BERKELEY – Paleoanthropologists from the University of California, Berkeley, and the Cleveland Museum of Natural History have found more fossils of a nearly 6-million-year-old human ancestor first reported three years ago, cementing its importance as the earliest hominid to appear after the human line diverged from the line leading to modern chimpanzees.
When first reported in the journal Nature in 2001, the hominid was named Ardipithecus ramidus kadabba, a subspecies of a younger hominid, Ardipithecus ramidus, also from the Middle Awash region of Ethiopia. The new fossils - six teeth - provide enough evidence to designate the hominid a distinct species, Ardipithecus kadabba, rather than a subspecies of Ardipithecus ramidus.
"Ardipithecus kadabba may also represent the first species on the human branch of the family tree just after the evolutionary split between lines leading to modern chimpanzees and humans," said Yohannes Haile-Selassie, curator and head of physical anthropology at the Cleveland museum.
Haile-Selassie and coauthors Tim White of UC Berkeley and Gen Suwa of the University Museum at the University of Tokyo report their fossil finds in the March 5 issue of the journal Science.
Between 1997 and 2000, Haile-Selassie excavated 11 hominid fossils from at least five individuals who once lived in a wooded environment, now a dry, rocky area in the Afar rift of Ethiopia's Middle Awash region. He and White, along with geologist Giday WoldeGabriel of Los Alamos National Laboratory, interpreted the bones as those of a bipedal hominid about the size of a chimpanzee living between 5.2 and 5.8 million years ago.
The six new teeth were found during a one-month excavation in 2002 at one site, Asa Koma ("Red Hill") Locality 3, that yielded a fragment of an arm bone and an isolated tooth during the earlier surveys. Asa Koma is located along the western margin of the Middle Awash study area about 180 miles northeast of the Ethiopian capital, Addis Ababa.
The teeth sieved from the sediments include an upper canine, premolars from both jaws, and upper molars. All of these were from deposits sandwiched between volcanic horizons dated by the argon/argon method to between 5.54 and 5.77 million years. The dating work was done at the Berkeley Geochronology Center by Paul Renne, adjunct professor of earth and planetary science at UC Berkeley.
Though the scientists now have only 17 specimens, mostly teeth, from this hominid, the fossils nevertheless tell much about how these creatures lived. The teeth, in particular, are critical in differentiating between ape and human fossils.
"In all the great apes - and that includes fossil and modern - the large, tusk-like, projecting, shearing canine teeth are used as weapons, and in most of them the main use is in males fighting with other males for access to estrus females," White said. "The earliest hominids lack that adaptation, showing much smaller canines that are not at all chimpanzee-like."
In the apes, the upper canine is continually honed against the lower third premolar to keep it sharp. Human canines lack that function, White said.
The implication of this dental difference is that the newly evolved hominids were living in a radically different, less competitive social structure than seen in modern chimps, he said. Anthropologist Owen Lovejoy proposed in the 1980s that reduced canines among early hominids showed that males had become more involved in the parenting process, and that the carrying of infants and food was strong selective pressure toward a musculoskeletal system adapted to walking on two legs.
The new fossils show the most primitive canines ever found among hominids.
"We see wear facets on the premolar in Ardipithecus kadabba which A. ramidus doesn't have," Haile-Selassie said. "We don't really know whether some A. kadabba individuals had functional honing - we just have one sample here - but we're saying that the primitive shape and the presence of this facet on the upper canine and the lower premolar distinguishes A. ramidus from A. kadabba, so that it becomes a species of its own."
The researchers note two other sites that have yielded fossil hominids from the same 5- to 6-million-year period. One group of fossils found in 2002 in Chad has been named Sahelanthropus tchadensis, while others found at a Kenyan site in 2000 have been dubbed Orrorin tugenensis. All of these fossils are sufficiently similar that they should be included in the same genus as Ardipithecus kadbba, the team argues.
The Middle Awash field work was funded by the National Science Foundation and The Institute of Geophysics and Planetary Physics at Los Alamos National Laboratory.
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