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Ardipithecus Ramidus
Image courtesy Science/AAAS
Editor's note: At the request of illustrator J.H. Matternes and Science/AAAS, we are removing the illustrations of Ardipithecus ramidus that were once featured here. The illustrations may be viewed in a scientific paper.
October 1, 2009--In 1994 a research team led by Tim White of the University of California, Berkeley; Berhane Asfaw, former director of the National Museum of Ethiopia; and Giday WoldeGabriel of the Los Alamos National Laboratory announced the discovery of the first fossils of a new human ancestor, Ardipithecus ramidus. The researchers presented tantalizing evidence that the species was a biped living in woodland conditions more than a million years before the famous "Lucy" fossil of the species Australopithecus afarensis.
The research, to be published in an October 2, 2009, special issue of the journal Science, reveals that our earliest ancestors underwent a previously unknown phase of evolution, shedding new light on the nature of the last common ancestor of chimpanzees and humans.
What did our earliest known ancestors look like? The skeletal material associated with Ardipithecus ramidus now provides the best evidence. (Above, a digital reconstruction of skull parts from two individuals is shown.)
"Ardi" stood about 47 inches (120 centimeters) tall and weighed about 110 pounds (50 kilograms). The face of "Ardi" did not project as much as those of modern apes, but was not as flat and massive as the later australopithecines. Researchers who studied the species suggest this difference is related to the small size of the species' incisor teeth compared to those of chimps. Based on the relatively small size of its brow ridge and canine teeth, scientists suggest this fossil is of a female.
While A. ramidus was quite apelike in appearance, researchers who studied the fossil skeleton suggest the species lacked the adaptations of living apes for climbing vertically, hanging from branches, and walking on its knuckles. Instead, it was a "careful climber" in the trees, and supported its weight on the palms of its hands while using its divergent big toe for grasping.
The feet of the new ancestor were more like those of Old World monkeys, which have feet that are more rigid and provide a better lever for walking. The lower part of A. ramidus's pelvis, however, retained an apelike condition, probably to accommodate strong leg muscles still useful for climbing.
Read more about Ardi >>October 1, 2009
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Tooth Comparison
Image courtesy Science/AAAS
This row of images shows the male canine condition of Ardipithecus ramidus (center digital image) intermediate in size between humans (left) and chimpanzees. The reduced size of canine teeth is an indication of a shift in social behavior away from male-male aggression, and is one of the hallmarks of the human lineage.
Finding the trait in Ardipithecus suggests that a social structure involving more cooperation between males evolved very early after the divergence between the chimpanzee and human lineages. A. ramidus' molars and premolars were small compared to those of later australopithecines and had thinner enamel, reflecting differences in diet.
In addition to the partial skeleton found in 1994, bone fragments of at least 35 other Ardipithecus ramidus individuals were excavated at the Aramis site in the Middle Awash region of the Afar Depression in Ethiopia.
While its massive forearms and long, curved fingers show A. ramidus was a climber, the upper portion of its pelvis had evolved away from an apelike condition to accommodate strong muscles that assisted in walking upright. This more human-like pelvis shape can be seen in later human ancestors, such as Australopithecus afarensis.October 1, 2009
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science cover ardipithecus ramidus
Image courtesy Science/AAAS
So fragile they would turn to dust at a touch, the bones of the A. ramidus skeleton took 15 years to extricate from their surrounding rock, analyze, and compare with those of other individuals of the species found in the same deposits.
"Ardi," the nickname for the newly revealed A. ramidus skeleton, predates "Lucy," the well-known Australopithecus afarensis fossil, by more than a million years.October 1, 2009
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Middle Awash
Map by National Geographic magazine staff
The Middle Awash study area, where the Ardipithecus bones were found, is on the Awash River about 140 miles (230 kilometers) from the Ethiopian capital of Addis Ababa.
It's part of the parched Afar rift valley, where the famous "Lucy" skeleton was found farther to the north.
The valley was formed by the wrenching apart of three massive sections of Earth's crust. Volcanoes, earthquakes, and erosion associated with the rifting have had the effect of first burying, and then much later disgorging to the surface, fossils formed millions of years before.October 1, 2009
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Excavation
Photograph courtesy Dr. Tim White
The bones of the "Ardi" skeleton were extremely fragile, requiring unusually meticulous excavation techniques (pictured, the scientists at work) and years of preparation in the laboratory.
In addition to the hominid bones themselves, scientists uncovered a wealth of other specimens from the site, including fragments of large mammals, tiny shrews, plant seeds, pollen, and even fossilized dung beetle balls.
All this information helped confirm that Ardi lived in a woodland environment.October 1, 2009
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Middle Awash
Photograph courtesy Dr. Tim White
Today, the Middle Awash research area (pictured) where Ardipithecus was found is a parched desert. When the species lived 4.4 million years ago, the landscape was radically different: a moist, cool woodland with patches of thicker forest.
Geological faulting and erosion over millennia brought the bones to within reach of the excavation team.
Read more about Ardi >>October 1, 2009
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