier and we've learned a great deal. So that's one big hope, is a breakthrough to get beyond the 6 million-year mark.

The other interesting thing as far as Ardipithecus is concerned is to try to understand that transition, from that grade of hominid into the Australopithecus grade - where did it happen, when did it happen, how did it happen, why did it happen?

And then, the other great transition, getting into the genus Homo - stone tools appear at 2.6 [million years ago], hominids expand beyond Africa by 2 million years ago. What's going on there? Very interesting time there because you have multiple lineages of hominids. Very specialized Australopithecus that's still there, still evolving, still just being Australopithecus and then these other forms that have larger brains, that have stone-tool technology and, interestingly that are starting to get involved with scavenging from large-mammal carcasses.

That puts you into a whole different adaptive framework. I think that's right at the base of genus Homo, that's when we take those last steps in becoming this bipedal, technological, old-world, global primate, as it were.

EWL:: I wonder if you expand beyond communicating to an audience of other scientists and other researchers. What's the take-away message that you would like people to come away with from this research?

 

[Palmela]

TDW: I think the most important thing in the broader sense is that we now no longer have to guess about where we came from, that we now have an evidentiary basis for understanding that we didn't get here in the form we see today: We evolved.

That's bigger than Ardipithecus. That's the record of this kilometer of rocks with superimposed fossils - that we can now track our evolution back in time, that we now really understand it much better than Darwin could have, and the way that we got here is through the fossil record. The way that we move forward and better understand it is through more of this field research in Africa.

EWL:: Are there any other important points you'd like to make before we close out here this afternoon?

TDW: The most important point to be made here is that this kind of research is not done by one or two people anymore. It's not about riding a camel across the desert, finding a skull and having a “Eureka!” moment. It's about the coordinated efforts of very diverse scientists with special skills, special abilities.

It's about the support of a country like Ethiopia that treasures its antiquities, that's built the infrastructure. It's about the Ethiopians involved in this research, not [just] as fossil finders, but as fossil finders and qualified geologists, qualified physical anthropologists, and not just qualified - these guys are pushing global knowledge about human evolution forward and they're doing so in a developing country.

So Ethiopia really has taken the lead in this. But it's not Ethiopia alone. This is the study of all of our origins and this Middle Awash research team reflects that reality. We have people from all over the world who are working on these remains, on the context of these remains and the authorship of this Science package is all kinds of men and women from all over the planet who are doing all kinds of things, from CT scanning the skull to working on the fossil pollen to figuring out the isotopics of the soils.

These people are all dedicated scientists and until you integrate all of these different data that we've been able to squeeze out of these ancient rocks, you can't get a real picture of the biology there.

I think it's very important to understand in the context of modern science that this is a big integrated, multidisciplinary, international endeavor and that it takes sustained funding, which the National Science Foundation has given us, and it takes sustained support, which our institutions have all given us and which the Ethiopian government has provided over many decades.

That's the only way that you can get these kinds of high-resolution data to really tell us what happened in our past.

EWL:: Dr. Tim White, your work is fascinating. Thank you for your time. Talk to you later.

TDW: Thanks very much.

30 September 2009


http://www.aaas.org/news/releases/2009/1001sp_ardi_interview.shtml

 

[Palmela]

Who Found What, And When?

The first Ardipithecus fossil (a single upper molar) was found by Dr. Gen Suwa in 1992. The species Ar. Ramidus was first named in 1994 (in Nature) on the basis of this and other fragments found nearby.
Then, on November 11, 1994, Yohannes Haile-Selassie found the first piece (hand bone) of the new Ardipithecus skeleton. This skeleton was recovered through careful excavations between 1994 and 1997.
The skeleton is partial, its bones disarticulated and scattered, with larger elements often having been broken into smaller pieces prior to being covered with sediment. More than 125 pieces of this individual were found.
Estimation of the “percentage” representation of a skeleton is not very meaningful, but the most important parts of any early hominid skeleton are the skull, teeth, arms, hands, pelvis, legs, and feet. All these were recovered for this individual, but many of the larger elements were damaged and required restoration.
The team found many other fossils of other individuals of the same hominid species, a total of 110 specimens representing a minimum of 36 different individuals, including youngsters, males, and females (of which the “Ardi” skeleton is only one; ARA-VP-6/500 is her catalog number). As in many fossil assemblages, teeth are common.
The team also found thousands of bones from dozens of animal and plant species, many new to science, and all informative about “Ardi's” environment.

http://www.aaas.org/news/releases/2009/1001sp_ardi_backgrounder.shtml

 

[Phronesis]

Hopefully they did not do reconstructions based on a few teeth and material that they refer to as "roadkill".

