(01) Ardipithecus kadabba The Smithsonian Institution's Human Origins Program

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2 (01) Ardipithecus kadabba The Smithsonian Institution's Human Origins Program Ardipithecus kadabba Where Lived: Eastern Africa (Middle Awash Valley, Ethiopia) When Lived: Between about 5.8 and 5.2 million years ago Ardipithecus kadabba was bipedal (walked upright), probably similar in body and brain size to a modern chimpanzee, and had canines that resemble those in later hominins but that still project beyond the tooth row. This early human species is only known in the fossil record by a few post-cranial bones and sets of teeth. One bone from the large toe has a broad, robust appearance, suggesting its use in bipedal pushoff. Year of Discovery: 1997 History of Discovery: species In 2002, six teeth were discovered in the Middle Awash at the site Asa Koma. The dental wear patterns con rmed the early human fossils were unique and not a subspecies of A. ramidus. Based on these teeth, paleoanthropologists Yohannes Haile-Selassie, Gen Suwa, and Tim White allocated the fossils in 2004 to a new species they named Ardipithecus kadabba ( kadabba means oldest ancestor in the Afar language).

3 (01) Ardipithecus kadabba The Smithsonian Institution's Human Origins Program We don t know everything about early humans but we keep learning more! Paleoanthropologists are constantly in the field, excavating new areas with groundbreaking technology, and continually filling in some of the gaps about our understanding of human evolution. When he found a piece of lower jaw lying on the ground in the Middle Awash region of Ethiopia 1997, paleoanthropologist Yohannes Haile-Selassie didn t realize that he had uncovered a new Below are some of the still unanswered questions about Ardipithecus kadabba that may be answered with future discoveries: 1. Was Ar. kadabba routinely bipedal? So far, the evidence for Ar. kadabba s upright walking comes from an single toe bone that dates to 5.2 million years old and was found 10 miles away from the other Ar. kadabba specimens. 2. If Ar. kadabba walked upright, what was its gait like? 3. Did bipedalism independently develop in the Ardipithecus lineage? Or is Ar. kadabba somehow related to Orrorin tugenensis and Sahelanthropus tchadensis, two earlier human species? 4. How is the Ardipithecus lineage related the Australopithecus lineage? 5. How big were the average male and female Ar. kadabba individuals? Was there a high level of sexual dimorphism in this early human species? First paper: Haile-Selassie, Y., Suwa, G., White, T.D., Late Miocene teeth from Middle Awash, Ethiopia, and early hominid dental evolution. Science 303, How They Survived: Instead of eating mostly fruit and soft leaves like chimpanzees, there is evidence that Ardipithecus kadabba ate a variety of fibrous foods.

4 (01) Ardipithecus kadabba The Smithsonian Institution's Human Origins Program How do we know they ate fibrous foods? The back teeth of Ardipithecus kadabba are larger than a chimpanzee s, but its front teeth are narrower. This evidence suggests this species did most of its chewing in the back of its mouth. This type of chewing would focus on hard-to-eat foods like fibrous nuts. Evolutionary Tree Information: Scientists originally considered Ardipithecus kadabba to be a subspecies of the later Ardipithecus ramidus, then renamed as its own distinct species based on dental differences.

5 (02) Ardipithecus kadabba - Australian Museum This early hominin lived over 5 million years ago in East Africa. Background of discovery Age 5.8 to 5.2 million years ago Important fossil discoveries Eleven specimens, from 5 localities in Ethiopia, were discovered between 1997 and They represent at least 5 individuals and include teeth, jaws, hand, toe, arm and collar bones. The type specimen is a right lower jaw fragment, ALA-VP- 2/10. Most of the fossils date to million years old, however one of the toe bones is dated at 5.2 million years old. There is some concern over the classi cation of the toe bone to this species, as it was found 15 kilometres away from the other fossils and is younger in age. They were classified as a subspecies Ardipthecus ramidus kadabba. In 2002, six teeth were found at Asa Koma in the Middle Awash. They date to between 5.6 and 5.8 million years old. Distinct features of these teeth led the nders to place all the fossils into a new species Ardipithecus kadabba rather than a subspecies of Ardipithecus ramidus. What the name means The name is derived from the local Afar language. Ardi means ground or oor, and is combined with the Latinised Greek word pithecus, meaning ape. The species name kadabba means oldest ancestor in the Afar language. Distribution Eastern Africa in the Middle Awash, Ethiopia

6 (02) Ardipithecus kadabba - Australian Museum Relationships with other species The scientists that discovered the remains claim this species is a direct human ancestor and the earliest species yet discovered on the human branch of the family tree. Those that discovered Orrorin tugenensis dispute this claim as they believe their nd is a better candidate for direct human ancestry. Some scientists assign these remains to the subspecies Ardipithecus ramidus kadabba, because it shares many similarities to Ardipithecus ramidus, but has more primitive, or ape-like, teeth features. Key physical features Brain similar in size to modern chimpanzees Body size and shape similar to modern chimpanzees Limbs the structure of the toe bones suggests that this species may have been bipedal. However, some scientists debate whether this fossil should be included with this species as it was found about 15 kilometres away from the other fossils and is dated several hundred thousand years younger. Jaws and teeth some primitive dental features such as thick tooth enamel and relatively large canines compared to humans. some features of the teeth show a movement away from the primitive ape-like condition, such as molars that are larger than those of chimpanzees, a tendency towards incisiform lower canines and hominin-like upper pre-molars.

