Ardi - human origins
last common ancestor evolution
The Ardi fossil was painstakingly recovered over a number of years
after November 1994 following on from an initial discovery of teeth and
small bones
from what appeared to be a hominid species that had been made in 1992.
The initial discovery and subsequent scientific recovery being made in
Ethiopia's Afar Rift, where erosion had exposed 4.4 million-year-old
clay
sediment, in what had been an ancient river valley
but was now a semi-arid desert region lying to the north and east of
Addis Ababa, the Ethiopian capital.
This is not the first time that a celebrated fossil of an early
hominid species had been discovered in Ethiopia as the very important
Lucy fossil, of an individual from a species
known to science as Australopithecus afarensis and dating from some 3.2
million years ago, was unearthed in 1974 only 74 kilometres from where
the fossil remains of Ardi were found.
This means that, in terms of inquiry into the course of possible human evolution / the origins of our species, the Ardi fossil
has displaced Lucy as being from the earliest "reliably known about" possible close human ancestor species.
According to Todd Disotell, an anthropologist at New York University's Center for the Study of Human Origins. "This
is as important as Lucy...we have a much better picture,
which allows us to improve upon our estimate of the last common
ancestor...Ardi brings us to within 1.6 million years of the last common
ancestor."
Similarly David Pilbeam, curator of paleoanthropology at Harvard's Peabody Museum of Archaeology and Ethnology, has said:-.
"This is one of the most important discoveries for the study of human
evolution...It is relatively complete in that it preserves head,
hands, feet and some critical parts in between.
It represents a genus plausibly ancestral to Australopithecus" and begins “to fill in the temporal and structural ‘space’ between the apelike common ancestor
and Australopithecus
- itself ancestral to our genus Homo."
Ardipithecus is seen as having had a mix of "primitive" traits, shared
with its predecessors, and "derived" traits, which it shares exclusively
with later hominids.
One of the authors of one of the scientific papers appearing in Science magazine has stated that the Ardi fossil is "so rife with anatomical surprises, that no one
could have imagined it without direct fossil evidence".
To quote Tim White of the University of California, Berkeley, a
co-leader of the Middle Awash research team that discovered and studied
the new Ardipithecus ramidus fossils:-
The problem was that these pieces had been very broken at the time
that they were deposited. They were in very bad shape. They were at that
point in time cemented into a very hard silky matrix so those had to be
removed from the field, encased in plaster jackets and hardened with a
consolidant. Very, very fragile.
It took us many, many years to clean the bones in the National Museum of
Ethiopia and then set about to restore this skeleton to its original
dimensions and form;
and then study it and compare it with all the other fossils that are
known from Africa and elsewhere, as well as with the modern age. This is
not an ordinary fossil.
"In Ardipithecus we have an unspecialized form that hasn't evolved very far in the direction of Australopithecus.
So when you go from head to toe, you're seeing a mosaic creature, that is neither
chimpanzee, nor is it human. It is Ardipithecus."
"Darwin said we have to be really careful. The only way we're really
going to know what this last common ancestor looked like is to go and
find it.
Well, we have not found it. But we have come closer than we have ever
come...And, just like Darwin appreciated, evolution of the ape lineages
and the human lineage has been going on independently since the time
those lines split,
since that last common ancestor we shared."
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Skull :
The face had a projecting muzzle, giving her an ape-like appearance. The
details of the bottom of the skull,
where nerves and blood vessels enter the brain, indicate that Ardi's
brain was positioned in a way similar to modern humans. Ardi's face and
her brain
were also relatively small at about the size of
those of a modern female chimpanzee.
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Teeth :
The skull features smallish, relatively human-like teeth. Her tooth
types, the associated tooth enamel thickness, and certain isotope
residues all point to a somwhat varied,
rather than a strictly fruit-eating diet.
The absence of long canine teeth, which could have been used for threat or fight, could indicate that Ardipithecus,
all of 4.4 million years ago, had adapted in-group social behaviours in which threat-displays were not reliant
on canine teeth. The fossil set shows that males were close in size to females again possibly suggesting something about
the dynamics of Ardipithecus' group life and pairing behaviours.
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Lower Back :
Ardi had a mobile lower back with more vertebrae than the stiff backs of arboreal African apes.
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Pelvis :
The pelvis and hip show the gluteal muscles were positioned so she could
walk upright whilst also being a capable climber of trees. The hipbone
includes structures unique to hominids like us,
but not found in modern apes.
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Hands and Wrists :
Ardi's hands feature a large and well-muscled opposable thumb.
