Several studies on the genetic
information carried in our DNA and RNA point to chimpanzees being more
closely related to humans than to gorillas. However, many researchers
point out that chimpanzees and gorillas share several highly specialized
anatomical traits with one another. These may indicate that a closer relationship
exists between these two apes, with humans a slightly more distant evolutionary
cousin.
These anatomical traits are mainly centered around adaptations for
knuckle-walking. When gorillas and chimpanzees walk on the ground, they
usually do so quadrupedally (on four legs). They support the weight
of the front part of their body on the middle phalanges (the portion
of the finger between the first and second knuckle of their hands, hence
the term "knuckle-walking"). This is a highly specialized
behavior that requires many modifications of the bones of the wrist,
hand, and fingers to create a locking mechanism that is able to support
their body weight. (Try doing a push-up on your middle phalanges!)
A serious question is then raised: If chimpanzees and humans are more
closely related (as some genetic experiments suggest), did chimpanzees
and gorillas evolve the modifications for knuckle-walking independently?
To many paleontologists, this would seem a bit too coincidental.
Two researchers with the Smithsonian's Human Origins Program and George
Washington University have recently identified evidence in early humans
for a knuckle-walking ancestor. Using specimens of gorilla, chimpanzee,
human and other primates that do not knuckle-walk, they were able to
identify specific features of the radius, one of the bones in the forearm,
that are indicative of knuckle-walking. Specifically they identified
a ridge of bone that projects down into the wrist, stabilizing it when
weight is placed on it in chimpanzees and gorillas.
Identifying a feature indicative of knuckle-walking at the end of the
radius was important, because two early human species from near the
beginning of human evolutionary history have preserved specimens of
this bone: Australopithecus anamensis (KNM ER 20419) and Australopithecus
afarensis (AL 288-1, the famous "Lucy" specimen). Upon examination
of these specimens a clear ridge of bone was evident extending into
the wrist from the radius. This ridge was not as pronounced as it is
in modern chimpanzees and gorillas, which indicates that neither of
these early human species were active knuckle-walkers. Rather, it suggests
that these were traits inherited from a knuckle-walking ancestor. Examination
of later-occurring early human species (Australopithecus
africanus and Paranthropus robustus)
showed that by three million years ago this trait had been lost in the
human lineage.
This study implies that early humans evolved from an ancestor that
was originally a knuckle-walker, and this raises the possibility that
knuckle-walking evolved only once for gorillas, humans and chimpanzees.
While we have stopped using this form of locomotion, the other African
apes have retained it, and so they retain the similar anatomy that is
associated with it.