7 (02) Ardipithecus kadabba - Australian Museum Lifestyle Culture There is no evidence for any speci c cultural attributes, but they may have used simple tools similar to those used by modern chimpanzees, including: twigs, sticks and other plant materials that were easily shaped or modi ed. These may have been used for a variety of simple tasks including obtaining food. unmodi ed stones, that is stones that were not shaped or altered before being used. These tools may have been used to process hard foods such as nuts. Environment and diet Fossil evidence from the site indicates the area was a mosaic of woodland and grasslands with lakes, swamps and springs. The discovery that this species lived in a forest environment challenged the theory about what kind of environment fostered the evolution of bipedalism. Did bipedalism evolve to take advantage of new open grassland environments, as was once believed, or did it first evolve in the trees? The large back molars and narrower incisors (compared to chimpanzees) suggest that the diet included more fibrous foods than just fruit and leaves. Fran Dorey, Exhibition Project Coordinator Last Updated: 27 October

8 (03) Ardipithecus kadaba Essay Becoming Human Ardipithecus kadaba Ardipithecus kadabba is an early hominin species recovered from sediments in the Middle Awash Valley of Ethiopia dated to between 5.2 and 5.8 million years ago. These fossils are of particular importance because fragments from both the skull and body have been found and are argued to demonstrate some of the earliest signs of bipedalism and hominin dental morphology. As one of the oldest species of human ancestors, Ar. kadabba helps to push back the origin of hominins into the late Miocene Epoch (roughly 11.6 to 5.3 million years ago). There are relatively few Ar. kadabba fossils and the majority are teeth and fragments of the mandible (lower jaw). Ar. kadabba teeth are noticeably more primitive (meaning they retain traits found in their ancestors, in this case represented by living apes) than Australopithecus afarensis and Australopithecus anamensis. The incisors (front teeth) are smaller than extant apes but are wider than those of australopiths and later hominins. Ar. kadabba canines are primitive in size i.e., slightly smaller than female chimpanzee canines. The upper canines of Au. afarensis, Au. anamensis, and Ardipithecus ramidus, in contrast, are smaller than those of female chimpanzees, exhibiting a shift to smaller canines found in all later hominins. The shape of the upper canine in Ar. kadabba, however, is relatively derived (meaning it di ers from that found in their ancestors and more closely resemble later hominins) with a more symmetrical, or circular, crown (enamel-covered part of the tooth, that protrudes above the gum); this shape di ers from other early hominin species e.g., Orrorin tugenensis which exhibits the primitive, ape-like shape. The other Ar. kadabba teeth are similar in many general features with its suggested descendant, Ar. ramidus (see below), including overall size, proportions and wear patterns. The Ar. kadabba dentition, however, retains some primitive features e.g., thinner dental enamel and a more

9 (03) Ardipithecus kadaba Essay Becoming Human asymmetrical lower third premolar (cheek teeth that lie between the canines and the molars) crown shape. The Ar. kadabba mandible is small yet broad, and the overall shape is similar to other early hominin species such as Sahelanthropus tchadensis and Ar. ramidus. When viewed from the side, the front of the mandible is relatively primitive in shape, retreating backward from top to bottom. The mandibular ramus (the vertical plates of bone at the rear of the mandible, behind the teeth) has a weaker outward are than later hominin species. Ar. kadabba postcrania ( the parts of the skeleton other than the skull) are limited to fragmentary pieces of the forearm, two nger bones, a clavicle (collar bone) fragment, and a bone from the fourth toe. At least ve individuals are represented among the eleven postcranial elements recovered. The forelimb (arm and hand) bones are fairly primitive, resembling living great apes. For example, the nger bones are relatively large with strongly-built joint surfaces (where two bones meet to form a joint) than later hominins and the ulna (bone of the forearm closest to the body) is more curved and ape-like. Another primitive feature that distinguishes this species from more derived hominin species is found in the morphology of the elbow joint, permitting increased mobility, characteristic of living apes and unlike the less mobile elbow of later hominins. Ar. kadabba has been argued to have walked bipedally based on the characteristics of a single bone from the fourth toe, speci cally the upward orientation of the joint surface closer to the rest of the foot. This feature is similar to the condition found, not only in Au. afarensis but also in other later hominins, including Homo sapiens; living great apes, on the other hand, have more downwardly-tilted joint surfaces. The morphology of fourth toe has led some researchers to suggest Ar. kadabba walked bipedally that this feature allowed this species to toe o (push o of the ground during bipedal walking, mostly with the big toe). However, as noted by other researchers, similar joint shape occurs in non-hominin Miocene apes (e.g., Sivapithecus) which may or may not have walked bipedally along limbs when in trees. In addition, the Ar. kadabba toe bone is dated to several hundred thousand years before the rest of the fossils, and was found in a locality 16 km from the main site. This fact has caused some researchers to question the validity of grouping the toe bone with the rest of the Ar. kadabba fossils.