Her wrists were flexible like ours, not stiff like those of chimpanzees
and gorillas,
which often support their heavy upper bodies whilst 'walking' on their
knuckles. Ardi lacked the powerful
upper torso and stiff wrists needed to swing from branch to branch as
easily
as modern apes but could easily cling
to trees with her hands and feet suggesting that Ardipithecus had to be a
careful climber and walked or rested on top of branches
placing weight as necessary on the palms of its hands.
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Feet :
Ardi had an opposable large toe, which was kept in alignment by a little bone, but which could have been useful
for gripping branches.
She also had a relatively stiff outer foot, more like ours than an ape's. This
rigidity and the guided alignment of the large toe suggests
a transition between tree climbing and upright walking.
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Sources :
Science, skeletal structure redrawn from sketch by J.H. Matternes
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Whereas Lucy had feet much like ours Ardi's foot had a long, curved big
toe that would help her grip tree limbs but the other four toes were
stiffer and flat which would have been helpful for walking
on two legs. Ardi's pelvis fits like the foot, is something of a hybrid
- the hip bones are low and curved and somewhat adapted to bi-pedalism,
or walking upright on two legs, without obviously shambling like a
chimp.
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A caricature of Darwin from an issue of
the satirical magazine The Hornet of 1871.
This being the year of the publication of
Darwin's Descent of Man wherein human origins
are suggested as being demonstrably due
to evolutionary processes over time.
Whilst Thomas Henry Huxley had already had
a book published on this theme the faith-views
of many in Queen Victoria's England, and beyond,
were further challenged by Darwin's new book.
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The Ardi fossil discovery undermines the previously prevailing idea in
terms of human origins that we evolved from chimpanzees, our closest
living genetic relative, and that our earliest ancestors
would therefore resemble a chimp. The research team now says that our
evolutionary paths branched from a yet more distant - last common
ancestor - dating from more than 6 million years ago.
This last common ancestor was probably somewhat hominid in form and
it was only after this branching that chimps and gorillas adapted the
large upper bodies, stiff wrists and short torsos that help them swing
through the trees.
According to a prominent member of the Ardi research team, Dr. C. Owen Lovejoy, an anthropologist at Kent State University:-
"People often think we evolved from apes, but no, apes in many ways
evolved from us, it has been a popular idea to think humans are modified
chimpanzees.
From studying Ardipithecus ramidus, or 'Ardi,' we learn that we cannot
understand or model human evolution from chimps and gorillas."
Ardi is “turning evolution on its head” ... “we’re going to have to rewrite
the textbooks on human origins.”
"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."
Other prominent researchers concur:-
"We can't say this species was a direct
ancestor of modern humans, so we have to be careful. But it suggests that the direction
of early hominids was away from the chimp."
Tim White
"People think that modern chimpanzees have not changed and therefore, our common ancestor looked like a chimp ...
As if it was only the human branch that evolved. In fact, Ardi is more primitive than a chimpanzee."
Tim White
"What is fascinating is that Ardipithecus is closer to our last common ancestor than is the modern chimpanzee."
Christoph Zollikofer - Professor of Anthropology at the University of Zurich
"This find is far more important than Lucy. It shows that the last common ancestor with chimps didn’t look like a chimp,
or a human, or some funny thing in between."
Alan Walker - a paleontologist from Penn State University
In terms of Human origins it may be that Ardi was tending towards our
branch of life, and was physically and behaviourally committed to
spending
much time walking upright on the ground.
"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,"
C. Owen Lovejoy
"She has opposable great toes and she has a pelvis that allows her to
negotiate tree branches rather well, so half of her life is spent in
the trees; she would have nested in trees and
occasionally fed in trees, but when she was on the ground she walked
upright pretty close to how you and I walk.
"
C. Owen Lovejoy
The researchers suggest that "whatever made Ardipethicus ramidus come
down from the trees had to be phenomenally powerful" in terms
of survival benefits that followed from habitual bipedalism.
Such survival benefits could come in terms of safety, the winning of
foods for nutrition or, perhaps, greater success in courtships and
breeding.
It seems that ground living bipedalism would not have been dramatically
safer in an environment where predators probably took a regular toll of
victims
so it follows that strong long-term survival advantages must have
flowed
in terms of nutrition or breeding success.
Ardipethicus ramidus lived all of 4.4 million years ago - this is way, way, back in time.
In terms of human evolution
Ardipithecus' body structure shows very limited objective or undisputable transition toward many uniquely human features.
Humans have unique feet, ankles and musculature, unique hip structure,
unique teeth and crania, totally unique cognitive abilities, a distinct
“gut structure,”
a full commitment to upright walking, unique vocal apparatus, a
“precipitous reduction of olfactory receptors", and an “unusually
energy-thirsty brain.”
"With Ardipithecus, we have to bear in mind this was a species that lived 4.4 million years ago,
and a lot has happened since then in human evolution, when it comes to behavior."
Tim White
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