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Oldest 'Human' Skeleton Found -- Disproves 'Missing Link'

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    Message 1 of 1 , Oct 1, 2009
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      By Jamie Shreeve
      National Geographic Magazine
      October 1, 2009


      [Visit the link above for additional links, articles, and illustrations
      related to this important story. --DS]

      Move over, Lucy. And kiss the missing link goodbye.

      Scientists today announced the discovery of the oldest fossil skeleton of a
      human ancestor. The find reveals that our forebears underwent a previously
      unknown stage of evolution more than a million years before Lucy, the iconic
      early human ancestor specimen that walked the Earth 3.2 million years ago.

      The centerpiece of a treasure trove of new fossils, the skeleton -- assigned
      to a species called Ardipithecus ramidus -- belonged to a small-brained,
      110-pound (50-kilogram) female nicknamed "Ardi."

      The fossil puts to rest the notion, popular since Darwin's time, that a
      chimpanzee-like missing link -- resembling something between humans and
      today's apes -- would eventually be found at the root of the human family
      tree. Indeed, the new evidence suggests that the study of chimpanzee anatomy
      and behavior -- long used to infer the nature of the earliest human
      ancestors -- is largely irrelevant to understanding our beginnings.

      Ardi instead shows an unexpected mix of advanced characteristics and of
      primitive traits seen in much older apes that were unlike chimps or gorillas
      (interactive: Ardi's key features). As such, the skeleton offers a window on
      what the last common ancestor of humans and living apes might have been

      Announced at joint press conferences in Washington, D.C., and Addis Ababa,
      Ethiopia, the analysis of the Ardipithecus ramidus bones will be published
      in a collection of papers tomorrow in a special edition of the journal
      Science, along with an avalanche of supporting materials published online.

      "This find is far more important than Lucy," said Alan Walker, a
      paleontologist from Pennsylvania State University who was not part of the
      research. "It shows that the last common ancestor with chimps didn't look
      like a chimp, or a human, or some funny thing in between."

      Ardi Surrounded by Family

      The Ardipithecus ramidus fossils were discovered in Ethiopia's harsh Afar
      desert at a site called Aramis in the Middle Awash region, just 46 miles (74
      kilometers) from where Lucy's species, Australopithecus afarensis, was found
      in 1974. Radiometric dating of two layers of volcanic ash that tightly
      sandwiched the fossil deposits revealed that Ardi lived 4.4 million years

      Older hominid fossils have been uncovered, including a skull from Chad at
      least six million years old and some more fragmentary, slightly younger
      remains from Kenya and nearby in the Middle Awash.

      While important, however, none of those earlier fossils are nearly as
      revealing as the newly announced remains, which in addition to Ardi's
      partial skeleton include bones representing at least 36 other individuals.

      "All of a sudden you've got fingers and toes and arms and legs and heads and
      teeth," said Tim White of the University of California, Berkeley, who
      co-directed the work with Berhane Asfaw, a paleoanthropologist and former
      director of the National Museum of Ethiopia, and Giday WoldeGabriel, a
      geologist at Los Alamos National Laboratory in New Mexico.

      "That allows you to do something you can't do with isolated specimens,"
      White said. "It allows you to do biology."

      Ardi's Weird Way of Moving

      The biggest surprise about Ardipithecus's biology is its bizarre means of
      moving about.

      All previously known hominids -- members of our ancestral lineage -- walked
      upright on two legs, like us. But Ardi's feet, pelvis, legs, and hands
      suggest she was a biped on the ground but a quadruped when moving about in
      the trees.

      Her big toe, for instance, splays out from her foot like an ape's, the
      better to grasp tree limbs. Unlike a chimpanzee foot, however,
      Ardipithecus's contains a special small bone inside a tendon, passed down
      from more primitive ancestors, that keeps the divergent toe more rigid.
      Combined with modifications to the other toes, the bone would have helped
      Ardi walk bipedally on the ground, though less efficiently than later
      hominids like Lucy. The bone was lost in the lineages of chimps and

      According to the researchers, the pelvis shows a similar mosaic of traits.
      The large flaring bones of the upper pelvis were positioned so that Ardi
      could walk on two legs without lurching from side to side like a chimp. But
      the lower pelvis was built like an ape's, to accommodate huge hind limb
      muscles used in climbing.

      Even in the trees, Ardi was nothing like a modern ape, the researchers say.

      Modern chimps and gorillas have evolved limb anatomy specialized to climbing
      vertically up tree trunks, hanging and swinging from branches, and
      knuckle-walking on the ground.

      While these behaviors require very rigid wrist bones, for instance, the
      wrists and finger joints of Ardipithecus were highly flexible. As a result
      Ardi would have walked on her palms as she moved about in the trees -- more
      like some primitive fossil apes than like chimps and gorillas.