10 (03) Ardipithecus kadaba Essay Becoming Human The evolutionary relationships between Ar. kadabba and other early hominin species are of great interest to paleoanthropologists. A more recent hominin taxon, Ar. ramidus, which is also found in the Middle Awash of Ethiopia, is argued by some researchers to be a direct descendant of Ar. kadabba because these two species share many features, such as relatively thin tooth enamel and larger canines. Some researchers further suggest that Ar. kadabba represents the earliest species in a single lineage of descent of East African hominin taxa, starting with Ar. kadabba to Ar. ramidus to Au. anamensis and ending with Au. afarensis. These species are all found in eastern Africa and some morphological trends, such as the reduction in canine size and premolar morphology, support this hypothesis, but the dates o f Ar. ramidus and Au. anamensis (see essay on Ar. ramidus) cast some doubt on this explanation. Ar. kadabba appears to have occupied a closed, densely-wooded habitat close to permanent sources of water (e.g., lakes and/or rivers) with swampy conditions and oodplain grasslands. This reconstruction is based on the fossil remains of non-hominin animals found in the same layers as the Ar. kadabba material and suggests strongly against the once proposal, called the "savannah hypothesis", that bipedalism initially evolved in open grassland environments.

11 (04) Time - One Giant Step for Mankind 07/23/2001 The region of Ethiopia called the Middle Awash, some 140 miles northeast of the capital of Addis Ababa, is a hot, harsh and inhospitable place-a rocky desert punctuated by tree-lined rivers, the occasional lake and patches of lava that are slowly being buried by sediments ushed out of the hills by the torrential rains that come along twice a year. But between 5 million and 6 million years ago, the landscape here was very di erent. The same tectonic forces that racked the region with earthquakes and volcanic eruptions had also thrust the land up as much as a mile higher than it is today. As a result, the area was cooler and wetter and overgrown with trees, bushes and patches of grass. These fertile woodlands were rich in wildlife. Primitive elephants, giant bears, horses, rhinos, pigs, rats and monkeys lived here, along with dozens of other mammal species long since extinct. And it was here too that nature indulged in what was perhaps her greatest evolutionary experiment. For it was in eastern Africa at about this time that a new type of primate arosean animal not so di erent from its apelike ancestors except in one crucial respect: this creature stood on two legs instead of scurrying along chimplike on all fours. Its knucklewalking cousins would stay low to the ground and never get much smarter. But while it wouldn't happen until millions of years in the future, this new primate's evolutionary descendants would eventually develop a large, complex brain. And from that would spring all of civilization, from Mesopotamia to Mozart to Who Wants to Be a Millionaire. That's the broad outline, anyway. While this view of human evolution has generally been accepted by scientists for decades, no one has yet been able to say precisely when that rst evolutionary step on the road to humanity happened, nor what might have triggered it.

12 (04) Time - One Giant Step for Mankind But a discovery reported last week in the journal Nature has brought paleontologists tantalizingly close to answering both these questions. Working as part of an international team led by U.S. and Ethiopian scientists, a graduate student named Yohannes Haile- Selassie (no relation to the Emperor), enrolled at the University of California, Berkeley, has found the remains of what appears to be the most ancient human ancestor ever discovered. It's a chimp-size creature that lived in the Ethiopian forests between 5.8 million and 5.2 million years ago-nearly a million and a half years earlier than the previous record holder and very close to the time when humans and chimps rst went their separate evolutionary ways. "Having a fossil in this region of time, very near the divergence point, is really exciting," says anthropologist C. Owen Lovejoy of Ohio's Kent State University. "Going all the way back to Darwin, people have speculated how, when and why humans stood up on two legs. For paleontologists, this find is a dream come true." As is often the case with discoveries like this, Haile-Selassie was not speci cally looking for the things he found. He had set out to better understand how the ancient ecosystems worked and evolved. "I didn't even think about nding hominids," he says. "All I wanted to do was collect enough vertebrate bones so that I could write my dissertation." In December 1997, though, at a place called Alayla, he spotted a piece of jawbone lying on the rock-strewn ground. "I picked up the mandible less than ve minutes after we got there," he recalls, "but didn't realize I had something really special until a year later, when we found some more bones and I started the serious analysis." In all, the team eventually found 11 specimens-from at least ve di erent individuals-in a cluster of sites, including Haile-Selassie's partial lower jaw with associated teeth, several hand and foot bones, and pieces of three arm bones and a collarbone. Luckily, the fossils were trapped in sediments that were sandwiched between layers of volcanic ash, whose age can be accurately gauged by a technique known as argon-argon dating. (This layering is still visible in places that have not been so heavily eroded, enabling the scientists to trace the area's geologic history.) The verdict, con rmed by a second dating

