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MARGINALIA

Hunting the First Hominid

Pat Shipman

A Tale of Two Trophies

Yohannes Haile-Selassie of the University of California, Berkeley, described a likely contender from Ethiopia in July of 2001. His material comes from Ethiopian sediments between 5.2 million and 5.8 million years old and is called Ardipithecus ramidus kadabba, a new subspecies. Ardipithecus means "root ape" in the Afar language, and the species has been explicitly proposed to be a "root species" ancestral to all later hominids.

Haile-Selassie's specimens include more than 20 teeth, some associated with a mandible or lower jaw; substantial pieces of two left humeri, or upper arm bones; a partial ulna from the same forearm as one humerus; a partial clavicle or collarbone; a half of one finger bone; and a complete toe bone.

No ironclad evidence of bipedality in any Ardipithecus specimen has yet been published. In this collection, the only evidence about habitual patterns of locomotion comes from the single toe bone. Its weight-bearing surface faces downward as in bipeds, not inward as in apes. Any jury might be suspicious that Ardipithecus was bipedal, but none of the really telltale body parts—pelvis, complete femur, tibia, or ankle bones—has yet been recovered. The preserved bones of the arm, finger and shoulder closely resemble those of Lucy and may have been used in grasping and tree-climbing. Ardipithecus is as yet too poorly known for its relative brain size or sexual dimorphism in body size to be assessed.Figure 2. Click to Enlarge Image

The other candidate that has already been bagged is a 6 million-year-old find from the Tugen Hills of Kenya called Orrorin tugenensis. Its generic name is derived from the Tugen language and means "original man"—a claim as bold as "root ape." Found by a joint French-Kenyan team headed by Brigitte Senut of the Centre nationale de la recherche scientifique, the Orrorin fossils include a few teeth, some embedded in a jaw fragment; a partial humerus; a finger bone; and substantial parts of three femurs, or thigh bones.

The femurs, which might provide definitive evidence of bipedality, are incomplete. The sole evidence for bipedality lies in the head of the femur in Orrorin, which is proportionately larger than Lucy's. One reason to evolve a large-headed femur is to dissipate the forces produced by bipedalism. The team concludes that Orrorin evolved bipedalism separately from Lucy (and from other species of Australopithecus), making Orrorin the only known ancestor of Homo. Australopithecus is displaced to an extinct side-branch of the hominid lineage.

Their surprising conclusion is not universally accepted. Skeptics reply that the femoral differences between Orrorin and Australopithecus might disappear if Orrorin's femur were compared with that of a large male individual rather than with the diminutive Lucy.

As in Ardipithecus, the bones from the upper limb of Orrorin show tree-climbing adaptations. Neither relative brain size nor body size dimorphism can be evaluated in Orrorin.

Where Orrorin and Ardipithecus differ are in their teeth. Orrorin appears to have thick enamel, like a hominid or an orang-utan, and Ardipithecus seems to have thin enamel, like other apes. This dental comparison might resolve the question "Who is the First Hominid?" in favor of Orrorin, except that Orrorin's canine teeth imply the opposite conclusion. Orrorin's single known canine is sizable and pointed and wears like an ape's canine. In contrast, the several known canines of Ardipithecus are all small-crowned and flat-wearing, like a hominid's canines.Figure 3. Six-million-year-old fossils . . .Click to Enlarge Image

Puzzlingly, Ardipithecus and Orrorin show different mosaics of hominid and ape features. Both may be bipeds, although Ardipithecus is a biped in the manner of Australopithecus and Orrorin is not. If one of these two newly announced species is the First Hominid, then the other must be banished to the ape lineage. The situation is deliciously complex and confusing.

It is also humbling. We thought we knew an ape from a person; we thought we could even identify a man in an ape suit or an ape in a tuxedo for what they were. Humans have long prided themselves on being very different from apes—but pride goeth before a fall. In this case, embarrassingly, we can't tell the ape from the hominid even though we have teeth, jaws, and arm and leg bones.

Paleoanthropologists must seriously reconsider the defining attributes of apes and hominids while we wait for new fossils. In the meantime, we should ponder our complicity, too, for we have been guilty of expecting evolution to be much simpler than it ever is.

© Pat Shipman




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