#1Oct 1st, 2009
An international team of scientists unveiled Thursday the results of 15 years of
study of one of the oldest known human ancestors, Ardipithecus ramidus, which
they say overturns much of what we know about human evolution.
And surprisingly, it's also rewriting the story of our relation to gorillas and
chimpanzees, our closest living relatives, and their development as well.
Yohannes Haile-Selassie, one of the authors involved in the research and the man
who discovered the first pieces of the most complete Ardipithecus ramidus
specimen, nicknamed Ardi by the researchers, says the findings represent a
complete rewrite about what is known about human and ape evolution, and give
new insight into how we became bipedal.
"What we are seeing . . . is something we never expected to find in the human
lineage," he says, his voice buoyant on the phone from the Cleveland Museum of
Natural History, where he is Curator/Head Physical Anthropology.
"'It's a revelation, and you can imagine how much it's going to change how we think
about the earliest parts of our evolution."
The peer-reviewed findings appear Friday in a special edition of the online journal
Science, published by the American Association for the Advancement of Science.
They are also being announced Thursday morning in simultaneous news conferences
in Washington, D.C. and Addis Ababa, Ethiopia.
The story of Ardi takes us back 4.4 million years to a corner of northeast Ethiopia
that today is a desert where erosion constantly exposes fossils from the dawn of
humankind.
In all, scientists have discovered fossilized bones and teeth in the area representing
three dozen individual Ardipithecus specimens, including much of Ardi's skull, pelvis,
lower arms and feet.
Until now, Haile-Selassie says, much of what we knew about our ancient past
derived from comparisons with the other apes, and especially chimps, and from
Ardi's younger 'sister' — Lucy, the 3.2-million-year-old specimen of another hominid
species, Australopithecus afarensis, discovered in 1974, also in Ethiopia.
Lucy's discovery showed that human forebears walked upright that long ago.
But Ardi, Haile-Selassie says, shows our first erect steps took place more than a
million years earlier and that is much closer to the last common ancestor (or LCA)
that the human line shares with the ape line after the two split some six million
years ago.
Until now, it has been assumed chimps and gorillas have retained many of the
supposed traits of that last common ancestor, among them knuckle-walking and
climbing ability.
Now, Haile-Selassie says, we know that isn't true.
Ardi shows that unlike modern apes, which are knuckle-walkers, her species — and
by extension all the ancestors of all apes and humans — descended from a common
ancestor that in turn was not a knuckle-walker, he says.
Through analyzing Ardi's teeth, pelvic bones, hands and feet, the researchers
determined Ardipithecus had a mixture of primitive traits, shared with its older
relatives, and traits that only later hominids — like Lucy and us — have.
However, they also found many of those traits do not appear in modern apes,
leading to the conclusion that apes have evolved significantly since the split with
the last common human-ape ancestor.
For one thing, says Haile-Selassie, chimps and gorillas developed less flexible wrists
that make knuckle-walking and swinging from branches possible only after they
split from the line that led to humans.
The researchers say the surprising findings mean chimps and gorillas
have specialized greatly since then and are poor models for a common ancestor and
for understanding human abilities such as walking.
On that last point, Haile-Selassie says Ardipithecus's woodland habitat and her
ability to walk upright — though not as well as later hominids — also "falsifies" the
long-accepted notion that bipedalism originated when our ancestors needed to see
further in open grassland — or savannah — in order to avoid predators.
Ardi's species was already walking before our first ancestors ventured onto the
savannah, he says.
Back in 1994, when he found the first fragments of Ardi — half a finger bone and
then the other half nearby — Haile-Selassie recalls he "was excited because he knew
he had some human forerunner on his hands.
"There's something that we call 'hominid fever,' " he says with a chuckle. "When
you're in the field you want to find a hominid." But he had no inkling of the
significance of his discovery.
The pieces were so fragmented and fragile that it has taken 15 years to dig out,
clean, assemble and analyze the partial skeleton of Ardi. "You just touch it, it turns
to powder," he says.
That in itself was quite a feat, says David Pilbeam, Harvard University Henry Ford II
Professor of Social Sciences and Curator of Paleoanthropology in the Peabody
Museum of Archeology and Ethnology.
"The find itself is extraordinary, as were the enormous labours that went into the
reconstruction of a skeleton shattered almost beyond repair, and particularly the
skull," says Pilbeam, who is not connected with the study team, but was given
access to the embargoed material for comment prior to publication.
Pilbeam doesn't accept all the authors' conclusions, but he says the work "is one of
the most important discoveries for the study of human evolution."
Altogether, 47 different authors, in 11 detailed papers and further summaries
contributed to the study of Ardipithecus ramidus and its environment.
The primary authors include Tim White of the University of California, Berkeley;
Berhane Asfaw of Rift Valley Research Service in Addis Ababa; Giday WoldeGabriel
of Los Alamos National Laboratory; Gen Suwa of the University of Tokyo and C.
Owen Lovejoy of Kent State University.
The studies are available by logging onto --
Who was Ardipithecus ramidus?
-They were creatures with long arms, made for climbing, short legs and rigid feet
that enabled them to walk upright on the ground — despite having an opposable
toe. Yohannes Haile-Selassie, of the Cleveland Museum of Natural History, says
they would have probably walked only short distances, given that they had to 'toe
off' painfully on the second toe, unlike Lucy and all other hominids we know of who
pushed off on the big toe.
-The brain was small, similar to a chimp's. They stood about four feet and weighed
about 50 kilograms, with long-fingered, flexible hands and wrists and would have
moved about in the trees by walking along branches supporting their weight on
their palms.
-The pelvis had features that supported upright walking, as well, and the gluteal
muscles allowed it to walk without shifting the centre of mass from side to side,
researchers say.
-The males' canine teeth were similar in size to the females' — unlike modern apes
— and far smaller and blunter than those of the apes From that, researchers infer
the males were less aggressive than male chimps and gorillas, who use their large,
sharp canines in conflicts.
-Ardipithecus would have been as adept on the ground as in the trees that at that
time grew in the Afar Rift's then temperate climate. (The climate is inferred by
the presence of fossilized animals, wood, seeds and other plant materials from
such an environment, which also give clues to Ardipithecus's omnivorous diet.)
According to Haile-Selassie, even 150 years ago, Charles Darwin understood that
although chimps are our closest living relatives, we cannot look to them for clues
about our evolution, that there had to be a common ancestor.
Ardi isn't that ancestor but she is the closest to finding it that we have come so
far, the research shows.
Dolphins with a brain size equivalent to ours surely could have evolved into an underwater civilization instead of just chasing fish around forever.