13 (04) Time - One Giant Step for Mankind method and by the other primitive animals found with the hominid remains: most of the fossils are between 5.6 million and 5.8 million years old, although one toe bone is a few hundred thousand years younger. It was the detailed anatomy of these fragmentary fossils, especially the teeth, that convinced Haile-Selassie that he had discovered a new human ancestor. Although apelike, the lower canines and upper premolars, in particular, display certain traits found only in the teeth of later hominids-the term scientists use to describe ourselves and our non-ape ancestors. They also di er in shape from the teeth of all known fossil and modern apes. Even the way in which the teeth had been worn down was telling. Explains Haile-Selassie's thesis adviser, Berkeley paleontologist Tim White: "Apes all sharpen their upper canines as they chew. Hominids don't." The new creature's back teeth are larger than a chimp's too, while the front teeth are narrower, suggesting that its diet included a variety of brous foods, rather than the fruits and soft leaves that chimps prefer. When Haile-Selassie compared the newly discovered bones and teeth with those of Ardipithecus ramidus, a 4.4 million-year-old hominid found in the Middle Awash in the early 1990s that was the previous record holder, he realized that the two creatures were very similar. But the older one's teeth, while di erent from an ape's, do have a number of characteristics that are decidedly more apelike than those of the younger hominid. On the basis of these minor but distinctive di erences, Haile-Selassie decided to classify the new human ancestor as a subspecies, or variant, of ramidus and has given it the name Ardipithecus ramidus kadabba. (The name is derived from the local Afar language. Ardi means ground or oor; ramid means root; and kadabba means basal family ancestor. In accordance with the sometimes bizarre nomenclature of science, the younger creature now gets renamed Ardipithecus ramidus.) Haile-Selassie and his colleagues haven't collected enough bones yet to reconstruct with great precision what kadabba looked like. But they do know it was about the size of

14 (04) Time - One Giant Step for Mankind modern common chimpanzees, which when standing average about 4 ft. tall. That makes it roughly the same size as its close relative A. ramidus ramidus and about 20% taller than Lucy, the famous 3.2 million-year-old human ancestor discovered about 50 miles away in 1974 that is even further along the evolutionary track. The size of kadabba's brain and the relative proportions of its arms and legs were probably chimplike as well. But unlike a chimp or any of the other modern apes that amble along on four limbs, kadabba almost certainly walked upright much of the time. The inch-long toe bone makes that clear. Two-legged primates (modern humans included) propel themselves forward by leaving the front part of their foot on the ground and lifting the heel. This movement, referred to as toeing o, causes the bones in the middle of the foot to take on a distinctive shape-a shape that is readily apparent in the ancient toe bone. "If you compare a chimp's foot bones with its hand bones, they look the same because they're used for the same thing"-that is, for grasping-haile-selassie explains. "Hominid ngers and toes don't look alike at all." Exactly how this hominid walked is still something of a mystery, though with a di erent skeletal structure, its gait would have been unlike ours. Details of kadabba's lifestyle remain speculative too, but many of its behaviors undoubtedly resembled those of chimpanzees today. It probably still spent some time in trees. It probably lived in large social groups that would include both sexes. And rather than competing with one another for mates, the males may well have banded together to defend the troop against predators, forage for food and even hunt for game. But that kadabba walked upright at all is hugely significant. Paleontologists have suspected for nearly 200 years that bipedalism was probably the key evolutionary transition that split the human line o from the apes, and fossil discoveries as far back as Java Man in the 1890s supported that notion. The astonishingly complete skeleton of Lucy, with its clearly apelike skull but upright posture, cemented the idea a quarter-century ago.

15 (04) Time - One Giant Step for Mankind What's been much tougher to pin down is just why two-leggedness arose. The conventional wisdom has long focused on the fact that eastern Africa became signi cantly dryer about the time that humans rst evolved. The change would have tended to favor grasslands over forests, and, so went the theory, our ancestors changed to take advantage of the new conditions. We learned to walk upright so that we could see over the tall grasses to spot predators coming; an upright posture, moreover, would o er a much smaller target for the oppressive heat of the grassland sun, and a larger target for cooling breezes. The only trouble with this theory is that it's wrong. The earliest humans, it turns out, didn't live in grasslands. Dry climate or not, a companion paper published last week in Nature shows on the basis of the other fossilized ora and fauna, as well as the chemistry of the ancient soil, that Ardipithecus ramidus kadabba lived in a well-forested environment. That's also the case with other extremely ancient hominids found during the past several years, including Ardipithecus ramidus and a species called Orrorin tugenensis, announced last December by French and Kenyan researchers. And while the ability to walk on two legs probably started out as an increasingly frequent behavior, evolution demands an explanation for why it persisted. On rst blush, bipedalism just doesn't make much sense. For our earliest ancestors, it would have been slower than walking on all fours, while requiring the same amount of energy. Says Lovejoy bluntly: "It's unnatural. It's bizarre." Yet the advantages of walking upright were somehow so great that the behavior endured through thousands of generations. Indeed, the anatomy of our ancestors underwent all sorts of basic changes to accommodate this new way of moving. Many of the changes help the body stay balanced by stabilizing the weight-bearing leg and keeping the upper torso centered over the feet. Lovejoy, who studies the anatomy and biomechanics of locomotion, thinks the changes may have improved coordination as well. "To walk upright in a habitual way, you have to do so in synchrony," he says. "If the ligaments and muscles are out of synch, that leads to injuries. And then you'd be cheetah meat." By far the most crucial changes, according to Lovejoy, were those in the spine. The distance between chest and pelvis is longer in humans than in apes, allowing the lower spine to