Oldest Human Femur Wades Into Controversy. Science. 2004 Sep 24;305(5692):1885. Gibbons A.
PARIS. Tempers flared last week in a sweltering salon at the French Academy of Sciences here as scientists hotly debated the attributes of anthropology's most famous thighbone, the 6-million-year-old femur of an ancient Kenyan hominid called Orrorin tugenensis. More than 100 scholars packed the academy's opulent, wood-paneled Grande Salle to witness the first face-to-face gathering of the discoverers of the three oldest putative hominids. In talks and a panel discussion, the researchers discussed whether Orrorin and other contenders for the title of earliest human ancestor walked upright and in what manner. Bipedalism is a traditional hallmark of membership in the human family rather than being an ancestor of chimpanzees gorillas, or quadrupedal apes. The speakers were particularly interested in learning more about Orrorin's legs. Paleontologist Brigitte Senut of the National Museum of Natural History in Paris presented recently published computed tomography (CT) scans of Orrorina's thighbone (Science, 3 September, p. 1450). According to Senut, the scans sho that the bone is thicker on the bottom of the subhorizontal neck of the femur, indicating that weight was put on the top of the bone. Other features also suggest that the hips were stabilized in a manner similar to those of modern humans. In fact, Senut proposed that Orrorina's gait was more humanlike than that of the 2- to 4-million-year-old australopithecines. If so, australopithecines would be bumped off the direct line to humans a dramatic revision of our prehistory.
But paleoanthropologist Tim White of the University of California, Berkeley, immediately attacked this view of Orrorin. He said that the resolution of the CT scans was so poor that it was impossible to be certain of the pattern of bone thickness. CT scan expert James Ohman of Liverpool John Moores University in the U.K., who was not a the meeting, agreed that the published scans were taken at the wrong angle. White further grilled Senut about the fossil analysis, asking if her team had directly measured the internal structure of the bone at a preexisting break, a more reliable means of gathering the data than CT scanning. Senut responded that colleagues had suggested doing the scans to make her case stronger an added in an interview that the bone was broken in a zigzag pattern that made it difficult to photograph. In her view, other features on the bone make it clear that Orrorin had
walked uprighta'so there was no need to unglue the bone and measure it. White accepts that Orrorin walked upright and so is one of the first members of the hominid family. But he says Senut has offered little evidence as to Orrorinas gait. "Was it human, an Australopithecus pattern, or something different he asked. Even standard x-rays would help answer that question. As the discussion grew more heated, White called Senutss displacement of australopithecines "une position créationniste," because it suggests that Orrorinas femur was quite modern 6 million years ago, rather than evolving in stages. Senut declared indignantly that she is not a creationist and then asked White to provide his own evidence about the mysterious Ardipithecus ramidus. A partial skeleton of that 4.4-million-year-old species was discovered by White's team, the Middle Awash Research Project, in Ethiopia from 1994 to 1996, but the bones remain unpublished. White responded by projecting images of the Ardipithecus skull for the first time in public. The CT scans were startling: The skull was so crushed that the top of the vault was smashed almost to the base, forming a slab of hundreds of chalky pieces. White described it as "road kill"..The reconstruction uses microCT scans to reassemble the specimen. This is the most fragile hominid skeleton ever found," says White. We are very sorry it's taken us this long to do, but I think you want the right answer instead of the quick answer.ANN GIBBONS

Maybe "road kill" is jargon for "good specimen" ?

 

[Geriatrix]

Whoa Wall Of Text!
Brain shut down imminent...

 

[w1z4rd]

Thanks for the posts, interesting read.

Hooray for South Africa,

And then we were able - because of all discoveries in South Africa and the Lucy discovery and so forth - to characterize the Australopithecus grade very, very well. There are a number of different species. Some of them became so specialized that their back teeth expanded into great big chewing platforms and their front teeth became very diminished and their faces became really flat and then about a million years ago, they went extinct.

So it seems there have being a lot of findings of species associated with this?

And now we've found Ardipithecus. We have a good many individuals.