Washington Post Staff Writer
Thursday, October 1, 2009 10:33 AM
Ardi lived 4.4 million years ago in the woodlands of East Africa. She
spent most of her time in the trees. She stood about four feet tall,
weighed 110 pounds, and had long arms, short legs, and a grasping big
toe that was perfect for clambering branch to branch. She ate in the
trees, raised her offspring in the trees, slept in the trees.
But sometimes she came down to the ground, and stood upright. She
could walk on two legs. She was, in a sense, taking baby steps on a
journey that would change the world.
"Ardi" is the nickname given to a remarkable, shattered skeleton that
an international team of scientists believes is a major breakthrough
in the study of human origins. The skeletal remains were painstakingly
recovered from the Ethiopian desert along with bones from at least 35
other members of a species scientists call Ardipithecus ramidus. The
15-year investigation of Ardipithecus culminated Thursday in the
publication of a raft of papers in the online edition of the journal
Science, as well as dual press conferences in Washington and Addis
Ababa, Ethiopia.
"This is huge. This is the biggest discovery really since the 'Lucy'
skeleton of the 1970s," said Carol Ward, a University of Missouri
paleoanthropologist who was not involved with the research but had
been given a preview so that she could offer an independent
assessment.
Human origins is a field with high stakes and small bones, and the
elaborate roll-out of the Ardipithecus research probably will trigger
debate about the message contained in fossils so fragile they had to
be excavated with dental picks and porcupine quills. If the scientists
who found Ardi are correct, she represents a transitional figure,
almost a hybrid -- a tree creature who could carry food in her arms as
she explored the woodland floor on two legs.
Ardi lived more than a million years before Lucy, the name given to a
3.2 million-year-old skeleton found in 1974 that is the best example
of Australopithecus afarensis, a small-brained primate that had fully
adapted to a bipedal life and had expanded its habitat beyond the
forest into the savannah of Africa.
Unlike Ardi, she lacked the grasping big toe that extends laterally
from the foot. Lucy's big toe pointed forward, aligned with the other
toes, and was used for propulsion. Ardi and Lucy had different teeth,
with Lucy's enlarged molars more adapted to a wide-ranging diet on the
savannah. But Ardi and Lucy had rather similar faces, skulls, hands,
and pelvises.
The scientists who found Ardi do not contend that Ardi necessarily
evolved into Lucy, or that Ardipithecus ramidus was necessarily a
direct human ancestor. The human family of primates could have
splintered into multiple species along the way, with some winding up
as genetic dead ends. If that were the case, Ardi would be more of a
distant cousin to human beings rather than a direct forebear.
"The individual, Ardi, that female individual, is she our ancestor?"
said Tim White, a University of California at Berkeley
paleoanthropologist who led the research team. "And the answer is,
probably not. If she didn't have any kids, tough luck, she's nobody's
ancestor."
The Ardi team, however, does make the case that the genus
Ardipithecus, which could have encompassed a number of species, is
ancestral to the genus Australopithecus. Thus the general body plan of
Ardi would evolve into the general body plan of Lucy, and on down the
line until the genus Homo appears.
"The Ardipithecus genus gave rise to Australiopithecus even though we
can't say exactly what species did. Maybe ramidus did. But certainly
something like ramidus did," White said.
White and colleagues found the first signs of Ardipithecus in 1994 in
what is known as the Middle Awash, a treeless desert that 4 million
years ago would have been much wetter, teeming with birds, reptiles,
primates and thickly covered with fig and palm trees. A key moment
came Nov. 5, 1994, when a Berkeley graduate student, Yohannes Haile-
Selassie of Ethiopia, found fragments of two finger bones. Further
digging turned up scraps of a pelvis, feet, hands, chips from a skull.
By January 1995 the scientists realized they'd found a paleontological
treasure, a partial skeleton, broken up and ravaged by time. This was
Ardi.
The scientists found scores of other specimens, from both males and
females, though the bones were for the most part scattered and
isolated. Although Ardipithecus quickly entered the paleontology
lexicon in the mid-1990s, and scientists knew that this was
potentially a major discovery, it was not until Thursday -- and after
some complaints by fellow scientists over how long the process was
taking -- that White and his colleagues produced a detailed
description of the species.
"Ardi tells us twice as much as Lucy did. We have hands and feet, a
more complete environment, a more complete skeleton, it's older, it's
more primitive, it shows us the process of transformation from common
ancestor to hominid," said C. Owen Lovejoy, an anthropologist at Kent
State University who was part of the Ardi team.
The origin of the human species via evolution from earlier primates is
beyond scientific dispute. Even when the fossil record of Africa was
virtually nonexistent, Charles Darwin argued that human beings
probably evolved from African primates. Field work over the past
century confirmed Darwin's hypothesis, which was bolstered further by
laboratory analysis of the genetic codes of humans, chimpanzees and
other primates. The fine details of human origin, however, has become
sketchier, and more subject to interpretation and debate, as the
researchers dig deeper into the past and the fossils become scarcer,
more fragmentary and in many cases more enigmatic.
Scientists continue to search for the "Last Common Ancestor,"
sometimes abbreviated as the LCA. This is the creature to which both
modern humans and modern chimpanzees can trace their ancestry. Many
scientists believe the common ancestor lived about 7 million years
ago. The new research on Ardi suggests that this ancestor didn't look
nearly as much like a modern chimpanzee as had been previously
suspected. Rather, the ancestor would have looked more like
Ardipithecus. This suggests that chimpanzees, far from being time
machines for visiting the distant past, have themselves evolved
significantly, including developing such skills as suspending from
branches and knuckle-walking.
"The common ancestor looked like Ardi. It's the chimp and gorilla that
have evolved enormously, not hominids. Hominids have concentrated
their evolution in two things -- upright walking and brain. Everything
else is pretty primitive," Lovejoy said.
In the Ardipithecus genus, the males are not significantly different
in size from the females. The males also lack the dagger-like teeth
that male chimps use to fight one another for access to ovulating
females. Lovejoy argues that this is a sign of a different social
organization. The males, he argues, pair-bonded with females, and
supplied them with food. The upright walking would have made food
transport easier. Lovejoy sees male parental investment in the
survival of offspring as a hallmark of the human lineage.
"The road to becoming human didn't start with a big brain. The road to
becoming human began with setting the social conditions that would
allow for the expansion of the big brain," Lovejoy said, reiterating a
hypothesis he developed long before the discovery of Ardi.
Andrew Hill, a Yale anthropologist, said he didn't think there was
enough evidence to support Lovejoy's conclusions about the domestic
relations of male and female Ardipithecines. But he said the newly
described hominid is a "very satisfactory animal" that "reinforces the
accumulating evidence that these things probably evolved and really
lived in woodland conditions rather than savannahs."
David Pilbeam, a Harvard paleontologist, noted that there has been
some impatience in the scientific community as White and his team
conducted the Ardipithecus analysis, but he suggested that the wait
was worth it: "This is an extraordinary achievement, of discovery,
recovery, reconstitution, description and analysis, which will keep
many others busy for at least another 15 years."
Ardi did not look like a human by any stretch. She had a small head
relative to her body size. There is no way to read her mind and
measure her sense of self, her awareness of her place in the universe.
But if the scientists are correct, her path in life proved to be
fruitful over time, and the planet witnessed the rise of a new animal
that could run on two legs, invent tools, tame fire, and perhaps
eventually -- with much digging and scraping -- decipher its own
origin.
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