16 (04) Time - One Giant Step for Mankind curve, which locates the upper body over the pelvis for balance. The pelvis grew broader, meanwhile, and humans developed a hip joint and associated muscles that stabilize the pelvis. Explains Lovejoy: "That's why a chimp sways from side to side as it walks upright and humans don't." Changes also had to take place in the femur, or thighbone. For example, the femoral neck-- the bent portion at the top of the bone--is broader in humans than it is in apes, which improves balance. The human knee is specialized for walking upright too: to compensate for the thighbone's being at an angle, there's a lump, or groove, at the end of the femur that prevents the patella from sliding o the joint. "A chimp doesn't have this groove because there is no angulation between the hip and the knee," Lovejoy says. "This change says you're a biped." Finally, there's the foot. "What's important here is the arch," Lovejoy says. "It's a really important shock absorber. It's like wearing a good pair of running shoes." In order to create that arch, the chimp's opposable great toe became aligned with the others, and the toe's muscles and ligaments, which had been used for grasping and climbing, were repositioned under the foot. "The shape of the big toe is indicative of this. You can see it in Lucy's species," Lovejoy says, but not in the bone Haile-Selassie found, because it's from a di erent toe. "What we can see [in the new discovery's foot] is that the base of the bone adjacent to the knuckle has a distinct angle, showing that the creature walked step after step after step with its heel off the ground, using the front of its foot as a platform." That's how it walked. Why it walked is tougher to understand, since motivation leaves behind no physical remains. But armed with knowledge about our ancestors' physical attributes and the environment that surrounded them, scientists have come up with several theories. Anthropologist Henry McHenry, of the University of California, Davis, for example, champions the idea that climate variation was part of the picture after all.

17 (04) Time - One Giant Step for Mankind When Africa dried out, say McHenry and his colleague Peter Rodman, the change left patches of forest widely spaced between open savannah. The rst hominids lived mostly in these forest refuges but couldn't nd enough food in any one place. Learning to walk on two legs helped them travel long distances over ground to the next woodsy patch, and thus to more food. Meave Leakey, head of paleontology at the National Museums of Kenya and a member of the world's most famous fossil-hunting family, suspects the change in climate rewarded bipedalism for a di erent reason. Yes, the dryer climate made for more grassland, but our early ancestors, she argues, spent much of their time not in dense forest or on the savannah but in an environment with some trees, dense shrubbery and a bit of grass. "And if you're moving into more open country with grasslands and bushes and things like this, and eating a lot of fruits and berries coming o low bushes, there is a hell of an advantage to be able to reach higher. That's why the gerenuk [a type of antelope] evolved its long neck and stands on its hind legs, and why the gira e evolved its long neck. There's strong pressure to be able to reach a wider range of levels." But for Kent State's Lovejoy, the real answer is sex. Males who were best at walking upright would get more of it, leading to more o spring who were good on two legs, who in turn got more sex. His reasoning, first proposed nearly two decades ago, goes like this: like many modern Americans, monkeys and apes of both genders work outside the home--in the latter case, searching for food. Early humans, though, discovered the Leave It to Beaver strategy: if males handled the breadwinning, females could stay closer to home and devote more time to rearing the children, thus giving them a better shot at growing up strong and healthy. And if you're going to bring home the bacon, or the Miocene equivalent, it helps to have your hands free to carry it. Over time, female apes would choose to mate only with those males who brought them food--presumably the ones who were best adapted for upright walking. Is that the way it actually happened? Maybe, but we may never know for sure. Leakey, for one, is unconvinced. "There are all sorts of hypotheses," she says, "and they are all fairy tales really because you can't prove anything."