      "What Ardi tells us is there was this vast intermediate stage in our
      evolution that nobody knew about," said Owen Lovejoy, an anatomist at Kent
      State University in Ohio, who analyzed Ardi's bones below the neck. "It
      changes everything."

      Against All Odds, Ardi Emerges

      The first, fragmentary specimens of Ardipithecus were found at Aramis in
      1992 and published in 1994. The skeleton announced today was discovered that
      same year and excavated with the bones of the other individuals over the
      next three field seasons. But it took 15 years before the research team
      could fully analyze and publish the skeleton, because the fossils were in
      such bad shape.

      After Ardi died, her remains apparently were trampled down into mud by
      hippos and other passing herbivores. Millions of years later, erosion
      brought the badly crushed and distorted bones back to the surface.

      They were so fragile they would turn to dust at a touch. To save the
      precious fragments, White and colleagues removed the fossils along with
      their surrounding rock. Then, in a lab in Addis, the researchers carefully
      tweaked out the bones from the rocky matrix using a needle under a
      microscope, proceeding "millimeter by submillimeter," as the team puts it in
      Science. This process alone took several years.

      Pieces of the crushed skull were then CT-scanned and digitally fit back
      together by Gen Suwa, a paleoanthropologist at the University of Tokyo.

      In the end, the research team recovered more than 125 pieces of the
      skeleton, including much of the feet and virtually all of the hands -- an
      extreme rarity among hominid fossils of any age, let alone one so very

      "Finding this skeleton was more than luck," said White. "It was against all

      Ardi's World

      The team also found some 6,000 animal fossils and other specimens that offer
      a picture of the world Ardi inhabited: a moist woodland very different from
      the region's current, parched landscape. In addition to antelope and monkey
      species associated with forests, the deposits contained forest-dwelling
      birds and seeds from fig and palm trees.

      Wear patterns and isotopes in the hominid teeth suggest a diet that included
      fruits, nuts, and other forest foods.

      If White and his team are right that Ardi walked upright as well as climbed
      trees, the environmental evidence would seem to strike the death knell for
      the "savanna hypothesis" -- a long-standing notion that our ancestors first
      stood up in response to their move onto an open grassland environment.

      Sex for Food

      Some researchers, however, are unconvinced that Ardipithecus was quite so

      "This is a fascinating skeleton, but based on what they present, the
      evidence for bipedality is limited at best," said William Jungers, an
      anatomist at Stony Brook University in New York State.

      "Divergent big toes are associated with grasping, and this has one of the
      most divergent big toes you can imagine," Jungers said. "Why would an animal
      fully adapted to support its weight on its forelimbs in the trees elect to
      walk bipedally on the ground?"

      One provocative answer to that question -- originally proposed by Lovejoy in
      the early 1980s and refined now in light of the Ardipithecus discoveries --
      attributes the origin of bipedality to another trademark of humankind:
      monogamous sex.

      Virtually all apes and monkeys, especially males, have long upper canine
      teeth -- formidable weapons in fights for mating opportunities.

      But Ardipithecus appears to have already embarked on a uniquely human
      evolutionary path, with canines reduced in size and dramatically "feminized"
      to a stubby, diamond shape, according to the researchers. Males and female
      specimens are also close to each other in body size.

      Lovejoy sees these changes as part of an epochal shift in social behavior:
      Instead of fighting for access to females, a male Ardipithecus would supply
      a "targeted female" and her offspring with gathered foods and gain her
      sexual loyalty in return.

      To keep up his end of the deal, a male needed to have his hands free to
      carry home the food. Bipedalism may have been a poor way for Ardipithecus to
      get around, but through its contribution to the "sex for food" contract, it
      would have been an excellent way to bear more offspring. And in evolution,
      of course, more offspring is the name of the game.

      Two hundred thousand years after Ardipithecus, another species called
      Australopithecus anamensis appeared in the region. By most accounts, that
      species soon evolved into Australopithecus afarensis, with a slightly larger
      brain and a full commitment to a bipedal way of life. Then came early Homo,
      with its even bigger brain and budding tool use.

      Did primitive Ardipithecus undergo some accelerated change in the 200,000
      years between it and Australopithecus -- and emerge as the ancestor of all
      later hominids? Or was Ardipithecus a relict species, carrying its quaint
      mosaic of primitive and advanced traits with it into extinction?

      Study co-leader White sees nothing about the skeleton "that would exclude it
      from ancestral status." But he said more fossils would be needed to fully
      resolve the issue.

      Stony Brook's Jungers added, "These finds are incredibly important, and
      given the state of preservation of the bones, what they did was nothing
      short of heroic.

      But this is just the beginning of the story."


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