Anatomy Of An Ancestor

KEY TRAITS link putative hominids Ardipithecus ramidus kadabba, Orrorin and Sahelanthropus to humans and distinguish them from apes such as chimpanzees. The fossils exhibit primitive apelike characteristics, too, as would be expected of creatures this ancient. For instance, the A. r. kadabba toe bone has a humanlike upward tilt to its joint surface, but the bone is long and curves downward like a chimp's does (which somewhat obscures the joint's cant). Likewise, Sahelanthropus has a number of apelike traits—its small braincase among them—but is more humanlike in the form of the canines and the projection of the lower face. (Reconstruction of the Sahelanthropus cranium, which is CRANIUM

distorted, will give researchers a better Modern human understanding of its morphology.) The Orrorin

Sahelanthropus Chi femur has a long neck and a groove carved out by the obturator externus muscle—traits typically associated with habitual bipedalism and therefore with humans—but the distribution of cortical bone in the femoral neck may be more like that of a quadrupedal ape.

TOE BONE Modern human A. r. kadabba Chimpanzee

-Joint-1 Joint surface surface cants downward

I cants upward

Sahelanthropus

Chimpanzee

Chimpanzee

Moderately projecting lower face

Strongly projecting lower face

Moderately projecting lower face

Strongly projecting lower face

© C. OWEN LOVEJOY\ßr/// Atlanta (human, A. r. kadabba and chimpanzee toe bones); CHRISTIAN SIDOR New York College of Osteopathic Medicine (human skull and humanfemur); MISSION PALÉOANTHROPOLOGIQUE FRANCO-TCHADIENNE (Sahelanthropus skull); © 1996 DAVID L. BRILL\DIVISION OF MAMMALS, NATIONAL MUSEUM OF NATURAL HISTORY, SMITHSONIAN INSTITUTION (chimpanzee skull); GAMMA (Orrorin femur); C. OWEN LOVEJOY Kent State University (chimpanzeefemur)

Large, sharp

Standing Tall

THE FIRST HOMINID CLUE to come from beyond the 4.4-million-year mark was announced in the spring of 2001. Paleontologists Martin Pickford and Brigitte Senut of the National Museum of Natural History in Paris found in Kenya's Tugen Hills the six-million-year-old remains of a creature they called Orrorin tugenensis. To date, the researchers have amassed 21 specimens, including bits of jaw, isolated teeth, finger and arm bones, and some partial upper leg bones, or femurs. According to Pickford and Senut, Orrorin exhibits several characteristics that clearly align it with the hominid fam-ily—notably those suggesting that, like all later members of our group, it walked on two legs. "The femur is remarkably humanlike," Pickford observes. It has a long femoral neck, which would have placed the shaft at an angle relative to the lower leg (thereby stabilizing the hip), and a groove on the back of that femoral neck, where a muscle known as the obturator ex-ternus pressed against the bone during upright walking. In other respects, Orrorin was a primitive animal: its canine teeth are large and pointed relative to human canines, and its arm and finger bones retain adaptations for climbing. But the femur characteristics signify to Pickford and Senut that when it was on the ground, Orrorin walked like a man.

In fact, they argue, Orrorin appears to have had a more humanlike gait than the much younger Lucy did. Breaking with paleoanthropological dogma, the team posits that Orrorin gave rise to Homo via the proposed genus Praeanthropus (which comprises a subset of the fossils currently assigned to A. afaren-sis and A. anamensis), leaving Lucy and her kin on an evolutionary sideline. Ardipithecus, they believe, was a chimpanzee ancestor.

Not everyone is persuaded by the femur argument. C. Owen Lovejoy of Kent State University counters that published computed tomography scans through Orrorin's femoral neck— which Pickford and Senut say reveal humanlike bone structure—actually show a chimplike distribution of cortical bone, an important indicator of the strain placed on that part of the femur during locomotion. Cross sections of A. afarensis's femoral neck, in contrast, look entirely human, he states. Love-joy suspects that Orrorin was frequently—but not habitually— bipedal and spent a significant amount of time in the trees. That wouldn't exclude it from hominid status, because full-blown bipedalism almost certainly didn't emerge in one fell swoop. Rather Orrorin may have simply not yet evolved the full complement of traits required for habitual bipedalism. Viewed that way, Orrorin could still be on the ancestral line, albeit further removed from Homo than Pickford and Senut would have it.

Better evidence of early routine bipedalism, in Lovejoy's view, surfaced a few months after the Orrorin report, when Berkeley graduate student Yohannes Haile-Selassie announced the discovery of slightly younger fossils from Ethiopia's Middle Awash region. Those 5.2-million- to 5.8-million-year-old remains, which have been classified as a subspecies of Ardi-pithecus ramidus, A. r. kadabba, include a complete foot phalanx, or toe bone, bearing a telltale trait. The bone's joint is angled in precisely the way one would expect if A. r. kadabba "toed off" as humans do when walking, reports Lovejoy, who has studied the fossil.

Other workers are less impressed by the toe morphology. "To me, it looks for all the world like a chimpanzee foot phalanx," comments David Begun of the University of Toronto, noting from photographs that it is longer, slimmer and more curved than a biped's toe bone should be. Clarification may come when White and his collaborators publish findings on an as yet undescribed partial skeleton of Ardipithecus, which White says they hope to do within the next year or two.

Differing anatomical interpretations notwithstanding, if either Orrorin or A. r. kadabba were a biped, that would not only push the origin of our strange mode of locomotion back by nearly 1.5 million years, it would also lay to rest a popular idea about the conditions under which our striding gait evolved. Received wisdom holds that our ancestors became bipedal on the African savanna, where upright walking may have kept the blistering sun off their backs, given them access

0 0

Post a comment