18 (04) Time - One Giant Step for Mankind If paleontologists argue about why bipedalism evolved, they're even more contentious over the organization of the human family tree. According to Haile-Selassie and his colleagues, the picture looks pretty straightforward from about 5.8 million years ago to the present. First comes Ardipithecus ramidus kadabba, the newest nd. Then, more than a million years later, its descendant, the newly renamed Ardipithecus ramidus ramidus, appears. After that comes a new genus, called Australopithecus (where Lucy belongs), and finally, about 2 million years ago, the first members of the human genus Homo. But not everyone buys the story. Indeed, the French and Kenyan team that presented a 6 million-year-old fossil last December insists that theirs, known as Orrorin tugenensis (or, more familiarly, Millennium Man because it was announced in 2000), is the true human ancestor and that Ardipithecus is nothing more than a monkey's uncle--or a chimp's greatgreat-grandfather, anyway. They even dismiss Lucy and her close kin, about as rmly entrenched in the human lineage as you can get, as evolutionary dead ends that left no living descendants. No one disputes that this competing ancestor is 6 million years old and thus more ancient than Ardipithecus. What's still to be proved is that it's a hominid. Says Leakey: "If you read their paper, almost everything they say about the teeth suggests it's more apelike." And when they get to the femur, she says, they present no evidence disproving that it walked on all fours. Haile-Selassie makes precisely the same point. But Brigitte Senut of the National Museum of Natural History in Paris and Martin Pickford, chairman of paleoanthropology and prehistory at the College de France, coleaders of the team that found Orrorin, dismiss the criticisms. Additional fossils found just last March, they say, along with the more detailed analysis they now have in hand of the earlier bones, will prove their case. "We are absolutely delighted about it," says Senut. "We had the possibility to show the evidence to some colleagues in South Africa recently, and just looking at the cast they said, 'Fantastic, it's a biped! And a better biped than Lucy.'"

19 (04) Time - One Giant Step for Mankind Even if they're right, though, establishing the precise path of human descent might be very hard. For most of the past 6 million years, multiple hominid species roamed the earth at the same time--including a mere 30,000 years ago, when modern humans and Neanderthals still coexisted. We still can't gure out exactly how Neanderthals relate to the human family; it's all the more di cult to know where these newly discovered species, with far fewer fossil remains to study, belong. In the case of Ardipithecus, says Donald Johanson, professor of anthropology and director of the Institute of Human Origins at Arizona State University (and the man who discovered Lucy back in 1974), "when you put 5.5 million-year-old fossils together with 4.4 million-year-old ones as members of the same species, you're not taking into consideration that these could be twigs on a tree. Everything's been forced into a straight line." Beyond that, he's dubious about categorizing the 5.2 million-year-old toe bone with the rest of the fossils: not only is it separated in time by several hundred thousand years, but it was also found some 10 miles away from the rest. If Orrorin turns out to be a hominid, the same skepticism will apply to any claims about its pivotal position on the family tree. According to University of Tokyo paleontologist Gen Suwa, a co-discoverer of the 4.4. million-year-old Ardipithecus ramidus ramidus, Orrorin could well be ancestral to the new Ardipithecus remains, rather than the other way around."there is nothing in the fossils," he says, "that would preclude such a position. But which side of the chimp-hominid split Orrorin occupies can be determined only by further analyses and new nds." Indeed, suggests Haile-Selassie, while Orrorin may be one of the earliest chimps or an ape that became extinct, it could also turn out to be the last common ancestor of humans and chimps--a creature paleontologists have been dreaming of nding for decades. One of the most intriguing questions the new discoveries raise, says Bernard Wood, a professor of human origins at George Washington University, is whether bipedalism should still be considered the de ning characteristic of being human. After all, all birds have wings, but not all creatures with wings are birds. It's already clear that eastern Africa

20 (04) Time - One Giant Step for Mankind was bubbling with evolutionary experiments 6 million years ago. Maybe two-legged walking evolved independently in several branches of the primate family. Says Wood: "This might be the rst example of a creature it's not possible to label as hominid ancestor or chimp ancestor. But that doesn't make it the last common ancestor of both. I think it's going to be very hard to pin the tail on that donkey." In the end, that may be the most exciting thing about these latest discoveries from the human race's birthing ground. Not that long ago, paleontologists were pretty certain we started on the road to becoming human by standing upright on the grassy savannah. Now that science is actually bringing in hard evidence, the story is getting more complicated-- and more interesting. Clearly, there are still plenty of questions to ask, and plenty of surprises left to uncover, in the ancient sediments of eastern Africa. --With reporting by Simon Robinson/Nairobi Time Magazine, July 23,

21 (05) Science - The Earliest Hominins--Is Less More? 03/05/2004 David R. Begun Science, Vol. 303, Issue 5663, pp DOI: /science Fossil and molecular evidence are converging on a consensus that the human lineage diverged from that of the chimpanzee between 6 and 8 million years ago (Ma). Recent fossil discoveries in Africa are consistent with this conclusion and are beginning to paint a picture of the pattern of speciation that led to the origin of our lineage. On page 1503 of this issue, Haile-Selassie et al. (1) announce a new late Miocene African hominid (2) species, the third in the past 3 years (1, 3, 4). Ardipithecus kadabba is a nearly 6-million-yearold hominin (2) that Haile-Selassie and colleagues elevate to the species level from Ardipithecus ramidus kadabba. The new discovery renews discussion of the number of taxa that lived shortly after the human lineage diverged from that of chimpanzees, and may shed light on the very nature of the rst humans (2). Although Haile-Selassie and co-workers argue that all known African hominids between 7 and 4.4 Ma may belong to one genus and see little taxonomic diversity near hominin origins, they may in fact have evidence of more diversity than previously recognized (see the gure left). What you see is what you get. Two perspectives on hominin evolution driven by di erent models of the origin and evolution of diversity in the fossil record. (above)

