Hunting the First Hominid
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.
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
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.
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