22 (05) Science - The Earliest Hominins--Is Less More? A clean view of hominin evolution depicts an evolving lineage of Miocene taxa with progressively less apelike canines and a primitive form of bipedalism (A). Miocene A. kadabba is ancestral to A. ramidus, which is broadly ancestral to a somewhat larger diversity of younger Pliocene hominins (Au. anamensis, Au. afarensis, Au. africanus) (B) that is increasingly winnowed to one genus, Homo (C). (Right) A messy view depicts a series of adaptive radiations, migrations, and adaptive innovations. A new kind of hominid radiates into a number of taxa with new adaptations (facultative bipedalism, megadontia, canine reduction, or some combination of all three) (A). One of these radiates into the impressive diversity of Plio-Pleistocene hominins (B). This second radiation yields two terminal clades, one hypermegadont (robust australopithecines) and the other hypercerebral (Homo) (C). The degree of messiness in chimpanzee (Pan) evolution is unknown. Most recent accounts of human evolution are either messy or clean". The new evidence, discovered in the Middle Awash region of Ethiopia, is a handful of teeth including premolars and canines that are indeed distinct from A. ramidus (1, 5, 6). But the relationship of this older and smaller sample of specimens to A. ramidus is uncertain. A. ramidus from the Middle Awash study area (locality of Aramis) has been dated at 4.4 Ma, whereas the morphologically distinctive A. kadabba sample from Asa Koma and several other localities also in the Middle Awash study area is older, 5.6 to 5.8 Ma (one specimen, an enigmatic foot phalanx, is dated to 5.2 Ma) (7, 8). On the basis of their interpretation of a gradual transformation from apelike to humanlike canine/premolar morphology, and the expectation of high levels of diversity within genera or even species, Haile-Selassie et al. argue that A. kadabba represents an early phase in the development of human anterior teeth. They suggest that A. kadabba represents a transition between chimpanzees (with large, dagger-like canines) and A. ramidus, Australopithecus, and Homo (with unimpressive canines that wear down from the tips). Two other late Miocene hominins, Orrorin tugenensis and Sahelanthropus tchadensis, show predictable

23 (05) Science - The Earliest Hominins--Is Less More? variability in this evolving lineage. This is a plausible scenario.however, Ardipithecus, Orrorin, and Sahelanthropus o er evidence of striking diversity. Orrorin has modernlooking thickly enameled molars and robust jaws but has postcranial morphology that is distinct from that of Pliocene hominins (the epoch following the Miocene when clearly bipedal humans are found) (3, 9). Sahelanthropus is also quite distinct from Pliocene hominins in craniofacial and dental morphology (4, 10). Do these samples represent stages in early hominin evolution or discrete lineages? In other words, how many taxa are represented by these fossil samples? Could all of these fossils belong to the same genus, or even the same species? This is implied by Haile-Selassie et al. but seems unlikely to me. Comparisons among these taxa are di cult because they preserve little in common. Nonetheless, we know that Orrorin has mandibular and molar morphology similar to that of australopithecines, but female canines resembling those of apes (3, 11). Sahelanthropus has a unique combination of super cially modern-looking facial and periorbital morphology, primitive-looking teeth and braincase, and a probably homologous hominin suite of features of the basicranium (4, 10). A. kadabba is distinctive in having canines that appear to partly hone or sharpen against the lower rst premolar (P3 by paleontological convention), not as much as in apes but more than in humans. In crown cross section and shape, however, the canines are more hominin-like than the more chimpanzee-like Orrorin canines. But Orrorin has australopithecine-like thickly enameled teeth, usually associated with a diet requiring powerful chewing forces, whereas Sahelanthropus, A. ramidus, and A. kadabba have intermediate-thickness enamel (less thick than in other hominins, but much thicker than in African apes) (5). Miocene hominids range from thinly enameled chimpanzee-like molars, as in Dryopithecus, to Australopithecus-like thickly enameled molars, as in Sivapithecus and Ouranopithecus, a n d it is unclear which among these or the intermediate condition in Ardipithecus and Sahelanthropus represents the ancestral condition for hominins. Differences in enamel thickness often distinguish taxa at the genus level and usually re ect dietary adaptation rather than phylogeny.

24 (05) Science - The Earliest Hominins--Is Less More? Orrorin is said to share hominin synapomorphies (newly evolved, shared characteristics indicating a recent common ancestry) of the lower limb related to bipedalism (3, 9), but this is far from established. The most important features suggesting bipedalism either are found in nonbipeds (the groove for the obturator externus muscle) or appear di erent enough from the morphology of known bipeds to call the functional homology of these features into question (the linea aspera, which is unusually laterally displaced in Orrorin, or the asymmetric femoral neck cortical bone, which is less asymmetric than in known bipeds) (12). A. kadabba also lacks synapomorphies of A. ramidus and Australopithecus in canine, premolar, and last molar morphology, which suggests that the Asa Koma and other older samples belong to another genus (note that A. ramidus was originally named Australopithecus ramidus because of an understandable hesitation to name a new human genus, and was later elevated to a new genus when no Australopithecus-Homo synapomorphies were discovered). It is tempting to see evidence of anagenesis (unilinear evolution) in the late Miocene hominin record in part because continuity is suggested by claims for some evidence of bipedalism in all known taxa. The evidence from Orrorin is ambiguous (see above), whereas that from Sahelanthropus is indirect, based only on the position of the foramen magnum. The region is severely distorted in the only cranial specimen of Sahelanthropus, and even the describers recognize the uncertainty (4). A. kadabba is interpreted as a biped on the basis of a single toe bone, a foot proximal phalanx, with a dorsally oriented proximal joint surface, as in more recent hominins (6) However, the same joint con guration occurs in the de nitely nonbipedal late Miocene hominid Sivapithecus (13), and the length and curvature of this bone closely resembles those of a chimpanzee or bonobo. In addition, the specimen is 400,000 to 600,000 years younger than the rest of the A. kadabba sample, 800,000 years older than A. ramidus, and from a locality that is geographically much closer to Aramis than to Asa Koma. It may or may not be from a biped, and if it is, which biped? Another issue is the canine/premolar complex. Tall crowned interlocking canines are usually associated with agonistic (aggressive) displays typical of primate societies characterized by strong male competition (14). A shorter canine crown in humans is

25 (05) Science - The Earliest Hominins--Is Less More? interpreted as an indication of greater levels of male coalitionary behavior, or at least reduced competitiveness. Chimpanzee males are intermediate between humans and most other great ape males in canine crown height and competition levels, and bonobos are intermediate between chimpanzees and humans. But the remarkable reduction of canine crown height in humans is thought to signal a dramatic increase in the degree of male cooperation, cited as a hallmark of human origins [e.g. (15)]. The diversity of canine/premolar morphology described by Haile-Selassie et al., from chimpanzee-like to humanlike, represents a striking and unlikely degree for a single genus, especially in an anatomical complex strongly correlated to an aspect of socioecology that is considered to be a primary distinction between apes and humans. Both clade (synapomorphies) and grade (diet, locomotion, and socioecology) criteria suggest more rather than fewer taxa among the earliest possible hominins. Why the di erent interpretations? Evidence is scarce and fragmentary, and uncertainty predominates. Interpretations rely especially heavily on past experience to make sense of incomplete evidence. Haile-Selassie and colleagues interpret diversity in fossil hominids in terms of variability and gradual evolutionary change in an evolving lineage. Others see cladistic diversity as opposed to ancestor-descendant relations (see the gure). Ancestordescendant relations must exist, but adaptive radiation and cladogenesis also must exist, or organic diversity would be the same today as it was at the beginning of biological evolution. Rather than a single lineage, the late Miocene hominin fossil record may sample an adaptive radiation, from a source either in Eurasia or yet undiscovered in Africa (16), the rst of several radiations during the course of human evolution (see the gure). Regardless, the level of uncertainty in the available direct evidence at this time renders irreconcilable di erences of opinion inevitable. The solution is in the mantra of all paleontologists: We need more fossils!

26 (05) Science - The Earliest Hominins--Is Less More? References and Notes 1. Y. Haile-Selassie, G. Suwa, T. D. White, Science 303, 1503 (2004). 2. The terms hominid, hominin, and even human are in ux among paleoanthropologists. Here I refer to all great apes and humans as hominids (a family), and all taxa more closely related to living humans than to chimpanzees as hominins (a tribe). In this context, human is synonymous with hominin. 3. B. Senut et al., C. R. Acad. Sci. Paris Ser. IIA 332, 137 (2001). 4. M. Brunet et al., Nature 418, 145 (2002). 5. T. White, G. Suwa, B. Asfaw, Nature 371, 306 (1994). 6. Y. Haile-Selassie, Nature 412, 178 (2001). 7. G. WoldeGabriel et al., Nature 371, 330 (1994). 8. G. WoldeGabriel et al., Nature 412, 175 (2001). 9. M. Pickford, B. Senut, D. Gommery, J. Treil, C. R. Palevol 1, 191 (2002). 10. M. H. Wolpoff, B. Senut, M. Pickford, J. Hawks, Nature 419, 581 (2002). 11. Teeth do not have gender, but morphologists can distinguish within hominoids between the canines of males, which are tall crowned and pointed, and the canines of females, which are shorter crowned and less pointed. 12. The linea aspera is a crest on the back of the femur for the main origin of the quadriceps muscle and is typically more pronounced in humans than in apes. 13. M. D. Rose, J. Hum. Evol. 15, 333 (1986). 14. J. M. Plavcan, in Reconstructing Behavior in the Primate Fossil Record, J. M. Plavcan, R. F. Kay, W. L. Jungers, C. P. van Schaik, Eds. (Kluwer Academic, New York, 2002), pp C. O. Lovejoy, Science 211, 341 (1981). 16. D. R. Begun, in Hominoid Evolution and Climatic Change in Europe. Volume 2. Phylogeny of the Neogene Hominoid Primates of Eurasia, L. de Bonis, G. Koufos, P. Andrews, Eds. (Cambridge Univ. Press, Cambridge, 2001), pp